Fatty Acids

Abstract

Fatty acids are aliphatic monocarboxylic acids, and those considered lipids have 10 or more carbons. Generally, fatty acids occur in nature as a homologous series ranging in chain length from C₁₀ to C₃₆. However, fatty acids commonly range from C₁₄ to C₂₀ in most organisms with the even-numbered homologues being predominant. Fatty acids with 16 and 18 carbon atoms are most common and quantitatively important. Fatty acids may be saturated (C_nH_2n+1COOH) or unsaturated [mono-(C_nH_2n-1COOH) and poly-(C_nH_2n-xCOOH)], they may have odd-numbered carbon chains, or they may be substituted with methyl, oxygen (keto, epoxy), or hydroxyl functions. Cyclic (cyclopropyl- and cyclopentyl-) fatty acids also occur in some organisms. The principal product of the fatty acid synthetase in plants, animals, and microorganisms contains 16 carbons (see Chapter 4). Most other fatty acids are formed by elongation, desaturation, and substitution, or combinations of these modifications. Palmitic acid (C₁₆) is the predominant saturated fatty acid of most organisms, and oleic (C_18:1), linoleic (C_18:2) and linolenic (C_18:3) are the major unsaturated acids.