7.1. Structure and Functions of the Major Human Lipids
Lipids are organic molecules with very diverse chemical structures and biological functions, but they have one physical property in common: relative insolubility in water. The latter governs their unique functions. Lipids are the major structural compounds of all membranes in all living systems, serve as fuels and hormones and hormone-like signal molecules.
Lipids include the following classes: fatty acids, triacylglycerols, phospholipids, sphingolipids, glycolipids, steroids, fat-soluble vitamins, eicosanoids (Table 7.1).
Fatty acid consists of an alkyl chain with a terminal carboxyl group (Fig. 7.1). Most of the lipids found in the body contain fatty acids bound by an ester bond to hydroxyl group of glycerol or cholesterol. Most of the fatty acids in the human lipids have an even number of carbon atoms, usually between 16 and 20 (Table 7.2). The carbon atoms of the fatty acid are numbered starting with the carboxyl carbon. Alternatively, carbon atoms are designated by Greek letters. The б-carbon atom is the next to the carboxyl carbon, в-carbon atom is the next to б-carbon atom and etc. The last carbon atom in the chain is the щ-carbon. The designations are needed for the classification of fatty acids. Fatty acids may be saturated and unsaturated. Saturated fatty acids have no double bonds in the chain. Their general formula is CH3-(CH2)n-COOH.