... be discerned only faintly with a transmission electron microscope. One of the key roles of the membrane is to maintain the cell potential. Phospholipid molecules in the cell membrane are "fluid," in the sense that they are 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 ...

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