Organic compounds containing a carboxyl group (-COOH) and an amino group (-NH2). About 30 amino acids are known. They are fundamental constituents of living matter because protein molecules are made up of many amino acid molecules combined together. Amino acids are synthesized by green plants and some bacteria, but some (arginine, histidine, lysine. threonine, methionine, isoleucine, leucine, valine, phenylalanine, tryptophane) cannot be synthesized by animals and therefore are essential constituents of their diet. Proteins from specific plants may lack certain amino acids, so a vegetarian diet must include a wide range of plant products.