Before considering phospholipids and their role in the structure of biomembranes, we briefly review the properties of fatty acids. Like glucose, fatty acids are an important energy source for many cells and are stored in the form of triacyl-glycerols within adipose tissue (Chapter 8). Fatty acids also are precursors for phospholipids and many other lipids with a variety of functions (Chapter 18).
Fatty acids consist of a hydrocarbon chain attached to a carboxyl group (—COOH). They differ in length, although the predominant fatty acids in cells have an even number of carbon atoms, usually 14, 16, 18, or 20. The major fatty acids in phospholipids are listed in Table 2-3. Fatty acids often are designated by the abbreviation Cx:y, where x is the number of carbons in the chain and y is the number of double bonds. Fatty acids containing 12 or more carbon atoms are nearly insoluble in aqueous solutions because of their long hydrophobic hydrocarbon chains.
Fatty acids with no carbon-carbon double bonds are said to be saturated; those with at least one double bond are un-saturated. Unsaturated fatty acids with more than one carboncarbon double bond are referred to as polyunsaturated. Two
"essential" polyunsaturated fatty acids, linoleic acid (C18:2) and linolenic acid (C18:3), cannot be synthesized by mammals and must be supplied in their diet. Mammals can synthesize other common fatty acids. Two stereoisomeric configurations, cis and trans, are possible around each carbon-carbon double bond:
h2cx CH2 HH
A cis double bond introduces a rigid kink in the otherwise flexible straight chain of a fatty acid (Figure 2-18). In general, the fatty acids in biological systems contain only cis double bonds.
Fatty acids can be covalently attached to another molecule by a type of dehydration reaction called esterification, in which the OH from the carboxyl group of the fatty acid and a H from a hydroxyl group on the other molecule are lost. In the combined molecule formed by this reaction, the portion derived from the fatty acid is called an acyl group, or fatty acyl group. This is illustrated by triacylglycerols, which contain three acyl groups esterfied to glycerol:
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