Amino acids are organic compounds which contain both an amino group (NH2) and a carboxyl (COOH) group. There are at least 25 amino acids within the body. They vary in the nature of the third of four groups around the central carbon atom. The majority of these are constituents of proteins; some occur freely or in smaller units such as peptides. Essential amino acids cannot be synthesized within the body.

The functions of amino acids largely overlap with those of proteins. Additionally, some amino acids can act as transmitter substances e.g. tryptophan and gamma-amino butyric acid.