Lipids
in Biological Membranes
Biological membranes contain an interesting array of molecules. These include glycerophospholipids, sphingolipids, cholesterol, proteins, carbohydrates, and some molecules which are conjugates containing molecules from two or more of these groups (Figure 10.10).
The primary structural requirement for a molecule to reside in a lipid bilayer is an amphiphilic nature (i.e., part polar, part nonpolar). Molecules are oriented within the lipid bilayer so that their nonpolar portion is inserted into (and sometimes through) the nonpolar core of the bilayer. The polar portion associates with the polar outsides of the bilayer.
Membrane proteins have their polypeptide chains folded so that the nonpolar regions associate with each other in a cluster. This nonpolar unit then sticks into the nonpolar part of the bilayer with the remaining polar parts sticking out. Some protein molecules, such as bacteriorhodopsin, actually extend though both sides of the lipid bilayer.
The presence of bulky nonpolar molecules, such as cholesterol, disrupts the regularity of a lipid bilayer.
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