Biology Assignment: Plasma Membrane Structure and Function

This report provides a detailed overview of the plasma membrane, the outer boundary of both prokaryotic and eukaryotic cells. It begins by describing the membrane's structure, emphasizing the phospholipid bilayer composed of amphipathic molecules, integral and peripheral proteins, and cholesterol, highlighting its semi-permeable nature. The report then explains the membrane's function in regulating the concentration of substances and discusses various transport mechanisms. It differentiates between passive transport (simple diffusion) and active transport, illustrating how oxygen and sodium, respectively, move across the membrane. The report also mentions the sodium-potassium pump and symporters. References are provided to support the information.

Plasma membrane is a biological and outer membrane of cell found in both prokaryotes and
eukaryotes. It is semi permeable in nature and acts as a barrier between inner cell and outer cell
surface. This membrane is composed of tightly packed phospholipid bilayer which permits
movement of only selected molecules through it. A single molecule of phospholipid consists of
a group of phosphate known as head on one end and on another end is the lipid tail which is
made by 2 side by side chains of fatty acids. Head of phosphate group is negatively charged, that
makes it hydrophilic and polar in nature so it is attracted to water of both intracellular and
extracellular environment (Yeagle, 2009). Similarly, tails are nonpolar in nature i.e. it is
hydrophobic in nature which repels water. So, the membrane has hydrophobic interior and
hydrophilic exterior. As it is made up of 2 layers of phospholipids, tails of one-layer face towards
the tail of other layer and these assembles at the boundary of two layers.
Phospholipids are amphipathic molecules as it contains hydrophobic as well as hydrophilic
regions. Some of lipid tails contains saturated fatty acids and other contains unsaturated fatty
acids and because of which tails are continuously moving (Soult, 2016). The lipid bilayer forms
the basic structure of the plasma membrane and consist of several proteins which are of two
types: integral protein and other is called peripheral protein. Integral proteins like channel
protein, cell recognition proteins, glycoproteins etc are present in the cell. Glycoproteins act as a
receptor and are used for cell recognition. Many lipids with short carbohydrate chains are also
attached on the membrane on extracellular side.
Peripheral protein is present on the outer or inner surface of the phospholipid bilayer and can
also be found on the surface of integral protein which serves specific purpose for the cell. It also
consists of lipids like cholesterol which helps to maintain the fluidity of the membrane.
One of the most important function of the plasma membrane is that it helps in the regulation of
the concentration of substances like fatty acids, amino acids, wastes like carbon dioxide and ions
like sodium, potassium etc inside the cell.
Plasma membrane allows only few non-polar substances like oxygen, alcohol, carbon dioxide to
pass through the lipid bilayer as the tails are non-polar in nature. But other water-soluble
substances like amino acids, glucose and electrolytes need some support to pass through it as
they are resisted by the hydrophobic tails of membrane. Each substance needs either passive
transport or active transport systems to move across the membrane.
Passive transport is a process of movement of any molecules across the plasma membrane
without use of any cellular energy via concentration gradient or electrical gradient (Cooper,
2000). In contrast to this, active transport is a process in which energy in the form of ATP
(adenosine triphosphate) is required for movement of substance through the membrane against
concentration gradient (NJ & MJ, 2015). It also often needs carrier proteins for the transport
process.

Oxygen easily diffuses across the plasma membrane from outer cellular surface to inner cellular
surface using its own intrinsic kinetic energy as it is more concentrated outside the membrane.
As we know that cells use oxygen very rapidly during metabolism, the oxygen concentration
inside the cell is very low in comparison to outside the cell. So, oxygen from the interstitial fluid
diffuses by the process of simple diffusion through the cell membrane inside the cell. Oxygen
can directly pass through the membrane by passive diffusion as it simply dissolves in the lipid
bilayer and diffuses across the layer (Albert, Johnson, Raff, & Roberts, 2002). Then it dissolves
in the aqueous solution inside the cell on the other side of membrane. It does not require any
membrane protein for transportation. The direction of transport is simply determined by the
concentration gradient. In this process, external source of energy or carrier protein is not
required.
Sodium being the charged molecule cannot pass through the plasma membrane. For the
movement of such molecules across the membrane requires specific transport system is used
which helps to control the entry and exit of most molecules in the cell. It also needs specific
protein called channel protein to transport molecules like potassium, sodium, calcium etc,
through the membrane. These proteins form small open pores in the lipid membrane and allows
these molecules of appropriate charge and size to pass through the membrane. One of the most
common active transport is called sodium potassium pump. In this process ATP is used to force
to pass potassium ions inside the cell and sodium out from the cell. This pump helps to maintain
high amount of sodium outside the cell. Whenever cell needs sodium ion, pump opens a passive
sodium channel and due to concentration gradient sodium enters inside the cell. There are
membrane proteins called symporters that helps in secondary active transports.

References
Albert, B., Johnson, A. J., Raff, M., & Roberts, K. (2002). Molecular Biology of the Cell. NCBI.
Cooper, G. M. (2000). The Cell, 2nd edition: A Molecular Approach. NCBI.
NJ, Y., & MJ, H. (2015). Getting Across the Cell Membrane: An Overview for Small Molecules, Peptides,
and Proteins. NCBI, 29-53.
Soult, A. (2016, December 7). Phospholipids in Cell Membranes. Retrieved from ChemistryLibreTexts:
https://chem.libretexts.org/LibreTexts/University_of_Kentucky/UK:_CHE_103_-
_Chemistry_for_Allied_Health_(Soult)/Chapters/Chapter_14:_Biological_Molecules/
14.3:_Phospholipids_in_Cell_Membranes
The Plasma Membrane: Structure. (2104, July 8). Retrieved from Anatomy & Physiology:
http://anatomyandphysiologyi.com/the-plasma-membrane-structure/
Yeagle. (2009, December 10). Cell Membrane Features. Retrieved from Research article:
https://www.researchgate.net/publication/227992332_Cell_Membrane_Features