There are three major kinds of membrane lipids: phospholipids, glycolipids, and sterols. Both phospholipids and glycolipids readily associate spontaneously to form a lipid bilayer (Figure 2-2). Cellular membranes behave as two-dimensional, semifluid structures, allowing embedded protein molecules to constantly move about rather freely by lateral diffusion. The fluidity of prokaryotic membranes is regulated by varying the number of double bonds in, and the lengths of, the fatty acid chains of the lipid molecules constituting the membrane. In animals, the quantity of the sterol lipid cholesterol is a key regulator of membrane fluidity.
The plasma membrane is a selective filter that controls the entry of nutrients and other molecules needed for cellular processes. Waste products of metabolism pass out of the cell through this membrane. Due to their composition, membranes have a low permeability for ions and most polar molecules, thus these molecules must pass through channels formed from integral membrane proteins. If a substance is moving against its concentration gradient (i.e., from an area of lower concentration to an area of higher concentration), then energy must be expended. This is termed active transport.
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