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FIGURE 8.2 • Structures of two unusual fatty acids: lactobacillic acid, a fatty acid containing a cyclopropane ring, and tuberculostearic acid, a branched-chain fatty acid.

prostaglandins, a class of compounds that exert hormone-like effects in many physiological processes (discussed in Chapter 25).

In addition to unsaturated fatty acids, several other modified fatty acids are found in nature. Microorganisms, for example, often contain branched-chain fatty acids, such as tuberculostearic acid (Figure 8.2). When these fatty acids are incorporated in membranes, the methyl group constitutes a local structural perturbation in a manner similar to the double bonds in unsaturated fatty acids (see Chapter 9). Some bacteria also synthesize fatty acids containing cyclic structures such as cyclopropane, cyclopropene, and even cyclopentane rings.

8.2 • Triacylglycerols

A significant number of the fatty acids in plants and animals exist in the form of triacylglycerols (also called triglycerides). Triacylglycerols are a major energy reserve and the principal neutral derivatives of glycerol found in animals. These molecules consist of a glycerol esterified with three fatty acids (Figure 8.3). If all three fatty acid groups are the same, the molecule is called a simple tri-acylglycerol. Examples include tristearoylglycerol (common name tristearin) and trioleoylglycerol (triolein). Mixed triacylglycerols contain two or three different fatty acids. Triacylglycerols in animals are found primarily in the adipose tissue (body fat), which serves as a depot or storage site for lipids. Mono-acylglycerols and diacylglycerols also exist, but are far less common than the triacylglycerols. Most natural plant and animal fat is composed of mixtures of simple and mixed triacylglycerols.

Acylglycerols can be hydrolyzed by heating with acid or base or by treatment with lipases. Hydrolysis with alkali is called saponification and yields salts of free fatty acids and glycerol. This is how soap (a metal salt of an acid derived from fat) was made by our ancestors. One method used potassium hydroxide (potash) leached from wood ashes to hydrolyze animal fat (mostly triacylglyc-erols). (The tendency of such soaps to be precipitated by Mg2+ and Ca2+ ions in hard water makes them less useful than modern detergents.) When the fatty acids esterified at the first and third carbons of glycerol are different, the sec-

FIGURE 8.3 • Triacylglycerols are formed from glycerol and fatty acids.

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