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, now 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 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 the amino acids 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!
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 different amino acids together in a different order 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 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:
The molecules formed in these condensation reactions are called dipeptides since they are formed from two
amino acids. These dipeptide molecules
contain one amide
or peptide link as shown in the image above.
The dipeptide molecule formed still has reactive amino and
acidic functional groups on each
end of the molecule 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. 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 down to a three letter abbreviation or sometimes even 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:
