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Fatty Acid. Any of a group of compounds that consist of linear hydrocarbon (q.v.) chains varying in length from four to 26 carbon atoms (q.v.), with a carboxyl group (-COOH) attached at one end, and a methyl group (-CH3) at the other. They are the major constituents of fats (q.v.) and oils and largely establish their chemical and physical properties. A fatty acid is said to be saturated when every carbon atom in the chain is combined with two hydrogen atoms to form a methylene group (-CH2), and to be unsaturated (q.v.) when some of the carbon atoms lack one hydrogen atom and form a double bond (q.v.) with an adjacent carbon atom. Fatty acids with two or more such double bonds are referred to as polyunsaturated fatty acids (q.v.).