A level Biology exam revision resources written by A level Examiners


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.

Figure 1

This diagram shows that at the centre of an amino acid is a carbon atom, this is termed the Biochemistry-carbon. To this atom the following are attached:

  • a hydrogen atom
  • an amino group
  • a carboxyl group
  • a variable R group

The variable R group is the portion that is different between different amino acids. At A-level it is not necessary to learn any of the structures of specific R groups. Largely it is the R groups rather than the other components of the amino acids that determine the properties of protein molecules.

Proteins are formed by the condensation of amino acids. Two amino acids combine by condensation to form a dipeptide, this is shown in figure 2 below.

Figure 2

Further condensation reactions then create polypeptides as shown in figure 3, below. Many polypeptide chains are hundreds of amino acid monomers long.

Figure 3

Share this knowledge


Back To Top Back To Top