Anatomy and Transport Across Plasma Membrane

This article discusses the anatomy of plasma membrane and the transport of oxygen and sodium ions across it. It explains the fluid mosaic model and the passive diffusion of polar molecules. It also describes the Na+/K+ ATPase pump and the symporter for sodium ion transport. The article is relevant to bioscience and is suitable for students studying anatomy and physiology.

Running head: BIOSCIENCE
Bioscience
Name of the Student
Name of the University
Author Note

1
BIOSCIENCE
Anatomy of plasma membrane
The plasma membrane is defined as the boundary of the cell, which separates the
components of the cell from the extra-cellular components. It is made up of protein and lipid
molecule that forms tough, thin, pliable and hydrophobic barrier around the cell (Luckey et
al., 2014). In other words it can be said that cellular membranes or plasma membrane is
composed of two layers of phospholipid molecule. This bipartite layer has hydrophilic end
and hydrophobic end. The hydrophilic ends are directed towards the inner and the outer
surface of the membrane whereas the hydrophobic ends are buried within the interior of the
membrane (Rodwell et al., 2015).
This arrangement of the plasma membrane is known as fluid mosaic model as
proposed by Singer and Nicolson (Rodwell et al., 2015). According to the fluid mosaic
model, the hydrocarbon chain of phospholipid bilayer forms hydrophobic core that restricts
diffusion of water soluble solutes through the membrane. The polar head group of the
phospholipid has hydrophilic tail that remains embedded within the membrane (Lodish et al.,
2000).
Transport of Oxygen across plasma membrane
Gases such as oxygen (O2) and CO2 (carbon di-oxide) and other small uncharged
molecules which are polar in nature (urea and ethanol) are transported through the plasma
membrane via passive diffusion. These molecules can diffuse across the plasma membrane
without the requirement of transport proteins. Moreover, metabolic energy is not expended
due to the movement of these polar molecules occur from a domain of high concentration to a
region of low concentration, down the chemical concentration gradient. From the theory of
thermodynamics it can be stated that such transportation reactions are spontaneous because it

2
BIOSCIENCE
has positive change in entropy (delta S) and negative change in Gibbs free energy (delta G)
(Lodish et al., 2000).
Transport of sodium across the plasma membrane
The transport of sodium ions (Na+) takes place along with potassium ions (K+)
through P-class ion pump known as Na+/K+ ATPase. In the E1 configuration of Na+/K+
ATPase have three high-affinity sodium ions binding sites along with two low affinity
potassium ion binding sites at the cytosolic face. Hence three molecules of Na+ are
transported across the concentration in the expense ATP breakdown. While face on the
exterior surface of the plasma membrane, Na+/K+ ATPase has two high affinity K+ ion
binding sites, upon releasing 3 Na+, 2K+ binds to its respective binding sites and is released
inside the cell (across its concentration gradient) along with the release of the phosphate ions
formed as a result of ATP to ADP conversion during Na+ ion transport (Lodish et al., 2000).
Transport of Sodium ions inside the cell takes place along with glucose molecule via the help
of symporter (Lodish et al., 2000).

3
BIOSCIENCE
References
Lodish, H., Berk, A., Zipursky, S. L., Matsudaira, P., Baltimore, D., & Darnell, J. (2000).
Molecular cell biology 4th edition. National Center for Biotechnology Information,
Bookshelf.
Luckey, M. (2014). Membrane structural biology: with biochemical and biophysical
foundations. Cambridge University Press.
Rodwell, V., Bender, D., Botham, K. M., Kennelly, P. J., & Weil, P. A. (2015). Harpers
Illustrated Biochemistry 30th Edition. McGraw Hill Professional.