A tripeptide is a peptide consisting of three amino acids joined by peptide bonds. The exact nature and function of proteins is determined by the amino acids present and the order they occur. Simple examples are the tripeptides made by combining either three glycine  molecules or two glycines and one alanine. The three glycine tripeptide can have only one structure: glycine-glycine-glycine. The other tripeptides can have different orders: glycine-alanine-glycine, glycine-glycine-alanine, and alanine-glycine-glycine. Each is a distinct molecule.