Amino acids are organic molecules that contain an amine group, a carboxyl group, a central carbon atom called the alpha carbon, and a variable side chain. There are 20 common amino acids that differ in their side chains and physical/chemical properties. Amino acids can polymerize through peptide bonds between their carboxyl and amine groups to form polypeptides. Polypeptides are linear chains of amino acids that can further fold into three-dimensional protein structures and carry out biological functions.