In this chemistry, an amino acid (AA) is any molecule that contains both amino and carboxylic acid functional groups. In molecular biology, and in this web site, this term is used to refer to α-amino acids: those amino acids in which the amino (-NH2) and carboxylic (-COOH) groups are attached to the same α-carbon atom. Each AA also has a side chain (-R). There are 20 AAs commonly found in animals. More than 100 less common AAs also occur in biological systems, particularly in plants. Every AA except Gly can occur as either of two optically active stereo isomers, D or L; the more common isomer in nature is the L-form. When the carboxyl C-atom of one AA covalently binds to the amino N-atom of another AA with the release of H2O, a peptide bond is formed.