APA Dictionary of Psychology

an organic compound that contains an amino group (–NH₂) and a carboxyl group (–COOH). Twenty amino acids are constituents of proteins; nine of these are essential amino acids, that is, they cannot be synthesized by the body and must be obtained from foods. Other amino acids (e.g., glutamic acid, glycine) are neurotransmitters or precursors to neurotransmitters.