Contain Complex Mixtures of Lipids

Two main types of lipids occur in biological membranes phospholipids and sterols. The predominant phospholipids in most membranes are phosphoglycerides, which are phosphate esters of the three-carbon alcohol, glycerol. A typical structure is that of phosphatidylcholine (lecithin)  

Here R, and R2 are long, fatty acid side chains. The parent fatty acids RjC02H and R2C02H usually have an even number of carbon atoms 16- and 18-carbon acids are the most common. The acid esterified to the hydroxyl group on C-l of the glycerol (that at the top of phosphatidylcholine is drawn above) usually has a fully saturated chain, whereas the acid attached at C-2 often has one or more double bonds, which are almost always cis double bonds. Table 17.1 lists some of the fatty acids commonly found in these positions. A phosphatidylcholine that has palmitic acid esterified at both the C-l and C-2 positions of the glycerol is known by 

In addition to phosphoglycerides, membranes from animal cells usually contain a second group of phospholipids, the sphingolipids. Sphingomyelin, which is representative of this group, has the structure 

The second major type of lipid found in some biological membranes is cholesterol. Cholesterol (fig. 17.5) is an isoprenoid compound with four fused rings, a short aliphatic chain, and a single hydroxyl group. It occurs in membranes both in its free form and esterified with long-chain fatty acids. Table 17.3 compares the lipid compositions of mem- 

Fatty Acids Frequently Found in Membrane Phospholipids 

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