natural polymerisation

Chemistry higher tier

There are many naturally occurring polymers for example starch, cellulose, silk, protein and DNA are but a few of the many polymers found in living organisms. Let's start by looking at proteins. Proteins are found in all living organisms and there are many different types of proteins, from the proteins found in muscle, skin, tendons and enzymes to the proteins found in spider webs and silk. All these different proteins have one thing in common; they are all polymers made from monomers called an amino acids.

There are around 20 or so amino acids found in most proteins. Amino acids as the name suggests contain two different functional groups; an amino group (-NH2) and a carboxyl group (-COOH). You will probably have met the carboxyl group before; it is the functional group found on all carboxylic acids. The amino group is a basic group and so will readily react with an acidic carboxyl group on another amino acid. The structure of a typical amino acid molecule is shown below.

amino acid general structure.

There are just over twenty common amino acids found in living organisms and they all have the basic structure shown above. The only difference between amino acids is the structure of the R- group. The simplest amino acid is one where the side group R is simply a hydrogen atom. This gives an amino acid called glycine (gyl for short). If the R- group is a -CH3 group then an amino acid called alanine is formed. The structures of glycine and alanine are shown below. A quick search on Google will show the structure of all common amino acids if you care to look!

Structure of the amino acids alanine and glycine.

You can clearly see the only difference between these two amino acids is in the side chain -R. The twenty or so common amino acids can link together to form a vast number of different proteins. As a simple example think of the number of words you can make from the 26 letters in the alphabet; well by linking the 20 or so amino acids together in a different order and in polymer chains of different lengths you can end up with an almost limitless number of possible protein structures.
The amino acids link together in a condensation reaction (one where a small molecule; usually water is lost) to form an amide or peptide link. For example the amino acids alanine and glycine can link together to form a dipeptide molecule as shown below:

There are two possible ways these two amino acid molecules can react with each other to form a dipeptide:

This is outlined in the diagram below:

The amino acids alanine and glycine forming a dipeptide.

The molecule formed is called a dipeptide since it was formed from two amino acids. This molecule contains one amide or peptide link as shown in the image. The dipeptide molecule formed still has reactive amino and acid groups on each end and so it can react further with more amino-acids molecules to form more peptide links. In fact thousands or even hundreds of thousands of these amino-acid monomers can react to form a giant polymer called a protein. The order in which the amino acids link together will determine the type of protein formed. Smaller numbers of amino acids can link to form large molecules called polypeptides, these can contain up to and around 50 or so amino acids all linked together by amide or peptide bonds.

Amino acid polymerisation

The image below shows how one amino acid molecule can polymerise to form a polypeptide molecule. Amino acids polymerising to form a polypeptide.

Key Points