B1.2.3

• There are twenty different amino acids that are used by ribosomes to make polypeptides.
• Essential amino acids cannot be synthesized in sufficient quantities, so they must be obtained from the diet. Non-essential amino acids can be synthesised by an animal using metabolic pathways that transform one amino acid into another.
• Nine of 20 amino acids are essential in humans, the others are non-essential however several become essential in special circumstances. Some amino acids can be made from others.