The peptide bonds in dipeptides, polypeptides and proteins can be broken by hydrolysis. A water molecule is added across each peptide bond: the -OH joins the carbon to reform a carboxyl group and the -H joins the nitrogen to reform an amine group.Complete hydrolysis of a polypeptide gives its individual amino acids. Hydrolysis is the reverse of the condensation reaction that formed the peptide bonds.In living organisms the reaction is catalysed by protease enzymes. For example, pepsin in the stomach and trypsin in the small intestine digest proteins into shorter peptides, and peptidases complete the breakdown to amino acids that can be absorbed.In the laboratory, proteins can be hydrolysed by heating them with hydrochloric acid (about 6 mol dm-3) for several hours. The amino acids released can then be separated and identified by chromatography.