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Asymmetry of Cell Membrane
The cell membrane is an asymmetric structure. That means that the two sides of membrane are structurally and functionally different. This difference has to do with (1) the difference in composition of lipids, proteins and carbohydrates (2) the difference in the orientation and positioning of the proteins and (3) the difference in the enzymatic activities of the two sides of the membrane. The asymmetric nature of the membrane is crucial for the proper functioning of the cell. So how does this asymmetry arise? When proteins and lipids are synthesized in the cell, they are inserted into the membrane in an asymmetric fashion. This asymmetry is retained for long periods of time because the proteins do not rotate from one side to the other (a process called transverse diffusion) and because all membranes are created and elongated from pre-existing asymmetric membranes. Since lipids such as phospholipids do rotate, the absolute asymmetry of the lipids is not retained (as the case is with proteins) but rather changes over time. However, since some lipids do not rotate (i.e. glycolipids) and those that do rotate do so very slowly, lipid asymmetry contributes to the asymmetry of the membrane. For instance, the outer layer of the membrane of red blood cells contains a high concentration of sphingomyelin and phosphatidylcholine while the inner layer of the membrane contains a high concentration of phosphatidylethanolamine and phosphatidylserine.
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