important component of lipids (fat-soluble components of living cells) in plants, animals, and microorganisms. Generally, a fatty acid consists of a straight chain of an even number of carbon atoms, with hydrogen atoms along the length of the chain and at one end of the chain and a carboxyl group (−COOH) at the other end. It is this carboxyl group that makes it an acid (carboxylic acid). If the carbon-to-carbon bonds are all single, the acid is saturated; if any of the bonds is double or triple, the acid is unsaturated and is more reactive. A few fatty acids have branched chains; others contain ring structures (e.g., prostaglandins). Fatty acids are not found in a free state in nature; commonly they exist in combination with the alcohol glycerol in the form of triglyceride (q.v.).

The most widely distributed fatty acid is oleic acid, which is abundant in some vegetable oils (e.g., olive, palm, peanut, and sunflower seed) and which makes up about 46 percent of human fat.

Many animals cannot synthesize one or more of the fatty acids and must ingest them in foods. Two such derived fatty acids are linoleic and linolenic acids; these, and sometimes arachidonic acid, which can be synthesized from linolenic, are required by all mammals and are called essential fatty acids.

Soaps are the sodium and potassium salts of fatty acids.