Building block of protein that contains carbon, hydrogen, oxygen, nitrogen,(all four are present in ALL amino acids) and traces of other elements(type varies). There are 20 types of amino acids with the distinguishing feature being the variable element. On the diagram for amino acid there is the amino group on the left, and the carboxyl group on the right. The r represents the variable element that changes throughout the types. There are two types that contain sulphur as the R 1. Methionine 2. Cysteine. Different types of Amino acid can react together via condensation reaction to form a polypeptide. Polypeptides can then be conformed into 4 different types:primary, secondary, tertiary, quateneary