Akash Dixit

Functions and Related Structure of the Plasma Membrane

One of the oldest and most fundamental component of all living things is the plasma membrane.The membrane is composed of two layers of phospholipids, held together by hydrophobic interactions. It serves as a barrier between the cytoplasm and organelles inside the cell and the outside environment. Proteins on the surface of the cell can selectively allow molecules to enter or leave the cell, either passively through diffusion, or by using the cell's energy to actively pump the molecules in our out of the cell. The cumulative effect of the cell membrane is that the cell is able to maintain homeostasis, or a fairly stable internal environment in terms of temperature pH, and ion concentration.

The membrane itself is made of two layers of phospholipids. A phospholipid is composed of a polar phosphate group connected by a glycerol molecule to two nonpolar fatty acid chains. The polar and nonpolar ends of the molecule make it amphiphilic, where the polar half is hydrophilic and the nonpolar tails are hydrophobic. When exposed to water, the amphiphilic structure of the molecules drives them to arrange into a two layer sheet with the polar heads on the outside facing the water, and the fatty acid chains on the inside, protected from the water. In cells, this sheet forms a sphere, encompassing the cytoplasm and organelles of the cell. A recent study used computer models to “observe the entire process at atomic detail with realistic lipids.” and found “Starting from random solutions, bilayers are formed spontaneously on time scales of 10−100 ns”[1] 

Figure 1[2]

The rigidity of the bilayer can be modulated by altering the ratio of saturated fatty acids to unsaturated fatty acids. A saturated fatty acid has no double bonds between carbons, and therefore, they form fairly straight chains. The straight chains are able to stack together neatly and solidify easily, making the cell membrane rigid. Unsaturated fatty acids do contain double bonds between carbons, which cause a kink in the chain. The kinks serve to disrupt that stacking process, keeping the cell membrane fluid, even at lower temperatures. A recent study on conducted on scallops was able to show “a simple but very strong relationship between (membrane) fluidity and a polyunsaturated fatty acid, eicosapentaenoic acid”[3] and that “ under variable conditions, organisms can exploit the the chemical diversity among membrane lipids to defend the physical properties of the membrane”[3]. 

Aside from phospholipids, the plasma membrane is contains a number of proteins. Some going all the way through the membrane, and some associated with one side or another. These proteins are diverse in their structures and tasks, some being involved in movement of the cell, some relay sensory information about the outside environment, and some act as gateways through which they can selectively allow molecules to pass through the membrane. The way many of these gateway proteins allow specific molecules through is by having a binding site on one side of the membrane. Research demonstrates that “the transporter exposes its substrate binding site(s) to one side of the membrane or the other during transport catalysis, requiring a substantial conformational change of the carrier protein”[4] The binding site is shaped such that only a particular molecule can bond to it, commonly known as a “lock and key” mechanism. When a molecule is fit into the binding site, the protein changes shape, and allows the molecule through.

A number of the functions of the cell membrane are driven by its structure. The shape of the phospholipid molecules drive them to spontaneously arrange themselves into a wall when exposed to water. The shape of the fatty acid tail changes the fluidity of the membrane, and can be manipulated to allow the cell to retain a stable shape despite changing temperatures. The structure and folding of massive proteins create locks that can only open to molecules of a specific shape.

Works Cited

[1] Marrink, S J, E Lindahl, O Edholm, and A E Mark. "Simulation of the Spontaneous Aggregation of Phospholipids into Bilayers." Journal of the American Chemical Society

[2] Avissar, Yael. Biology. OpenStax College, 30 May 2013. Web. 10 Mar. 2016.

[3] Hall, Jonathon M, Christopher C Parrish, and Raymond J Thompson. "Eicosapentaenoic Acid  Regulates Scallop (Placopecten Magellanicus) Membrane Fluidity in Response to Cold." Biological Bulletin, 202.3 (2002): 201-203.

[4] Forrest, Lucy R, Reinhard Krämer, and Christine Ziegler. "The Structural Basis of Secondary Active Transport Mechanisms." BBA - Bioenergetics, 1807.2 (2011): 167-188.