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 or produced by living organisms. Let's start by looking at proteins. Proteins are found in all living organisms; 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 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.
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 different amino acids is the structure of the R group. The simplest amino acid is one where the side group R is a hydrogen atom. This gives an amino acid called glycine (gly for short). If the R group is a –CH3 group then an amino acid called alanine is formed. The structures of the amino acids glycine and alanine are shown below.
You can clearly see the only difference between these two amino acids is 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 different amino acids together in different orders 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; that is, a reaction where a small molecule — usually water — is lost to form an amide (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 to form a dipeptide:
This is outlined in the diagram below:
The molecules formed in these
condensation reactions are called
dipeptides since they are formed from two
amino acids.
These dipeptide molecules contain one
amide (peptide) link, as shown above.
The dipeptide molecule formed still has reactive amino and acidic functional groups on each end and can therefore react further with more amino acid molecules to form more peptide links. In fact, hundreds or even thousands of these amino acid monomers can react to form a large polymer called a protein. The order in which the amino acids link together determines the type of protein formed. Smaller numbers of amino acids can link to form molecules called polypeptides, which contain up to around 50 amino acids joined by peptide (amide) bonds. Chains longer than about 50 amino acids are usually referred to as proteins. The image below shows how a large polypeptide molecule is formed from nine different amino acids reacting together in a condensation reaction.
The names of amino acids are often shortened to a three-letter abbreviation, or sometimes a single letter; for example, the amino acid glycine can be shortened to “gly” and alanine to “ala”. The order or sequence in which the amino acids in a polypeptide or protein polymer join or link together is referred to as its primary structure; this is outlined in the image below:
