Notes on Fatty Acids (With Diagram)

The fatty acids are an important and also abundant group of lipids. In cells, the fatty acids only sparingly occur freely; instead, they are esterified to other com­ponents and form the saponifiable lipids.

A fatty acid molecule may be either saturated or unsat­urated. The saturated fatty acids consist of long hy­drocarbon chains terminating in a carboxyl group and conform to the general formula;

CH₃-(CH₂)_B-COOH

In nearly all naturally occurring fatty acids, n is an even number from 2 to 22. In the saturated fatty acids most commonly found in animal tissues, n is either 12 (i.e., myristic acid), 14 (i.e., palmitic acid), or 16 (i.e., stearic acid).

In unsaturated fatty acids, at least two but usually no more than six of the carbon atoms of the hydrocarbon chain are linked together by double bonds.

The two most common unsaturated fatty acids are oleic acid and linoleic acid, depicted in Figure 6-1 along with the saturated fatty acid stearic acid. In all the common saturated and unsaturated fatty acids, the long hydrocarbon chain is unbranched. Unsatu­rated fatty acids predominate in the saponifiable lip­ids of higher plants and in animals that live at low temperatures. Lipids in the tissues of animals inhabit­ing warm climates contain larger quantities of satu­rated fatty acids.

Some animals, especially mammals, are unable to synthesize certain fatty acids and there­fore require them in their diets. The essential fatty acids, like linoleic acid, must be obtained from plant material. Fatty acids found in bacterial cells are usu­ally of short chain length and, when unsaturated, have only one double bond.

Fatty acid molecules contain both hydrophilic and hydrophobic parts. In their dissociated states (shown in the formulas of Fig. 6-1), the carboxyl ends of the molecules are mildly soluble in water, whereas the long hydrocarbon chains repel water.

In neutral solutions, salts of the fatty acids form small spherical droplets or micelles in which the dissociated carboxyl groups occur at the surface, and the hydrophobic car­bon chains project toward the center. The most of the fatty acids that are found in cells and tissues are esterified to other molecules. Because the ester link­ages can be hydrolyzed by the addition of alkali, these lipids are called saponifiable lipids.