Proteins contain the elements carbon, hydrogen, oxygen, nitrogen and sometimes sulphur. They are the most complex and diverse group of macromolecules, which have a huge range of different functions in living organisms, including structural and physiological functions.
Their complexity and diversity, is due to the structure of proteins. Proteins are polymers made from amino acid monomer joined by condensation reactions forming peptide bonds. The number of different amino acids, and the variable nature of the polymer chain gives the immense variety of protein types. The test for the presence of proteins is the Biuret test.
Amino acids are the monomers that proteins are constructed from. Generally, unlike carbohydrates, the monomers do not have biological functions in their own right. They are simply used to construct the polymers. There are about 20 different types of amino acid, but they can all be represented with one general structural formula. Figure 1, below, shows the general formula for an amino acid.