Detailed structure of cell membrane

Phospholipid molecules in the cell membrane are "fluid," in the sense of free to diffuse and exhibit rapid lateral diffusion. Lipid rafts and caveolae are examples of cholesterol-enriched microdomains in the cell membrane. Many proteins are not free to diffuse. The cytoskeleton undergirds the cell membrane and provides anchoring points for integral membrane proteins. Anchoring restricts them to a particular cell face or surface for example, the "apical" surface of epithelial cells that line the vertebrate gut and limits how far they may diffuse within the bilayer. Rather than presenting always a formless and fluid contour, the plasma membrane surface of cells may show structure. Returning to the example of epithelial cells in the gut, the apical surfaces of many such cells are dense with involutions, all similar in size. The finger-like projections, called microvilli, increase cell surface area and facilitate the absorption of molecules from the outside. Synapses are another example of highly-structured membrane.

New material is incorporated into the membrane, or deleted from it, by a variety of mechanisms.
(i) Fusion of intracellular vesicles with the membrane not only excretes the contents of the vesicle, but also incorporates the vesicle membrane's components into the cell membrane. The membrane may form blebs that pinch off to become vesicles.
(ii) If a membrane is continuous with a tubular structure made of membrane material, then material from the tube can be drawn into the membrane continuously.
(iii) Although the concentration of membrane components in the aqueous phase is low (stable membrane components have low solubility in water), exchange of molecules with this small reservoir is possible. In all cases, the mechanical tension in the membrane has an effect on the rate of exchange. In some cells, usually having a smooth shape, the membrane tension and area are interrelated by elastic and dynamical mechanical properties, and the time-dependent interrelation is sometimes called homeostasis, area regulation or tension regulation.


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