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15

The Molecules of Life

Fig. 15.8 Some naturally occurring lipids and membrane components: 1, a fatty acid; 2, phos-

phatidic acid; 3, phosphatidylethanolamine; 4, phosphatidylcholine; 5, cardiolipin (diphosphatidyl-

glycerol); 6, cholesterol

A large number of natural lipids are known and found in natural membranes; both

the head groups and tails can be varied. A small selection is shown in Fig. 15.8. The

lipid repertoire of a cell or organism is called the “lipidome”. This diversity allows

the shape, fluidity, permeability, affinity for macromolecules, and so on of mem-

branes to be adjusted. The biosynthesis of lipids and other membrane components

such as cholesterol is, of course, carried out by enzymes, but the regulation of their

abundance and activity is not well understood, and the importance of their variety

has probably been underestimated. Most enzymes are attached to membranes, and