Exchange of materials across the Cell Membrane

This essay discusses the movements of substances such as glucose, water, ions, food, hormones and oxygen across the cell membrane through specialized proteins. It focuses on diffusion, osmosis, active transport and phagocytosis and how they are related to the structure of the plasma membrane.

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Exchange of materials across the Cell Membrane
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Exchange of materials across the Cell Membrane
Introduction
The role of a cell is to move substances such as glucose, water, ions, food, hormones and
oxygen across the cell membrane (AlHarbi et al., 2015). Most of these materials cannot pass
directly through the cell membrane; therefore, their transport occurs through specialized proteins.
In general, only small non-polar molecules pass through the cell membrane. For example, gases
like oxygen or carbon dioxide can move through the membrane freely. Different types of
movements that happen across the cell membrane include diffusion, osmosis, active transport
and phagocytosis. This essay will focus on these movements and how they are related to the
structure of the plasma membrane.
Diffusion
Diffusion is the net passive movement of particles from an area of higher medium to that of a
lower medium. The flow of these molecules continues until there is a uniform concentration
throughout. Diffusion is a process that works best with gases or liquids. It is faster in gases as
particles move faster than in fluids where particles move slowly (Chung et al., 2015). Diffusion
is as a result of the collision of molecules, and an example of this is when food coloring
molecules interact with water molecules, causing the colorings to move around the glass
randomly. The molecules spread out over space as collisions continue. Diffusion occurs when
particles are spread and free to move. Besides, diffusion is an essential activity in human organs
where chemicals and gases move in and out of cells. Human organs where diffusion occurs
include; the lungs where oxygen and carbon dioxide move in and out and in kidneys where
movement of water, salts and waste products occurs (Chung et al., 2015). Change of calcium

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from food into cells in the intestines is also an example of places diffusion occurs in human
organs.
Importance of diffusion
 It helps molecules enter the body and wastes get eliminated.
 It helps in movement of digested food materials from the intestine through the blood.
Osmosis
It is the process by which water molecules pass through a semipermeable membrane from a
solution of high to low concentrations (Dixon, 2015). Osmosis is passive transport since it does
not require the application of energy. An examples of a perfect solvent is water while a living
tissue can be used as a semipermeable membrane. Osmosis balances the concentrations of solute
on interior and exterior sides of a membrane. It occurs when a partially permeable membrane
separates two solutions. Osmosis accounts for many functions that maintain life. Presence of cell
wall in plant cells makes them remain turgid when placed in pure distilled water. Unlike the

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plant cells, blood cells swell and ruptures due to the absence of the cell wall. Osmosis is useful in
plants, and it helps them absorb water from the soil through their root hair cells (Evans, 2018).
Living things also benefit from osmosis in the following ways, uptake of nutrients and minerals
by the cells which is essential to the survival of a cell and getting rid of their metabolic waste
products such as urea. Water dissolves many essential molecules and nutrients that one needs to
live. So if water could not diffuse, humans would not get critical molecules to live.
Osmosis can be reversed and it’s a method of getting clean water from dirty water or salt water
by using pressure to force water out of a membrane (Evans, 2018). Reverse osmosis can be
demonstrated by filtering polluted water using pressure. Filtered water is very beneficial to
humans in that drinking of purified water minimizes the exposure to contaminants, and it
potentially reduces allergy symptoms. Although reverse osmosis water filters reduce a pretty
broad spectrum of contaminants such as lead, mercury, calcium, iron, dissolved salts, cysts, and
asbestos, it does not remove some solvents, pesticides, volatile organic chemicals (VOCs) such
as ions and metals, e.g., Chlorine and Radon.
Active transport
Active transport is the movement of molecules from their medium of lower concentration to the
medium of higher concentration across a membrane and against the concentration gradient or
other obstructing factors (Jaroszek & Dydo, 2016). Active transport needs cellular energy to
make its movement. Active transport is of two types which are primary and secondary active
transport. Primary active transport uses Adenosine triphosphate while secondary active transport
uses an electrochemical gradient. Primary active transport includes the Sodium-potassium pump
and Anti-port pump while Secondary active transport comprises of Exocytosis and Endocytosis.

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Phagocytosis
It is the ingestion of bacteria or other phagocytes and amoeboid protozoans. Phagocytosis is a
primary mechanism used to get rid of cell debris and pathogens (Logan, 2015). For instance,
when a macrophage digests a pathogenic microorganism, the pathogen is trapped in a phagosome
which then combines with a lysosome to form a phagolysosome.
How the Exchange of Materials across the Cell Membrane is related to its Structure
The plasma membrane is flexible and comprises of different types of molecules making it a fluid
mosaic. The phospholipids form the basic structure of the cell membrane named the lipid bilayer.
Cholesterol molecules are dispersed in the lipid layer and keeps the membrane fluid consistent.
The arrangement of the bilayer permits small, uncharged substances, e.g., hydrophobic
molecules, Oxygen, and carbon dioxide to penetrate the cell membrane, down their concentration
gradient by simple diffusion. Membrane proteins are essential in transporting substances across
the cell membrane and works as enzymes or receptors (Murphy, McDevitt & Engler, 2014).
Since the cell membrane is selectively permeable; it moderates the entry and exit of molecules
into and out of the cell. A physical wall is formed between the cell and the external environment
as it only permits selected important molecules to pass through and prevent unwanted substances
at the same time.
Cell membrane moderates the entry and exit of molecules into and out of the cell. Crucial
molecules permitted by the cell membrane are; water, amino acids, oxygen and glucose (Oztas,
2014).

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Conclusion
The cell membrane or plasma membrane is present in all living things. Its main task is to
separate the outside of the cell from the inside. All cells have cell membranes which separate the
cytoplasm from its surroundings. In general, the cell membrane takes charge of the movement of
substances in and out of the cell, and it is located on the outside of a cell.

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References
AlHarbi, N.N., Treagust, D.F., Chandrasegaran, A.L. and Won, M., 2015. Influence of particle
theory conceptions on pre-service science teachers’ understanding of osmosis and
diffusion. Journal of Biological Education, 49(3), pp.232-245.
Chung, J., Torta, F., Masai, K., Lucast, L., Czapla, H., Tanner, L.B., Narayanaswamy, P., Wenk,
M.R., Nakatsu, F. and De Camilli, P., 2015. PI4P/phosphatidylserine countertransport at ORP5-
and ORP8-mediated ER–plasma membrane contacts. Science, 349(6246), pp.428-432.
Dixon, E.M., 2015. Ninth grade students' understanding of diffusion and osmosis after
participation in direct or inquiry-based lessons: a mixed methods study (Doctoral dissertation).
Evans, E.A., 2018. Mechanics and Thermodynamics of Biomembranes: 0. CRC press.
Jaroszek, H. and Dydo, P., 2016. Diffusion and osmosis in potassium nitrate synthesis by
electrodialysis metathesis. Architecture Civil Engineering Environment, 9(4), pp.115-121.
Logan, B., Penn State Research Foundation, 2015. Materials and configurations for scalable
microbial fuel cells. U.S. Patent 8,962,165.
Murphy, W.L., McDevitt, T.C. and Engler, A.J., 2014. Materials as stem cell regulators. Nature
materials, 13(6), p.547.
Oztas, F., 2014. How do high school students know diffusion and osmosis? High school
students’ difficulties in understanding diffusion & osmosis. Procedia-Social and Behavioral
Sciences, 116, pp.3679-3682.