Amino Acids (AQA GCSE Chemistry)

Higher tier only

• Amino acids are small molecules containing the amino, NH₂, and carboxylic acid, COOH, functional groups
• The NH₂ group is basic and behaves in a similar way to ammonia
• The COOH group is acidic and is called a carboxyl group
• There are twenty naturally occurring amino acids and they all have the same general structure

The general structure of an amino acid 

The structure of naturally occurring amino acids have an amino group on the second carbon along from the carboxyl group. The R represents a varying side group.

Polypeptides

• Polypeptides are condensation polymers which are formed from amino acid monomers joined together by peptide links
• In polypeptides the amino group on one amino acid links to the acid group of an adjacent amino acid

Peptide link

Diagram showing a peptide link which holds proteins together

• This pattern continues to form the polymer chain with one water molecule being lost per peptide link
• Proteins are one or more long-chains of polypeptides
• Most proteins contain at least 20 different amino acids
• Polypeptides and proteins can contain different amino acids in their chains. The order of the amino acids determines the proteins structure and properties

Formation of a peptide bond

Diagram showing condensation polymerisation to produce a protein