Cellular Changes with Aging

The provided content discusses cellular diversity and aging. Cellular diversity refers to the difference in size and shape among cells in the human body. Cells can be classified into about 200 different cell types, and their shapes and sizes are related to their functions in the body. The content also explores the process of aging at the cellular level. Aging is a normal process that produces observable changes in structure and function, increasing vulnerability to environmental stress and disease. Factors such as telomeres, glucose, and free radicals contribute to the aging process. Additionally, the content touches on certain diseases, including Progeria and Werner Syndrome, which are characterized by rapid aging and accelerated cellular decline.

ANATOMY AND PHYSIOLOGY DEFINED
OBJECTIVE
• Define anatomy and physiology, and name several subspecialties of these sciences.
Two branches of science—anatomy and physiology—provide the foundation for understanding the body's
parts and functions. Anatomy (a‐NAT‐ō‐mē; ana‐ = up; ‐tomy = process of cutting) is the science of
body structures and the relationships among them. It was first studied by dissection (dis‐SEK‐shun; dis‐ =
apart; ‐section = act of cutting), the careful cutting apart of body structures to study their relationships. Today,
a variety of imaging techniques (see Table 1.3) also contribute to the advancement of anatomical knowledge.
Whereas anatomy deals with structures of the body, physiology (fiz′‐ē‐OL‐ō‐jē; physio‐ = nature; ‐logy =
study of) is the science of body functions—how the body parts work. Table 1.1 describes several subspecialties
of anatomy and physiology.
Table 1.1 Selected Subspecialties of Anatomy and Physiology
SUBSPECIALTIES OF ANATOMY STUDY OF
Embryology
(em'‐brē‐OL‐ō‐jē; embry‐ = embryo; ‐
logy = study of)
The first eight weeks of development after fertilization of a
human egg.
Developmental biology The complete development of an individual from fertilization
to death.
Cell biology Cellular structure and functions.
Histology
(hiss'‐TOL‐ō‐jē; hist‐ = tissue)
Microscopic structure of tissues.
Gross anatomy Structures that can be examined without a microscope.
Systemic anatomy Structure of specific systems of the body such as the nervous
or respiratory systems.
Regional anatomy Specific regions of the body such as the head or chest.
Surface anatomy Surface markings of the body to understand internal anatomy
through visualization and palpation (gentle touch).
Radiographic anatomy
(rā'‐dē‐ō‐GRAF‐ik; radio‐ = ray; ‐
graphic = to write)
Body structures that can be visualized with x‐rays.
Pathological anatomy Structural changes (gross to microscopic) associated with

(path'‐ō‐LOJ‐i‐kal; path‐ = disease) disease.
SUBSPECIALTIES OF
PHYSIOLOGY STUDY OF
Neurophysiology
(NOOR‐ō‐fiz‐ē‐ol'‐ō‐jē; neuro‐ = nerve)
Functional properties of nerve cells.
Endocrinology
(en'‐dō‐kri‐NOL‐ō‐jē; endo‐ = within; ‐
crin = secretion)
Hormones (chemical regulators in the blood) and how they
control body functions.
Cardiovascular physiology
(kar‐dē‐ō‐VAS‐kū‐lar; cardi‐ = heart; ‐
vascular = blood vessels)
Functions of the heart and blood vessels.
Immunology
(im'‐ū‐NOL‐ō‐jē; immun‐ = not
susceptible)
The body's defenses against disease‐causing agents.
Respiratory physiology
(RES‐pir‐a‐to'‐rē; respira‐ = to breathe)
Functions of the air passageways and lungs.
Renal physiology
(RĒ‐nal; ren‐ = kidney)
Functions of the kidneys.
Exercise physiology Changes in cell and organ functions due to muscular activity.
Pathophysiology
(PATH‐ō‐fiz‐ē‐ol'‐ō‐jē)
Functional changes associated with disease and aging.
Because structure and function are so closely related, you will learn about the human body by studying its
anatomy and physiology together. The structure of a part of the body often reflects its functions. For example,
the bones of the skull join tightly to form a rigid case that protects the brain. The bones of the fingers are more
loosely joined to allow a variety of movements. The walls of the air sacs in the lungs are very thin, permitting
rapid movement of inhaled oxygen into the blood. The lining of the urinary bladder is much thicker to prevent
the escape of urine into the pelvic cavity, yet its construction allows for considerable stretching.
Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.
1.2 LEVELS OF STRUCTURAL ORGANIZATION
AND BODY SYSTEMS

OBJECTIVES
• Describe the body's six levels of structural organization.
• List the 11 systems of the human body, representative organs present in each, and their general
functions.
The levels of organization of a language—letters, words, sentences, paragraphs, and so on—can be compared
to the levels of organization of the human body. Your exploration of the human body will extend from atoms
and molecules to the whole person. From the smallest to the largest, six levels of organization will help you to
understand anatomy and physiology: the chemical, cellular, tissue, organ, system, and organismal levels of
organization (Figure 1.1).
1. Chemical level. This very basic level can be compared to the letters of the alphabet and
includes atoms, the smallest units of matter that participate in chemical reactions, and molecules, two
or more atoms joined together. Certain atoms, such as carbon (C), hydrogen (H), oxygen (O), nitrogen
(N), phosphorus (P), calcium (Ca), and sulfur (S), are essential for maintaining life. Two familiar
molecules found in the body are deoxyribonucleic acid (DNA), the genetic material passed from one
generation to the next, and glucose, commonly known as blood sugar. Chapters 2 and 25 focus on the
chemical level of organization.
2. Cellular level. Molecules combine to form cells, the basic structural and functional units of an
organism that are composed of chemicals. Just as words are the smallest elements of language that
make sense, cells are the smallest living units in the human body. Among the many kinds of cells in
your body are muscle cells, nerve cells, and epithelial cells. Figure 1.1 shows a smooth muscle cell, one
of the three types of muscle cells in the body. The cellular level of organization is the focus of
Chapter 3.
3. Tissue level. Tissues are groups of cells and the materials surrounding them that work together to
perform a particular function, similar to the way words are put together to form sentences. There are
just four basic types of tissues in your body: epithelial tissue, connective tissue, muscular tissue, and
nervous tissue. Epithelial tissue covers body surfaces, lines hollow organs and cavities, and forms
glands. Connective tissue connects, supports, and protects body organs while distributing blood vessels
to other tissues. Muscular tissue contracts to make body parts move and generates heat. Nervous
tissue carries information from one part of the body to another through nerve impulses.
Chapter 4 describes the tissue level of organization in greater detail. Shown in Figure 1.1 is smooth
muscle tissue, which consists of tightly packed smooth muscle cells.
4. Organ level. At the organ level different types of tissues are joined together. Similar to the
relationship between sentences and paragraphs, organs are structures that are composed of two or
more different types of tissues; they have specific functions and usually have recognizable shapes.
Examples of organs are the stomach, skin, bones, heart, liver, lungs, and brain. Figure 1.1 shows how
several tissues make up the stomach. The stomach's outer covering is a layer of epithelial tissue and
connective tissue that reduces friction when the stomach moves and rubs against other organs.
Underneath are three layers of a type of muscular tissue called smooth muscle tissue, which contracts to
churn and mix food and then push it into the next digestive organ, the small intestine. The innermost
lining is an epithelial tissue layer that produces fluid and chemicals responsible for digestion in the
stomach.

5. System level. A system (or chapter in our language analogy) consists of related organs (paragraphs)
with a common function. An example of the system level, also called the organ‐system level, is the
digestive system, which breaks down and absorbs food. Its organs include the mouth, salivary glands,
pharynx (throat), esophagus (food tube), stomach, small intestine, large intestine, liver, gallbladder, and
pancreas. Sometimes an organ is part of more than one system. The pancreas, for example, is part of
both the digestive system and the hormone‐producing endocrine system.
6. Organismal level. An organism (OR‐ga‐nizm), any living individual, can be compared to a book in
our analogy. All the parts of the human body functioning together constitute the total organism.
Figure 1.1 Levels of structural organization in the human body.
The levels of structural organization are chemical, cellular, tissue, organ, system, and organismal. Kevin
Somerville; Rubberball Productions/Getty Images
Which level of structural organization is composed of two or more different types of
tissues that work together to perform a specific function?
In the chapters that follow, you will study the anatomy and physiology of the body systems. Table 1.2 lists the
components and introduces the functions of these systems. You will also discover that all body systems
influence one another. As you study each of the body systems in more detail, you will discover how they work
together to maintain health, provide protection from disease, and allow for reproduction of the human species.
Table 1.2 The Eleven Systems of the Human Body
INTEGUMENTARY SYSTEM (Chapter 5) SKELETAL SYSTEM (Chapters 6–9)
Components: Skin and associated structures, such
as hair, fingernails and toenails, sweat
glands, and oil glands.
Functions: Protects body; helps regulate body
temperature; eliminates some wastes; helps make
vitamin D; detects sensations such as touch, pain,
warmth, and cold.
Components: Bones and joints of the body and
their associated cartilages.
Functions: Supports and protects body; provides
surface area for muscle attachments; aids body
movements; houses cells that produce blood cells;
stores minerals and lipids (fats).
MUSCULAR SYSTEM (CHAPTERS 10, 11) NERVOUS SYSTEM (CHAPTERS 12–17)
Components: Specifically, skeletal muscle tissue—
muscle usually attached to bones (other muscle
tissues include smooth and cardiac).
Functions: Participates in body movements, such as
walking; maintains posture; produces heat.
Components: Brain, spinal cord, nerves, and
special sense organs, such as eyes and ears.
Functions: Generates action potentials (nerve
impulses) to regulate body activities; detects
changes in body's internal and external

environments, interprets changes, and responds by
causing muscular contractions or glandular
secretions.
ENDOCRINE SYSTEM (CHAPTER 18) CARDIOVASCULAR SYSTEM (CHAPTERS 19–21)
Components: Hormone‐producing glands (pineal
gland, hypothalamus, pituitary gland, thymus,
thyroid gland, parathyroid glands, adrenal
glands, pancreas, ovaries, and testes) and
hormone‐producing cells in several other organs.
Functions: Regulates body activities by releasing
hormones (chemical messengers transported in
blood from endocrine gland or tissue to target
organ).
Components: Blood, heart, and blood vessels.
Functions: Heart pumps blood through blood
vessels; blood carries oxygen and nutrients to cells
and carbon dioxide and wastes away from cells and
helps regulate acid–base balance, temperature, and
water content of body fluids; blood components
help defend against disease and repair damaged
blood vessels.
LYMPHATIC SYSTEM AND IMMUNITY (Chapter 22) RESPIRATORY SYSTEM (CHAPTER 23)
Components: Lymphatic fluid and vessels; spleen,
thymus, lymph nodes, and tonsils; cells that carry
out immune responses (B cells, T cells, and others).
Functions: Returns proteins and fluid to blood;
carries lipids from gastrointestinal tract to blood;
contains sites of maturation and proliferation of B
cells and T cells that protect against disease‐causing
microbes.
Components: Lungs and air passageways such as
the pharynx (throat), larynx (voice box), trachea
(windpipe), and bronchial tubes leading into and
out of lungs.
Functions: Transfers oxygen from inhaled air to
blood and carbon dioxide from blood to exhaled air;
helps regulate acid–base balance of body fluids; air
flowing out of lungs through vocal cords produces
sounds.
DIGESTIVE SYSTEM (CHAPTER 24) URINARY SYSTEM (CHAPTER 26)
Components: Organs of gastrointestinal tract, a
long tube that includes the mouth,
pharynx (throat), esophagus (food tube), stomach,
small and large intestines, and anus; also includes
accessory organs that assist in digestive processes,
such as salivary glands, liver,
gallbladder, and pancreas.
Functions: Achieves physical and chemical
breakdown of food; absorbs nutrients; eliminates
solid wastes.
Components: Kidneys, ureters, urinary
bladder, and urethra.
Functions: Produces, stores, and eliminates urine;
eliminates wastes and regulates volume and
chemical composition of blood; helps maintain the
acid–base balance of body fluids; maintains body's
mineral balance; helps regulate production of red
blood cells.

REPRODUCTIVE SYSTEMS (CHAPTER 28)
Components: Gonads (testes in males
and ovaries in females) and associated organs
(uterine tubes, uterus, vagina, and mammary
glands in females and epididymides, ductus
deferens, seminal vesicles, prostate, and penis in
males).
Functions: Gonads produce gametes (sperm or
oocytes) that unite to form a new organism; gonads
also release hormones that regulate reproduction
and other body processes; associated organs
transport and store gametes; mammary glands
produce milk.
DNA Illustrations
Examples
Anatomy Overview: The Integumentary System
Anatomy Overview: The Skeletal System
Anatomy Overview: The Muscular System
Anatomy Overview: The Nervous System
Anatomy Overview: The Endocrine System
Anatomy Overview: The Cardiovascular System
Anatomy Overview: The Lymphatic and Immune Systems
Anatomy Overview: The Respiratory System
Anatomy Overview: The Digestive System
Anatomy Overview: The Urinary System
Anatomy Overview: The Reproductive System
CLINICAL CONNECTION Noninvasive Diagnostic Techniques
Health‐care professionals and students of anatomy and physiology commonly use several noninvasive
diagnostic techniques to assess certain aspects of body structure and function. A noninvasive diagnostic
technique is one that does not involve insertion of an instrument or device through the skin or a body
opening. In inspection, the examiner observes the body for any changes that deviate from normal. For

example, a physician may examine the mouth cavity for evidence of disease. Following inspection, one
or more additional techniques may be employed. In palpation (pal‐PĀ‐shun; palp‐ = gently touching)
the examiner feels body surfaces with the hands. An example is palpating the abdomen to detect enlarged
or tender internal organs or abnormal masses. In auscultation (aws‐kul‐TĀ‐shun; auscult‐ = listening)
the examiner listens to body sounds to evaluate the functioning of certain organs, often using a
stethoscope to amplify the sounds. An example is auscultation of the lungs during breathing to check for
crackling sounds associated with abnormal fluid accumulation. In percussion (pur‐KUSH‐un; percus‐ =
beat through) the examiner taps on the body surface with the fingertips and listens to the resulting echo.
For example, percussion may reveal the abnormal presence of fluid in the lungs or air in the intestines. It
may also provide information about the size, consistency, and position of an underlying structure. An
understanding of anatomy is important for the effective application of most of these diagnostic
techniques.
1.3 CHARACTERISTICS OF THE LIVING HUMAN
ORGANISM
OBJECTIVES
• Define the important life processes of the human body.
• Define homeostasis and explain its relationship to interstitial fluid.
Basic Life Processes
Certain processes distinguish organisms, or living things, from nonliving things. Following are the six most
important life processes of the human body:
1. Metabolism (me‐TAB‐ō‐lizm) is the sum of all the chemical processes that occur in the body. One
phase of metabolism is catabolism (ka‐TAB‐ō‐lizm; catabol‐ = throwing down; ‐ism = a condition),
the breakdown of complex chemical substances into simpler components. The other phase of
metabolism is anabolism (a‐NAB‐ō‐lizm; anabol‐ = a raising up), the building up of complex chemical
substances from smaller, simpler components. For example, digestive processes catabolize (split)
proteins in food into amino acids. These amino acids are then used to anabolize (build) new proteins
that make up body structures such as muscles and bones.
2. Responsiveness is the body's ability to detect and respond to changes. For example, an increase in body
temperature during a fever represents a change in the internal environment (within the body), and
turning your head toward the sound of squealing brakes is a response to a change in the external
environment (outside the body) to prepare the body for a potential threat. Different cells in the body
respond to environmental changes in characteristic ways. Nerve cells respond by generating electrical
signals known as nerve impulses (action potentials). Muscle cells respond by contracting, which
generates force to move body parts.

3. Movement includes motion of the whole body, individual organs, single cells, and even tiny structures
inside cells. For example, the coordinated action of leg muscles moves your whole body from one place
to another when you walk or run. After you eat a meal that contains fats, your gallbladder contracts and
releases bile into the gastrointestinal tract to help digest them. When a body tissue is damaged or
infected, certain white blood cells move from the bloodstream into the affected tissue to help clean up
and repair the area. Inside the cell, various parts, such as secretory vesicles (see Figure 3.20), move
from one position to another to carry out their functions.
4. Growth is an increase in body size that results from an increase in the size of existing cells, an increase
in the number of cells, or both. In addition, a tissue sometimes increases in size because the amount of
material between cells increases. In a growing bone, for example, mineral deposits accumulate between
bone cells, causing the bone to grow in length and width.
5. Differentiation (dif′‐er‐en‐shē‐Ā‐shun) is the development of a cell from an unspecialized to a
specialized state. Such precursor cells, which can divide and give rise to cells that undergo
differentiation, are known as stem cells. As you will see later in the text, each type of cell in the body
has a specialized structure and function that differs from that of its precursor (ancestor) cells. For
example, red blood cells and several types of white blood cells all arise from the same unspecialized
precursor cells in red bone marrow. Also through differentiation, a single fertilized human egg (ovum)
develops into an embryo, and then into a fetus, an infant, a child, and finally an adult.
6. Reproduction (rē‐prō‐DUK‐shun) refers either to (1) the formation of new cells for tissue growth,
repair, or replacement, or (2) the production of a new individual. In humans, the former process occurs
continuously throughout life, which continues from one generation to the next through the latter
process, the fertilization of an ovum by a sperm cell.
When any one of the life processes ceases to occur properly, the result is death of cells and tissues, which may
lead to death of the organism. Clinically, loss of the heartbeat, absence of spontaneous breathing, and loss of
brain functions indicate death in the human body.
CLINICAL CONNECTION Autopsy
An autopsy (AW‐top‐sē = seeing with one's own eyes) or necropsy is a postmortem (after death)
examination of the body and dissection of its internal organs to confirm or determine the cause of death. An
autopsy can uncover the existence of diseases not detected during life, determine the extent of injuries, and
explain how those injuries may have contributed to a person's death. It also may provide more information
about a disease, assist in the accumulation of statistical data, and educate health‐care students. Moreover, an
autopsy can reveal conditions that may affect offspring or siblings (such as congenital heart defects).
Sometimes an autopsy is legally required, such as during a criminal investigation. It may also be useful in
resolving disputes between beneficiaries and insurance companies about the cause of death.
1.4 HOMEOSTASIS
OBJECTIVES

• Define homeostasis.
• Describe the components of a feedback system.
• Contrast the operation of negative and positive feedback systems.
• Explain how homeostatic imbalances are related to disorders.
Homeostasis (hō′‐mē‐ō‐STĀ‐sis; homeo‐ = sameness; ‐stasis = standing still) is the condition of
equilibrium (balance) in the body's internal environment due to the constant interaction of the body's many
regulatory processes. Homeostasis is a dynamic condition. In response to changing conditions, the body's
equilibrium can shift among points in a narrow range that is compatible with maintaining life. For example,
the level of glucose in blood normally stays between 70 and 110 milligrams of glucose per 100 milliliters of
blood.* Each structure, from the cellular level to the system level, contributes in some way to keeping the
internal environment of the body within normal limits.
Homeostasis and Body Fluids
An important aspect of homeostasis is maintaining the volume and composition of body fluids, dilute,
watery solutions containing dissolved chemicals that are found inside cells as well as surrounding them.
The fluid within cells is intracellular fluid (intra‐ = inside), abbreviated ICF. The fluid outside body cells
is extracellular fluid (extra‐ = outside), abbreviated ECF. The ECF that fills the narrow spaces between
cells of tissues is known as interstitial fluid (in′‐ter‐STISH‐al; inter‐ = between). As you progress with
your studies, you will learn that the ECF differs depending on where it occurs in the body: ECF within
blood vessels is termed blood plasma, within lymphatic vessels it is called lymph, in and around the brain
and spinal cord it is known as cerebrospinal fluid, in joints it is referred to as synovial fluid, and the ECF
of the eyes is called aqueous humor and vitreous body.
The proper functioning of body cells depends on precise regulation of the composition of the interstitial
fluid surrounding them. Because of this, interstitial fluid is often called the body's internal environment.
The composition of interstitial fluid changes as substances move back and forth between it and blood
plasma. Such exchange of materials occurs across the thin walls of the smallest blood vessels in the body,
the blood capillaries. This movement in both directions across capillary walls provides needed materials,
such as glucose, oxygen, ions, and so on, to tissue cells. It also removes wastes, such as carbon dioxide,
from interstitial fluid.
Control of Homeostasis
Homeostasis in the human body is continually being disturbed. Some disruptions come from the external
environment in the form of physical insults such as the intense heat of a hot summer day or a lack of
enough oxygen for that two‐mile run. Other disruptions originate in the internal environment, such as a
blood glucose level that falls too low when you skip breakfast. Homeostatic imbalances may also occur due
to psychological stresses in our social environment—the demands of work and school, for example. In most
cases the disruption of homeostasis is mild and temporary, and the responses of body cells quickly restore
balance in the internal environment. However, in some cases the disruption of homeostasis may be intense
and prolonged, as in poisoning, overexposure to temperature extremes, severe infection, or major surgery.
Fortunately, the body has many regulating systems that can usually bring the internal environment back into
balance. Most often, the nervous system and the endocrine system, working together or independently,
provide the needed corrective measures. The nervous system regulates homeostasis by sending electrical
signals known as nerve impulses (action potentials) to organs that can counteract changes from the

balanced state. The endocrine system includes many glands that secrete messenger molecules
called hormones into the blood. Nerve impulses typically cause rapid changes, but hormones usually work
more slowly. Both means of regulation, however, work toward the same end, usually through negative
feedback systems.
Examples
Animation: Communication, Regulation and Homeostasis
Feedback Systems
The body can regulate its internal environment through many feedback systems. A feedback
system or feedback loop is a cycle of events in which the status of a body condition is monitored,
evaluated, changed, remonitored, reevaluated, and so on. Each monitored variable, such as body
temperature, blood pressure, or blood glucose level, is termed a controlled condition. Any disruption that
changes a controlled condition is called a stimulus. A feedback system includes three basic components: a
receptor, a control center, and an effector (Figure 1.2).
1. A receptor is a body structure that monitors changes in a controlled condition and sends input to a
control center. This pathway is called an afferent pathway (AF‐er‐ent; af‐ = toward; ‐ferrent =
carried), since the information flows toward the control center. Typically, the input is in the form of
nerve impulses or chemical signals. For example, certain nerve endings in the skin sense
temperature and can detect changes, such as a dramatic drop in temperature.
2. A control center in the body, for example, the brain, sets the range of values within which a
controlled condition should be maintained (set point), evaluates the input it receives from receptors,
and generates output commands when they are needed. Output from the control center typically
occurs as nerve impulses, or hormones or other chemical signals. This pathway is called an efferent
pathway (EF‐er‐ent; ef‐ = away from), since the information flows away from the control center. In
our skin temperature example, the brain acts as the control center, receiving nerve impulses from the
skin receptors and generating nerve impulses as output.
3. An effector (e‐FEK‐tor) is a body structure that receives output from the control center and
produces a response or effect that changes the controlled condition. Nearly every organ or tissue in
the body can behave as an effector. When your body temperature drops sharply, your brain (control
center) sends nerve impulses (output) to your skeletal muscles (effectors). The result is shivering,
which generates heat and raises your body temperature.

Figure 1.2 Operation of a feedback system.
The dashed return arrow symbolizes negative feedback.
The three basic components of a feedback system are the receptor, control center, and effector.
Imagineering
What is the main difference between negative and positive feedback systems?
A group of receptors and effectors communicating with their control center forms a feedback system that
can regulate a controlled condition in the body's internal environment. In a feedback system, the response of
the system “feeds back” information to change the controlled condition in some way, either negating it
(negative feedback) or enhancing it (positive feedback).

Negative Feedback Systems
A negative feedback system reverses a change in a controlled condition. Consider the regulation of blood
pressure. Blood pressure (BP) is the force exerted by blood as it presses against the walls of blood vessels.
When the heart beats faster or harder, BP increases. If some internal or external stimulus causes blood
pressure (controlled condition) to rise, the following sequence of events occurs
(Figure 1.3). Baroreceptors (the receptors), pressure‐sensitive nerve cells located in the walls of certain
blood vessels, detect the higher pressure. The baroreceptors send nerve impulses (input) to the brain
(control center), which interprets the impulses and responds by sending nerve impulses (output) to the heart
and blood vessels (the effectors). Heart rate decreases and blood vessels dilate (widen), which cause BP to
decrease (response). This sequence of events quickly returns the controlled condition—blood pressure—to
normal, and homeostasis is restored. Notice that the activity of the effector causes BP to drop, a result that
negates the original stimulus (an increase in BP). This is why it is called a negative feedback system.

Figure 1.3 Homeostatic regulation of blood pressure by a negative feedback system.
Note that the response is fed back into the system, and the system continues to lower blood pressure until there is
a return to normal blood pressure (homeostasis).
If the response reverses the stimulus, a system is operating by negative feedback.
Imagineering

What would happen to heart rate if some stimulus caused blood pressure to decrease?
Would this occur by way of positive or negative feedback?
Examples
Animation: Negative Feedback Control of Blood Pressure
Animation: Negative Feedback Control of Temperature
Positive Feedback Systems
Unlike a negative feedback system, a positive feedback system tends to strengthen or reinforce a change
in one of the body's controlled conditions. In a positive feedback system, the response affects the controlled
condition differently than in a negative feedback system. The control center still provides commands to an
effector, but this time the effector produces a physiological response that adds to or reinforces the initial
change in the controlled condition. The action of a positive feedback system continues until it is interrupted
by some mechanism.
Normal childbirth provides a good example of a positive feedback system (Figure 1.4). The first
contractions of labor (stimulus) push part of the fetus into the cervix, the lowest part of the uterus, which
opens into the vagina. Stretch‐sensitive nerve cells (receptors) monitor the amount of stretching of the
cervix (controlled condition). As stretching increases, they send more nerve impulses (input) to the brain
(control center), which in turn releases the hormone oxytocin (output) into the blood. Oxytocin causes
muscles in the wall of the uterus (effector) to contract even more forcefully. The contractions push the fetus
farther down the uterus, which stretches the cervix even more. The cycle of stretching, hormone release,
and ever‐stronger contractions is interrupted only by the birth of the baby. Then, stretching of the cervix
ceases and oxytocin is no longer released.

Figure 1.4 Positive feedback control of labor contractions during birth of a baby.
The solid return arrow symbolizes positive feedback.

If the response enhances or intensifies the stimulus, a system is operating by positive feedback.
Imagineering
Why do positive feedback systems that are part of a normal physiological response
include some mechanism that terminates the system?
Examples
Animation: Positive Feedback Control of Labor
Another example of positive feedback is what happens to your body when you lose a great deal of blood.
Under normal conditions, the heart pumps blood under sufficient pressure to body cells to provide them
with oxygen and nutrients to maintain homeostasis. Upon severe blood loss, blood pressure drops and blood
cells (including heart cells) receive less oxygen and function less efficiently. If the blood loss continues,
heart cells become weaker, the pumping action of the heart decreases further, and blood pressure continues
to fall. This is an example of a positive feedback cycle that has serious consequences and may even lead to
death if there is no medical intervention. As you will see in Chapter 19, blood clotting is also an example of
a positive feedback system.
These examples suggest some important differences between positive and negative feedback systems.
Because a positive feedback system continually reinforces a change in a controlled condition, some event
outside the system must shut it off. If the action of a positive feedback system is not stopped, it can “run
away” and may even produce life‐threatening conditions in the body. The action of a negative feedback
system, by contrast, slows and then stops as the controlled condition returns to its normal state. Usually,
positive feedback systems reinforce conditions that do not happen very often, and negative feedback
systems regulate conditions in the body that remain fairly stable over long periods.
Homeostatic Imbalances
You've seen homeostasis defined as a condition in which the body's internal environment remains relatively
stable. The body's ability to maintain homeostasis gives it tremendous healing power and a remarkable
resistance to abuse. The physiological processes responsible for maintaining homeostasis are in large part
also responsible for your good health.
For most people, lifelong good health is not something that happens effortlessly. The many factors in this
balance called health include the following:
• The environment and your own behavior.
• Your genetic makeup.
• The air you breathe, the food you eat, and even the thoughts you think.
The way you live your life can either support or interfere with your body's ability to maintain homeostasis
and recover from the inevitable stresses life throws your way.
Many diseases are the result of years of poor health behavior that interferes with the body's natural drive to
maintain homeostasis. An obvious example is smoking‐related illness. Smoking tobacco exposes sensitive
lung tissue to a multitude of chemicals that cause cancer and damage the lung's ability to repair itself.
Because diseases such as emphysema and lung cancer are difficult to treat and are very rarely cured, it is

much wiser to quit smoking—or never start—than to hope a doctor can “fix” you once you are diagnosed
with a lung disease. Developing a lifestyle that works with, rather than against, your body's homeostatic
processes helps you maximize your personal potential for optimal health and well‐being.
As long as all of the body's controlled conditions remain within certain narrow limits, body cells function
efficiently, homeostasis is maintained, and the body stays healthy. Should one or more components of the
body lose their ability to contribute to homeostasis, however, the normal balance among all of the body's
processes may be disturbed. If the homeostatic imbalance is moderate, a disorder or disease may occur; if it
is severe, death may result.
Examples
Animation: Homeostatic Relationships
A disorder is any abnormality of structure or function. Disease is a more specific term for an illness
characterized by a recognizable set of signs and symptoms. A local disease affects one part or a limited
region of the body (for example, a sinus infection); a systemic disease affects either the entire body or
several parts of it (for example, influenza). Diseases alter body structures and functions in characteristic
ways. A person with a disease may experience symptoms, subjective changes in body functions that are not
apparent to an observer. Examples of symptoms are headache, nausea, and anxiety. Objective changes that
a clinician can observe and measure are called signs. Signs of disease can be either anatomical, such as
swelling or a rash, or physiological, such as fever, high blood pressure, or paralysis.
The science that deals with why, when, and where diseases occur and how they are transmitted among
individuals in a community is known as epidemiology (ep′‐i‐dē‐mē‐OL‐ō‐jē; epi‐ = upon; ‐demi =
people). Pharmacology (far′‐ma‐KOL‐ō‐jē; pharmac‐ = drug) is the science that deals with the effects and
uses of drugs in the treatment of disease.
CLINICAL CONNECTION Diagnosis of Disease
Diagnosis (di'‐ag‐NŌ‐sis; dia‐ = through; ‐gnosis = knowledge) is the science and skill of distinguishing
one disorder or disease from another. The patient's symptoms and signs, his or her medical history, a
physical exam, and laboratory tests provide the basis for making a diagnosis. Taking a medical
history consists of collecting information about events that might be related to a patient's illness. These
include the chief complaint (primary reason for seeking medical attention), history of present illness,
past medical problems, family medical problems, social history, and review of symptoms. A physical
examination is an orderly evaluation of the body and its functions. This process includes the noninvasive
techniques of inspection, palpation, auscultation, and percussion that you learned about earlier in the
chapter, along with measurement of vital signs (temperature, pulse, respiratory rate, and blood pressure),
and sometimes laboratory tests.
1.5 BASIC ANATOMICAL TERMINOLOGY
OBJECTIVES

• Describe the anatomical position.
• Relate the anatomical names and the corresponding common names for various regions of the human
body.
• Define the anatomical planes, anatomical sections, and directional terms used to describe the human
body.
• Outline the major body cavities, the organs they contain, and their associated linings.
Scientists and health‐care professionals use a common language of special terms when referring to body
structures and their functions. The language of anatomy they use has precisely defined meanings that allow us
to communicate clearly and precisely. For example, is it correct to say, “The wrist is above the fingers”? This
might be true if your upper limbs (described shortly) are at your sides. But if you hold your hands up above
your head, your fingers would be above your wrists. To prevent this kind of confusion, anatomists use a
standard anatomical position and a special vocabulary for relating body parts to one another.
Body Positions
Descriptions of any region or part of the human body assume that it is in a standard position of reference called
the anatomical position (an′‐a‐TOM‐i‐kal). In the anatomical position, the subject stands erect facing the
observer, with the head level and the eyes facing directly forward. The feet are flat on the floor and directed
forward, and the upper limbs are at the sides with the palms turned forward (Figure 1.5). In the anatomical
position, the body is upright. Two terms describe a reclining body. If the body is lying face down, it is in
the prone position. If the body is lying face up, it is in the supine position.

Figure 1.5
The anatomical position.
The anatomical names and corresponding common names (in parentheses) are indicated for specific body regions.
For example, the cephalic region is the head.
In the anatomical position, the subject stands erect facing the observer with the head level and the eyes
facing forward. The feet are flat on the floor and directed forward, and the upper limbs are at the sides with the palms
facing forward.

Molly Borman
What is the usefulness of defining one standard anatomical position?
Regional Names
The human body is divided into several major regions that can be identified externally. The principal regions
are the head, neck, trunk, upper limbs, and lower limbs (Figure 1.5). The head consists of the skull and face.
The skull encloses and protects the brain; the face is the front portion of the head that includes the eyes, nose,
mouth, forehead, cheeks, and chin. The neck supports the head and attaches it to the trunk. The trunk consists
of the chest, abdomen, and pelvis. Each upper limb attaches to the trunk and consists of the shoulder, armpit,
arm (portion of the limb from the shoulder to the elbow), forearm (portion of the limb from the elbow to the
wrist), wrist, and hand. Each lower limb also attaches to the trunk and consists of the buttock, thigh (portion of
the limb from the buttock to the knee), leg (portion of the limb from the knee to the ankle), ankle, and foot.
The groin is the area on the front surface of the body marked by a crease on each side, where the trunk
attaches to the thighs.
Examples
Real Anatomy Viewpoint: Major Regions of the Body
Figure 1.5 shows the anatomical and common names of major parts of the body. The anatomical term in the
form of an adjective for each part appears first and is followed in parentheses by the corresponding common
name. For example, if you receive a tetanus shot in your gluteal region, it is an injection in your buttock.
Because the anatomical term for a body part usually is based on a Greek or Latin word, it may look different
from the common name for the same part or area. For example, the Latin word for axilla (ak‐SIL‐a) is the
common name armpit. Thus, the axillary nerve is one of the nerves passing within the armpit. You will learn
more about the Greek and Latin word roots of anatomical and physiological terms as you read this book.
Directional Terms
To locate various body structures, anatomists use specific directional terms, words that describe the position
of one body part relative to another. Several directional terms are grouped in pairs that have opposite
meanings, such as anterior (front) and posterior (back). Exhibit 1.A and Figure 1.6 present the main directional
terms.
Exhibit 1.A Directional Terms (Figure 1.6)
OBJECTIVE
• Define each directional term used to describe the human body.
OVERVIEW
Most of the directional terms used to describe the relationship of one part of the body to another can be
grouped into pairs that have opposite meanings. For example, superior means toward the upper part of the
body, and inferior means toward the lower part of the body. It is important to understand that directional
terms have relative meanings; they make sense only when used to describe the position of one structure
relative to another. For example, your knee is superior to your ankle, even though both are located in the

inferior half of the body. Study the directional terms below and the example of how each is used. As you
read the examples, look at Figure 1.6 to see the location of each structure.
DIRECTIONAL TERM DEFINITION EXAMPLE OF USE
Superior (soo′‐PĒR‐ē‐or)
(cephalic or cranial)
Toward the head, or the upper part
of a structure.
The heart is superior to the
liver.
Inferior (in′‐FĒR‐ē‐or)
(caudal)
Away from the head, or the lower
part of a structure.
The stomach is inferior to the
lungs.
Anterior (an‐TĒR‐ē‐or)
(ventral)*
Nearer to or at the front of the
body.
The sternum (breastbone) is
anterior to the heart.
Posterior (pos‐TĒR‐ē‐or) (do
rsal)
Nearer to or at the back of the
body.
The esophagus (food tube) is
posterior to the trachea
(windpipe).
Medial (MĒ‐dē‐al) Nearer to the midline.† The ulna is medial to the
radius.
Lateral (LAT‐er‐al) Farther from the midline. The lungs are lateral to the
heart.
Intermediate (in′‐ter‐MĒ‐dē‐
at)
Between two structures. The transverse colon is
intermediate between the
ascending and descending
colons.
Ipsilateral (ip‐si‐LAT‐er‐al) On the same side of the body as
another structure.
The gallbladder and ascending
colon are ipsilateral.
Contralateral (CON‐tra‐lat‐
er‐al)
On the opposite side of the body
from another structure.
The ascending and descending
colons are contralateral.
Proximal (PROK‐si‐mal) Nearer to the attachment of a limb
to the trunk; nearer to the
origination of a structure.
The humerus (arm bone) is
proximal to the radius.
Distal (DIS‐tal) Farther from the attachment of a
limb to the trunk; farther from the
origination of a structure.
The phalanges (finger bones)
are distal to the carpals (wrist
bones).
Superficial (soo′‐per‐FISH‐
al) (external)
Toward or on the surface of the
body.
The ribs are superficial to the
lungs.
Deep (internal) Away from the surface of the
body.
The ribs are deep to the skin of
the chest and back.

Figure 1.6
Directional terms.
Directional terms precisely locate various parts of the body relative to one another. Kevin

Somerville/Imagineering
Is the radius proximal to the humerus? Is the esophagus anterior to the trachea? Are the
ribs superficial to the lungs? Is the urinary bladder medial to the ascending colon? Is the
sternum lateral to the descending colon?
Planes and Sections
You will also study parts of the body relative to planes, imaginary flat surfaces that pass through the body
parts (Figure 1.7). A sagittal plane (SAJ‐i‐tal; sagitt‐ = arrow) is a vertical plane that divides the body or an
organ into right and left sides. More specifically, when such a plane passes through the midline of the body or
an organ and divides it into equal right and left sides, it is called a midsagittal plane or a median
plane. The midline is an imaginary vertical line that divides the body into equal left and right sides. If the
sagittal plane does not pass through the midline but instead divides the body or an organ into unequal right and
left sides, it is called a parasagittal plane (para‐ = near). A frontal or coronal plane (kō‐RŌ‐nal; corona =
crown) divides the body or an organ into anterior (front) and posterior (back) portions. A transverse
plane divides the body or an organ into superior (upper) and inferior (lower) portions. Other names for a
transverse plane are a cross‐sectional or horizontal plane. Sagittal, frontal, and transverse planes are all at
right angles to one another. An oblique plane (ō‐BLĒK), by contrast, passes through the body or an organ at
an oblique angle (any angle other than a 90‐degree angle).

Figure 1.7 Planes through the human body.
Frontal, transverse, sagittal, and oblique planes divide the body in specific ways. DNA Illustrations
Which plane divides the heart into anterior and posterior portions?
When you study a body region, you often view it in section. A section is a cut of the body or one of its organs
made along one of the planes just described. It is important to know the plane of the section so you can
understand the anatomical relationship of one part to another. Figure 1.8a–c indicates how three different
sections—transverse, frontal, and midsagittal—provide different views of the brain.

Figure 1.8 Planes and sections through different parts of the brain.
The diagrams (left) show the planes, and the photographs (right) show the resulting sections. Note: The arrows in the
diagrams indicate the direction from which each section is viewed. This aid is used throughout the book to indicate
viewing perspectives.
Planes divide the body in various ways to produce sections.
Imagineering; (a) Dissection Shawn Miller; Photograph Mark Nielsen. (b) Dissection Shawn Miller; Photograph Mark
Nielsen. (c) Dissection Shawn Miller; Photograph Mark Nielsen
Which plane divides the brain into unequal right and left portions?

Body Cavities
Body cavities are spaces within the body that help protect, separate, and support internal organs. Bones,
muscles, ligaments, and other structures separate the various body cavities from one another. Here we discuss
several body cavities (Figure 1.9).
Figure 1.9
Body cavities.
The dashed line in (a) indicates the border between the abdominal and pelvic cavities.
The major cavities of the trunk are the thoracic and abdominopelvic cavities.
Imagineering
In which cavities are the following organs located: urinary bladder, stomach, heart, small
intestine, lungs, internal female reproductive organs, thymus, spleen, liver? Use the following
symbols for your responses: T = thoracic cavity, A = abdominal cavity, or P = pelvic cavity.
The cranial bones form a hollow space of the head called the cranial cavity (KRĀ‐nē‐al), which contains the
brain. The bones of the vertebral column (backbone) form the vertebral (spinal) canal (VER‐te‐bral), which

contains the spinal cord. The cranial cavity and vertebral canal are continuous with one another. Three layers
of protective tissue, the meninges (me‐NIN‐jēz), and a shock‐absorbing fluid surround the brain and spinal
cord.
The major body cavities of the trunk are the thoracic and abdominopelvic cavities. The thoracic cavity (thor‐
AS‐ik; thorac‐ = chest) or chest cavity (Figure 1.10) is formed by the ribs, the muscles of the chest, the
sternum (breastbone), and the thoracic portion of the vertebral column. Within the thoracic cavity are
the pericardial cavity (per′‐i‐KAR‐dē‐al; peri‐ = around; ‐cardial = heart), a fluid‐filled space that surrounds
the heart, and two fluid‐filled spaces called pleural cavities (PLOOR‐al; pleur‐ = rib or side), one around each
lung. The central part of the thoracic cavity is an anatomical region called the mediastinum (mē′‐dē‐as‐TĪ‐
num; media‐ = middle; ‐stinum = partition). It is between the lungs, extending from the sternum to the
vertebral column and from the first rib to the diaphragm (Figure 1.10a, b). The mediastinum contains all
thoracic organs except the lungs themselves. Among the structures in the mediastinum are the heart,
esophagus, trachea, thymus, and several large blood vessels that enter and exit the heart. The diaphragm (DĪ‐
a‐fram = partition or wall) is a dome‐shaped muscle that separates the thoracic cavity from the abdominopelvic
cavity.

Figure 1.10
The thoracic cavity.
The dashed lines indicate the borders of the mediastinum. Note: When transverse sections are viewed inferiorly (from
below), the anterior aspect of the body appears on top and the left side of the body appears on the right side of the
illustration.
The thoracic cavity contains three smaller cavities and the mediastinum.
What is the name of the cavity that surrounds the heart? Which cavities surround the lungs?
The abdominopelvic cavity (ab‐dom′‐i‐nō‐PEL‐vik; see Figure 1.9) extends from the diaphragm to the groin
and is encircled by the abdominal muscular wall and the bones and muscles of the pelvis. As the name
suggests, the abdominopelvic cavity is divided into two portions, even though no wall separates them
(Figure 1.11). The superior portion, the abdominal cavity (ab‐DOM‐i‐nal; abdomin‐ = belly), contains the
stomach, spleen, liver, gallbladder, small intestine, and most of the large intestine. The inferior portion,
the pelvic cavity (PEL‐vik; pelv‐ = basin), contains the urinary bladder, portions of the large intestine, and
internal organs of the reproductive system. Organs inside the thoracic and abdominopelvic cavities are
called viscera (VIS‐er‐a).
Figure 1.11 The abdominopelvic cavity.
The dashed lower line shows the approximate boundary between the abdominal and pelvic cavities.
The abdominopelvic cavity extends from the diaphragm to the groin.
Kevin Somerville

To which body systems do the organs shown here within the abdominal and pelvic
cavities belong? (Hint: Refer to Table 1.2.)
Thoracic and Abdominal Cavity Membranes
A membrane is a thin, pliable tissue that covers, lines, partitions, or connects structures. One example is a
slippery, double‐layered membrane called a serous membrane (SĒR‐us), which covers the viscera within the
thoracic and abdominal cavities and also lines the walls of the thorax and abdomen. The parts of a serous
membrane are (1) the parietal layer (pa‐RĪ‐e‐tal), a thin epithelium that lines the walls of the cavities, and (2)
the visceral layer (VIS‐er‐al), a thin epithelium that covers and adheres to the viscera within the cavities. A
small amount of lubricating fluid (serous fluid) between the two layers reduces friction, allowing the viscera to
slide somewhat during movements, such as when the lungs inflate and deflate during breathing.
The serous membrane of the pleural cavities is called the pleura (PLOO‐ra). The visceral pleura clings to the
surface of the lungs, and the anterior part of the parietal pleura lines the chest wall, covering the superior
surface of the diaphragm (see Figure 1.10a). In between is the pleural cavity, filled with a small amount of
lubricating fluid (see Figure 1.10). The serous membrane of the pericardial cavity is the pericardium (per′‐i‐
KAR‐dē‐um). The visceral pericardium covers the surface of the heart; the parietal pericardium lines the chest
wall. Between them is the pericardial cavity, filled with a small amount of lubricating fluid (see Figure 1.10).
The peritoneum (per‐i‐tō‐NĒ‐um) is the serous membrane of the abdominal cavity. The visceral
peritoneum covers the abdominal viscera, and the parietal peritoneum lines the abdominal wall, covering the
inferior surface of the diaphragm. Between them is the peritoneal cavity, which contains a small amount of
lubricating fluid. Most abdominal organs are located in the peritoneal cavity. Some are located between the
parietal peritoneum and the posterior abdominal wall. Such organs are said to be retroperitoneal (re′‐trō‐per‐i‐
tō‐NĒ‐al; retro‐ = behind). The kidneys, adrenal glands, pancreas, duodenum of the small intestine, ascending
and descending colons of the large intestine, and portions of the abdominal aorta and inferior vena cava are
retroperitoneal.
In addition to the major body cavities just described, you will also learn about other body cavities in later
chapters. These include the oral (mouth) cavity, which contains the tongue and teeth (see Figure 24.5);
the nasal cavity in the nose (see Figure 23.1); the orbital cavities (orbits), which contain the eyeballs (see
Figure 7.3); the middle ear cavities (middle ears), which contain small bones (see Figure 17.4); and
the synovial cavities, which are found in freely movable joints and contain synovial fluid (see Figure 9.3).
A summary of the major body cavities and their membranes is presented in the table included in Figure 1.9.
Examples
Anatomy Overview: Serous Membrane
Abdominopelvic Regions and Quadrants
To describe the location of the many abdominal and pelvic organs more easily, anatomists and clinicians use
two methods of dividing the abdominopelvic cavity into smaller areas. In the first method, two horizontal and
two vertical lines, aligned like a tic‐tac‐toe grid, partition this cavity into nine abdominopelvic
regions (Figure 1.12a,b). The top horizontal line, the subcostal line (sub‐ = under; costal = rib), is drawn just
inferior to the rib cage, across the inferior portion of the stomach; the bottom horizontal line,
the transtubercular line (trans‐too‐BER‐kū‐lar), is drawn just inferior to the tops of the hip bones. Two vertical
lines, the left and right midclavicular lines (mid‐kla‐VIK‐ū‐lar), are drawn through the midpoints of the
clavicles (collar bones), just medial to the nipples. The four lines divide the abdominopelvic cavity into a
larger middle section and smaller left and right sections. The names of the nine abdominopelvic regions
are right hypochondriac (hī′‐pō‐KON‐drē‐ak), epigastric (ep‐i‐GAS‐trik), left hypochondriac, right

lumbar, umbilical (um‐BIL‐i‐kal) left lumbar, right inguinal (iliac) (IN‐gwi‐nal), hypogastric
(pubic), and left inguinal (iliac).

Figure 1.12 Regions and quadrants of the abdominopelvic cavity.
The nine‐region designation is used for anatomical studies; the quadrant designation is used to locate the
site of pain, tumors, or some other abnormality. Kevin Somerville; (a) Andy Washnik
In which abdominopelvic region is each of the following found: most of the liver, ascending
colon, urinary bladder, and most of the small intestine? In which abdominopelvic quadrant
would pain from appendicitis (inflammation of the appendix) be felt?
The second method is simpler and divides the abdominopelvic cavity into quadrants (KWOD‐rantz; quad‐ =
one‐fourth), as shown in Figure 1.12c. In this method, a midsagittal line (the median line) and a transverse line
(the transumbilical line) are passed through the umbilicus (um‐BI‐li‐kus; umbilic‐ = navel) or belly button.
The names of the abdominopelvic quadrants are right upper quadrant (RUQ), left upper quadrant (LUQ),
right lower quadrant (RLQ), and left lower quadrant (LLQ). The nine‐region division is more widely used
for anatomical studies, and quadrants are more commonly used by clinicians for describing the site of
abdominopelvic pain, a tumor, or another abnormality.
1.6 MEDICAL IMAGING
OBJECTIVE
• Describe the principles and importance of medical imaging procedures in the evaluation of organ
functions and the diagnosis of disease.
Medical imaging refers to techniques and procedures used to create images of the human body. Various types
of medical imaging allow visualization of structures inside our bodies and are increasingly helpful for precise
diagnosis of a wide range of anatomical and physiological disorders. The grandparent of all medical imaging
techniques is conventional radiography (x‐rays), in medical use since the late 1940s. The newer imaging
technologies not only contribute to diagnosis of disease, but they also are advancing our understanding of
normal anatomy and physiology. Table 1.3 describes some commonly used medical imaging techniques. Other
imaging methods, such as cardiac catheterization, will be discussed in later chapters.
Table 1.3 Common Medical Imaging Procedures
RADIOGRAPHY
Procedure: A single barrage of x‐rays passes through the body, producing an image of interior structures on
x‐ray–sensitive film. The resulting two‐dimensional image is a radiograph (RĀ‐dē‐ō‐graf′), commonly
called an x‐ray.

Biophoto Associates/Photo Researchers
Comments: Relatively inexpensive, quick, and simple to perform; usually provides sufficient information for
diagnosis. X‐rays do not easily pass through dense structures, so bones appear white. Hollow structures, such
as the lungs, appear black. Structures of intermediate density, such as skin, fat, and muscle, appear as
varying shades of gray. At low doses, x‐rays are useful for examining soft tissues such as the
breast (mammography) and for determining bone density (bone densitometry).
Breast Cancer Unit, Kings College Hospital, London/Photo Researchers, Inc.

Zephyr/Photo Researchers, Inc.
It is necessary to use a substance called a contrast medium to make hollow or fluid‐filled structures visible
(appear white) in radiographs. X‐rays make structures that contain contrast media appear white. The medium
may be introduced by injection, orally, or rectally, depending on the structure to be imaged. Contrast x‐rays
are used to image blood vessels (angiography), the urinary system (intravenous urography), and the
gastrointestinal tract (barium contrast x‐ray).
Cardio‐Thoracic Centre, Freeman Hospital, Newcastle‐Upon‐Tyne/Photo Researchers, Inc.

CNRI/Science Photo Library/Photo Researchers, Inc.
Science Photo Library/Photo Researchers, Inc.
MAGNETIC RESONANCE IMAGING (MRI) DIFFUSION TENSOR IMAGING (DTI)
Procedure: The body is exposed to a high‐energy
magnetic field, which causes protons (small positive
particles within atoms, such as hydrogen) in body fluids
and tissues to arrange themselves in relation to the field.
Then a pulse of radio waves “reads” these ion patterns,
and a color‐coded image is assembled on a video monitor.
The result is a two‐ or three‐dimensional blueprint of
cellular chemistry.
Comments: Relatively safe, but cannot be used on
patients with metal in their bodies. Shows fine details for
soft tissues but not for bones. Most useful for
differentiating between normal and abnormal tissues.
Used to detect tumors and artery‐clogging fatty plaques;
reveal brain abnormalities; measure blood flow; and
detect a variety of musculoskeletal, liver, and kidney
disorders.
Procedure: Diffusion tensor imaging (DTI) is
a variation of MRI that tracks the movement of
water molecules along the length of axons, long
processes of nerve cells that form the white
matter in the brain. The white matter is
organized into bundles of axons called tracts
that connect one part of the brain with another.
DTI provides images of the brain's white matter.
Comments: Used in the diagnosis of addictions,
epilepsy, brain tumors, traumatic brain injury,
stroke, multiple sclerosis, and
neurodegenerative diseases.

Scott Camazine/Photo Researchers, Inc.
ISM/Phototake
COMPUTED TOMOGRAPHY (CT)
[formerly called computerized axial tomography
(CAT) scanning] CORONARY (CARDIAC) COMPUTED TOMOGRAPHY
ANGIOGRAPHY (CCTA) SCAN
Procedure: In this form of computer‐assisted
radiography, an x‐ray beam traces an arc at multiple
angles around a section of the body. The resulting
transverse section of the body, called a CT scan, is
shown on a video monitor.
Comments: Visualizes soft tissues and organs with
much more detail than conventional radiographs.
Differing tissue densities show up as various shades of
gray. Multiple scans can be assembled to build three‐
dimensional views of structures (described next).
Whole‐body CT scanning typically targets the torso
and appears to provide the most benefit in screening
for lung cancers, coronary artery disease, and kidney
cancers.
Procedure: In this form of computer‐assisted
radiography, an iodine‐containing contrast medium
is injected into a vein and a beta‐blocker is given to
decrease heart rate. Then, numerous x‐ray beams
trace an arc around the heart and a scanner detects
the x‐ray beams and transmits them to a computer,
which transforms the information into a three‐
dimensional image of the coronary blood vessels on
a monitor. The image produced is called a CCTA
scan and can be generated in less than 20 seconds.
Comments: Used primarily to determine if there
are any coronary artery blockages (for example,
atherosclerotic plaque or calcium) that may require
an intervention such as angioplasty or stent. The
CCTA scan can be rotated, enlarged, and moved at
any angle. The procedure can take thousands of
images of the heart within the time of a single
heartbeat, so it provides a great amount of detail
about the heart's structure and function.

Simon Fraser/Photo Researchers, Inc. ISM/Phototake
ULTRASOUND SCANNING
Procedure: High‐frequency sound waves produced by a
handheld wand reflect off body tissues and are detected
by the same instrument. The image, which may be still
or moving, is called a sonogram (SON‐ō‐gram) and is
shown on a video monitor.
Comments: Safe, noninvasive, painless, and uses no
dyes. Most commonly used to visualize the fetus during
pregnancy. Also used to observe the size, location, and
actions of organs and blood flow through blood
vessels (Doppler ultrasound).
Courtesy Andrew Joseph Tortora and Damaris Soler
POSITRON EMISSION TOMOGRAPHY (PET) RADIONUCLIDE SCANNING
Procedure: A substance that emits positrons
(positively charged particles) is injected into
the body, where it is taken up by tissues. The
collision of positrons with negatively charged
electrons in body tissues produces gamma rays
(similar to x‐rays) that are detected by gamma
cameras positioned around the subject. A
computer receives signals from the gamma
Procedure: A radionuclide (radioactive substance) is
introduced intravenously into the body and carried by the
blood to the tissue to be imaged. Gamma rays emitted by
the radionuclide are detected by a gamma camera outside
the subject, and the data are fed into a computer. The
computer constructs a radionuclide image and displays it in
color on a video monitor. Areas of intense color take up a
lot of the radionuclide and represent high tissue activity;

cameras and constructs a PET scan image,
displayed in color on a video monitor. The
PET scan shows where the injected substance
is being used in the body. In the PET scan
image shown here, the black and blue colors
indicate minimal activity; the red, orange,
yellow, and white colors indicate areas of
increasingly greater activity.
Comments: Used to study the physiology of
body structures, such as metabolism in the
brain or heart.
Department of Nuclear Medicine, Charing Cross Hospital /Photo
Researchers, Inc.
areas of less intense color take up smaller amounts of the
radionuclide and represent low tissue activity. Single‐
photon‐emission computerized tomography (SPECT)
scanning is a specialized type of radionuclide scanning
that is especially useful for studying the brain, heart, lungs,
and liver.
Comments: Used to study activity of a tissue or organ,
such as searching for malignant tumors in body tissue or
scars that may interfere with heart muscle activity.
SIU/Visuals Unlimited
Dept. of Nuclear Medicine, Charing Cross Hospital/Photo Researchers, Inc.
ENDOSCOPY

Procedure: Endoscopy involves the visual examination of the inside of
body organs or cavities using a lighted instrument with lenses called
an endoscope. The image is viewed through an eyepiece on the
endoscope or projected onto a monitor.
Comments: Examples include colonoscopy (used to examine the interior
of the colon, which is part of the large intestine), laparoscopy (used to
examine the organs within the abdominopelvic cavity),
and arthroscopy (used to examine the interior of a joint, usually the
knee).
©Camal/Phototake
Examples
Medical Image: Cardiac PET Scan
Medical Image: Radiographs of Common Fractures
Chapter Review and Resource Summary
Review
1.1 Anatomy and Physiology Defined
1. Anatomy is the science of body structures and the relationships among structures; physiology is
the science of body functions.
2. Dissection is the careful cutting apart of body structures to study their relationships.
3. Some subspecialties of anatomy are embryology, developmental biology, cell biology,
histology, gross anatomy, systemic anatomy, regional anatomy, surface anatomy, radiographic
anatomy, and pathological anatomy (see Table 1.1).
4. Some subspecialties of physiology are neurophysiology, endocrinology, cardiovascular
physiology, immunology, respiratory physiology, renal physiology, exercise physiology, and
pathophysiology (see Table 1.1).
1.2 Levels of Structural Organization and Body Systems

Review
1. The human body consists of six levels of structural organization: chemical, cellular, tissue,
organ, system, and organismal.
2. Cells are the basic structural and functional living units of an organism and the smallest living
units in the human body.
3. Tissues are groups of cells and the materials surrounding them that work together to perform a
particular function.
4. Organs are composed of two or more different types of tissues; they have specific functions and
usually have recognizable shapes.
5. Systems consist of related organs that have a common function.
6. An organism is any living individual.
7. Table 1.2 introduces the 11 systems of the human organism: the integumentary, skeletal,
muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary, and
reproductive systems.
1.3 Characteristics of the Living Human Organism
1. All organisms carry on certain processes that distinguish them from nonliving things.
2. Among the life processes in humans are metabolism, responsiveness, movement, growth,
differentiation, and reproduction.
1.4 Homeostasis
1. Homeostasis is a condition of equilibrium in the body's internal environment produced by the
interplay of all the body's regulatory processes.
2. Body fluids are dilute, watery solutions. Intracellular fluid (ICF) is inside cells, and
extracellular fluid (ECF) is outside cells. Plasma is the ECF within blood vessels. Interstitial
fluid is the ECF that fills spaces between tissue cells; because it surrounds all body cells,
interstitial fluid is called the body's internal environment.
3. Disruptions of homeostasis come from external and internal stimuli and psychological stresses.
When disruption of homeostasis is mild and temporary, responses of body cells quickly restore

Review
balance in the internal environment. If disruption is extreme, regulation of homeostasis may
fail.
4. Most often, the nervous and endocrine systems acting together or separately regulate
homeostasis. The nervous system detects body changes and sends nerve impulses to counteract
changes in controlled conditions. The endocrine system regulates homeostasis by secreting
hormones.
5. Feedback systems include three components: (1) Receptors monitor changes in a controlled
condition and send input to a control center (afferent pathway). (2) The control center sets the
value (set point) at which a controlled condition should be maintained, evaluates the input it
receives from receptors (efferent pathway), and generates output commands when they are
needed. (3) Effectors receive output from the control center and produce a response (effect)
that alters the controlled condition.
6. If a response reverses the original stimulus, the system is operating by negative feedback. If a
response enhances the original stimulus, the system is operating by positive feedback.
7. One example of negative feedback is the regulation of blood pressure. If a stimulus causes
blood pressure (controlled condition) to rise, baroreceptors (pressure‐sensitive nerve cells, the
receptors) in blood vessels send impulses (input) to the brain (control center). The brain sends
impulses (output) to the heart (effector). As a result, heart rate decreases (response) and blood
pressure decreases to normal (restoration of homeostasis).
8. One example of positive feedback occurs during the birth of a baby. When labor begins, the
cervix of the uterus is stretched (stimulus), and stretch‐sensitive nerve cells in the cervix
(receptors) send nerve impulses (input) to the brain (control center). The brain responds by
releasing oxytocin (output), which stimulates the uterus (effector) to contract more forcefully
(response). Movement of the fetus further stretches the cervix, more oxytocin is released, and
even more forceful contractions occur. The cycle is broken with the birth of the baby.
9. Disruptions of homeostasis—homeostatic imbalances—can lead to disorders, diseases, and
even death. A disorder is a general term for any abnormality of structure or function. A disease
is an illness with a definite set of signs and symptoms.
10. Symptoms are subjective changes in body functions that are not apparent to an observer; signs
are objective changes that can be observed and measured.
1.5 Basic Anatomical Terminology
1. Descriptions of any region of the body assume the body is in the anatomical position, in which
the subject stands erect facing the observer, with the head level and the eyes facing directly
forward. The feet are flat on the floor and directed forward, and the upper limbs are at the
sides, with the palms turned forward. A body lying facedown is prone; a body lying faceup is
supine.

Review
2. Regional names are terms given to specific regions of the body. The principal regions are the
head, neck, trunk, upper limbs, and lower limbs. Within the regions, specific body parts have
anatomical names and corresponding common names. Examples are thoracic (chest), nasal
(nose), and carpal (wrist).
3. Directional terms indicate the relationship of one part of the body to another.
Exhibit 1.A summarizes commonly used directional terms.
4. Planes are imaginary flat surfaces that are used to divide the body or organs to visualize
interior structures. A midsagittal plane divides the body or an organ into equal right and left
sides. A parasagittal plane divides the body or an organ into unequal right and left sides. A
frontal plane divides the body or an organ into anterior and posterior portions. A transverse
plane divides the body or an organ into superior and inferior portions. An oblique plane passes
through the body or an organ at an oblique angle.
5. Sections are cuts of the body or its organs made along a plane. They are named according to
the plane along which the cut is made and include transverse, frontal, and sagittal sections.
6. Figure 1.9 summarizes body cavities and their membranes. Body cavities are spaces in the
body that help protect, separate, and support internal organs. The cranial cavity contains the
brain, and the vertebral canal contains the spinal cord. The meninges are protective tissues that
line the cranial cavity and vertebral canal. The diaphragm separates the thoracic cavity from
the abdominopelvic cavity. Viscera are organs within the thoracic and abdominopelvic
cavities. A serous membrane lines the wall of the cavity and adheres to the viscera.
7. The thoracic cavity is subdivided into three smaller cavities: a pericardial cavity, which
contains the heart, and two pleural cavities, each of which contains a lung. The central part of
the thoracic cavity is an anatomical region called the mediastinum. It is located between the
pleural cavities, extending from the sternum to the vertebral column and from the first rib to
the diaphragm. It contains all thoracic viscera except the lungs.
8. The abdominopelvic cavity is divided into a superior abdominal and an inferior pelvic cavity.
Viscera of the abdominal cavity include the stomach, spleen, liver, gallbladder, small intestine,
and most of the large intestine. Viscera of the pelvic cavity include the urinary bladder,
portions of the large intestine, and internal organs of the reproductive system.
9. Serous membranes line the walls of the thoracic and abdominal cavities and cover the organs
within them. They include the pleura, associated with the lungs; the pericardium, associated
with the heart; and the peritoneum, associated with the abdominal cavity.
10. To describe the location of organs more easily, the abdominopelvic cavity is divided into nine
regions: right hypochondriac, epigastric, left hypochondriac, right lumbar, umbilical, left
lumbar, right inguinal (iliac), hypogastric (pubic), and left inguinal (iliac). To locate the site of
an abdominopelvic abnormality in clinical studies, the abdominopelvic cavity is divided into
quadrants: right upper quadrant (RUQ), left upper quadrant (LUQ), right lower quadrant
(RLQ), and left lower quadrant (LLQ).

Review
1.6 Medical Imaging
1. Medical imaging refers to techniques and procedures used to create images of the human body.
They allow visualization of internal structures to diagnose abnormal anatomy and deviations
from normal physiology.
2. Table 1.3 summarizes and illustrates several medical imaging techniques.
2.1 How Matter is Organized
OBJECTIVES
• Identify the main chemical elements of the human body.
• Describe the structures of atoms, ions, molecules, free radicals, and compounds.
Chemical Elements
Matter exists in three states: solid, liquid, and gas. Solids, such as bones and teeth, are compact
and have a definite shape and volume. Liquids, such as blood plasma, have a definite volume and
assume the shape of their container. Gases, like oxygen and carbon dioxide, have neither a
definite shape nor volume. All forms of matter—both living and nonliving—are made up of a
limited number of building blocks called chemical elements. Each element is a substance that
cannot be split into a simpler substance by ordinary chemical means. Scientists now recognize
117 elements. Of these, 92 occur naturally on Earth. The rest have been produced from the
natural elements using particle accelerators or nuclear reactors. Each element is designated by a
chemical symbol, one or two letters of the element's name in English, Latin, or another
language. Examples of chemical symbols are H for hydrogen, C for carbon, O for oxygen, N for
nitrogen, Ca for calcium, and Na for sodium (natrium = sodium).*
Twenty‐six different chemical elements normally are present in your body. Just four elements,
called the major elements, constitute about 96% of the body's mass: oxygen, carbon, hydrogen,
and nitrogen. Eight others, the lesser elements, contribute about 3.6% to the body's mass:
calcium, phosphorus (P), potassium (K), sulfur (S), sodium, chlorine (Cl), magnesium (Mg), and
iron (Fe). An additional 14 elements—the trace elements—are present in tiny amounts.
Together, they account for the remaining about 0.4% of the body's mass. Several trace elements
have important functions in the body. For example, iodine is needed to make thyroid hormones.

The functions of some trace elements are unknown. Table 2.1 lists the main chemical elements
of the human body.
Examples
Anatomy Overview: Common Biomolecules
Table 2.1 Main Chemical Elements in the Body
CHEMICAL
ELEMENT
(SYMBOL)
% OF
TOTAL
BODY
MASS SIGNIFICANCE
MAJOR
ELEMENTS
(about 96)
Oxygen (O) 65.0 Part of water and many organic (carbon‐containing)
molecules; used to generate ATP, a molecule used by cells to
temporarily store chemical energy.
Carbon (C) 18.5 Forms backbone chains and rings of all organic molecules:
carbohydrates, lipids (fats), proteins, and nucleic acids (DNA
and RNA).
Hydrogen (H) 9.5 Constituent of water and most organic molecules; ionized
form ( ) makes body fluids more acidic.
Nitrogen (N) 3.2 Component of all proteins and nucleic acids.
LESSER
ELEMENTS
(about 3.6)
Calcium (Ca) 1.5 Contributes to hardness of bones and teeth; ionized form (
) needed for blood clotting, release of some hormones,
contraction of muscle, and many other processes.
Phosphorus (P) 1.0 Component of nucleic acids and ATP; required for normal
bone and tooth structure.
Potassium (K) 0.35
Ionized form ( ) is the most plentiful cation (positively
charged particle) in intracellular fluid; needed to generate
action potentials.

Sulfur (S) 0.25 Component of some vitamins and many proteins.
Sodium (Na) 0.2
Ionized form ( ) is the most plentiful cation in
extracellular fluid; essential for maintaining water balance;
needed to generate action potentials.
Chlorine (Cl) 0.2
Ionized form ( ) is the most plentiful anion (negatively
charged particle) in extracellular fluid; essential for
maintaining water balance.
Magnesium (Mg) 0.1
Ionized form ( ) needed for action of many enzymes,
molecules that increase the rate of chemical reactions in
organisms.
Iron (Fe) 0.005
Ionized forms ( and ) are part of hemoglobin
(oxygen‐carrying protein in red blood cells) and some
enzymes.
TRACE
ELEMENTS
(about 0.4) Aluminum (Al), boron (B), chromium (Cr), cobalt (Co),
copper (Cu), fluorine (F), iodine (I), manganese (Mn),
molybdenum (Mo), selenium (Se), silicon (Si), tin (Sn),
vanadium (V), and zinc (Zn).
Structure of Atoms
Each element is made up of atoms, the smallest units of matter that retain the properties and
characteristics of the element. Atoms are extremely small. Two hundred thousand of the largest
atoms would fit on the period at the end of this sentence. Hydrogen atoms, the smallest atoms,
have a diameter less than 0.1 nanometer ( ), and the largest atoms are only five times larger.
Dozens of different subatomic particles compose individual atoms. However, only three types
of subatomic particles are important for understanding the chemical reactions in the human body:
protons, neutrons, and electrons (Figure 2.1). The dense central core of an atom is its nucleus.
Within the nucleus are positively charged protons ( ) and uncharged (neutral) neutrons (
). The tiny, negatively charged electrons ( ) move about in a large space surrounding
the nucleus. They do not follow a fixed path or orbit but instead form a negatively charged
“cloud” that envelops the nucleus (Figure 2.1a).
Figure 2.1 Two representations of the structure of an atom.
Electrons move about the nucleus, which contains neutrons and protons. (a) In the electron cloud
model of an atom, the shading represents the chance of finding an electron in regions outside the

nucleus. (b) In the electron shell model, filled circles represent individual electrons, which are
grouped into concentric circles according to the shells they occupy. Both models depict a carbon
atom, with six protons, six neutrons, and six electrons.
An atom is the smallest unit of matter that retains the properties and characteristics of its
element.
Imagineering
How are the electrons of carbon distributed between the first and second
electron shells?
Even though their exact positions cannot be predicted, specific groups of electrons are most
likely to move about within certain regions around the nucleus. These regions, called electron
shells, are depicted as simple circles around the nucleus. Because each electron shell can hold a
specific number of electrons, the electron shell model best conveys this aspect of atomic
structure (Figure 2.1b). The first electron shell (nearest the nucleus) never holds more than 2
electrons. The second shell holds a maximum of 8 electrons, and the third can hold up to 18
electrons. The electron shells fill with electrons in a specific order, beginning with the first shell.
For example, notice in Figure 2.2 that sodium (Na), which has 11 electrons total, contains 2
electrons in the first shell, 8 electrons in the second shell, and 1 electron in the third shell. The
most massive element present in the human body is iodine, which has a total of 53 electrons: 2 in
the first shell, 8 in the second shell, 18 in the third shell, 18 in the fourth shell, and 7 in the fifth
shell.
Figure 2.2 Atomic structures of several stable atoms.
The atoms of different elements have different atomic numbers because they have different
numbers of protons. Imagineering
Which four of these elements are present most abundantly in living
organisms?
The number of electrons in an atom of an element always equals the number of protons. Because
each electron and proton carries one charge, the negatively charged electrons and the positively
charged protons balance each other. Thus, each atom is electrically neutral; its total charge is
zero.
Examples
Animation: Atomic Structure and the Basis of Bonds
Atomic Number and Mass Number
The number of protons in the nucleus of an atom is an atom's atomic number. Figure 2.2 shows
that atoms of different elements have different atomic numbers because they have different
numbers of protons. For example, oxygen has an atomic number of 8 because its nucleus has 8
protons, and sodium has an atomic number of 11 because its nucleus has 11 protons.

The mass number of an atom is the sum of its protons and neutrons. Because sodium has 11
protons and 12 neutrons, its mass number is 23 (Figure 2.2). Although all atoms of one element
have the same number of protons, they may have different numbers of neutrons and thus
different mass numbers. Isotopes are atoms of an element that have different numbers of
neutrons and therefore different mass numbers. In a sample of oxygen, for example, most atoms
have 8 neutrons, and a few have 9 or 10, but all have 8 protons and 8 electrons. Most isotopes are
stable, which means that their nuclear structure does not change over time. The stable isotopes of
oxygen are designated 16O, 17O, and 18O (or O‐16, O‐17, and O‐18). As you may already have
determined, the numbers indicate the mass number of each isotope. As you will discover shortly,
the number of electrons of an atom determines its chemical properties. Although the isotopes of
an element have different numbers of neutrons, they have identical chemical properties because
they have the same number of electrons.
Certain isotopes called radioactive isotopes are unstable; their nuclei decay (spontaneously
change) into a stable configuration. Examples are H‐3, C‐14, O‐15, and O‐19. As they decay,
these atoms emit radiation—either subatomic particles or packets of energy—and in the process
often transform into a different element. For example, the radioactive isotope of carbon, C‐14,
decays to N‐14. The decay of a radioisotope may be as fast as a fraction of a second or as slow as
millions of years. The half‐life of an isotope is the time required for half of the radioactive atoms
in a sample of that isotope to decay into a more stable form. The half‐life of C‐14, which is used
to determine the age of organic samples, is about 5730 years; the half‐life of I‐131, an important
clinical tool, is 8 days.
CLINICAL CONNECTION Harmful and Beneficial Effects of Radiation
Radioactive isotopes may have either harmful or helpful effects. Their radiations can break
apart molecules, posing a serious threat to the human body by producing tissue damage and/or
causing various types of cancer. Although the decay of naturally occurring radioactive
isotopes typically releases just a small amount of radiation into the environment, localized
accumulations can occur. Radon‐222, a colorless and odorless gas that is a naturally occurring
radioactive breakdown product of uranium, may seep out of the soil and accumulate in
buildings. It is not only associated with many cases of lung cancer in smokers but has also
been implicated in many cases of lung cancer in nonsmokers. Beneficial effects of certain
radioisotopes include their use in medical imaging procedures to diagnose and treat certain
disorders. Some radioisotopes can be used as tracers to follow the movement of certain
substances through the body. Thallium‐201 is used to monitor blood flow through the heart
during an exercise stress test. Iodine‐131 is used to detect cancer of the thyroid gland and to
assess its size and activity, and may also be used to destroy part of an overactive thyroid
gland. Cesium‐137 is used to treat advanced cervical cancer, and iridium‐192 is used to treat
prostate cancer.
Atomic Mass
The standard unit for measuring the mass of atoms and their subatomic particles is a dalton, also
known as an atomic mass unit (amu). A neutron has a mass of 1.008 daltons, and a proton has a
mass of 1.007 daltons. The mass of an electron, at 0.0005 dalton, is almost 2000 times smaller
than the mass of a neutron or proton. The atomic mass (also called the atomic weight) of an
element is the average mass of all its naturally occurring isotopes. Typically, the atomic mass of
an element is close to the mass number of its most abundant isotope.

Ions, Molecules, and Compounds
As we discussed, atoms of the same element have the same number of protons. The atoms of
each element have a characteristic way of losing, gaining, or sharing their electrons when
interacting with other atoms to achieve stability. The way that electrons behave enables atoms in
the body to exist in electrically charged forms called ions, or to join with each other into complex
combinations called molecules. If an atom either gives up or gains electrons, it becomes an ion.
An ion is an atom that has a positive or negative charge because it has unequal numbers of
protons and electrons. Ionization is the process of giving up or gaining electrons. An ion of an
atom is symbolized by writing its chemical symbol followed by the number of its positive (+) or
negative (–) charges. Thus, stands for a calcium ion that has two positive charges because it
has lost two electrons.
When two or more atoms share electrons, the resulting combination is called a molecule (MOL‐
e‐kūl). A molecular formula indicates the elements and the number of atoms of each element that
make up a molecule. A molecule may consist of two atoms of the same kind, such as an oxygen
molecule (Figure 2.3a). The molecular formula for a molecule of oxygen is O2. The subscript 2
indicates that the molecule contains two atoms of oxygen. Two or more different kinds of atoms
may also form a molecule, as in a water molecule (H2O). In H2O one atom of oxygen shares
electrons with two atoms of hydrogen.
Figure 2.3 Atomic structures of an oxygen molecule and a superoxide free radical.
A free radical has an unpaired electron in its outermost electron shell. Imagineering
What substances in the body can inactivate oxygen‐derived free radicals?
A compound is a substance that contains atoms of two or more different elements. Most of the
atoms in the body are joined into compounds. Water (H2O) and sodium chloride (NaCl),
common table salt, are compounds. However, a molecule of oxygen (O2) is not a compound
because it consists of atoms of only one element.
A free radical is an atom or group of atoms with an unpaired electron in the outermost shell. A
common example is superoxide, which is formed by the addition of an electron to an oxygen
molecule (Figure 2.3b). Having an unpaired electron makes a free radical unstable, highly
reactive, and destructive to nearby molecules. Free radicals become stable by either giving up
their unpaired electron to, or taking on an electron from, another molecule. In so doing, free
radicals may break apart important body molecules.
CLINICAL CONNECTION Free Radicals and Antioxidants
There are several sources of free radicals, including exposure to ultraviolet radiation in
sunlight, exposure to x‐rays, and some reactions that occur during normal metabolic
processes. Certain harmful substances, such as carbon tetrachloride (a solvent used in dry
cleaning), also give rise to free radicals when they participate in metabolic reactions in the
body. Among the many disorders, diseases, and conditions linked to oxygen‐derived free
radicals are cancer, atherosclerosis, Alzheimer disease, emphysema, diabetes mellitus,

cataracts, macular degeneration, rheumatoid arthritis, and deterioration associated with aging.
Consuming more antioxidants—substances that inactivate oxygen‐derived free radicals—is
thought to slow the pace of damage caused by free radicals. Important dietary antioxidants
include selenium, zinc, beta‐carotene, and vitamins C and E. Red, blue, or purple fruits and
vegetables contain high levels of antioxidants.
2.2 Chemical Bonds
OBJECTIVES
• Describe how valence electrons form chemical bonds.
• Distinguish among ionic, covalent, and hydrogen bonds.
The forces that hold together the atoms of a molecule or a compound are chemical bonds. The
likelihood that an atom will form a chemical bond with another atom depends on the number of
electrons in its outermost shell, also called the valence shell. An atom with a valence shell
holding eight electrons is chemically stable, which means it is unlikely to form chemical bonds
with other atoms. Neon, for example, has eight electrons in its valence shell, and for this reason
it does not bond easily with other atoms. The valence shell of hydrogen and helium is the first
electron shell, which holds a maximum of two electrons. Because helium has two valence
electrons, it too is stable and seldom bonds with other atoms. Hydrogen, on the other hand, has
only one valence electron (see Figure 2.2), so it binds readily with other atoms.
The atoms of most biologically important elements do not have eight electrons in their valence
shells. Under the right conditions, two or more atoms can interact in ways that produce a
chemically stable arrangement of eight valence electrons for each atom. This chemical principle,
called the octet rule (octet = set of eight), helps explain why atoms interact in predictable ways.
One atom is more likely to interact with another atom if doing so will leave both with eight
valence electrons. For this to happen, an atom either empties its partially filled valence shell, fills
it with donated electrons, or shares electrons with other atoms. The way that valence electrons
are distributed determines what kind of chemical bond results. We will consider three types of
chemical bonds: ionic bonds, covalent bonds, and hydrogen bonds.
Examples
Animation: Chemical Bonding
Ionic Bonds
As you have already learned, when atoms lose or gain one or more valence electrons, ions are
formed. Positively and negatively charged ions are attracted to one another—opposites attract.
The force of attraction that holds together ions with opposite charges is an ionic bond. Consider
sodium and chlorine atoms, the components of common table salt. Sodium has one valence

electron (Figure 2.4a). If sodium loses this electron, it is left with the eight electrons in its second
shell, which becomes the valence shell. As a result, however, the total number of protons (11)
exceeds the number of electrons (10). Thus, the sodium atom has become a cation (KAT‐ī‐on),
or positively charged ion. A sodium ion has a charge of 1+ and is written Na+. By contrast,
chlorine has seven valence electrons (Figure 2.4b). If chlorine gains an electron from a
neighboring atom, it will have a complete octet in its third electron shell. After gaining an
electron, the total number of electrons (18) exceeds the number of protons (17), and the chlorine
atom has become an anion (AN‐ī‐on), a negatively charged ion. The ionic form of chlorine is
called a chloride ion. It has a charge of 1− and is written Cl−. When an atom of sodium donates
its sole valence electron to an atom of chlorine, the resulting positive and negative charges pull
both ions tightly together, forming an ionic bond (Figure 2.4c). The resulting compound is
sodium chloride, written NaCl.
In general, ionic compounds exist as solids, with an orderly, repeating arrangement of the ions,
as in a crystal of NaCl (Figure 2.4d). A crystal of NaCl may be large or small—the total number
of ions can vary—but the ratio of Na+ to Cl− is always 1:1. In the body, ionic bonds are found
mainly in teeth and bones, where they give great strength to these important structural tissues.
An ionic compound that breaks apart into positive and negative ions in solution is called an
electrolyte (e‐LEK‐trō‐līt). Most ions in the body are dissolved in body fluids as electrolytes, so
named because their solutions can conduct an electric current. (In Chapter 27 we will discuss the
chemistry and importance of electrolytes.) Table 2.2 lists the names and symbols of common
ions in the body.
Table 2.2 Common Ions in the Body
CATIONS ANIONS
NAME SYMBOL NAME SYMBOL
Hydrogen ion H+ Fluoride ion F−
Sodium ion Na+ Chloride ion Cl−
Potassium ion K+ Iodide ion I−
Ammonium ion NH4+ Hydroxide ion OH−
Magnesium ion Mg2+ Bicarbonate ion HCO3−
Calcium ion Ca2+ Oxide ion O2−
Iron(II) ion Fe2+ Sulfate ion SO42−
Iron(III) ion Fe3+ Phosphate ion PO43−
Figure 2.4 Ions and ionic bond formation.
(a) A sodium atom can have a complete octet of electrons in its outermost shell by losing one
electron. (b) A chlorine atom can have a complete octet by gaining one electron. (c) An ionic
bond may form between oppositely charged ions. (d) In a crystal of NaCl, each Na+ is

surrounded by six Cl−. In (a), (b), and (c), the electron that is lost or accepted is colored red.
An ionic bond is the force of attraction that holds together oppositely charged ions.
Imagineering
What are cations and anions?
Covalent Bonds
When a covalent bond forms, two or more atoms share electrons rather than gaining or losing
them. Atoms form a covalently bonded molecule by sharing one, two, or three pairs of valence
electrons. The larger the number of electron pairs shared between two atoms, the stronger the
covalent bond. Covalent bonds may form between atoms of the same element or between atoms
of different elements. They are the most common chemical bonds in the body, and the
compounds that result from them form most of the body's structures.
A single covalent bond results when two atoms share one electron pair. For example, a
molecule of hydrogen forms when two hydrogen atoms share their single valence electrons
(Figure 2.5a), which allows both atoms to have a full valence shell at least part of the time. A
double covalent bond results when two atoms share two pairs of electrons, as happens in an
oxygen molecule (Figure 2.5b). A triple covalent bond occurs when two atoms share three pairs
of electrons, as in a molecule of nitrogen (Figure 2.5c). Notice in the structural formulas for
covalently bonded molecules in Figure 2.5 that the number of lines between the chemical
symbols for two atoms indicates whether the bond is a single (─), double (═), or triple (≡)
covalent bond.
Figure 2.5 Covalent bond formation.
The red electrons are shared equally in (a)–(d) and unequally in (e). In writing the structural
formula of a covalently bonded molecule, each straight line between the chemical symbols for
two atoms denotes a pair of shared electrons. In molecular formulas, the number of atoms in each
molecule is noted by subscripts.
In a covalent bond, two atoms share one, two, or three pairs of valence electrons.
Imagineering
What is the principal difference between an ionic bond and a covalent bond?
The same principles of covalent bonding that apply to atoms of the same element also apply to
covalent bonds between atoms of different elements. The gas methane (CH4) contains covalent
bonds formed between the atoms of two different elements, one carbon and four hydrogens
(Figure 2.5d). The valence shell of the carbon atom can hold eight electrons but has only four of
its own. The single electron shell of a hydrogen atom can hold two electrons, but each hydrogen
atom has only one of its own. A methane molecule contains four separate single covalent bonds.
Each hydrogen atom shares one pair of electrons with the carbon atom.
In some covalent bonds, two atoms share the electrons equally—one atom does not attract the
shared electrons more strongly than the other atom. This type of bond is a nonpolar covalent

bond. The bonds between two identical atoms are always nonpolar covalent bonds (Figure 2.5a–
c). The bonds between carbon and hydrogen atoms are also nonpolar, such as the four C─H
bonds in a methane molecule (Figure 2.5d).
In a polar covalent bond, the sharing of electrons between two atoms is unequal—the nucleus of
one atom attracts the shared electrons more strongly than the nucleus of the other atom. When
polar covalent bonds form, the resulting molecule has a partial negative charge near the atom that
attracts electrons more strongly. This atom has greater electronegativity, the power to attract
electrons to itself. At least one other atom in the molecule then will have a partial positive
charge. The partial charges are indicated by a lowercase Greek delta with a minus or plus sign: δ−
or δ+. A very important example of a polar covalent bond in living systems is the bond between
oxygen and hydrogen in a molecule of water (Figure 2.5e); in this molecule, the nucleus of the
oxygen atom attracts the electrons more strongly than do the nuclei of the hydrogen atoms, so the
oxygen atom is said to have greater electronegativity. Later in the chapter, we will see how polar
covalent bonds allow water to dissolve many molecules that are important to life. Bonds between
nitrogen and hydrogen and those between oxygen and carbon are also polar bonds.
Hydrogen Bonds
The polar covalent bonds that form between hydrogen atoms and other atoms can give rise to a
third type of chemical bond, a hydrogen bond (Figure 2.6). A hydrogen bond forms when a
hydrogen atom with a partial positive charge (δ+) attracts the partial negative charge (δ−) of
neighboring electronegative atoms, most often larger oxygen or nitrogen atoms. Thus, hydrogen
bonds result from attraction of oppositely charged parts of molecules rather than from sharing of
electrons as in covalent bonds, or the loss or gain of electrons as in ionic bonds. Hydrogen bonds
are weak compared to ionic and covalent bonds. Thus, they cannot bind atoms into molecules.
However, hydrogen bonds do establish important links between molecules or between different
parts of a large molecule, such as a protein or nucleic acid (both discussed later in this chapter).
Figure 2.6
Hydrogen bonding among water molecules.
Each water molecule forms hydrogen bonds (indicated by dotted lines) with three to four
neighboring water molecules.
Hydrogen bonds occur because hydrogen atoms in one water molecule are attracted to the
partial negative charge of the oxygen atom in another water molecule.
Imagineering
Why would you expect ammonia (NH3) to form hydrogen bonds with water
molecules?
The hydrogen bonds that link neighboring water molecules give water considerable cohesion, the
tendency of like particles to stay together. The cohesion of water molecules creates a very high
surface tension, a measure of the difficulty of stretching or breaking the surface of a liquid. At
the boundary between water and air, water's surface tension is very high because the water
molecules are much more attracted to one another than they are attracted to molecules in the air.
This is readily seen when a spider walks on water or a leaf floats on water. The influence of
water's surface tension on the body can be seen in the way it increases the work required for
breathing. A thin film of watery fluid coats the air sacs of the lungs. So, each inhalation must

have enough force to overcome the opposing effect of surface tension as the air sacs stretch and
enlarge when taking in air.
Even though single hydrogen bonds are weak, very large molecules may contain thousands of
these bonds. Acting collectively, hydrogen bonds provide considerable strength and stability and
help determine the three‐dimensional shape of large molecules. As you will see later in this
chapter, a large molecule's shape determines how it functions.
2.3 Chemical Reactions
OBJECTIVES
• Define a chemical reaction.
• Describe the various forms of energy.
• Compare exergonic and endergonic chemical reactions.
• Describe the role of activation energy and catalysts in chemical reactions.
• Describe synthesis, decomposition, exchange, and reversible reactions.
A chemical reaction occurs when new bonds form or old bonds break between atoms. Chemical
reactions are the foundation of all life processes, and as we have seen, the interactions of valence
electrons are the basis of all chemical reactions. Consider how hydrogen and oxygen molecules
react to form water molecules (Figure 2.7). The starting substances—two H2 and one O2—are
known as the reactants. The ending substances—two molecules of H2O—are the products. The
arrow in the figure indicates the direction in which the reaction proceeds. In a chemical reaction,
the total mass of the reactants equals the total mass of the products. Thus, the number of atoms of
each element is the same before and after the reaction. However, because the atoms are
rearranged, the reactants and products have different chemical properties. Through thousands of
different chemical reactions, body structures are built and body functions are carried out. The
term metabolism refers to all the chemical reactions occurring in the body.
Figure 2.7 The chemical reaction between two hydrogen molecules (H2) and one oxygen
molecule (O2) to form two molecules of water (H2O).
Note that the reaction occurs by breaking old bonds and making new bonds.
The number of atoms of each element is the same before and after a chemical reaction.
Imagineering

Why does this reaction require two molecules of H2?
Forms of Energy and Chemical Reactions
Each chemical reaction involves energy changes. Energy (en‐ = in; ‐ergy = work) is the capacity
to do work. Two principal forms of energy are potential energy, energy stored by matter due to
its position, and kinetic energy, the energy associated with matter in motion. For example, the
energy stored in water behind a dam or in a person poised to jump down some steps is potential
energy. When the gates of the dam are opened or the person jumps, potential energy is converted
into kinetic energy. Chemical energy is a form of potential energy that is stored in the bonds of
compounds and molecules. The total amount of energy present at the beginning and end of a
chemical reaction is the same. Although energy can be neither created nor destroyed, it may be
converted from one form to another. This principle is known as the law of conservation of
energy. For example, some of the chemical energy in the foods we eat is eventually converted
into various forms of kinetic energy, such as mechanical energy used to walk and talk.
Conversion of energy from one form to another generally releases heat, some of which is used to
maintain normal body temperature.
Energy Transfer in Chemical Reactions
Chemical bonds represent stored chemical energy, and chemical reactions occur when new
bonds are formed or old bonds are broken between atoms. The overall reaction may either
release energy or absorb energy. Exergonic reactions (ex‐ = out) release more energy than they
absorb. By contrast, endergonic reactions (end‐ = within) absorb more energy than they release.
A key feature of the body's metabolism is the coupling of exergonic reactions and endergonic
reactions. Energy released from an exergonic reaction often is used to drive an endergonic one.
In general, exergonic reactions occur as nutrients, such as glucose, are broken down. Some of the
energy released may be trapped in the covalent bonds of adenosine triphosphate (ATP), which
we describe more fully later in this chapter. If a molecule of glucose is completely broken down,
the chemical energy in its bonds can be used to produce as many as 38 molecules of ATP. The
energy transferred to the ATP molecules is then used to drive endergonic reactions needed to
build body structures, such as muscles and bones. The energy in ATP is also used to do the
mechanical work involved in the contraction of muscle or the movement of substances into or
out of cells.
Activation Energy
Because particles of matter such as atoms, ions, and molecules have kinetic energy, they are
continuously moving and colliding with one another. A sufficiently forceful collision can disrupt
the movement of valence electrons, causing an existing chemical bond to break or a new one to
form. The collision energy needed to break the chemical bonds of the reactants is called the
activation energy of the reaction (Figure 2.8). This initial energy “investment” is needed to start
a reaction. The reactants must absorb enough energy for their chemical bonds to become
unstable and their valence electrons to form new combinations. Then, as new bonds form, energy
is released to the surroundings.
Figure 2.8 Activation energy.
Activation energy is the energy needed to break chemical bonds in the reactant molecules so

a reaction can start. Imagineering
Why is the reaction illustrated here exergonic?
Both the concentration of particles and the temperature influence the chance that a collision will
occur and cause a chemical reaction.
• Concentration. The more particles of matter present in a confined space, the greater the
chance that they will collide (think of people crowding into a subway car at rush hour). The
concentration of particles increases when more are added to a given space or when the
pressure on the space increases, which forces the particles closer together so that they
collide more often.
• Temperature. As temperature rises, particles of matter move about more rapidly. Thus,
the higher the temperature of matter, the more forcefully particles will collide, and the
greater the chance that a collision will produce a reaction.
Catalysts
As we have seen, chemical reactions occur when chemical bonds break or form after atoms, ions,
or molecules collide with one another. Body temperature and the concentrations of molecules in
body fluids, however, are far too low for most chemical reactions to occur rapidly enough to
maintain life. Raising the temperature and the number of reacting particles of matter in the body
could increase the frequency of collisions and thus increase the rate of chemical reactions, but
doing so could also damage or kill the body's cells.
Substances called catalysts solve this problem. Catalysts are chemical compounds that speed up
chemical reactions by lowering the activation energy needed for a reaction to occur (Figure 2.9).
The most important catalysts in the body are enzymes, which we will discuss later in this
chapter.
Figure 2.9 Comparison of energy needed for a chemical reaction to proceed with a catalyst
(blue curve) and without a catalyst (red curve).
Catalysts speed up chemical reactions by lowering the activation energy. Imagineering
Does a catalyst change the potential energies of the products and reactants?
A catalyst does not alter the difference in potential energy between the reactants and the
products. Rather, it lowers the amount of energy needed to start the reaction.
For chemical reactions to occur, some particles of matter—especially large molecules—not only
must collide with sufficient force, but they must hit one another at precise spots. A catalyst helps
to properly orient the colliding particles. Thus, they interact at the spots that make the reaction

happen. Although the action of a catalyst helps to speed up a chemical reaction, the catalyst itself
is unchanged at the end of the reaction. A single catalyst molecule can assist one chemical
reaction after another.
Types of Chemical Reactions
After a chemical reaction takes place, the atoms of the reactants are rearranged to yield products
with new chemical properties. In this section we will look at the types of chemical reactions
common to all living cells. Once you have learned them, you will be able to understand the
chemical reactions so important to the operation of the human body that are discussed throughout
the book.
Examples
Animation: Types of Reactions and Equilibrium
Synthesis Reactions—Anabolism
When two or more atoms, ions, or molecules combine to form new and larger molecules, the
processes are called synthesis reactions. The word synthesis means “to put together.” A
synthesis reaction can be expressed as follows:
One example of a synthesis reaction is the reaction between two hydrogen molecules and one
oxygen molecule to form two molecules of water (see Figure 2.7). Another example of a
synthesis reaction is the formation of ammonia from nitrogen and hydrogen:
All the synthesis reactions that occur in your body are collectively referred to as anabolism (a‐
NAB‐ō‐lizm). Overall, anabolic reactions are usually endergonic because they absorb more
energy than they release. Combining simple molecules like amino acids (discussed shortly) to
form large molecules such as proteins is an example of anabolism.
Decomposition Reactions—Catabolism
Decomposition reactions split up large molecules into smaller atoms, ions, or molecules. A
decomposition reaction is expressed as follows:
The decomposition reactions that occur in your body are collectively referred to as catabolism
(ka‐TAB‐ō‐lizm). Overall, catabolic reactions are usually exergonic because they release more
energy than they absorb. For instance, the series of reactions that break down glucose to pyruvic
acid, with the net production of two molecules of ATP, are important catabolic reactions in the
body. These reactions will be discussed in Chapter 25.
Exchange Reactions
Many reactions in the body are exchange reactions; they consist of both synthesis and
decomposition reactions. One type of exchange reaction works like this:

The bonds between A and B and between C and D break (decomposition), and new bonds then
form (synthesis) between A and D and between B and C. An example of an exchange reaction is
Notice that the ions in both compounds have “switched partners”: The hydrogen ion (H+) from
HCl has combined with the bicarbonate ion (HCO3−) from NaHCO3, and the sodium ion (Na+)
from NaHCO3 has combined with the chloride ion (Cl−) from HCl.
Reversible Reactions
Some chemical reactions proceed in only one direction, from reactants to products, as previously
indicated by the single arrows. Other chemical reactions may be reversible. In a reversible
reaction, the products can revert to the original reactants. A reversible reaction is indicated by
two half‐arrows pointing in opposite directions:
Some reactions are reversible only under special conditions:
In that case, whatever is written above or below the arrows indicates the condition needed for the
reaction to occur. In these reactions, AB breaks down into A and B only when water is added,
and A and B react to produce AB only when heat is applied. Many reversible reactions in the
body require catalysts called enzymes. Often, different enzymes guide the reactions in opposite
directions.
Oxidation–Reduction Reactions
You will learn in Chapter 25 that chemical reactions called oxidation–reduction reactions are
essential to life, since they are the reactions that break down food molecules to produce energy.
These reactions are concerned with the transfer of electrons between atoms and molecules.
Oxidation refers to the loss of electrons, and in the process the oxidized substance releases
energy. Reduction refers to the gain of electrons, and in the process the reduced substance gains
energy. Oxidation–reduction reactions are always parallel; when one substance is oxidized,
another is reduced at the same time. When a food molecule, such as glucose, is oxidized, the
energy produced is used by a cell to carry out its various functions.
2.4 Inorganic Compounds and Solutions
OBJECTIVES
• Describe the properties of water and those of inorganic acids, bases, and salts.
• Distinguish among solutions, colloids, and suspensions.

• Define pH and explain the role of buffer systems in homeostasis.
Most of the chemicals in your body exist in the form of compounds. Biologists and chemists
divide these compounds into two principal classes: inorganic compounds and organic
compounds. Inorganic compounds usually lack carbon and are structurally simple. Their
molecules also have only a few atoms and cannot be used by cells to perform complicated
biological functions. They include water and many salts, acids, and bases. Inorganic compounds
may have either ionic or covalent bonds. Water makes up 55–60% of a lean adult's total body
mass; all other inorganic compounds combined add 1–2%. Examples of inorganic compounds
that contain carbon are carbon dioxide (CO2), bicarbonate ion (HCO3−), and carbonic acid
(H2CO3). Organic compounds always contain carbon, usually contain hydrogen, and always
have covalent bonds. Most are large molecules and many are made up of long chains of carbon
atoms. Organic compounds make up the remaining 38–43% of the human body.
Water
Water is the most important and abundant inorganic compound in all living systems. Although
you might be able to survive for weeks without food, without water you would die in a matter of
days. Nearly all the body's chemical reactions occur in a watery medium. Water has many
properties that make it such an indispensable compound for life. We have already mentioned the
most important property of water, its polarity—the uneven sharing of valence electrons that
confers a partial negative charge near the one oxygen atom and two partial positive charges near
the two hydrogen atoms in a water molecule (see Figure 2.5e). This property alone makes water
an excellent solvent for other ionic or polar substances, gives water molecules cohesion (the
tendency to stick together), and allows water to resist temperature changes.
Examples
Anatomy Overview: Water
Water as a Solvent
In medieval times people searched in vain for a “universal solvent,” a substance that would
dissolve all other materials. They found nothing that worked as well as water. Although it is the
most versatile solvent known, water is not the universal solvent sought by medieval alchemists.
If it were, no container could hold it because it would dissolve all potential containers! What
exactly is a solvent? In a solution, a substance called the solvent dissolves another substance
called the solute. Usually there is more solvent than solute in a solution. For example, your sweat
is a dilute solution of water (the solvent) plus small amounts of salts (the solutes).
The versatility of water as a solvent for ionized or polar substances is due to its polar covalent
bonds and its bent shape, which allows each water molecule to interact with several neighboring
ions or molecules. Solutes that are charged or contain polar covalent bonds are hydrophilic
(hydro‐ = water; ‐philic = loving), which means they dissolve easily in water. Common examples
of hydrophilic solutes are sugar and salt. Molecules that contain mainly nonpolar covalent bonds,
by contrast, are hydrophobic (‐phobic = fearing). They are not very water soluble. Examples of
hydrophobic compounds include animal fats and vegetable oils.
To understand the dissolving power of water, consider what happens when a crystal of a salt
such as sodium chloride (NaCl) is placed in water (Figure 2.10). The electronegative oxygen

atom in water molecules attracts the sodium ions (Na+), and the electropositive hydrogen atoms
in water molecules attract the chloride ions (Cl−). Soon, water molecules surround and separate
Na+ and Cl− ions from each other at the surface of the crystal, breaking the ionic bonds that held
NaCl together. The water molecules surrounding the ions also lessen the chance that Na+ and Cl−
will come together and re‐form an ionic bond.
Figure 2.10 How polar water molecules dissolve salts and polar substances.
When a crystal of sodium chloride is placed in water, the slightly negative oxygen end (red) of
water molecules is attracted to the positive sodium ions (Na+), and the slightly positive hydrogen
portions (gray) of water molecules are attracted to the negative chloride ions (Cl−). In addition to
dissolving sodium chloride, water also causes it to dissociate, or separate into charged particles,
which is discussed shortly.
Water is a versatile solvent because its polar covalent bonds, in which electrons are shared
unequally, create positive and negative regions.
Imagineering
Table sugar (sucrose) easily dissolves in water but is not an electrolyte. Is it
likely that all the covalent bonds between atoms in table sugar are nonpolar
bonds? Why or why not?
The ability of water to form solutions is essential to health and survival. Because water can
dissolve so many different substances, it is an ideal medium for metabolic reactions. Water
enables dissolved reactants to collide and form products. Water also dissolves waste products,
which allows them to be flushed out of the body in the urine.
Examples
Animation: Water and Fluid Flow
Water in Chemical Reactions
Water serves as the medium for most chemical reactions in the body and participates as a
reactant or product in certain reactions. During digestion, for example, decomposition reactions
break down large nutrient molecules into smaller molecules by the addition of water molecules.
This type of reaction is called hydrolysis (hī‐DROL‐i‐sis; ‐lysis = to loosen or break apart).
Hydrolysis reactions enable dietary nutrients to be absorbed into the body. By contrast, when two
smaller molecules join to form a larger molecule in a dehydration synthesis reaction (de‐ =
from, down, or out; hydra‐ = water), a water molecule is one of the products formed. As you will
see later in the chapter, such reactions occur during synthesis of proteins and other large
molecules (for example, see Figure 2.21).
Thermal Properties of Water
In comparison to most substances, water can absorb or release a relatively large amount of heat
with only a modest change in its own temperature. For this reason, water is said to have a high
heat capacity. The reason for this property is the large number of hydrogen bonds in water. As
water absorbs heat energy, some of the energy is used to break hydrogen bonds. Less energy is

then left over to increase the motion of water molecules, which would increase the water's
temperature. The high heat capacity of water is the reason it is used in automobile radiators; it
cools the engine by absorbing heat without its own temperature rising to an unacceptably high
level. The large amount of water in the body has a similar effect: It lessens the impact of
environmental temperature changes, helping to maintain the homeostasis of body temperature.
Water also requires a large amount of heat to change from a liquid to a gas. Its heat of
vaporization is high. As water evaporates from the surface of the skin, it removes a large
quantity of heat, providing an important cooling mechanism.
Water as a Lubricant
Water is a major component of mucus and other lubricating fluids throughout the body.
Lubrication is especially necessary in the chest (pleural and pericardial cavities) and abdomen
(peritoneal cavity), where internal organs touch and slide over one another. It is also needed at
joints, where bones, ligaments, and tendons rub against one another. Inside the gastrointestinal
tract, mucus and other watery secretions moisten foods, which aids their smooth passage through
the digestive system.
Solutions, Colloids, and Suspensions
A mixture is a combination of elements or compounds that are physically blended together but
not bound by chemical bonds. For example, the air you are breathing is a mixture of gases that
includes nitrogen, oxygen, argon, and carbon dioxide. Three common liquid mixtures are
solutions, colloids, and suspensions.
Once mixed together, solutes in a solution remain evenly dispersed among the solvent molecules.
Because the solute particles in a solution are very small, a solution looks clear and transparent.
A colloid differs from a solution mainly because of the size of its particles. The solute particles
in a colloid are large enough to scatter light, just as water droplets in fog scatter light from a car's
headlight beams. For this reason, colloids usually appear translucent or opaque. Milk is an
example of a liquid that is both a colloid and a solution: The large milk proteins make it a
colloid, whereas calcium salts, milk sugar (lactose), ions, and other small particles are in
solution.
The solutes in both solutions and colloids do not settle out and accumulate on the bottom of the
container. In a suspension, by contrast, the suspended material may mix with the liquid or
suspending medium for some time, but eventually it will settle out. Blood is an example of a
suspension. When freshly drawn from the body, blood has an even, reddish color. After blood
sits for a while in a test tube, red blood cells settle out of the suspension and drift to the bottom
of the tube (see Figure 19.1a). The upper layer, the liquid portion of blood, appears pale yellow
and is called blood plasma. Blood plasma is both a solution of ions and other small solutes and a
colloid due to the presence of larger plasma proteins.
The concentration of a solution may be expressed in several ways. One common way is by a
mass per volume percentage, which gives the relative mass of a solute found in a given volume
of solution. For example, you may have seen the following on the label of a bottle of wine:
“Alcohol 14.1% by volume.” Another way expresses concentration in units of moles per liter
(mol/L), which relate to the total number of molecules in a given volume of solution. A mole is
the amount of any substance that has a mass in grams equal to the sum of the atomic masses of
all its atoms. For example, 1 mole of the element chlorine (atomic mass = 35.45) is 35.45 grams
and 1 mole of the salt sodium chloride (NaCl) is 58.44 grams (22.99 for Na + 35.45 for Cl). Just
as a dozen always means 12 of something, a mole of anything has the same number of particles:
6.023 × 1023. This huge number is called Avogadro's number. Thus, measurements of substances

that are stated in moles tell us about the numbers of atoms, ions, or molecules present. This is
important when chemical reactions are occurring because each reaction requires a set number of
atoms of specific elements. Table 2.3 describes these ways of expressing concentration.
Table 2.3 Percentage and Molarity
DEFINITION EXAMPLE
Percentage (mass per volume)
Number of grams of a substance
per 100 milliliters (mL) of
solution
To make a 10% NaCl solution, take 10 g of NaCl and
add water to make a total of 100 mL of solution.
Molarity: 5 moles (mol) per liter
A 1 molar (1 M) solution = 1
mole of a solute in 1 liter of
solution
To make a 1 molar (1 M) solution of NaCl, dissolve 1
mole of NaCl (58.44 g) in enough water to make a total
of 1 liter of solution.
Inorganic Acids, Bases, and Salts
When inorganic acids, bases, or salts dissolve in water, they dissociate (dis′‐sō‐sē‐ĀT); that is,
they separate into ions and become surrounded by water molecules. An acid (Figure 2.11a) is a
substance that dissociates into one or more hydrogen ions (H+) and one or more anions. Because
H+ is a single proton with one positive charge, an acid is also referred to as a proton donor. A
base, by contrast (Figure 2.11b), removes H+ from a solution and is therefore a proton acceptor.
Many bases dissociate into one or more hydroxide ions (OH−) and one or more cations.
Figure 2.11 Dissociation of inorganic acids, bases, and salts.
Dissociation is the separation of inorganic acids, bases, and salts into ions in a
solution. Imagineering
The compound CaCO3 (calcium carbonate) dissociates into a calcium ion
(Ca2+) and a carbonate ion (CO32‐). Is it an acid, a base, or a salt? What about
H2SO4, which dissociates into two H+ and one SO42‐?
Examples
Animation: Acids and Bases
A salt, when dissolved in water, dissociates into cations and anions, neither of which is H+ or
OH− (Figure 2.11c). In the body, salts such as potassium chloride are electrolytes that are
important for carrying electrical currents (ions flowing from one place to another), especially in
nerve and muscular tissues. The ions of salts also provide many essential chemical elements in
intracellular and extracellular fluids such as blood, lymph, and the interstitial fluid of tissues.

Acids and bases react with one another to form salts. For example, the reaction of hydrochloric
acid (HCl) and potassium hydroxide (KOH), a base, produces the salt potassium chloride (KCl)
and water (H2O). This exchange reaction can be written as follows:
Acid–Base Balance: The Concept of pH
To ensure homeostasis, intracellular and extracellular fluids must contain almost balanced
quantities of acids and bases. The more hydrogen ions (H+) dissolved in a solution, the more
acidic the solution; the more hydroxide ions (OH−), the more basic (alkaline) the solution. The
chemical reactions that take place in the body are very sensitive to even small changes in the
acidity or alkalinity of the body fluids in which they occur. Any departure from the narrow limits
of normal H+ and OH− concentrations greatly disrupts body functions.
A solution's acidity or alkalinity is expressed on the pH scale, which extends from 0 to 14
(Figure 2.12). This scale is based on the concentration of H+ in moles per liter. A pH of 7 means
that a solution contains one ten‐millionth (0.0000001) of a mole of hydrogen ions per liter. The
number 0.0000001 is written as in scientific notation, which indicates that the number is 1
with the decimal point moved seven places to the left. To convert this value to pH, the negative
exponent (–7) is changed to a positive number (7). A solution with a H+ concentration of 0.0001
(10−4) moles per liter has a pH of 4; a solution with a H+ concentration of 0.000000001 (10−9)
moles per liter has a pH of 9; and so on. It is important to realize that a change of one whole
number on the pH scale represents a tenfold change in the number of H+. A pH of 6 denotes 10
times more H+ than a pH of 7, and a pH of 8 indicates 10 times fewer H+ than a pH of 7 and 100
times fewer H+ than a pH of 6.
Figure 2.12 The pH scale.
A pH below 7 indicates an acidic solution—more H+ than OH−. A pH above 7 indicates a basic
(alkaline) solution; that is, there are more OH− than H+.
The lower the numerical value of the pH, the more acidic is the solution because the H+
concentration becomes progressively greater. The higher the pH, the more basic the solution.
Imagineering
At pH 7 (neutrality), the concentrations of H+ and OH− are equal (10−7
mol/liter). What are the concentrations of H+ and OH− at pH 6? Which pH is
more acidic, 6.82 or 6.91? Which pH is closer to neutral, 8.41 or 5.59?
The midpoint of the pH scale is 7, where the concentrations of H+ and OH− are equal. A
substance with a pH of 7, such as pure water, is neutral. A solution that has more H+ than OH− is
an acidic solution and has a pH below 7. A solution that has more OH− than H+ is a basic
(alkaline) solution and has a pH above 7.
Maintaining pH: Buffer Systems

Although the pH of body fluids may differ, as we have discussed, the normal limits for each fluid
are quite narrow. Table 2.4 shows the pH values for certain body fluids along with those of some
common substances outside the body. Homeostatic mechanisms maintain the pH of blood
between 7.35 and 7.45, which is slightly more basic than pure water. You will learn in Chapter
27 that if the pH of blood falls below 7.35, a condition called acidosis occurs, and if the pH rises
above 7.45, it results in a condition called alkalosis; both conditions can seriously compromise
homeostasis. Saliva is slightly acidic, and semen is slightly basic. Because the kidneys help
remove excess acid from the body, urine can be quite acidic.
Table 2.4 pH Values of Selected Substances
SUBSTANCE* pH VALUE
•Gastric juice (found in the stomach) 1.2–3.0
Lemon juice 2.3
Vinegar 3.0
Carbonated soft drink 3.0–3.5
Orange juice 3.5
•Vaginal fluid 3.5–4.5
Tomato juice 4.2
Coffee 5.0
•Urine 4.6–8.0
•Saliva 6.35–6.85
Milk 6.8
Distilled (pure) water 7.0
•Blood 7.35–7.45
•Semen (fluid containing sperm) 7.20–7.60
•Cerebrospinal fluid (fluid associated with nervous system) 7.4
•Pancreatic juice (digestive juice of the pancreas) 7.1–8.2
•Bile (liver secretion that aids fat digestion) 7.6–8.6
Milk of magnesia 10.5
Lye (sodium hydroxide) 14.0

Even though strong acids and bases are continually taken into and formed by the body, the pH of
fluids inside and outside cells remains almost constant. One important reason is the presence of
buffer systems, which function to convert strong acids or bases into weak acids or bases. Strong
acids (or bases) ionize easily and contribute many H+ (or OH−) to a solution. Therefore, they can
change pH drastically, which can disrupt the body's metabolism. Weak acids (or bases) do not
ionize as much and contribute fewer H+ (or OH−). Hence, they have less effect on the pH. The
chemical compounds that can convert strong acids or bases into weak ones are called buffers.
They do so by removing or adding protons (H+).
One important buffer system in the body is the carbonic acid–bicarbonate buffer system.
Carbonic acid (H2CO3) can act as a weak acid, and the bicarbonate ion (HCO3−) can act as a weak
base. Hence, this buffer system can compensate for either an excess or a shortage of H+. For
example, if there is an excess of H+ (an acidic condition), HCO3− can function as a weak base and
remove the excess H+, as follows:
If there is a shortage of H+ (an alkaline condition), by contrast, H2CO3 can function as a weak
acid and provide needed H+ as follows:
2.5 Organic Compounds
OBJECTIVES
• Describe the functional groups of organic molecules.
• Identify the building blocks and functions of carbohydrates, lipids, and proteins.
• Describe the structure and functions of deoxyribonucleic acid (DNA), ribonucleic acid
(RNA), and adenosine triphosphate (ATP).
Many organic molecules are relatively large and have unique characteristics that allow them to
carry out complex functions. Important categories of organic compounds include carbohydrates,
lipids, proteins, nucleic acids, and adenosine triphosphate (ATP).
Carbon and Its Functional Groups
Carbon has several properties that make it particularly useful to living organisms. For one thing,
it can form bonds with one to thousands of other carbon atoms to produce large molecules that
can have many different shapes. Due to this property of carbon, the body can build many
different organic compounds, each of which has a unique structure and function. Moreover, the
large size of most carbon‐containing molecules and the fact that some do not dissolve easily in
water make them useful materials for building body structures.

Organic compounds are usually held together by covalent bonds. Carbon has four electrons in its
outermost (valence) shell. It can bond covalently with a variety of atoms, including other carbon
atoms, to form rings and straight or branched chains. Other elements that most often bond with
carbon in organic compounds are hydrogen, oxygen, and nitrogen. Sulfur and phosphorus are
also present in organic compounds. The other elements listed in Table 2.1 are present in a
smaller number of organic compounds.
The chain of carbon atoms in an organic molecule is called the carbon skeleton. Many of the
carbons are bonded to hydrogen atoms, yielding a hydrocarbon. Also attached to the carbon
skeleton are distinctive functional groups, other atoms or molecules bound to the hydrocarbon
skeleton. Each type of functional group has a specific arrangement of atoms that confers
characteristic chemical properties on the organic molecule attached to it. Table 2.5 lists the most
common functional groups of organic molecules and describes some of their properties. Because
organic molecules often are big, there are shorthand methods for representing their structural
formulas. Figure 2.13 shows two ways to indicate the structure of the sugar glucose, a molecule
with a ring‐shaped carbon skeleton that has several hydroxyl groups attached.
Table 2.5 Major Functional Groups of Organic Molecules
NAME AND
STRUCTURAL
FORMULA*
OCCURRENCE AND SIGNIFICANCE
Alcohols contain an ─OH group, which is polar and hydrophilic due to
its electronegative O atom. Molecules with many ─OH groups dissolve
easily in water.
Thiols have an ─SH group, which is polar and hydrophilic due to its
electronegative S atom. Certain amino acids (for example, cysteine)
contain ─SH groups, which help stabilize the shape of proteins.
Ketones contain a carbonyl group within the carbon skeleton. The
carbonyl group is polar and hydrophilic due to its electronegative O
atom.
Aldehydes have a carbonyl group at the end of the carbon skeleton.
Carboxylic acids contain a carboxyl group at the end of the carbon
skeleton. All amino acids have a ─COOH group at one end. The
negatively charged form predominates at the pH of body cells and is
hydrophilic.
Esters predominate in dietary fats and oils and also occur in our body as
triglycerides. Aspirin is an ester of salicylic acid, a pain‐relieving
molecule found in the bark of the willow tree.
Phosphates contain a phosphate group (─PO42−), which is very
hydrophilic due to the dual negative charges. An important example is
adenosine triphosphate (ATP), which transfers chemical energy

between organic molecules during chemical reactions.
Amines have an ─NH2 group, which can act as a base and pick up a
hydrogen ion, giving the amino group a positive charge. At the pH of
body fluids, most amino groups have a charge of 1+. All amino acids
have an amino group at one end.
Figure 2.13 Alternative ways to write the structural formula for glucose.
In standard shorthand, carbon atoms are understood to be at locations where two bond lines
intersect, and single hydrogen atoms are not indicated. Imagineering
How many hydroxyl groups does a molecule of glucose have? How many
carbon atoms are part of glucose's carbon skeleton?
Small organic molecules can combine into very large molecules that are called macromolecules
(macro‐ = large). Macromolecules are usually polymers (poly‐ = many; ‐mers = parts). A
polymer is a large molecule formed by the covalent bonding of many identical or similar small
building‐block molecules called monomers (mono‐ = one). Usually, the reaction that joins two
monomers is a dehydration synthesis. In this type of reaction, a hydrogen atom is removed from
one monomer and a hydroxyl group is removed from the other to form a molecule of water (see
Figure 2.15a). Macromolecules such as carbohydrates, lipids, proteins, and nucleic acids are
assembled in cells via dehydration synthesis reactions.
Molecules that have the same molecular formula but different structures are called isomers (Ī‐so‐
merz; iso‐ = equal or the same). For example, the molecular formulas for the sugars glucose and
fructose are both C6H12O6. The individual atoms, however, are positioned differently along the
carbon skeleton (see Figure 2.15a), giving the sugars different chemical properties.
Carbohydrates
Carbohydrates include sugars, glycogen, starches, and cellulose. Even though they are a large
and diverse group of organic compounds and have several functions, carbohydrates represent
only 2–3% of your total body mass. In humans and animals, carbohydrates function mainly as a
source of chemical energy for generating ATP needed to drive metabolic reactions. Only a few
carbohydrates are used for building structural units. One example is deoxyribose, a type of sugar
that is a building block of deoxyribonucleic acid (DNA), the molecule that carries inherited
genetic information.
Carbon, hydrogen, and oxygen are the elements found in carbohydrates. The ratio of hydrogen to
oxygen atoms is usually 2:1, the same as in water. Although there are exceptions, carbohydrates
generally contain one water molecule for each carbon atom. This is the reason they are called
carbohydrates, which means “watered carbon.” The three major groups of carbohydrates, based
on their sizes, are monosaccharides, disaccharides, and polysaccharides (Table 2.6).
Table 2.6 Major Carbohydrate Groups
TYPE OF CARBOHYDRATE EXAMPLES

Monosaccharides (simple sugars that
contain from 3 to 7 carbon atoms)
Glucose (the main blood sugar).
Fructose (found in fruits).
Galactose (in milk sugar).
Deoxyribose (in DNA).
Ribose (in RNA).
Disaccharides (simple sugars fromed
from the combination of two
monosaccharides by dehydration
synthesis)
Sucrose (table sugar) = glucose + fructose.
Lactose (milk sugar) = glucose + galactose.
Maltose = glucose + glucose.
Polysaccharides (from tens to
hundreds of monosaccharides joined
by dehydration synthesis)
Glycogen (stored form of carbohydrates in animals).
Starch (stored form of carbohydrates in plants and
main carbohydrates in food). Cellulose (part of cell
walls in plants that cannot be digested by humans but
aids movement of food through intestines).
Examples
Anatomy Overview: Carbohydrates
Monosaccharides and Disaccharides: The Simple Sugars
Monosaccharides and disaccharides are known as simple sugars. The monomers of
carbohydrates, monosaccharides (mon′‐ō‐SAK‐a‐rīds; sacchar‐ = sugar), contain from three to
seven carbon atoms. They are designated by names ending in “‐ose” with a prefix that indicates
the number of carbon atoms. For example, monosaccharides with three carbons are called trioses
(tri‐ = three). There are also tetroses (four‐carbon sugars), pentoses (five‐carbon sugars), hexoses
(six‐carbon sugars), and heptoses (seven‐carbon sugars). Examples of pentoses and hexoses are
illustrated in Figure 2.14. Cells throughout the body break down the hexose glucose to produce
ATP.
Figure 2.14 Monosaccharides.
The structural formulas of selected monosaccharides are shown.
Monosaccharides are the monomers used to build carbohydrates.
Imagineering
Which of these monosaccharides are hexoses?
A disaccharide (dī‐SAK‐a‐rīd; di‐ = two) is a molecule formed from the combination of two
monosaccharides by dehydration synthesis (Figure 2.15). For example, molecules of the
monosaccharides glucose and fructose combine to form a molecule of the disaccharide sucrose
(table sugar), as shown in Figure 2.15a. Glucose and fructose are isomers. As you learned earlier
in the chapter, isomers have the same molecular formula, but the relative positions of the oxygen

and carbon atoms are different, causing the compounds to have different chemical properties.
Notice that the formula for sucrose is C12H22O11, not C12H24O12, because a molecule of water is
removed as the two monosaccharides are joined.
Figure 2.15 Disaccharides.
(a) The structural and molecular formulas for the monosaccharides glucose and fructose and the
disaccharide sucrose. In dehydration synthesis (read from left to right), two smaller molecules,
glucose and fructose, are joined to form a larger molecule of sucrose. Note the loss of a water
molecule. In hydrolysis (read from right to left), the addition of a water molecule to the larger
sucrose molecule breaks the disaccharide into two smaller molecules, glucose and fructose.
Shown in (b) and (c) are the structural formulas of the disaccharides lactose and maltose,
respectively.
A disaccharide consists of two monosaccharides that have combined by dehydration
synthesis.
Imagineering
How many carbon atoms are there in fructose? In sucrose?
Disaccharides can also be split into smaller, simpler molecules by hydrolysis. A molecule of
sucrose, for example, may be hydrolyzed into its components, glucose and fructose, by the
addition of water. Figure 2.15a also illustrates this reaction.
CLINICAL CONNECTION Artificial Sweeteners
Some individuals use artificial sweeteners to limit their sugar consumption for medical
reasons, while others do so to avoid calories that might result in weight gain. Examples of
artificial sweeteners include aspartame (trade names NutraSweet® and Equal®), saccharin
(Sweet 'N Low®), and sucralose (Splenda®). Aspartame is 200 times sweeter than sucrose and
it adds essentially no calories to the diet because only small amounts of it are used to produce
a sweet taste. Saccharin is about 400 times sweeter than sucrose, and sucralose is 600 times
sweeter than sucrose. Both saccharin and sucralose have zero calories because they pass
through the body without being metabolized. Artificial sweeteners are also used as sugar
substitutes because they do not cause tooth decay. In fact, studies have shown that using
artificial sweeteners in the diet helps reduce the incidence of dental cavities.
Polysaccharides
The third major group of carbohydrates is the polysaccharides (pol′‐ē‐SAK‐a‐rīds). Each
polysaccharide molecule contains tens or hundreds of monosaccharides joined through
dehydration synthesis reactions. Unlike simple sugars, polysaccharides usually are insoluble in
water and do not taste sweet. The main polysaccharide in the human body is glycogen, which is
made entirely of glucose monomers linked to one another in branching chains (Figure 2.16). A
limited amount of carbohydrates is stored as glycogen in the liver and skeletal muscles. Starches
are polysaccharides formed from glucose by plants. They are found in foods such as pasta and
potatoes and are the major carbohydrates in the diet. Like disaccharides, polysaccharides such as

glycogen and starches can be broken down into monosaccharides through hydrolysis reactions.
For example, when the blood glucose level falls, liver cells break down glycogen into glucose
and release it into the blood, making it available to body cells, which break it down to synthesize
ATP. Cellulose is a polysaccharide formed from glucose by plants that cannot be digested by
humans but does provide bulk to help eliminate feces.
Figure 2.16 Part of a glycogen molecule, the main polysaccharide in the human body.
Glycogen is made up of glucose monomers and is the stored form of carbohydrate in the
human body. Imagineering
Which body cells store glycogen?
Lipids
A second important group of organic compounds is lipids (lip‐ = fat). Lipids make up 18–25%
of body mass in lean adults. Like carbohydrates, lipids contain carbon, hydrogen, and oxygen.
Unlike carbohydrates, they do not have a 2:1 ratio of hydrogen to oxygen. The proportion of
electronegative oxygen atoms in lipids is usually smaller than in carbohydrates, so there are
fewer polar covalent bonds. As a result, most lipids are insoluble in polar solvents such as water;
they are hydrophobic. Because they are hydrophobic, only the smallest lipids (some fatty acids)
can dissolve in watery blood plasma. To become more soluble in blood plasma, other lipid
molecules join with hydrophilic protein molecules. The resulting lipid/protein complexes are
termed lipoproteins. Lipoproteins are soluble because the proteins are on the outside and the
lipids are on the inside.
The diverse lipid family includes fatty acids, triglycerides (fats and oils), phospholipids (lipids
that contain phosphorus), steroids (lipids that contain rings of carbon atoms), eicosanoids (20‐
carbon lipids), and a variety of other lipids, including fat‐soluble vitamins (vitamins A, D, E, and
K) and lipoproteins. Table 2.7 introduces the various types of lipids and highlights their roles in
the human body.
Examples
Anatomy Overview: Lipids
Table 2.7 Types of Lipids in the Body
TYPE OF LIPID FUNCTIONS
Fatty acids Used to synthesize triglycerides and phospholipids or catabolized to
generate adenosine triphosphate (ATP).
Triglyceride (fats and
oils)
Protection, insulation, energy storage.

Phospholipids Major lipid component of cell membranes.
Steroids
Cholesterol Minor component of all animal cell membranes; precursor of bile
salts, vitamin D, and steroid hormones.
Bile salts Needed for digestion and absorption of dietary lipids.
Vitamin D Help regulate calcium level in body; needed for bone growth and
repair.
Adrenocortical
hormones
Help regulate metabolism, resistance to stress, and salt and water
balance.
Sex hormones Stimulate reproductive functions and sexual characteristics.
Eicosanoids
(prostaglandins and
leukotrienes)
Have diverse effects on modifying responses to hormones, blood
clotting, inflammation, immunity, stomach acid secretion, airway
diameter, lipid breakdown, and smooth muscle contraction.
Other lipids
Carotenes Needed for synthesis of vitamin A (used to make visual pigments in
eye); function as antioxidants.
Vitamin E Promotes wound healing, prevents tissue scarring, contributes to
normal structure and function of nervous system, and functions as
antioxidant.
Vitamin K Required for synthesis of blood‐clotting proteins.
Lipoproteins Transport lipids in blood, carry triglycerides and cholesterol to
tissues, and remove excess cholesterol from blood.
Fatty Acids
Among the simplest lipids are the fatty acids, which are used to synthesize triglycerides and
phospholipids. Fatty acids can also be catabolized to generate adenosine triphosphate (ATP). A
fatty acid consists of a carboxyl group and a hydrocarbon chain (Figure 2.17a). Fatty acids can
be either saturated or unsaturated. A saturated fatty acid contains only single covalent bonds
between the carbon atoms of the hydrocarbon chain. Because they lack double bonds, each
carbon atom of the hydrocarbon chain is saturated with hydrogen atoms (see, for example,
palmitic acid in Figure 2.17a). An unsaturated fatty acid contains one or more double covalent
bonds between the carbon atoms of the hydrocarbon chain. Thus, the fatty acid is not completely
saturated with hydrogen atoms (see, for example, oleic acid in Figure 2.17a). The unsaturated
fatty acid has a kink (bend) at the site of the double bond. If the fatty acid has just one double
bond in the hydrocarbon chain, it is monounsaturated and it has just one kink. If a fatty acid has

more than one double bond in the hydrocarbon chain, it is polyunsaturated and it contains more
than one kink.
Figure 2.17 Fatty acid structure and triglyceride synthesis.
Shown in (a) are the structures of a saturated fatty acid and an unsaturated fatty acid. Each time a
glycerol and a fatty acid are joined in dehydration synthesis (b), a molecule of water is removed.
An ester linkage joins the glycerol to each of the three molecules of fatty acids, which vary in
length and in number and location of double bonds between carbon atoms (C═C). Shown here
(c) is a triglyceride molecule that contains two saturated fatty acids and a monounsaturated fatty
acid. The kink (bend) in the oleic acid occurs at the double bond.
One glycerol and three fatty acids are the building blocks of triglycerides.
Imagineering
Does the oxygen in the water molecule removed during dehydration synthesis
come from the glycerol or from a fatty acid?
Triglycerides
The most plentiful lipids in your body and in your diet are the triglycerides (trī‐GLI‐ser‐īds; tri‐
= three), also known as triacylglycerols. A triglyceride consists of two types of building blocks,
a single glycerol molecule and three fatty acid molecules. A three‐carbon glycerol molecule
forms the backbone of a triglyceride (Figure 2.17b, c). Three fatty acids are attached by
dehydration synthesis reactions, one to each carbon of the glycerol backbone. The chemical bond
formed where each water molecule is removed is an ester linkage (see Table 2.5). The reverse
reaction, hydrolysis, breaks down a single molecule of a triglyceride into three fatty acids and
glycerol.
Triglycerides can be either solids or liquids at room temperature. A fat is a triglyceride that is a
solid at room temperature. The fatty acids of a fat are mostly saturated. Because these saturated
fatty acids lack double bonds in their hydrocarbon chains, they can closely pack together and
solidify at room temperature. A fat that mainly consists of saturated fatty acids is called a
saturated fat. Although saturated fats occur mostly in meats (especially red meats) and nonskim
dairy products (whole milk, cheese, and butter), they are also found in a few plant products, such
as cocoa butter, palm oil, and coconut oil. Diets that contain large amounts of saturated fats are
associated with disorders such as heart disease and colorectal cancer.
An oil is a triglyceride that is a liquid at room temperature. The fatty acids of an oil are mostly
unsaturated. Recall that unsaturated fatty acids contain one or more double bonds in their
hydrocarbon chains. The kinks at the sites of the double bonds prevent the unsaturated fatty acids
of an oil from closely packing together and solidifying. The fatty acids of an oil can be either
monounsaturated or polyunsaturated. Monounsaturated fats contain triglycerides that mostly
consist of monounsaturated fatty acids. Olive oil, peanut oil, canola oil, most nuts, and avocados
are rich in triglycerides with monounsaturated fatty acids. Polyunsaturated fats contain
triglycerides that mostly consist of polyunsaturated fatty acids. Corn oil, safflower oil, sunflower
oil, soybean oil, and fatty fish (salmon, tuna, and mackerel) contain a high percentage of

polyunsaturated fatty acids. Both monounsaturated and polyunsaturated fats are believed to
decrease the risk of heart disease.
Triglycerides are the body's most highly concentrated form of chemical energy. Triglycerides
provide more than twice as much energy per gram as do carbohydrates and proteins. Our
capacity to store triglycerides in adipose (fat) tissue is unlimited for all practical purposes.
Excess dietary carbohydrates, proteins, fats, and oils all have the same fate: They are deposited
in adipose tissue as triglycerides.
CLINICAL CONNECTION Fatty Acids in Health and Disease
As its name implies, a group of fatty acids called essential fatty acids (EFAs) is essential to
human health. However, they cannot be made by the human body and must be obtained from
foods or supplements. Among the more important EFAs are omega‐3 fatty acids, omega‐6
fatty acids, and cis‐fatty acids.
Omega‐3 and omega‐6 fatty acids are polyunsaturated fatty acids that are believed to work
together to promote health. They may have a protective effect against heart disease and stroke
by lowering total cholesterol, raising HDL (high‐density lipoproteins or “good cholesterol”)
and lowering LDL (low‐density lipoproteins or “bad cholesterol”). In addition, omega‐3 and
omega‐6 fatty acids decrease bone loss by increasing calcium utilization by the body; reduce
symptoms of arthritis due to inflammation; promote wound healing; improve certain skin
disorders (psoriasis, eczema, and acne); and improve mental functions. Primary sources of
omega‐3 fatty acids include flaxseed, fatty fish, oils that have large amounts of
polyunsaturated fatty acids, fish oils, and walnuts. Primary sources of omega‐6 fatty acids
include most processed foods (cereals, breads, white rice), eggs, baked goods, oils with large
amounts of polyunsaturated fatty acids, and meats (especially organ meats, such as liver).
Note in Figure 2.17a that the hydrogen atoms on either side of the double bond in oleic acid
are on the same side of the unsaturated fatty acid. Such an unsaturated fatty acid is called a
cis‐fatty acid. Cis‐fatty acids are nutritionally beneficial unsaturated fatty acids that are used
by the body to produce hormonelike regulators and cell membranes. However, when cis‐fatty
acids are heated, pressurized, and combined with a catalyst (usually nickel) in a process called
hydrogenation, they are changed to unhealthy trans‐fatty acids. In trans‐fatty acids the
hydrogen atoms are on opposite sides of the double bond of an unsaturated fatty acid.
Hydrogenation is used by manufacturers to make vegetable oils solid at room temperature and
less likely to turn rancid. Hydrogenated or trans‐fatty acids are common in commercially
baked goods (crackers, cakes, and cookies), salty snack foods, some margarines, and fried
foods (donuts and french fries). When oil is used for frying and if the oil is reused (like in fast
food french fry machines), cis‐fatty acids are converted to trans‐fatty acids. If a product label
contains the words hydrogenated or partially hydrogenated, then the product contains trans‐
fatty acids. Among the adverse effects of trans‐fatty acids are an increase in total cholesterol,
a decrease in HDL, an increase in LDL, and an increase in triglycerides. These effects, which
can increase the risk of heart disease and other cardiovascular diseases, are similar to those
caused by saturated fats.
Phospholipids
Like triglycerides, phospholipids have a glycerol backbone and two fatty acid chains attached to
the first two carbons. In the third position, however, a phosphate group (PO43−) links a small
charged group that usually contains nitrogen (N) to the backbone (Figure 2.18). This portion of

the molecule (the “head”) is polar and can form hydrogen bonds with water molecules. The two
fatty acids (the “tails”), by contrast, are nonpolar and can interact only with other lipids.
Molecules that have both polar and nonpolar parts are said to be amphipathic (am‐fē‐PATH‐ik;
amphi‐ = on both sides; ‐pathic = feeling). Amphipathic phospholipids line up tail‐to‐tail in a
double row to make up much of the membrane that surrounds each cell (Figure 2.18c).
Figure 2.18 Phospholipids.
(a) In the synthesis of phospholipids, two fatty acids attach to the first two carbons of the
glycerol backbone. A phosphate group links a small charged group to the third carbon in
glycerol. In (b), the circle represents the polar head region, and the two wavy lines represent the
two nonpolar tails. Double bonds in the fatty acid hydrocarbon chain often form kinks in the tail.
Phospholipids are amphipathic molecules, having both polar and nonpolar regions.
Imagineering
Which portion of a phospholipid is hydrophilic, and which portion is
hydrophobic?
Steroids
The structure of steroids differs considerably from that of the triglycerides. Steroids have four
rings of carbon atoms (colored gold in Figure 2.19). Body cells synthesize other steroids from
cholesterol (Figure 2.19a), which has a large nonpolar region consisting of the four rings and a
hydrocarbon tail. In the body, the commonly encountered steroids, such as cholesterol, estrogens,
testosterone, cortisol, bile salts, and vitamin D, are known as sterols because they also have at
least one hydroxyl (alcohol) group (─OH). The polar hydroxyl groups make sterols weakly
amphipathic. Cholesterol is needed for cell membrane structure; estrogens and testosterone are
required for regulating sexual functions; cortisol is necessary for maintaining normal blood sugar
levels; bile salts are needed for lipid digestion and absorption; and vitamin D is related to bone
growth. In Chapter 10, we will discuss the use of anabolic steroids by athletes to increase muscle
size, strength, and endurance.
Figure 2.19 Steroids.
All steroids have four rings of carbon atoms. The individual rings are designated by the letters A,
B, C, and D.
Cholesterol, which is synthesized in the liver, is the starting material for synthesis of other
steroids in the body.
Imagineering
How is the structure of estradiol different from that of testosterone?

Other Lipids
Eicosanoids (ī‐KŌ‐sa‐noyds; eicosan‐ = twenty) are lipids derived from a 20‐carbon fatty acid
called arachidonic acid. The two principal subclasses of eicosanoids are the prostaglandins
(pros′‐ta‐GLAN‐dins) and the leukotrienes (loo′‐kō‐TRĪ‐ēnz). Prostaglandins have a wide
variety of functions. They modify responses to hormones, contribute to the inflammatory
response (Chapter 22), prevent stomach ulcers, dilate (enlarge) airways to the lungs, regulate
body temperature, and influence formation of blood clots, to name just a few. Leukotrienes
participate in allergic and inflammatory responses.
Other lipids also include fat‐soluble vitamins such as beta‐carotenes (the yellow‐orange pigments
in egg yolk, carrots, and tomatoes that are converted to vitamin A); vitamins D, E, and K; and
lipoproteins.
Proteins
Proteins are large molecules that contain carbon, hydrogen, oxygen, and nitrogen. Some proteins
also contain sulfur. A normal, lean adult body is 12–18% protein. Much more complex in
structure than carbohydrates or lipids, proteins have many roles in the body and are largely
responsible for the structure of body tissues. Enzymes are proteins that speed up most
biochemical reactions. Other proteins work as “motors” to drive muscle contraction. Antibodies
are proteins that defend against invading microbes. Some hormones that regulate homeostasis
also are proteins. Table 2.8 describes several important functions of proteins.
Table 2.8 Functions of Proteins
TYPE OF
PROTEIN FUNCTIONS
Structural Form structural framework of various parts of body.
Examples: collagen in bone and other connective tissues; keratin in skin,
hair, and fingernails.
Regulatory Function as hormones that regulate various physiological processes;
control growth and development; as neurotransmitters, mediate responses
of nervous system.
Examples: the hormone insulin (regulates blood glucose level); the
neurotransmitter known as substance P (mediates sensation of pain in
nervous system).
Contractile Allow shortening of muscle cells, which produces movement.
Examples: myosin; actin.
Immunological Aid responses that protect body against foreign substances and invading
pathogens.
Examples: antibodies; interleukins.
Transport Carry vital substances throughout body.
Example: hemoglobin (transports most oxygen and some carbon dioxide
in blood).
Catalytic Act as enzymes that regulate biochemical reactions.

Examples: salivary amylase; sucrase; ATPase.
Examples
Anatomy Overview: Proteins
Amino Acids and Polypeptides
The monomers of proteins are amino acids (a‐MĒ‐nō). Each of the 20 different amino acids has a
hydrogen (H) atom and three important functional groups attached to a central carbon atom
(Figure 2.20a): (1) an amino group (─NH2), (2) an acidic carboxyl group (─COOH), and (3) a
side chain (R group). At the normal pH of body fluids, both the amino group and the carboxyl
group are ionized (Figure 2.20b). The different side chains give each amino acid its distinctive
chemical identity (Figure 2.20c).
Figure 2.20 Amino acids.
(a) In keeping with their name, amino acids have an amino group (shaded blue) and a carboxyl
(acid) group (shaded red). The side chain (R group) is different in each amino acid. (b) At pH
close to 7, both the amino group and the carboxyl group are ionized. (c) Glycine is the simplest
amino acid; the side chain is a single H atom. Cysteine is one of two amino acids that contain
sulfur (S). The side chain in tyrosine contains a six‐carbon ring. Lysine has a second amino
group at the end of its side chain.
Body proteins contain 20 different amino acids, each of which has a unique side chain.
Imagineering
In an amino acid, what is the minimum number of carbon atoms? Of
nitrogen atoms?
A protein is synthesized in stepwise fashion—one amino acid is joined to a second, a third is
then added to the first two, and so on. The covalent bond joining each pair of amino acids is a
peptide bond. It always forms between the carbon of the carboxyl group (─COOH) of one
amino acid and the nitrogen of the amino group (─NH2) of another. As the peptide bond is
formed, a molecule of water is removed (Figure 2.21), making this a dehydration synthesis
reaction. Breaking a peptide bond, as occurs during digestion of dietary proteins, is a hydrolysis
reaction (Figure 2.21).
Figure 2.21
Formation of a peptide bond between two amino acids during dehydration
synthesis.
In this example, glycine is joined to alanine, forming a dipeptide (read from left to right).
Breaking a peptide bond occurs via hydrolysis (read from right to left).
Amino acids are the monomers used to build proteins.

Imagineering
What type of reaction takes place during catabolism of proteins?
When two amino acids combine, a dipeptide results. Adding another amino acid to a dipeptide
produces a tripeptide. Further additions of amino acids result in the formation of a chainlike
peptide (4–9 amino acids) or polypeptide (10–2000 or more amino acids). Small proteins may
consist of a single polypeptide chain with as few as 50 amino acids. Larger proteins have
hundreds or thousands of amino acids and may consist of two or more polypeptide chains folded
together.
Because each variation in the number or sequence of amino acids can produce a different protein,
a great variety of proteins is possible. The situation is similar to using an alphabet of 20 letters to
form words. Each different amino acid is like a letter, and their various combinations give rise to
a seemingly endless diversity of words (peptides, polypeptides, and proteins).
Levels of Structural Organization in Proteins
Proteins exhibit four levels of structural organization. The primary structure is the unique
sequence of amino acids that are linked by covalent peptide bonds to form a polypeptide chain
(Figure 2.22a). A protein's primary structure is genetically determined, and any changes in a
protein's amino acid sequence can have serious consequences for body cells. In sickle‐cell
disease, for example, a nonpolar amino acid (valine) replaces a polar amino acid (glutamate)
through two mutations in the oxygen‐carrying protein hemoglobin. This change of amino acids
diminishes hemoglobin's water solubility. As a result, the altered hemoglobin tends to form
crystals inside red blood cells, producing deformed, sickle‐shaped cells that cannot properly
squeeze through narrow blood vessels. The symptoms and treatment of sickle‐cell disease are
discussed in Disorders: Homeostatic Imbalances in Chapter 19.
Figure 2.22 Levels of structural organization in proteins.
(a) The primary structure is the sequence of amino acids in the polypeptide. (b) Common
secondary structures include alpha helixes and beta pleated sheets. For simplicity, the amino acid
side groups are not shown here. (c) The tertiary structure is the overall folding pattern that
produces a distinctive, three‐dimensional shape. (d) The quaternary structure in a protein is the
arrangement of two or more polypeptide chains relative to one another.
The unique shape of each protein permits it to carry out specific functions.
Imagineering
Do all proteins have a quaternary structure?
The secondary structure of a protein is the repeated twisting or folding of neighboring amino
acids in the polypeptide chain (Figure 2.22b). Two common secondary structures are alpha
helixes (clockwise spirals) and beta pleated sheets. The secondary structure of a protein is
stabilized by hydrogen bonds, which form at regular intervals along the polypeptide backbone.

The tertiary structure (TUR‐shē‐er′‐ē) refers to the three‐dimensional shape of a polypeptide
chain. Each protein has a unique tertiary structure that determines how it will function. The
tertiary folding pattern may allow amino acids at opposite ends of the chain to be close neighbors
(Figure 2.22c). Several types of bonds can contribute to a protein's tertiary structure. The
strongest but least common bonds, S─S covalent bonds called disulfide bridges, form between
the sulfhydryl groups of two monomers of the amino acid cysteine. Many weak bonds—
hydrogen bonds, ionic bonds, and hydrophobic interactions—also help determine the folding
pattern. Some parts of a polypeptide are attracted to water (hydrophilic), and other parts are
repelled by it (hydrophobic). Because most proteins in our body exist in watery surroundings, the
folding process places most amino acids with hydrophobic side chains in the central core, away
from the protein's surface. Often, helper molecules known as chaperones aid the folding process.
In those proteins that contain more than one polypeptide chain (not all of them do), the
arrangement of the individual polypeptide chains relative to one another is the quaternary
structure (KWA‐ter‐ner′‐ē; Figure 2.22d). The bonds that hold polypeptide chains together are
similar to those that maintain the tertiary structure.
Proteins vary tremendously in structure. Different proteins have different architectures and
different three‐dimensional shapes. This variation in structure and shape is directly related to
their diverse functions. In practically every case, the function of a protein depends on its ability
to recognize and bind to some other molecule. Thus, a hormone binds to a specific protein on a
cell in order to alter its function, and an antibody protein binds to a foreign substance (antigen)
that has invaded the body. A protein's unique shape permits it to interact with other molecules to
carry out a specific function.
On the basis of overall shape, proteins are classified as fibrous or globular. Fibrous proteins are
insoluble in water and their polypeptide chains form long strands that are parallel to each other.
Fibrous proteins have many structural functions. Examples include collagen (strengthens bones,
ligaments, and tendons), elastin (provides stretch in skin, blood vessels, and lung tissue), keratin
(forms structure of hair and nails and waterproofs the skin), dystrophin (reinforces parts of
muscle cells), fibrin (forms blood clots), and actin and myosin (are involved in contraction of
muscle cells, division in all cells, and transport of substances within cells). Globular proteins
are more or less soluble in water and their polypeptide chains are spherical (globular) in shape.
Globular proteins have metabolic functions. Examples include enzymes, which function as
catalysts; antibodies and complement proteins, which help protect us against disease;
hemoglobin, which transports oxygen; lipoproteins, which transport lipids and cholesterol;
albumins, which help regulate blood pH; membrane proteins, which transport substances into
and out of cells; and some hormones such as insulin, which helps regulate blood sugar level.
Homeostatic mechanisms maintain the temperature and chemical composition of body fluids,
which allow body proteins to keep their proper three‐dimensional shapes. If a protein encounters
an altered environment, it may unravel and lose its characteristic shape (secondary, tertiary, and
quaternary structure). This process is called denaturation. Denatured proteins are no longer
functional. Although in some cases denaturation can be reversed, a frying egg is a common
example of permanent denaturation. In a raw egg the soluble egg‐white protein (albumin) is a
clear, viscous fluid. When heat is applied to the egg, the protein denatures, becomes insoluble,
and turns white.
Enzymes
In living cells, most catalysts are protein molecules called enzymes (EN‐zīms). Some enzymes
consist of two parts—a protein portion, called the apoenzyme (ā′‐pō‐EN‐zīm), and a nonprotein

portion, called a cofactor. The cofactor may be a metal ion (such as iron, magnesium, zinc, or
calcium) or an organic molecule called a coenzyme. Coenzymes often are derived from vitamins.
The names of enzymes usually end in the suffix ‐ase. All enzymes can be grouped according to
the types of chemical reactions they catalyze. For example, oxidases add oxygen, kinases add
phosphate, dehydrogenases remove hydrogen, ATPases split ATP, anhydrases remove water,
proteases break down proteins, and lipases break down triglycerides.
Enzymes catalyze specific reactions. They do so with great efficiency and with many built‐in
controls. Three important properties of enzymes are as follows:
1. Enzymes are highly specific.
Each particular enzyme binds only to specific substrates—the reactant molecules on
which the enzyme acts. Of the more than 1000 known enzymes in your body, each has a
characteristic three‐dimensional shape with a specific surface configuration, which allows
it to recognize and bind to certain substrates. In some cases, the part of the enzyme that
catalyzes the reaction, called the active site, is thought to fit the substrate like a key fits in
a lock. In other cases the active site changes its shape to fit snugly around the substrate
once the substrate enters the active site. This change in shape is known as an induced fit.
Not only is an enzyme matched to a particular substrate; it also catalyzes a specific
reaction. From among the large number of diverse molecules in a cell, an enzyme must
recognize the correct substrate and then take it apart or merge it with another substrate to
form one or more specific products.
2. Enzymes are very efficient. Under optimal conditions; enzymes can catalyze reactions
at rates that are from 100 million to 10 billion times more rapid than those of similar
reactions occurring without enzymes. The number of substrate molecules that a single
enzyme molecule can convert to product molecules in one second is generally between 1
and 10,000 and can be as high as 600,000.
3. Enzymes are subject to a variety of cellular controls. Their rate of synthesis and their
concentration at any given time are under the control of a cell's genes. Substances within
the cell may either enhance or inhibit the activity of a given enzyme. Many enzymes have
both active and inactive forms in cells. The rate at which the inactive form becomes active
or vice versa is determined by the chemical environment inside the cell.
Enzymes lower the activation energy of a chemical reaction by decreasing the “randomness” of
the collisions between molecules. They also help bring the substrates together in the proper
orientation so that the reaction can occur. Figure 2.23 depicts how an enzyme works:
1. The substrates make contact with the active site on the surface of the enzyme molecule,
forming a temporary intermediate compound called the enzyme–substrate complex. In
this reaction the two substrate molecules are sucrose (a disaccharide) and water.
2. The substrate molecules are transformed by the rearrangement of existing atoms, the
breakdown of the substrate molecule, or the combination of several substrate molecules

into the products of the reaction. Here the products are two monosaccharides: glucose and
fructose.
3. After the reaction is completed and the reaction products move away from the enzyme,
the unchanged enzyme is free to attach to other substrate molecules.
Figure 2.23
How an enzyme works.
An enzyme speeds up a chemical reaction without being altered or
consumed. Imagineering; Courtesy T.A. Steitz, Yale University
Why is it that sucrase cannot catalyze the formation of sucrose from glucose
and fructose?
Sometimes a single enzyme may catalyze a reversible reaction in either direction, depending on
the relative amounts of the substrates and products. For example, the enzyme carbonic
anhydrase catalyzes the following reversible reaction:
During exercise, when more CO2 is produced and released into the blood, the reaction flows to
the right, increasing the amount of carbonic acid in the blood. Then, as you exhale CO2, its level
in the blood falls and the reaction flows to the left, converting carbonic acid to CO2 and H2O.
Nucleic Acids: Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA)
Nucleic acids (noo‐KLĒ‐ic), so named because they were first discovered in the nuclei of cells,
are huge organic molecules that contain carbon, hydrogen, oxygen, nitrogen, and phosphorus.
Nucleic acids are of two varieties. The first, deoxyribonucleic acid (DNA) (dē‐ok′‐sē‐rī‐bō‐nū‐
KLĒ‐ik), forms the inherited genetic material inside each human cell. In humans, each gene
(JĒN) is a segment of a DNA molecule. Our genes determine the traits we inherit, and by
controlling protein synthesis they regulate most of the activities that take place in body cells
throughout our lives. When a cell divides, its hereditary information passes on to the next
generation of cells. Ribonucleic acid (RNA), the second type of nucleic acid, relays instructions
from the genes to guide each cell's synthesis of proteins from amino acids.
A nucleic acid is a chain of repeating monomers called nucleotides. Each nucleotide of DNA
consists of three parts (Figure 2.24a):
1. Nitrogenous base. DNA contains four different nitrogenous bases, which contain atoms
of C, H, O, and N. In DNA the four nitrogenous bases are adenine (A), thymine (T),
cytosine (C), and guanine (G). Adenine and guanine are larger, double‐ring bases called
purines (PŪR‐ēnz); thymine and cytosine are smaller, single‐ring bases called
pyrimidines (pī‐RIM‐i‐deēnz). The nucleotides are named according to the base that is

present. For instance, a nucleotide containing thymine is called a thymine nucleotide, one
containing adenine is called an adenine nucleotide, and so on.
2. Pentose sugar. A five‐carbon sugar called deoxyribose attaches to each base in DNA.
3. Phosphate group. Phosphate groups (PO43−) alternate with pentose sugars to form the
“backbone” of a DNA strand; the bases project inward from the backbone chain (Figure
2.24b).
Figure 2.24 DNA molecule.
(a) A nucleotide consists of a base, a pentose sugar, and a phosphate group. (b) The paired bases
project toward the center of the double helix. The structure is stabilized by hydrogen bonds
(dotted lines) between each base pair. There are two hydrogen bonds between adenine and
thymine and three between cytosine and guanine.
Nucleotides are the monomers of nucleic acids.
Imagineering
Which bases always pair with each other?
In 1953, F.H.C. Crick of Great Britain and J.D. Watson, a young American scientist, published a
brief paper describing how these three components might be arranged in DNA. Their insights
into data gathered by others led them to construct a model so elegant and simple that the
scientific world immediately knew it was correct! In the Watson–Crick double helix model,
DNA resembles a spiral ladder (Figure 2.24b). Two strands of alternating phosphate groups and
deoxyribose sugars form the uprights of the ladder. Paired bases, held together by hydrogen
bonds, form the rungs. Because adenine always pairs with thymine, and cytosine always pairs
with guanine, if you know the sequence of bases in one strand of DNA, you can predict the
sequence on the complementary (second) strand. Each time DNA is copied, as when living cells
divide to increase their number, the two strands unwind. Each strand serves as the template or
mold on which to construct a new second strand. Any change that occurs in the base sequence of
a DNA strand is called a mutation. Some mutations can result in the death of a cell, cause cancer,
or produce genetic defects in future generations.
RNA, the second variety of nucleic acid, differs from DNA in several respects. In humans, RNA
is single‐stranded. The sugar in the RNA nucleotide is the pentose ribose, and RNA contains the
pyrimidine base uracil (U) instead of thymine. Cells contain three different kinds of RNA:
messenger RNA, ribosomal RNA, and transfer RNA. Each has a specific role to perform in
carrying out the instructions coded in DNA (see Figure 3.29).
CLINICAL CONNECTION DNA Fingerprinting
A technique called DNA fingerprinting is used in research and in courts of law to ascertain

whether a person's DNA matches the DNA obtained from samples or pieces of legal evidence
such as blood stains or hairs. In each person, certain DNA segments contain base sequences
that are repeated several times. Both the number of repeat copies in one region and the
number of regions subject to repeat are different from one person to another. DNA
fingerprinting can be done with minute quantities of DNA—for example, from a single strand
of hair, a drop of semen, or a spot of blood. It also can be used to identify a crime victim or a
child's biological parents and even to determine whether two people have a common ancestor.
Adenosine Triphosphate
Adenosine triphosphate (a‐DEN‐ō‐sēn trī‐FOS‐fāt) or ATP is the “energy currency” of living
systems (Figure 2.25). ATP transfers the energy liberated in exergonic catabolic reactions to
power cellular activities that require energy (endergonic reactions). Among these cellular
activities are muscular contractions, movement of chromosomes during cell division, movement
of structures within cells, transport of substances across cell membranes, and synthesis of larger
molecules from smaller ones. As its name implies, ATP consists of three phosphate groups
attached to adenosine, a unit composed of adenine and the five‐carbon sugar ribose.
Figure 2.25 Structures of ATP and ADP.
“Squiggles” (∼) indicate the two phosphate bonds that can be used to transfer energy. Energy
transfer typically involves hydrolysis of the last phosphate bond of ATP.
ATP transfers chemical energy to power cellular activities.
Imagineering
What are some cellular activities that depend on energy supplied by ATP?
When a water molecule is added to ATP, the third phosphate group (PO43−), symbolized by
in the following discussion, is removed, and the overall reaction liberates energy. The enzyme
that catalyzes the hydrolysis of ATP is called ATPase. Removal of the third phosphate group
produces a molecule called adenosine diphosphate (ADP) in the following reaction:
As noted previously, the energy supplied by the catabolism of ATP into ADP is constantly being
used by the cell. As the supply of ATP at any given time is limited, a mechanism exists to
replenish it: The enzyme ATP synthase catalyzes the addition of a phosphate group to ADP in the
following reaction:
Where does the cell get the energy required to produce ATP? The energy needed to attach a
phosphate group to ADP is supplied mainly by the catabolism of glucose in a process called
cellular respiration. Cellular respiration has two phases, anaerobic and aerobic:
1. Anaerobic phase. In a series of reactions that do not require oxygen, glucose is partially
broken down by a series of catabolic reactions into pyruvic acid. Each glucose molecule

that is converted into a pyruvic acid molecule yields two molecules of ATP.
2. Aerobic phase. In the presence of oxygen, glucose is completely broken down into
carbon dioxide and water. These reactions generate heat and 36 or 38 ATP molecules.
Examples
Animation: Enzyme Functions and ATP
Chapters 10 and 25 cover the details of cellular respiration.
In Chapter 1, you learned that the human body comprises various levels of organization; this
chapter has just showed you the alphabet of atoms and molecules that is the basis for the
language of the body. Now that you have an understanding of the chemistry of the human body,
you are ready to form words; in Chapter 3 you will see how atoms and molecules are organized
to form structures of cells and perform the activities of cells that contribute to homeostasis.
3.1 PARTS OF A CELL
OBJECTIVE
• Name and describe the three main parts of a cell.
Figure 3.1 provides an overview of the typical structures found in body cells. Most cells have many of the
structures shown in this diagram. For ease of study, we divide the cell into three main parts: plasma membrane,
cytoplasm, and nucleus.
Figure 3.1
Typical structures found in body cells.
The cell is the basic living, structural, and functional unit of the body. Tomo Narashima
What are the three principal parts of a cell?
1. The plasma membrane forms the cell's flexible outer surface, separating the cell's internal
environment (everything inside the cell) from the external environment (everything outside the cell). It
is a selective barrier that regulates the flow of materials into and out of a cell. This selectivity helps
establish and maintain the appropriate environment for normal cellular activities. The plasma
membrane also plays a key role in communication among cells and between cells and their external
environment.
2. The cytoplasm (SĪ‐tō‐plasm; ‐plasm = formed or molded) consists of all the cellular contents between
the plasma membrane and the nucleus. This compartment has two components: cytosol and

organelles. Cytosol (SĪ‐tō‐sol), the fluid portion of cytoplasm, contains water, dissolved solutes, and
suspended particles. Within the cytosol are several different types of organelles (or‐gan‐ELZ = little
organs). Each type of organelle has a characteristic shape and specific functions. Examples include the
cytoskeleton, ribosomes, endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, and
mitochondria.
3. The nucleus (NOO‐klē‐us = nut kernel) is a large organelle that houses most of a cell's DNA. Within
the nucleus, each chromosome (KRŌ‐mō‐sōm; chromo‐ = colored), a single molecule of DNA
associated with several proteins, contains thousands of hereditary units called genes that control most
aspects of cellular structure and function.
3.2 THE PLASMA MEMBRANE
OBJECTIVES
• Distinguish between cytoplasm and cytosol.
• Explain the concept of selective permeability.
• Define the electrochemical gradient and describe its components.
The plasma membrane, a flexible yet sturdy barrier that surrounds and contains the cytoplasm of a
cell, is best described by using a structural model called the fluid mosaic model. According to this
model, the molecular arrangement of the plasma membrane resembles a continually moving sea of fluid
lipids that contains a mosaic of many different proteins (Figure 3.2). Some proteins float freely like
icebergs in the lipid sea, whereas others are anchored at specific locations like islands. The membrane
lipids allow passage of several types of lipid‐soluble molecules but act as a barrier to the entry or exit of
charged or polar substances. Some of the proteins in the plasma membrane allow movement of polar
molecules and ions into and out of the cell. Other proteins can act as signal receptors or as molecules
that link the plasma membrane to intracellular or extracellular proteins.
Figure 3.2
The fluid mosaic arrangement of lipids and proteins in the plasma
membrane.
Membranes are fluid structures because the lipids and many of the proteins are free to rotate and move
sideways in their own half of the bilayer. Tomo Narashima
What is the glycocalyx?
Structure of the Plasma Membrane
The Lipid Bilayer
The basic structural framework of the plasma membrane is the lipid bilayer, two back‐to‐back layers

made up of three types of lipid molecules—phospholipids, cholesterol, and glycolipids (Figure 3.2).
About 75% of the membrane lipids are phospholipids, lipids that contain phosphorus. Present in
smaller amounts are cholesterol (about 20%), a steroid with an attached ─OH (hydroxyl) group, and
various glycolipids (about 5%), lipids with attached carbohydrate groups.
The bilayer arrangement occurs because the lipids are amphipathic (am‐fē‐PATH‐ik) molecules,
which means that they have both polar and nonpolar parts. In phospholipids (see Figure 2.18), the polar
part is the phosphate‐containing “head,” which is hydrophilic (hydro‐ = water; ‐philic = loving). The
nonpolar parts are the two long fatty acid “tails,” which are hydrophobic (‐phobic = fearing)
hydrocarbon chains. Because “like seeks like,” the phospholipid molecules orient themselves in the
bilayer with their hydrophilic heads facing outward. In this way, the heads face a watery fluid on either
side—cytosol on the inside and extracellular fluid on the outside. The hydrophobic fatty acid tails in
each half of the bilayer point toward one another, forming a nonpolar, hydrophobic region in the
membrane's interior.
Cholesterol molecules are weakly amphipathic (see Figure 2.19a) and are interspersed among the other
lipids in both layers of the membrane. The tiny ─OH group is the only polar region of cholesterol, and
it forms hydrogen bonds with the polar heads of phospholipids and glycolipids. The stiff steroid rings
and hydrocarbon tail of cholesterol are nonpolar; they fit among the fatty acid tails of the phospholipids
and glycolipids. The carbohydrate groups of glycolipids form a polar “head”; their fatty acid “tails” are
nonpolar. Glycolipids appear only in the membrane layer that faces the extracellular fluid, which is one
reason the two sides of the bilayer are asymmetric, or different.
Examples
Anatomy Overview: Plasma Membrane Structure
Arrangement of Membrane Proteins
Membrane proteins are classified as integral or peripheral according to whether they are firmly
embedded in the membrane (Figure 3.2). Integral proteins extend into or through the lipid bilayer
among the fatty acid tails and are firmly embedded in it. Most integral proteins are transmembrane
proteins, which means that they span the entire lipid bilayer and protrude into both the cytosol and
extracellular fluid. A few integral proteins are tightly attached to one side of the bilayer by covalent
bonding to fatty acids. Like membrane lipids, integral membrane proteins are amphipathic. Their
hydrophilic regions protrude into either the watery extracllular fluid or the cytosol, and their
hydrophobic regions extend among the fatty acid tails.
As their name implies, peripheral proteins (pe‐RIF‐er‐al) are not as firmly embedded in the
membrane. They are attached to the polar heads of membrane lipids or to integral proteins at the inner
or outer surface of the membrane.
Many integral proteins are glycoproteins, proteins with carbohydrate groups attached to the ends that
protrude into the extracellular fluid. The carbohydrates are oligosaccharides (oligo‐ = few; ‐
saccharides = sugars), chains of 2 to 60 monosaccharides that may be straight or branched. The
carbohydrate portions of glycolipids and glycoproteins form an extensive sugary coat called
the glycocalyx (glī‐kō‐KĀL‐iks). The pattern of carbohydrates in the glycocalyx varies from one cell to
another. Therefore, the glycocalyx acts like a molecular “signature” that enables cells to recognize one
another. For example, a white blood cell's ability to detect a “foreign” glycocalyx is one basis of the
immune response that helps us destroy invading organisms. In addition, the glycocalyx enables cells to
adhere to one another in some tissues and protects cells from being digested by enzymes in the
extracellular fluid. The hydrophilic properties of the glycocalyx attract a film of fluid to the surface of
many cells. This action makes red blood cells slippery as they flow through narrow blood vessels and
protects cells that line the airways and the gastrointestinal tract from drying out.
Functions of Membrane Proteins

Generally, the types of lipids in cellular membranes vary only slightly. In contrast, the membranes of
different cells and various intracellular organelles have remarkably different assortments of proteins
that determine many of the membrane's functions (Figure 3.3).
Figure 3.3 Functions of membrane proteins.
Membrane proteins largely reflect the functions a cell can perform. Imagineering
When stimulating a cell, the hormone insulin first binds to a protein in the
plasma membrane. This action best represents which membrane protein function?
• Some integral proteins form ion channels, pores or holes that specific ions, such as potassium
ions (K+), can flow through to get into or out of the cell. Most ion channels are selective; they
allow only a single type of ion to pass through.
• Other integral proteins act as carriers, selectively moving a polar substance or ion from one side
of the membrane to the other. Carriers are also known as transporters.
• Integral proteins called receptors serve as cellular recognition sites. Each type of receptor
recognizes and binds a specific type of molecule. For instance, insulin receptors bind the
hormone insulin. A specific molecule that binds to a receptor is called a ligand (LĪ‐gand; liga =
tied) of that receptor.
• Some integral proteins are enzymes that catalyze specific chemical reactions at the inside or
outside surface of the cell.
• Integral proteins may also serve as linkers that anchor proteins in the plasma membranes of
neighboring cells to one another or to protein filaments inside and outside the cell. Peripheral
proteins also serve as enzymes and linkers.
• Membrane glycoproteins and glycolipids often serve as cell‐identity markers. They may enable
a cell to (1) recognize other cells of the same kind during tissue formation or (2) recognize and
respond to potentially dangerous foreign cells. The ABO blood type markers are one example of
cell‐identity markers. When you receive a blood transfusion, the blood type must be compatible
with your own, or red blood cells may clump together.
In addition, peripheral proteins help support the plasma membrane, anchor integral proteins, and
participate in mechanical activities such as moving materials and organelles within cells, changing cell
shape in dividing and muscle cells, and attaching cells to one another.
Membrane Fluidity
Membranes are fluid structures; that is, most of the membrane lipids and many of the membrane
proteins easily rotate and move sideways in their own half of the bilayer. Neighboring lipid molecules
exchange places about 10 million times per second and may wander completely around a cell in only a
few minutes! Membrane fluidity depends both on the number of double bonds in the fatty acid tails of
the lipids that make up the bilayer, and on the amount of cholesterol present. Each double bond puts a

“kink” in the fatty acid tail (see Figure 2.18), which increases membrane fluidity by preventing lipid
molecules from packing tightly in the membrane. Membrane fluidity is an excellent compromise for the
cell; a rigid membrane would lack mobility, and a completely fluid membrane would lack the structural
organization and mechanical support required by the cell. Membrane fluidity allows interactions to
occur within the plasma membrane, such as the assembly of membrane proteins. It also enables the
movement of the membrane components responsible for cellular processes such as cell movement,
growth, division, and secretion, and the formation of cellular junctions. Fluidity allows the lipid bilayer
to self‐seal if torn or punctured. When a needle is pushed through a plasma membrane and pulled out,
the puncture site seals spontaneously, and the cell does not burst. This property of the lipid bilayer
allows a procedure called intracytoplasmic sperm injection to help infertile couples conceive a child;
scientists can fertilize an oocyte by injecting a sperm cell through a tiny syringe. It also permits
removal and replacement of a cell's nucleus in cloning experiments, such as the one that created Dolly,
the famous cloned sheep.
Despite the great mobility of membrane lipids and proteins in their own half of the bilayer, they seldom
flip‐flop from one half of the bilayer to the other, because it is difficult for hydrophilic parts of
membrane molecules to pass through the hydrophobic core of the membrane. This difficulty contributes
to the asymmetry of the membrane bilayer.
Because of the way it forms hydrogen bonds with neighboring phospholipid and glycolipid heads and
fills the space between bent fatty acid tails, cholesterol makes the lipid bilayer stronger but less fluid at
normal body temperature. At low temperatures, cholesterol has the opposite effect—it increases
membrane fluidity.
Membrane Permeability
The term permeable means that a structure permits the passage of substances through it,
while impermeable means that a structure does not permit the passage of substances through it. The
permeability of the plasma membrane to different substances varies. Plasma membranes permit some
substances to pass more readily than others. This property of membranes is termed selective
permeability (per′‐mē‐a‐BIL‐i‐tē).
The lipid bilayer portion of the membrane is permeable to nonpolar, uncharged molecules, such as
oxygen, carbon dioxide, and steroids, but is impermeable to ions and large, uncharged polar molecules
such as glucose. It is also slightly permeable to small, uncharged polar molecules such as water and
urea, a waste product from the breakdown of amino acids. The slight permeability to water and urea is
an unexpected property since they are polar molecules. These two small molecules are thought to pass
through the lipid bilayer in the following way. As the fatty acid tails of membrane phospholipids and
glycolipids randomly move about, small gaps briefly appear in the hydrophobic environment of the
membrane's interior. Water and urea molecules are small enough to move from one gap to another until
they have crossed the membrane.
Transmembrane proteins that act as channels and carriers increase the plasma membrane's permeability
to a variety of ions and uncharged polar molecules that, unlike water and urea molecules, cannot cross
the lipid bilayer unassisted. Channels and carriers are very selective. Each one helps a specific molecule
or ion to cross the membrane. Macromolecules, such as proteins, are so large that they are unable to
pass across the plasma membrane except by endocytosis and exocytosis (discussed later in this
chapter).
Gradients across the Plasma Membrane
The selective permeability of the plasma membrane allows a living cell to maintain different
concentrations of certain substances on either side of the plasma membrane. A concentration
gradient is a difference in the concentration of a chemical from one place to another, such as from the
inside to the outside of the plasma membrane. Many ions and molecules are more concentrated in either
the cytosol or the extracellular fluid. For instance, oxygen molecules and sodium ions (Na+) are more
concentrated in the extracellular fluid than in the cytosol; the opposite is true of carbon dioxide

molecules and potassium ions (K+).
The plasma membrane also creates a difference in the distribution of positively and negatively charged
ions between the two sides of the plasma membrane. Typically, the inner surface of the plasma
membrane is more negatively charged and the outer surface is more positively charged. A difference in
electrical charges between two regions constitutes an electrical gradient. Because it occurs across the
plasma membrane, this charge difference is termed the membrane potential.
As you will see shortly, the concentration gradient and electrical gradient are important because they
help move substances across the plasma membrane. In many cases a substance will move across a
plasma membrane down its concentration gradient. That is to say, a substance will move “downhill,”
from where it is more concentrated to where it is less concentrated, to reach equilibrium. Similarly, a
positively charged substance will tend to move toward a negatively charged area, and a negatively
charged substance will tend to move toward a positively charged area. The combined influence of the
concentration gradient and the electrical gradient on movement of a particular ion is referred to as
its electrochemical gradient.
3.3 TRANSPORT ACROSS THE PLASMA
MEMBRANE
OBJECTIVE
• Describe the processes that transport substances across the plasma membrane.
Transport of materials across the plasma membrane is essential to the life of a cell. Certain substances
must move into the cell to support metabolic reactions. Other substances that have been produced by
the cell for export or as cellular waste products must move out of the cell.
Substances generally move across cellular membranes via transport processes that can be classified as
passive or active, depending on whether they require cellular energy. In passive processes, a substance
moves down its concentration or electrical gradient to cross the membrane using only its own kinetic
energy (energy of motion). Kinetic energy is intrinsic to the particles that are moving. There is no input
of energy from the cell. An example is simple diffusion. In active processes, cellular energy is used to
drive the substance “uphill” against its concentration or electrical gradient. The cellular energy used is
usually in the form of adenosine triphosphate (ATP). An example is active transport. Another way that
some substances may enter and leave cells is an active process in which tiny, spherical membrane sacs
referred to as vesicles are used. Examples include endocytosis, in which vesicles detach from the
plasma membrane while bringing materials into a cell, and exocytosis, the merging of vesicles with the
plasma membrane to release materials from the cell.
Passive Processes
The Principle of Diffusion
Learning why materials diffuse across membranes requires an understanding of how diffusion occurs in
a solution. Diffusion (di‐FŪ‐zhun; diffus‐ = spreading) is a passive process in which the random mixing
of particles in a solution occurs because of the particles' kinetic energy. Both the solutes, the dissolved
substances, and the solvent, the liquid that does the dissolving, undergo diffusion. If a particular solute
is present in high concentration in one area of a solution and in low concentration in another area,
solute molecules will diffuse toward the area of lower concentration—they move down their

concentration gradient. After some time, the particles become evenly distributed throughout the
solution and the solution is said to be at equilibrium. The particles continue to move about randomly
due to their kinetic energy, but their concentrations do not change.
For example, when you place a crystal of dye in a water‐filled container (Figure 3.4), the color is most
intense in the area closest to the dye because its concentration is higher there. At increasing distances,
the color is lighter and lighter because the dye concentration is lower. Some time later, the solution of
water and dye will have a uniform color, because the dye molecules and water molecules have diffused
down their concentration gradients until they are evenly mixed in solution—they are at equilibrium (ē‐
kwi‐LIB‐rē‐um).
Figure 3.4 Principle of diffusion.
At the beginning of our experiment, a crystal of dye placed in a cylinder of water dissolves (a) and then
diffuses from the region of higher dye concentration to regions of lower dye concentration (b). At equilibrium
(c), the dye concentration is uniform throughout, although random movement continues.
In diffusion, a substance moves down its concentration gradient.
Andy Washnik
How would having a fever affect body processes that involve diffusion?
In this simple example, no membrane was involved. Substances may also diffuse through a membrane,
if the membrane is permeable to them. Several factors influence the diffusion rate of substances across
plasma membranes:
• Steepness of the concentration gradient. The greater the difference in concentration between
the two sides of the membrane, the higher is the rate of diffusion. When charged particles are
diffusing, the steepness of the electrochemical gradient determines the diffusion rate across the
membrane.
• Temperature. The higher the temperature, the faster the rate of diffusion. All of the body's
diffusion processes occur more rapidly in a person with a fever.
• Mass of the diffusing substance. The larger the mass of the diffusing particle, the slower its
diffusion rate. Smaller molecules diffuse more rapidly than larger ones.
• Surface area. The larger the membrane surface area available for diffusion, the faster is the
diffusion rate. For example, the air sacs of the lungs have a large surface area available for
diffusion of oxygen from the air into the blood. Some lung diseases, such as emphysema, reduce
the surface area. This slows the rate of oxygen diffusion and makes breathing more difficult.
• Diffusion distance. The greater the distance over which diffusion must occur, the longer it
takes. Diffusion across a plasma membrane takes only a fraction of a second because the
membrane is so thin. In pneumonia, fluid collects in the lungs; the additional fluid increases the
diffusion distance because oxygen must move through both the built‐up fluid and the membrane
to reach the bloodstream.
Now that you have a basic understanding of the nature of diffusion, we will consider three types of
diffusion: simple diffusion, facilitated diffusion, and osmosis.

Simple Diffusion
Simple diffusion is a passive process in which substances move freely through the lipid bilayer of the
plasma membranes of cells without the help of membrane transport proteins (Figure 3.5). Nonpolar,
hydrophobic molecules move across the lipid bilayer through the process of simple diffusion. Such
molecules include oxygen, carbon dioxide, and nitrogen gases; fatty acids; steroids; and fat‐soluble
vitamins (A, D, E, and K). Small, uncharged polar molecules such as water, urea, and small alcohols
also pass through the lipid bilayer by simple diffusion. Simple diffusion through the lipid bilayer is
important in the movement of oxygen and carbon dioxide between blood and body cells, and between
blood and air within the lungs during breathing. It also is the route for absorption of some nutrients and
excretion of some wastes by body cells.
Figure 3.5
Simple diffusion, channel‐mediated facilitated diffusion, and carrier‐
mediated facilitated diffusion.
In simple diffusion, a substance moves across the lipid bilayer of the plasma membrane without the
help of membrane transport proteins. In facilitated diffusion, a substance moves across the lipid bilayer aided
by a channel protein or a carrier protein. Imagineering
What types of molecules move across the lipid bilayer of the plasma membrane
via simple diffusion?
Facilitated Diffusion
Solutes that are too polar or highly charged to move through the lipid bilayer by simple diffusion can
cross the plasma membrane by a passive process called facilitated diffusion. In this process, an
integral membrane protein assists a specific substance across the membrane. The integral membrane
protein can be either a membrane channel or a carrier.
Channel‐Mediated Facilitated Diffusion
In channel‐mediated facilitated diffusion, a solute moves down its concentration gradient across the
lipid bilayer through a membrane channel (Figure 3.5). Most membrane channels are ion
channels, integral transmembrane proteins that allow passage of small, inorganic ions that are too
hydrophilic to penetrate the nonpolar interior of the lipid bilayer. Each ion can diffuse across the
membrane only at certain sites. In typical plasma membranes, the most numerous ion channels are
selective for K+ (potassium ions) or Cl− (chloride ions); fewer channels are available for Na+(sodium
ions) or Ca2+ (calcium ions). Diffusion of ions through channels is generally slower than free diffusion
through the lipid bilayer because channels occupy a smaller fraction of the membrane's total surface
area than lipids. Still, facilitated diffusion through channels is a very fast process: More than a million
potassium ions can flow through a K+ channel in one second!
A channel is said to be gated when part of the channel protein acts as a “plug” or “gate,” changing
shape in one way to open the pore and in another way to close it (Figure 3.6). Some gated channels
randomly alternate between the open and closed positions; others are regulated by chemical or
electrical changes inside and outside the cell. When the gates of a channel are open, ions diffuse into or
out of cells, down their electrochemical gradients. The plasma membranes of different types of cells
may have different numbers of ion channels and thus display different permeabilities to various ions.
Figure 3.6 Channel‐mediated facilitated diffusion of potassium ions (K+) through a gated

K+ channel.
A gated channel is one in which a portion of the channel protein acts as a gate to open or close the channel's
pore to the passage of ions.
Channels are integral membrane proteins that allow specific, small, inorganic ions to pass across the
membrane by facilitated diffusion.
Imagineering
Is the concentration of K+ in body cells higher in the cytosol or in the
extracellular fluid?
Carrier‐Mediated Facilitated Diffusion
In carrier‐mediated facilitated diffusion, a carrier (also called a transporter) moves a solute down its
concentration gradient across the plasma membrane (see Figure 3.5). Since this is a passive process, no
cellular energy is required. The solute binds to a specific carrier on one side of the membrane and is
released on the other side after the carrier undergoes a change in shape. The solute binds more often to
the carrier on the side of the membrane with a higher concentration of solute. Once the concentration is
the same on both sides of the membrane, solute molecules bind to the carrier on the cytosolic side and
move out to the extracellular fluid as rapidly as they bind to the carrier on the extracellular side and
move into the cytosol. The rate of carrier‐mediated facilitated diffusion (how quickly it occurs) is
determined by the steepness of the concentration gradient across the membrane.
The number of carriers available in a plasma membrane places an upper limit, called the transport
maximum, on the rate at which facilitated diffusion can occur. Once all of the carriers are occupied, the
transport maximum is reached, and a further increase in the concentration gradient does not increase the
rate of facilitated diffusion. Thus, much like a completely saturated sponge can absorb no more water,
the process of carrier‐mediated facilitated diffusion exhibits saturation.
Substances that move across the plasma membrane by carrier‐mediated facilitated diffusion include
glucose, fructose, galactose, and some vitamins. Glucose, the body's preferred energy source for
making ATP, enters many body cells by carrier‐mediated facilitated diffusion as follows (Figure 3.7):
1. Glucose binds to a specific type of carrier protein called the glucose transporter (GluT) on the
outside surface of the membrane.
2. As the transporter undergoes a change in shape, glucose passes through the membrane.
3. The transporter releases glucose on the other side of the membrane.
Figure 3.7 Carrier‐mediated facilitated diffusion of glucose across a plasma membrane.
The carrier protein binds to glucose in the extracellular fluid and releases it into the cytosol.
Carriers are integral membrane proteins that undergo changes in shape in order to move substances
across the membrane by facilitated diffusion.
Imagineering
Does insulin alter glucose transport by facilitated diffusion?

The selective permeability of the plasma membrane is often regulated to achieve homeostasis. For
instance, the hormone insulin, via the action of the insulin receptor, promotes the insertion of many
copies of glucose transporters into the plasma membranes of certain cells. Thus, the effect of insulin is
to elevate the transport maximum for facilitated diffusion of glucose into cells. With more glucose
transporters available, body cells can pick up glucose from the blood more rapidly. An inability to
produce or utilize insulin is called diabetes mellitus (Chapter 18).
Osmosis
Osmosis (oz‐MŌ‐sis) is a type of diffusion in which there is net movement of a solvent through a
selectively permeable membrane. Like the other types of diffusion, osmosis is a passive process. In
living systems, the solvent is water, which moves by osmosis across plasma membranes from an area
of higher water concentration to an area of lower water concentration. Another way to understand this
idea is to consider the solute concentration: In osmosis, water moves through a selectively permeable
membrane from an area of lower solute concentration to an area of higher solute concentration. During
osmosis, water molecules pass through a plasma membrane in two ways: (1) by moving between
neighboring phospholipid molecules in the lipid bilayer via simple diffusion, as previously described,
and (2) by moving through aquaporins (ak‐wa‐POR‐ins; aqua‐ = water), integral membrane proteins
that function as water channels.
Osmosis occurs only when a membrane is permeable to water but is not permeable to certain solutes. A
simple experiment can demonstrate osmosis. Consider a U‐shaped tube in which a selectively
permeable membrane separates the left and right arms of the tube. A volume of pure water is poured
into the left arm, and the same volume of a solution containing a solute that cannot pass through the
membrane is poured into the right arm (Figure 3.8a). Because the water concentration is higher on the
left and lower on the right, net movement of water molecules—osmosis—occurs from left to right, so
that the water is moving down its concentration gradient. At the same time, the membrane prevents
diffusion of the solute from the right arm into the left arm. As a result, the volume of water in the left
arm decreases, and the volume of solution in the right arm increases (Figure 3.8b).
Figure 3.8 Principle of osmosis.
Water molecules move through the selectively permeable membrane; the solute molecules in the right arm
cannot pass through the membrane. (a) As the experiment starts, water molecules move from the left arm
into the right arm, down the water concentration gradient. (b) After some time, the volume of water in the left
arm has decreased and the volume of solution in the right arm has increased. At equilibrium, there is no net
osmosis: Hydrostatic pressure forces just as many water molecules to move from right to left as osmosis
forces water molecules to move from left to right. (c) If pressure is applied to the solution in the right arm, the
starting conditions can be restored. This pressure, which stops osmosis, is equal to the osmotic pressure.
Osmosis is the movement of water molecules through a selectively permeable membrane.
Imagineering
Will the fluid level in the right arm rise until the water concentrations are the
same in both arms?
You might think that osmosis would continue until no water remained on the left side, but this
is not what happens. In this experiment, the higher the column of solution in the right arm becomes, the
more pressure it exerts on its side of the membrane. Pressure exerted in this way by a liquid, known
as hydrostatic pressure, forces water molecules to move back into the left arm. Equilibrium is reached
when just as many water molecules move from right to left due to the hydrostatic pressure as move

from left to right due to osmosis (Figure 3.8b).
To further complicate matters, the solution with the impermeable solute also exerts a force, called
the osmotic pressure. The osmotic pressure of a solution is proportional to the concentration of the
solute particles that cannot cross the membrane—the higher the solute concentration, the higher the
solution's osmotic pressure. Consider what would happen if a piston were used to apply more pressure
to the fluid in the right arm of the tube in Figure 3.8. With enough pressure, the volume of fluid in each
arm could be restored to the starting volume, and the concentration of solute in the right arm would be
the same as it was at the beginning of the experiment (Figure 3.8c). The amount of pressure needed to
restore the starting condition equals the osmotic pressure. So, in our experiment osmotic pressure is the
pressure needed to stop the movement of water from the left tube into the right tube. Notice that the
osmotic pressure of a solution does not produce the movement of water during osmosis. Rather it is the
pressure that would prevent such water movement.
Normally, the osmotic pressure of the cytosol is the same as the osmotic pressure of the interstitial fluid
outside cells. Because the osmotic pressure on both sides of the plasma membrane (which is selectively
permeable) is the same, cell volume remains relatively constant. When body cells are placed in a
solution having a different osmotic pressure than cytosol, however, the shape and volume of the cells
change. As water moves by osmosis into or out of the cells, their volume increases or decreases. A
solution's tonicity (tō‐NIS‐i‐tē; tonic = tension) is a measure of the solution's ability to change the
volume of cells by altering their water content.
Any solution in which a cell—for example, a red blood cell (RBC)—maintains its normal shape and
volume is an isotonic solution (ī′‐sō‐TON‐ik; iso‐ = same) (Figure 3.9). The concentrations of solutes
that cannot cross the plasma membrane are the same on both sides of the membrane in this solution. For
instance, a 0.9% NaCl solution (0.9 gram of sodium chloride in 100 mL of solution), called a normal
(physiological) saline solution, is isotonic for RBCs. The RBC plasma membrane permits the water to
move back and forth, but it behaves as though it is impermeable to Na+ and Cl−, the solutes. (Any
Na+ or Cl− ions that enter the cell through channels or transporters are immediately moved back out by
active transport or other means.) When RBCs are bathed in 0.9% NaCl, water molecules enter and exit
at the same rate, allowing the RBCs to keep their normal shape and volume.
Figure 3.9
Tonicity and its effects on red blood cells (RBCs).
The arrows indicate the direction and degree of water movement into and out of the cells. One example of an
isotonic solution for RBCs is 0.9% NaCl.
Cells placed in an isotonic solution maintain their shape because there is no net water movement into
or out of the cells.
Imagineering; David Phillips/Photo Researchers, Inc.
Will a 2% solution of NaCl cause hemolysis or crenation of RBCs? Why?
A different situation results if RBCs are placed in a hypotonic solution (hī′‐pō‐TON‐ik; hypo‐ = less
than), a solution that has a lower concentration of solutes than the cytosol inside the RBCs (Figure 3.9).
In this case, water molecules enter the cells faster than they leave, causing the RBCs to swell and
eventually to burst. The rupture of RBCs in this manner is called hemolysis (hē‐MOL‐i‐sis; hemo‐ =
blood; ‐lysis = to loosen or split apart); the rupture of other types of cells due to placement in a
hypotonic solution is referred to simply as lysis. Pure water is very hypotonic and causes rapid
hemolysis.
A hypertonic solution (hī′‐per‐TON‐ik; hyper‐ = greater than) has a higher concentration of solutes
than does the cytosol inside RBCs (Figure 3.9). One example of a hypertonic solution is a 2% NaCl

solution. In such a solution, water molecules move out of the cells faster than they enter, causing the
cells to shrink. Such shrinkage of cells is called crenation (kre‐NĀ‐shun).
CLINICAL CONNECTION Medical Uses of Isotonic, Hypertonic, and
Hypotonic Solutions
RBCs and other body cells may be damaged or destroyed if exposed to hypertonic or hypotonic
solutions. For this reason, most intravenous (IV) solutions, liquids infused into the blood of a vein,
are isotonic. Examples are isotonic saline (0.9% NaCl) and D5W, which stands for dextrose 5% in
water. Sometimes infusion of a hypertonic solution such as mannitol is useful to treat patients who
have cerebral edema, excess interstitial fluid in the brain. Infusion of such a solution relieves fluid
overload by causing osmosis of water from interstitial fluid into the blood. The kidneys then excrete
the excess water from the blood into the urine. Hypotonic solutions, given either orally or through
an IV, can be used to treat people who are dehydrated. The water in the hypotonic solution moves
from the blood into interstitial fluid and then into body cells to rehydrate them. Water and most
sports drinks that you consume to “rehydrate” after a workout are hypotonic relative to your body
cells.
Active Processes
Active Transport
Some polar or charged solutes that must enter or leave body cells cannot cross the plasma membrane
through any form of passive transport because they would need to move “uphill,” against their
concentration gradients. Such solutes may be able to cross the membrane by a process called active
transport. Active transport is considered an active process because energy is required for carrier
proteins to move solutes across the membrane against a concentration gradient. Two sources of cellular
energy can be used to drive active transport: (1) Energy obtained from hydrolysis of adenosine
triphosphate (ATP) is the source in primary active transport; (2) energy stored in an ionic
concentration gradient is the source in secondary active transport. Like carrier‐mediated facilitated
diffusion, active transport processes exhibit a transport maximum and saturation. Solutes actively
transported across the plasma membrane include several ions, such as Na+, K+, H+, Ca2+, I− (iodide
ions), and Cl−; amino acids; and monosaccharides. (Note that some of these substances also cross the
membrane via facilitated diffusion when the proper channel proteins or carriers are present.)
Primary Active Transport
In primary active transport, energy derived from hydrolysis of ATP changes the shape of a carrier
protein, which “pumps” a substance across a plasma membrane against its concentration gradient.
Indeed, carrier proteins that mediate primary active transport are often called pumps. A typical body
cell expends about 40% of the ATP it generates on primary active transport. Chemicals that turn off
ATP production—for example, the poison cyanide—are lethal because they shut down active transport
in cells throughout the body.
The most prevalent primary active transport mechanism expels sodium ions (Na+) from cells and brings
potassium ions (K+) in. Because of the specific ions it moves, this carrier is called the sodium‐
potassium pump. Because a part of the sodium‐potassium pump acts as an ATPase, an enzyme that
hydrolyzes ATP, another name for this pump is Na+/K+ATPase. All cells have thousands of sodium‐
potassium pumps in their plasma membranes. These sodium‐potassium pumps maintain a low
concentration of Na+ in the cytosol by pumping these ions into the extracellular fluid against the
Na+ concentration gradient. At the same time, the pumps move K+ into cells against the
K+ concentration gradient. Because K+ and Na+ slowly leak back across the plasma membrane down
their electrochemical gradients—through passive transport or secondary active transport—the sodium‐
potassium pumps must work nonstop to maintain a low concentration of Na+and a high concentration of
K+ in the cytosol.

Figure 3.10 depicts the operation of the sodium‐potassium pump:
1. Three Na+ in the cytosol bind to the pump protein.
2. Binding of Na+ triggers the hydrolysis of ATP into ADP, a reaction that also attaches a
phosphate group to the pump protein. This chemical reaction changes the shape of the
pump protein, expelling the three Na+ into the extracellular fluid. Now the shape of the pump
protein favors binding of two K+ in the extracellular fluid to the pump protein.
3. The binding of K+ triggers release of the phosphate group from the pump protein. This reaction
again causes the shape of the pump protein to change.
4. As the pump protein reverts to its original shape, it releases K+ into the cytosol. At this point, the
pump is again ready to bind three Na+, and the cycle repeats.
Figure 3.10 The sodium‐potassium pump (Na+/K+ATPase) expels sodium ions (Na+) and
brings potassium ions (K+) into the cell.
Sodium‐potassium pumps maintain a low intracellular concentration of sodium ions. Imagineering
What is the role of ATP in the operation of this pump?
The different concentrations of Na+ and K+ in cytosol and extracellular fluid are crucial for maintaining
normal cell volume and for the ability of some cells to generate electrical signals such as action
potentials. Recall that the tonicity of a solution is proportional to the concentration of its solute particles
that cannot penetrate the membrane. Because sodium ions that diffuse into a cell or enter through
secondary active transport are immediately pumped out, it is as if they never entered. In effect, sodium
ions behave as if they cannot penetrate the membrane. Thus, sodium ions are an important contributor
to the tonicity of the extracellular fluid. A similar condition holds for K+ in the cytosol. By helping to
maintain normal tonicity on each side of the plasma membrane, the sodium‐potassium pumps ensure
that cells neither shrink nor swell due to the movement of water by osmosis out of or into cells.
Secondary Active Transport
In secondary active transport, the energy stored in a Na+ or H+ concentration gradient is used to drive
other substances across the membrane against their own concentration gradients. Because a Na+ or
H+ gradient is established by primary active transport, secondary active transport indirectly uses energy
obtained from the hydrolysis of ATP.
The sodium‐potassium pump maintains a steep concentration gradient of Na+ across the plasma
membrane. As a result, the sodium ions have stored or potential energy, just like water behind a dam.
Accordingly, if there is a route for Na+ to leak back in, some of the stored energy can be converted to
kinetic energy (energy of motion) and used to transport other substances against their concentration
gradients. In essence, secondary active transport proteins harness the energy in the Na+ concentration
gradient by providing routes for Na+ to leak into cells. In secondary active transport, a carrier protein
simultaneously binds to Na+ and another substance and then changes its shape so that both substances
cross the membrane at the same time. If these transporters move two substances in the same direction

they are called symporters (sim‐PORT‐ers; sym‐ = same); antiporters (an′‐tē‐PORT‐ers), in contrast,
move two substances in opposite directions across the membrane (anti‐ = against).
Plasma membranes contain several antiporters and symporters that are powered by the Na+ gradient
(Figure 3.11). For instance, the concentration of calcium ions (Ca2+) is low in the cytosol because
Na+/Ca2+ antiporters eject calcium ions. Likewise, Na+/H+ antiporters help regulate the cytosol's pH
(H+ concentration) by expelling excess H+. By contrast, dietary glucose and amino acids are absorbed
into cells that line the small intestine by Na+/glucose and Na+/amino acid symporters (Figure 3.11b). In
each case, sodium ions are moving down their concentration gradient while the other solutes move
“uphill,” against their concentration gradients. Keep in mind that all these symporters and antiporters
can do their job because the sodium‐potassium pumps maintain a low concentration of Na+ in the
cytosol.
Figure 3.11 Secondary active transport mechanisms.
(a) Antiporters carry two substances across the membrane in opposite directions. (b) Symporters carry two
substances across the membrane in the same direction.
Secondary active transport mechanisms use the energy stored in an ionic concentration gradient (here,
for Na+). Because primary active transport pumps that hydrolyze ATP maintain the gradient, secondary active
transport mechanisms consume ATP indirectly.
Imagineering
What is the main difference between primary and secondary active transport
mechanisms?
CLINICAL CONNECTION Digitalis Increases Ca2+ in Heart Muscle
Cells
Digitalis often is given to patients with heart failure, a condition of weakened pumping action by
the heart. Digitalis exerts its effect by slowing the action of the sodium‐potassium pumps, which lets
more Na+ accumulate inside heart muscle cells. The result is a decreased Na+ concentration gradient
across the plasma membrane, which causes the Na+/Ca2+ antiporters to slow down. As a result, more
Ca2+ remains inside heart muscle cells. The slight increase in the level of Ca2+ in the cytosol of heart
muscle cells increases the force of their contractions and thus strengthens the force of the heartbeat.
Transport in Vesicles
A vesicle (VES‐i‐kul = little blister or bladder), as noted earlier, is a small, spherical sac. As you will
learn later in this chapter, a variety of substances are transported in vesicles from one structure to
another within cells. Vesicles also import materials from and release materials into extracellular fluid.
During endocytosis (en′‐dō‐sī‐TŌ‐sis; endo‐ = within), materials move into a cell in a vesicle formed
from the plasma membrane. In exocytosis (ek′‐sō‐sī‐TŌ‐sis; exo‐ = out), materials move out of a cell
by the fusion with the plasma membrane of vesicles formed inside the cell. Both endocytosis and
exocytosis require energy supplied by ATP. Thus, transport in vesicles is an active process.
Endocytosis
Here we consider three types of endocytosis: receptor‐mediated endocytosis, phagocytosis, and bulk‐
phase endocytosis. Receptor‐mediated endocytosis is a highly selective type of endocytosis by which
cells take up specific ligands. (Recall that ligands are molecules that bind to specific receptors.) A
vesicle forms after a receptor protein in the plasma membrane recognizes and binds to a particular
particle in the extracellular fluid. For instance, cells take up cholesterol‐containing low‐density
lipoproteins (LDLs), transferrin (an iron‐transporting protein in the blood), some vitamins, antibodies,

and certain hormones by receptor‐mediated endocytosis. Receptor‐mediated endocytosis of LDLs (and
other ligands) occurs as follows (Figure 3.12):
1. Binding. On the extracellular side of the plasma membrane, an LDL particle that contains
cholesterol binds to a specific receptor in the plasma membrane to form a receptor–LDL
complex. The receptors are integral membrane proteins that are concentrated in regions of the
plasma membrane called clathrin‐coated pits. Here, a protein called clathrin attaches to the
membrane on its cytoplasmic side. Many clathrin molecules come together, forming a basketlike
structure around the receptor–LDL complexes that causes the membrane to invaginate (fold
inward).
2. Vesicle formation. The invaginated edges of the membrane around the clathrin‐coated pit
fuse, and a small piece of the membrane pinches off. The resulting vesicle, known as a clathrin‐
coated vesicle, contains the receptor–LDL complexes.
3. Uncoating. Almost immediately after it is formed, the clathrin‐coated vesicle loses its clathrin
coat to become an uncoated vesicle. Clathrin molecules either return to the inner surface of the
plasma membrane or help form coats on other vesicles inside the cell.
4. Fusion with endosome. The uncoated vesicle quickly fuses with a vesicle known as
an endosome. Within an endosome, the LDL particles separate from their receptors.
5. Recycling of receptors to plasma membrane. Most of the receptors accumulate in elongated
protrusions of the endosome (the arms of the cross‐shaped vesicle at the center of the figure).
These pinch off, forming transport vesicles that return the receptors to the plasma membrane.
An LDL receptor is returned to the plasma membrane about 10 minutes after it enters a cell.
6. Degradation in lysosomes. Other transport vesicles, which contain the LDL particles, bud off
the endosome and soon fuse with a lysosome. Lysosomes contain many digestive enzymes.
Certain enzymes break down the large protein and lipid molecules of the LDL particle into
amino acids, fatty acids, and cholesterol. These smaller molecules then leave the lysosome. The
cell uses cholesterol for rebuilding its membranes and for synthesis of steroids, such as estrogen.
Fatty acids and amino acids can be used for ATP production or to build other molecules needed
by the cell.
Figure 3.12
Receptor‐mediated endocytosis of a low‐density lipoprotein (LDL)
particle.
Receptor‐mediated endocytosis imports materials that are needed by cells. Imagineering
What are several other examples of ligands that can undergo receptor‐mediated
endocytosis?
CLINICAL CONNECTION Viruses and Receptor
‐Mediated
Endocytosis

Although receptor‐mediated endocytosis normally imports needed materials, some viruses are able
to use this mechanism to enter and infect body cells. For example, the human immunodeficiency
virus (HIV), which causes acquired immunodeficiency syndrome (AIDS), can attach to a receptor
called CD4. This receptor is present in the plasma membrane of white blood cells called helper T
cells. After binding to CD4, HIV enters the helper T cell via receptor‐mediated endocytosis.
Phagocytosis (fag′‐ō‐sī‐TŌ‐sis; phago‐ = to eat) is a form of endocytosis in which the cell engulfs large
solid particles, such as worn‐out cells, whole bacteria, or viruses (Figure 3.13). Only a few body cells,
termed phagocytes (FAG‐ō‐sīts), are able to carry out phagocytosis. Two main types of phagocytes
are macrophages, located in many body tissues, and neutrophils, a type of white blood cell.
Phagocytosis begins when the particle binds to a plasma membrane receptor on the phagocyte, causing
it to extend pseudopods (SOO‐dō‐pods; pseudo‐ = false; ‐pods = feet), projections of its plasma
membrane and cytoplasm. Pseudopods surround the particle outside the cell, and the membranes fuse to
form a vesicle called a phagosome, which enters the cytoplasm. The phagosome fuses with one or more
lysosomes, and lysosomal enzymes break down the ingested material. In most cases, any undigested
materials in the phagosome remain indefinitely in a vesicle called a residual body. The residual bodies
are then either secreted by the cell via exocytosis or they remain stored in the cell as lipofuscin
granules.
Figure 3.13 Phagocytosis.
Pseudopods surround a particle, and the membranes fuse to form a phagosome.
Phagocytosis is a vital defense mechanism that helps protect the body from disease.
Imagineering; ©Omikron/Photo Researchers
What triggers pseudopod formation?
CLINICAL CONNECTION Phagocytosis and Microbes
The process of phagocytosis is a vital defense mechanism that helps protect the body from disease.
Through phagocytosis, macrophages dispose of invading microbes and billions of aged, worn‐out
red blood cells every day; neutrophils also help rid the body of invading microbes. Pus is a mixture
of dead neutrophils, macrophages, and tissue cells and fluid in an infected wound.
Most body cells carry out bulk‐phase endocytosis, also called pinocytosis (pi‐nō‐sī‐TŌ‐sis; pino‐ = to
drink), a form of endocytosis in which tiny droplets of extracellular fluid are taken up (Figure 3.14). No
receptor proteins are involved; all solutes dissolved in the extracellular fluid are brought into the cell.
During bulk‐phase endocytosis, the plasma membrane folds inward and forms a vesicle containing a
droplet of extracellular fluid. The vesicle detaches or “pinches off” from the plasma membrane and
enters the cytosol. Within the cell, the vesicle fuses with a lysosome, where enzymes degrade the
engulfed solutes. The resulting smaller molecules, such as amino acids and fatty acids, leave the
lysosome to be used elsewhere in the cell. Bulk‐phase endocytosis occurs in most cells, especially
absorptive cells in the intestines and kidneys.
Figure 3.14 Bulk‐phase endocytosis.
The plasma membrane folds inward, forming a vesicle.

Most body cells carry out bulk‐phase endocytosis, the nonselective uptake of tiny droplets of
extracellular fluid.
Imagineering
How do receptor‐mediated endocytosis and phagocytosis differ from bulk‐phase
endocytosis?
Exocytosis
In contrast with endocytosis, which brings materials into a cell, exocytosis releases materials from a
cell. All cells carry out exocytosis, but it is especially important in two types of cells: (1) secretory cells
that liberate digestive enzymes, hormones, mucus, or other secretions and (2) nerve cells that release
substances called neurotransmitters (see Figure 12.23). In some cases, wastes are also released by
exocytosis. During exocytosis, membrane‐enclosed vesicles called secretory vesicles form inside the
cell, fuse with the plasma membrane, and release their contents into the extracellular fluid.
Segments of the plasma membrane lost through endocytosis are recovered or recycled by exocytosis.
The balance between endocytosis and exocytosis keeps the surface area of a cell's plasma membrane
relatively constant. Membrane exchange is quite extensive in certain cells. In your pancreas, for
example, the cells that secrete digestive enzymes can recycle an amount of plasma membrane equal to
the cell's entire surface area in 90 minutes.
Transcytosis
Transport in vesicles may also be used to successively move a substance into, across, and out of a cell.
In this active process, called transcytosis (tranz′‐sī‐TŌ‐sis), vesicles undergo endocytosis on one side
of a cell, move across the cell, and then undergo exocytosis on the opposite side. As the vesicles fuse
with the plasma membrane, the vesicular contents are released into the extracellular fluid. Transcytosis
occurs most often across the endothelial cells that line blood vessels and is a means for materials to
move between blood plasma and interstitial fluid. For instance, when a woman is pregnant, some of her
antibodies cross the placenta into the fetal circulation via transcytosis.
Table 3.1 summarizes the processes by which materials move into and out of cells.
Table 3.1 Transport of Materials into and out of Cells
TRANSPORT PROCESS DESCRIPTION SUBSTANCES TRANSPORTED
PASSIVE
PROCESSES
Movement of substances down a
concentration gradient until equilibrium
is reached; do not require cellular
energy in the form of ATP.
Diffusion Movement of molecules or ions down a
concentration gradient due to their
kinetic energy until they reach
equilibrium.
Simple diffusion Passive movement of a substance down
its concentration gradient through the
lipid bilayer of the plasma membrane
without the help of membrane transport
proteins.
Nonpolar, hydrophobic
solutes: oxygen, carbon
dioxide, and nitrogen gases;
fatty acids; steroids; and fat‐
soluble vitamins. Polar
molecules such as water,

urea, and small alcohols.
Facilitated
diffusion
Passive movement of a substance down
its concentration gradient through the
lipid bilayer by transmembrane proteins
that function as channels or carriers.
Polar or charged solutes:
glucose; fructose; galactose;
some vitamins; and ions such
as K+, Cl−, Na+, and Ca2+.
Osmosis Passive movement of water molecules
across a selectively permeable
membrane from an area of higher to
lower water concentration until
equilibrium is reached.
Solvent: water in living
systems.
ACTIVE
PROCESSES
Movement of substances against a
concentration gradient; requires cellular
energy in the form of ATP.
Active Transport Active process in which a cell expends
energy to move a substance across the
membrane against its concentration
gradient by transmembrane proteins
that function as carriers.
Polar or charged solutes.
Primary active
transport
Active process in which a substance
moves across the membrane against its
concentration gradient by pumps
(carriers) that use energy supplied by
hydrolysis of ATP.
Na+, K+, Ca2+, H+, I−, Cl−, and
other ions.
Secondary active
transport
Coupled active transport of two
substances across the membrane using
energy supplied by a Na+ or
H+concentration gradient maintained by
primary active transport pumps.
Antiporters move Na+ (or H+) and
another substance in opposite directions
across the membrane; symporters move
Na+ (or H+) and another substance in
the same direction across the
membrane.
Antiport: Ca2+, H+ out of
cells. Symport: glucose,
amino acids into cells.
Transport in
Vesicles
Active process in which substances
move into or out of cells in vesicles that
bud from plasma membrane; requires
energy supplied by ATP.
Endocytosis Movement of substances into a cell in
vesicles.
Receptor‐
mediated
endocytosis
Ligand–receptor complexes trigger
infolding of a clathrin‐coated pit that
forms a vesicle containing ligands.
Ligands: transferrin, low‐
density lipoproteins (LDLs),
some vitamins, certain

hormones, and antibodies.
Phagocytosis “Cell eating”; movement of a solid
particle into a cell after pseudopods
engulf it to form a phagosome.
Bacteria, viruses, and aged or
dead cells.
Bulk‐phase
endocytosis
“Cell drinking”; movement of
extracellular fluid into a cell by
infolding of plasma membrane to form
a vesicle.
Solutes in extracellular fluid.
Exocytosis Movement of substances out of a cell in
secretory vesicles that fuse with the
plasma membrane and release their
contents into the extracellular fluid.
Neurotransmitters,
hormones, and digestive
enzymes.
Transcytosis Movement of a substance through a cell
as a result of endocytosis on one side
and exocytosis on the opposite side.
Substances, such as
antibodies, across endothelial
cells. This is a common route
for substances to pass
between blood plasma and
interstitial fluid.
3.4 CYTOPLASM
OBJECTIVE
• Describe the structure and function of cytoplasm, cytosol, and organelles.
Cytoplasm consists of all the cellular contents between the plasma membrane and the nucleus, and has two
components: (1) the cytosol and (2) organelles, tiny structures that perform different functions in the cell.
Cytosol
The cytosol (intracellular fluid) is the fluid portion of the cytoplasm that surrounds organelles (see
Figure 3.1) and constitutes about 55% of total cell volume. Although it varies in composition and consistency
from one part of a cell to another, cytosol is 75–90% water plus various dissolved and suspended components.
Among these are different types of ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, and waste
products, some of which we have already discussed. Also present in some cells are various organic molecules
that aggregate into masses for storage. These aggregations may appear and disappear at different times in the
life of a cell. Examples include lipid droplets that contain triglycerides, and clusters of glycogen molecules
called glycogen granules.
The cytosol is the site of many chemical reactions required for a cell's existence. For example, enzymes in
cytosol catalyze glycolysis, a series of 10 chemical reactions that produce two molecules of ATP from one
molecule of glucose (see Figure 25.4). Other types of cytosolic reactions provide the building blocks for
maintenance of cell structures and for cell growth.

The cytoskeleton is a network of protein filaments that extends throughout the cytosol (see Figure 3.1). Three
types of filamentous proteins contribute to the cytoskeleton's structure, as well as the structure of other
organelles. In the order of their increasing diameter, these structures are microfilaments, intermediate
filaments, and microtubules.
Microfilaments
Microfilaments (mī‐krō‐FIL‐a‐ments) are the thinnest elements of the cytoskeleton. They are composed of the
proteins actin and myosin and are most prevalent at the edge of a cell (Figure 3.15a). Microfilaments have two
general functions: They help generate movement and provide mechanical support. With respect to movement,
microfilaments are involved in muscle contraction, cell division, and cell locomotion, such as occurs during
the migration of embryonic cells during development, the invasion of tissues by white blood cells to fight
infection, or the migration of skin cells during wound healing.
Figure 3.15 Cytoskeleton.
The cytoskeleton is a network of three types of protein filaments—microfilaments, intermediate filaments, and
microtubules—that extend throughout the cytoplasm.
FUNCTIONS OF THE CYTOSKELETON
1. Serves as a scaffold that helps to determine a cell's shape and to organize the cellular contents.
2. Aids movement of organelles within the cell, of chromosomes during cell division, and of whole
cells such as phagocytes.
Imagineering; (a) Albert Tousson/Phototake; (b) Albert Tousson/Phototake; (c) Alexey Khodjakov/Photo
Researchers, Inc.
Which cytoskeletal component helps form the structure of centrioles, cilia, and flagella?
Microfilaments provide much of the mechanical support that is responsible for the basic strength and shapes of
cells. They anchor the cytoskeleton to integral proteins in the plasma membrane. Microfilaments also provide
mechanical support for cell extensions called microvilli (mī‐krō‐VIL‐ī; micro‐ = small; ‐villi = tufts of hair;
singular is microvillus), nonmotile, microscopic fingerlike projections of the plasma membrane. Within each
microvillus is a core of parallel microfilaments that supports it. Because they greatly increase the surface area
of the cell, microvilli are abundant on cells involved in absorption, such as the epithelial cells that line the
small intestine.
Intermediate Filaments
As their name suggests, intermediate filaments are thicker than microfilaments but thinner than microtubules
(Figure 3.15b). Several different proteins can compose intermediate filaments, which are exceptionally strong.
They are found in parts of cells subject to mechanical stress; they help stabilize the position of organelles such
as the nucleus and help attach cells to one another.
Microtubules
Microtubules (mī‐krō‐TOO‐būls′) are the largest of the cytoskeletal components and are long, unbranched
hollow tubes composed mainly of the protein tubulin. The assembly of microtubules begins in an organelle

called the centrosome (discussed shortly). The microtubules grow outward from the centrosome toward the
periphery of the cell (Figure 3.15c). Microtubules help determine cell shape. They also function in the
movement of organelles such as secretory vesicles, of chromosomes during cell division, and of specialized
cell projections, such as cilia and flagella.
Organelles
As noted earlier, organelles are specialized structures within the cell that have characteristic shapes, and they
perform specific functions in cellular growth, maintenance, and reproduction. Despite the many chemical
reactions going on in a cell at any given time, there is little interference among reactions because they are
confined to different organelles. Each type of organelle has its own set of enzymes that carry out specific
reactions, and serves as a functional compartment for specific biochemical processes. The numbers and types
of organelles vary in different cells, depending on the cell's function. Although they have different functions,
organelles often cooperate to maintain homeostasis. Even though the nucleus is a large organelle, it is
discussed in a separate section because of its special importance in directing the life of a cell.
Centrosome
The centrosome (SEN‐trō‐sōm), located near the nucleus, consists of two components: a pair of centrioles and
pericentriolar material (Figure 3.16a). The two centrioles (SEN‐trē‐ōls) are cylindrical structures, each
composed of nine clusters of three microtubules (triplets) arranged in a circular pattern (Figure 3.16b). The
long axis of one centriole is at a right angle to the long axis of the other (Figure 3.16c). Surrounding the
centrioles is pericentriolar material (per′‐ē‐sen′‐trē‐Ō‐lar), which contains hundreds of ring‐shaped
complexes composed of the protein tubulin. These tubulin complexes are the organizing centers for growth of
the mitotic spindle, which plays a critical role in cell division, and for microtubule formation in nondividing
cells. During cell division, centrosomes replicate so that succeeding generations of cells have the capacity for
cell division.
Figure 3.16 Centrosome.
Located near the nucleus, the centrosome consists of a pair of centrioles and pericentriolar material.
FUNCTIONS OF CENTROSOMES
1. The pericentriolar material of the centrosome contains tubulins that build microtubules in
nondividing cells.
2. The pericentriolar material of the centrosome forms the mitotic spindle during cell division.
Tomo Narashima; (c) David M. Phillips/Visuals Unlimited
If you observed that a cell did not have a centrosome, what could you predict about its
capacity for cell division?
Cilia and Flagella
Microtubules are the dominant components of cilia and flagella, which are motile projections of the cell
surface. Cilia (SIL‐ē‐a = eyelashes; singular is cilium) are numerous, short, hairlike projections that extend
from the surface of the cell (see Figures 3.1 and 3.17b). Each cilium contains a core of 20 microtubules

surrounded by plasma membrane (Figure 3.17a). The microtubules are arranged such that one pair in the center
is surrounded by nine clusters of two fused microtubules (doublets). Each cilium is anchored to a basal
body just below the surface of the plasma membrane. A basal body is similar in structure to a centriole and
functions in initiating the assembly of cilia and flagella.
(b) P. Motta/Photo Researchers, Inc.; (c) David M. Phillips/Visuals Unlimited
Figure 3.17 Cilia and flagella.
A cilium contains a core of microtubules with one pair in the center surrounded by nine clusters of doublet
microtubules.
FUNCTIONS OF CILIA AND FLAGELLA
1. Cilia move fluids along a cell's surface.
2. A flagellum moves an entire cell.
Tomo Narashima
What is the functional difference between cilia and flagella?
A cilium displays an oarlike pattern of beating; it is relatively stiff during the power stroke (oar digging into
the water), but more flexible during the recovery stroke (oar moving above the water preparing for a new
stroke) (Figure 3.17d). The coordinated movement of many cilia on the surface of a cell causes the steady
movement of fluid along the cell's surface. Many cells of the respiratory tract, for example, have hundreds of
cilia that help sweep foreign particles trapped in mucus away from the lungs. In cystic fibrosis, the extremely
thick mucous secretions that are produced interfere with ciliary action and the normal functions of the
respiratory tract.
CLINICAL CONNECTION Cilia and Smoking
The movement of cilia is also paralyzed by nicotine in cigarette smoke. For this reason, smokers cough
often to remove foreign particles from their airways. Cells that line the uterine (fallopian) tubes also have
cilia that sweep oocytes (egg cells) toward the uterus, and females who smoke have an increased risk of
ectopic (outside the uterus) pregnancy.
Flagella (fla‐JEL‐a = whip; singular is flagellum) are similar in structure to cilia but are typically much
longer. Flagella usually move an entire cell. A flagellum generates forward motion along its axis by rapidly

wiggling in a wavelike pattern (Figure 3.17e). The only example of a flagellum in the human body is a sperm
cell's tail, which propels the sperm toward the oocyte in the uterine tube (Figure 3.17c).
Ribosomes
Ribosomes (RĪ‐bō‐sōms; ‐somes = bodies) are the sites of protein synthesis. The name of these tiny structures
reflects their high content of one type of ribonucleic acid, ribosomal RNA (rRNA), but each one also includes
more than 50 proteins. Structurally, a ribosome consists of two subunits, one about half the size of the other
(Figure 3.18). The large and small subunits are made separately in the nucleolus, a spherical body inside the
nucleus. Once produced, the large and small subunits exit the nucleus separately, then come together in the
cytoplasm.
Figure 3.18 Ribosomes.
Ribosomes are the sites of protein synthesis.
FUNCTIONS OF RIBOSOMES
1. Ribosomes associated with endoplasmic reticulum synthesize proteins destined for insertion in the
plasma membrane or secretion from the cell.
2. Free ribosomes synthesize proteins used in the cytosol.
Tomo Narashima
Where are subunits of ribosomes synthesized and assembled?
Some ribosomes are attached to the outer surface of the nuclear membrane and to an extensively folded
membrane called the endoplasmic reticulum. These ribosomes synthesize proteins destined for specific
organelles, for insertion in the plasma membrane, or for export from the cell. Other ribosomes are “free” or
unattached to other cytoplasmic structures. Free ribosomes synthesize proteins used in the cytosol. Ribosomes
are also located within mitochondria, where they synthesize mitochondrial proteins.
Endoplasmic Reticulum
The endoplasmic reticulum (en′‐dō‐PLAS‐mik re‐TIK‐ū‐lum; ‐plasmic = cytoplasm; reticulum = network)
or ER is a network of membranes in the form of flattened sacs or tubules (Figure 3.19). The ER extends from
the nuclear envelope (membrane around the nucleus), to which it is connected, throughout the cytoplasm. The
ER is so extensive that it constitutes more than half of the membranous surfaces within the cytoplasm of most
cells.
Figure 3.19 Endoplasmic reticulum.
The endoplasmic reticulum is a network of membrane‐enclosed sacs or tubules that extend throughout the
cytoplasm and connect to the nuclear envelope.
FUNCTIONS OF ENDOPLASMIC RETICULUM

1. Rough ER synthesizes glycoproteins and phospholipids that are transferred into cellular organelles,
inserted into the plasma membrane, or secreted during exocytosis.
2. Smooth ER synthesizes fatty acids and steroids, such as estrogens and testosterone; inactivates or
detoxifies drugs and other potentially harmful substances; removes the phosphate group from
glucose‐6‐phosphate; and stores and releases calcium ions that trigger contraction in muscle cells.
Tomo Narashima; (b) D. W. Fawcett/Photo Researchers, Inc.
What are the structural and functional differences between rough and smooth ER?
Cells contain two distinct forms of ER, which differ in structure and function. Rough ER is continuous with
the nuclear membrane and usually is folded into a series of flattened sacs. The outer surface of rough ER is
studded with ribosomes, the sites of protein synthesis. Proteins synthesized by ribosomes attached to rough ER
enter spaces within the ER for processing and sorting. In some cases, enzymes attach the proteins to
carbohydrates to form glycoproteins. In other cases, enzymes attach the proteins to phospholipids, also
synthesized by rough ER. These molecules (glycoproteins and phospholipids) may be incorporated into the
membranes of organelles, inserted into the plasma membrane, or secreted via exocytosis. Thus rough ER
produces secretory proteins, membrane proteins, and many organellar proteins.
Smooth ER extends from the rough ER to form a network of membrane tubules (Figure 3.19). Unlike rough
ER, smooth ER does not have ribosomes on the outer surfaces of its membrane. However, smooth ER contains
unique enzymes that make it functionally more diverse than rough ER. Because it lacks ribosomes, smooth ER
does not synthesize proteins, but it does synthesize fatty acids and steroids, such as estrogens and testosterone.
In liver cells, enzymes of the smooth ER help release glucose into the bloodstream and inactivate or detoxify
lipid‐soluble drugs or potentially harmful substances, such as alcohol, pesticides, and carcinogens (cancer‐
causing agents). In liver, kidney, and intestinal cells a smooth ER enzyme removes the phosphate group from
glucose‐6‐phosphate, which allows the “free” glucose to enter the bloodstream. In muscle cells, the calcium
ions (Ca2+) that trigger contraction are released from the sarcoplasmic reticulum, a form of smooth ER.
CLINICAL CONNECTION Smooth ER and Drug Tolerance
One of the functions of smooth ER, as noted earlier, is to detoxify certain drugs. Individuals who repeatedly
take such drugs, such as the sedative phenobarbital, develop changes in the smooth ER in their liver cells.
Prolonged administration of phenobarbital results in increased tolerance to the drug; the same dose no
longer produces the same degree of sedation. With repeated exposure to the drug, the amount of smooth ER
and its enzymes increases to protect the cell from its toxic effects. As the amount of smooth ER increases,
higher and higher dosages of the drug are needed to achieve the original effect. This could result in an
increased possibility of overdose and increased drug dependence.
Golgi Complex
Most of the proteins synthesized by ribosomes attached to rough ER are ultimately transported to other regions
of the cell. The first step in the transport pathway is through an organelle called the Golgi complex (GOL‐jē).
It consists of 3 to 20 cisternae (sis‐TER‐nē = cavities; singular is cisterna), small, flattened membranous sacs
with bulging edges that resemble a stack of pita bread (Figure 3.20). The cisternae are often curved, giving the

Golgi complex a cuplike shape. Most cells have several Golgi complexes, and Golgi complexes are more
extensive in cells that secrete proteins, a clue to the organelle's role in the cell.
Figure 3.20 Golgi complex.
The opposite faces of a Golgi complex differ in size, shape, content, and enzymatic activities.
FUNCTIONS OF GOLGI COMPLEX
1. Modifies, sorts, packages, and transports proteins received from the rough ER.
2. Forms secretory vesicles that discharge processed proteins via exocytosis into extracellular fluid;
forms membrane vesicles that ferry new molecules to the plasma membrane; forms transport
vesicles that carry molecules to other organelles, such as lysosomes.
Tomo Narashima; (b) Biophoto Associates/Photo Researchers, Inc.
How do the entry and exit faces differ in function?
The cisternae at the opposite ends of a Golgi complex differ from each other in size, shape, and enzymatic
activity. The convex entry or cis face is a cisterna that faces the rough ER. The concave exit or trans face is a
cisterna that faces the plasma membrane. Sacs between the entry and exit faces are called medial
cisternae. Transport vesicles (described shortly) from the ER merge to form the entry face. From the entry
face, the cisternae are thought to mature, in turn becoming medial and then exit cisternae.
Different enzymes in the entry, medial, and exit cisternae of the Golgi complex permit each of these areas to
modify, sort, and package proteins into vesicles for transport to different destinations. The entry face receives
and modifies proteins produced by the rough ER. The medial cisternae add carbohydrates to proteins to form
glycoproteins and lipids to proteins to form lipoproteins. The exit face modifies the molecules further and then
sorts and packages them for transport to their destinations.
Proteins arriving at, passing through, and exiting the Golgi complex do so through maturation of the cisternae
and exchanges that occur via transfer vesicles (Figure 3.21):
1. Proteins synthesized by ribosomes on the rough ER are surrounded by a piece of the ER membrane,
which eventually buds from the membrane surface to form transport vesicles.
2. Transport vesicles move toward the entry face of the Golgi complex.
3. Fusion of several transport vesicles creates the entry face of the Golgi complex and releases proteins
into its lumen (space).
4. The proteins move from the entry face into one or more medial cisternae. Enzymes in the medial
cisternae modify the proteins to form glycoproteins, glycolipids, and lipoproteins. Transfer
vesicles that bud from the edges of the cisternae move specific enzymes back toward the entry face and
move some partially modified proteins toward the exit face.

5. The products of the medial cisternae move into the lumen of the exit face.
6. Within the exit face cisterna, the products are further modified and are sorted and packaged.
7. Some of the processed proteins leave the exit face and are stored in secretory vesicles. These vesicles
deliver the proteins to the plasma membrane, where they are discharged by exocytosis into the
extracellular fluid. For example, certain pancreatic cells release the hormone insulin in this way.
8. Other processed proteins leave the exit face in membrane vesicles that deliver their contents to the
plasma membrane for incorporation into the membrane. In doing so, the Golgi complex adds new
segments of plasma membrane as existing segments are lost and modifies the number and distribution
of membrane molecules.
9. Finally, some processed proteins leave the exit face in transport vesicles that will carry the proteins to
another cellular destination. For instance, transport vesicles carry digestive enzymes to lysosomes; the
structure and functions of these important organelles are discussed next.
Figure 3.21
Processing and packaging of proteins by the Golgi complex.
All proteins exported from the cell are processed in the Golgi complex. Imagineering
What are the three general destinations for proteins that leave the Golgi complex?
Lysosomes
Lysosomes (LĪ‐sō‐sōms; lyso‐ = dissolving; ‐somes = bodies) are membrane‐enclosed vesicles that form from
the Golgi complex (Figure 3.22). They can contain as many as 60 kinds of powerful digestive and hydrolytic
enzymes that can break down a wide variety of molecules once lysosomes fuse with vesicles formed during
endocytosis. Because lysosomal enzymes work best at an acidic pH, the lysosomal membrane includes active
transport pumps that import hydrogen ions (H+). Thus, the lysosomal interior has a pH of 5, which is 100 times
more acidic than the pH of the cytosol (pH 7). The lysosomal membrane also includes transporters that move
the final products of digestion, such as glucose, fatty acids, and amino acids, into the cytosol.
Figure 3.22 Lysosomes.
Lysosomes contain several types of powerful digestive enzymes.
FUNCTIONS OF LYSOSOMES
1. Digest substances that enter a cell via endocytosis and transport final products of digestion into
cytosol.
2. Carry out autophagy, the digestion of worn‐out organelles.

3. Implement autolysis, the digestion of an entire cell.
4. Accomplish extracellular digestion.
Tomo Narashima; (b) David M. Phillips/Visuals Unlimited
What is the name of the process by which worn‐out organelles are digested by
lysosomes?
Lysosomal enzymes also help recycle worn‐out cell structures. A lysosome can engulf another organelle,
digest it, and return the digested components to the cytosol for reuse. In this way, old organelles are
continually replaced. The process by which entire worn‐out organelles are digested is called autophagy (aw‐
TOF‐a‐jē; auto‐ = self; ‐phagy = eating). In autophagy, the organelle to be digested is enclosed by a membrane
derived from the ER to create a vesicle called an autophagosome (aw‐tō‐FĀ‐gō‐sōm); the vesicle then fuses
with a lysosome. In this way, a human liver cell, for example, recycles about half of its cytoplasmic contents
every week. Autophagy is also involved in cellular differentiation, control of growth, tissue remodeling,
adaptation to adverse environments, and cell defense. Lysosomal enzymes may also destroy the entire cell that
contains them, a process known as autolysis (aw‐TOL‐i‐sis). Autolysis occurs in some pathological conditions
and also is responsible for the tissue deterioration that occurs immediately after death.
As we just discussed, most lysosomal enzymes act within a cell. However, some operate in extracellular
digestion. One example occurs during fertilization. The head of a sperm cell releases lysosomal enzymes that
aid its penetration of the oocyte by dissolving its protective coating in a process called the acrosomal reaction
(see Section 29.1).
CLINICAL CONNECTION Tay
‐Sachs Disease
Some disorders are caused by faulty or absent lysosomal enzymes. For instance, Tay‐Sachs disease (TĀ‐
SAKS), which most often affects children of Ashkenazi (eastern European Jewish) descent, is an inherited
condition characterized by the absence of a single lysosomal enzyme called Hex A. This enzyme normally
breaks down a membrane glycolipid called ganglioside GM2 that is especially prevalent in nerve cells. As
the excess ganglioside GM2 accumulates, the nerve cells function less efficiently. Children with Tay‐Sachs
disease typically experience seizures and muscle rigidity. They gradually become blind, demented, and
uncoordinated and usually die before the age of 5. Tests can now reveal whether an adult is a carrier of the
defective gene.
Peroxisomes
Another group of organelles similar in structure to lysosomes, but smaller, are the peroxisomes (pe‐ROKS‐i‐
sōms; peroxi‐ = peroxide; ‐somes = bodies; see Figure 3.1). Peroxisomes, also called microbodies, contain
several oxidases, enzymes that can oxidize (remove hydrogen atoms from) various organic substances. For
instance, amino acids and fatty acids are oxidized in peroxisomes as part of normal metabolism. In addition,
enzymes in peroxisomes oxidize toxic substances, such as alcohol. Thus, peroxisomes are very abundant in the
liver, where detoxification of alcohol and other damaging substances occurs. A by‐product of the oxidation
reactions is hydrogen peroxide (H2O2), a potentially toxic compound, and associated free radicals such as
superoxide. However, peroxisomes also contain the enzyme catalase, which decomposes H2O2. Because
production and degradation of H2O2 occur within the same organelle, peroxisomes protect other parts of the

cell from the toxic effects of H2O2. Peroxisomes also contain enzymes that destroy superoxide. Without
peroxisomes, by‐products of metabolism could accumulate inside a cell and result in cellular death.
Peroxisomes can self‐replicate. New peroxisomes may form from preexisting ones by enlarging and dividing.
They may also form by a process in which components accumulate at a given site in the cell and then assemble
into a peroxisome.
Proteasomes
As you have just learned, lysosomes degrade proteins delivered to them in vesicles. Cytosolic proteins also
require disposal at certain times in the life of a cell. Continuous destruction of unneeded, damaged, or faulty
proteins is the function of tiny barrel‐shaped structures consisting of four stacked rings of proteins around a
central core called proteasomes (PRŌ‐tē‐a‐sōms = protein bodies). For example, proteins that are part of
metabolic pathways need to be degraded after they have accomplished their function. Such protein destruction
plays a part in negative feedback by halting a pathway once the appropriate response has been achieved. A
typical body cell contains many thousands of proteasomes, in both the cytosol and the nucleus. Discovered
only recently because they are far too small to discern under the light microscope and do not show up well in
electron micrographs, proteasomes were so named because they contain myriad proteases, enzymes that cut
proteins into small peptides. Once the enzymes of a proteasome have chopped up a protein into smaller
chunks, other enzymes then break down the peptides into amino acids, which can be recycled into new
proteins.
CLINICAL CONNECTION Proteasomes and Disease
Some diseases could result from failure of proteasomes to degrade abnormal proteins. For example, clumps
of misfolded proteins accumulate in brain cells of people with Parkinson disease and Alzheimer disease.
Discovering why the proteasomes fail to clear these abnormal proteins is a goal of ongoing research.
Mitochondria
Because they generate most of the ATP through aerobic (oxygen‐requiring) respiration, mitochondria (mī‐tō‐
KON‐drē‐a; mito‐ = thread; ‐chondria = granules; singular is mitochondrion) are referred to as the
“powerhouses” of the cell. A cell may have as few as a hundred or as many as several thousand mitochondria,
depending on its activity. Active cells, such as those found in the muscles, liver, and kidneys, which use ATP
at a high rate, have a large number of mitochondria. For example, regular exercise can lead to an increase in
the number of mitochondria in muscle cells, and this allows muscle cells to function more efficiently.
Mitochondria are usually located within the cell where oxygen enters the cell or where the ATP is used, for
example, among the contractile proteins in muscle cells.
A mitochondrion consists of an outer mitochondrial membrane and an inner mitochondrial
membrane with a small fluid‐filled space between them (Figure 3.23). Both membranes are similar in
structure to the plasma membrane. The inner mitochondrial membrane contains a series of folds
called mitochondrial cristae (KRIS‐tē = ridges). The central fluid‐filled cavity of a mitochondrion, enclosed
by the inner mitochondrial membrane, is the mitochondrial matrix. The elaborate folds of the cristae provide
an enormous surface area for the chemical reactions that are part of the aerobic phase of cellular
respiration, the reactions that produce most of a cell's ATP (see Chapter 25). The enzymes that catalyze these
reactions are located on the cristae and in the matrix of the mitochondria.
Figure 3.23 Mitochondria.
Within mitochondria, chemical reactions of aerobic cellular respiration generate ATP.
FUNCTIONS OF MITOCHONDRIA

1. Generate ATP through reactions of aerobic cellular respiration.
2. Play an important early role in apoptosis.
Tomo Narashima; (b) Don W. Fawcett/Visuals Unlimited
How do the mitochondrial cristae contribute to its ATP‐producing function?
Mitochondria also play an important and early role in apoptosis (ap′‐ōp‐TŌ‐sis or ap‐ō‐TŌ‐sis = a falling off),
the orderly, genetically programmed death of a cell. In response to stimuli such as large numbers of destructive
free radicals, DNA damage, growth factor deprivation, or lack of oxygen and nutrients, certain chemicals are
released from mitochondria following the formation of a pore in the outer mitochondrial membrane. One of the
chemicals released into the cytosol of the cell is cytochrome c, which while inside the mitochondria is involved
in aerobic cellular respiration. In the cytosol, however, cytochrome c and other substances initiate a cascade of
activation of protein‐digesting enzymes that bring about apoptosis.
Like peroxisomes, mitochondria self‐replicate, a process that occurs during times of increased cellular energy
demand or before cell division. Synthesis of some of the proteins needed for mitochondrial functions occurs on
the ribosomes that are present in the mitochondrial matrix. Mitochondria even have their own DNA, in the
form of multiple copies of a circular DNA molecule that contains 37 genes. These mitochondrial genes control
the synthesis of 2 ribosomal RNAs, 22 transfer RNAs, and 13 proteins that build mitochondrial components.
Although the nucleus of each somatic cell contains genes from both your mother and your father,
mitochondrial genes are inherited only from your mother. This is due to the fact that all mitochondria in a cell
are descendants of those that were present in the oocyte (egg) during the fertilization process. The head of a
sperm (the part that penetrates and fertilizes an oocyte) normally lacks most organelles, such as mitochondria,
ribosomes, endoplasmic reticulum, and the Golgi complex, and any sperm mitochondria that do enter the
oocyte are soon destroyed. Since all mitochondrial genes are inherited from the maternal parent, mitochondrial
DNA can be used to trace maternal lineage (in other words, to determine whether two or more individuals are
related through their mother's side of the family).
3.5 NUCLEUS
OBJECTIVE
• Describe the structure and function of the nucleus.
The nucleus is a spherical or oval‐shaped structure that usually is the most prominent feature of a cell
(Figure 3.24). Most cells have a single nucleus, although some, such as mature red blood cells, have none. In
contrast, skeletal muscle cells and a few other types of cells have multiple nuclei. A double membrane called
the nuclear envelope separates the nucleus from the cytoplasm. Both layers of the nuclear envelope are lipid
bilayers similar to the plasma membrane. The outer membrane of the nuclear envelope is continuous with
rough ER and resembles it in structure. Many openings called nuclear pores extend through the nuclear