Amino Acids and DNA (Triple)AQA GCSE Chemistry: Revision notes

Amino acids are small molecules that contain two different functional groups within the same molecule. Having two reactive functional groups means each amino acid can bond to another at both ends, allowing long chains to build up.

This structure is what allows amino acids to react together by condensation polymerisation.

Amino acids react by condensation polymerisation to produce polypeptides. Different amino acids can be combined together in the same chain, and the exact sequence and combination used determines which protein is produced.

This is why there is such a huge variety of proteins in living organisms, despite there being a limited number of different amino acids.

In condensation polymerisation generally, monomers with two functional groups join together, and a small molecule — usually water — is lost each time a new bond forms.

A well-known non-biological example is ethanediol reacting with hexanedioic acid to form a polyester, releasing water at each link. Amino acids follow the same basic pattern, but their two functional groups let them form polypeptide chains instead.

DNA (deoxyribonucleic acid) is a large molecule essential for life that encodes genetic instructions. Most DNA molecules are made of two polymer chains, built from four different monomers called nucleotides, twisted together into a double helix.

DNA is one of several important natural polymers, alongside proteins (from amino acids), starch and cellulose (from sugar monomers).

One of the two functional groups found in every amino acid is the carboxyl group, –COOH — the same functional group that defines a carboxylic acid.

Carboxylic acids are weak acids: they only partially ionise in aqueous solution, unlike strong acids such as hydrochloric acid, which fully ionise. This partial ionisation behaviour is a property of the –COOH group wherever it appears, including in amino acids.