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In chemistry, an amino acid is a molecule that contains both amine and carboxyl functional groups. In biochemistry, this term refers to alpha-amino acids with the general formula NH<sub>2</sub>CHRCOOH.[1] These are molecules where the amino and carboxylate groups are attached to the same carbon, which is called the α–carbon. The various alpha amino acids differ in which side chain (R group) is attached to their alpha carbon. This can vary in size from just a hydrogen atom in glycine, through a methyl group in alanine, to a large heterocyclic group in tryptophan.