Describe the structure of proteins.

Proteins are made up of chains of amino acids. Amino acids are made up of a central carbon atom, joined to one hydrogen atom, an amine group (NH2), a carboxyl group (COOH) and an R group. The R group varies between different amino acids. For example, in Glycine the group is a hydrogen atom, and in alanine it is a methyl group (CH3). There are 20 different amino acids. Amino acids undergo condensation reactions to join to other amino acids. These reactions form a peptide bond between the amine group of one amino acid, and the carboxyl group of the other. Two joined amino acids are a dipeptide, while many amino acids in a chain form a polypeptide. This is known as the primary structure. Polypeptides can then be folded by hydrogen bonds into either an alpha helix or a beta-pleated sheet. This is the secondary structure. The secondary structure can be folded into a complex 3D structure by other bonds such as disulphide bridges, ionic bonds and more hydrogen bonds. This is the tertiary structure. Several tertiary structures can be bonded together to from the quaternary structure. Tertiary and quaternary structures can also bind other nonpolypetide units, such as ions. These are known as prosthetic groups. The unique and complex structures of proteins allow them to carry out their specific functions.