In the liver these amino acids are deaminated to form ammonia.

The idea

Excess amino acids cannot be stored by the body, so the liver removes their nitrogen-containing amino group in a reaction called deamination. The amino group is converted into ammonia (NH₃), while the remaining carbon skeleton is respired to release energy or converted to carbohydrate or fat. For example, after a protein-rich meal, surplus amino acids absorbed from the small intestine travel in the blood to the liver, where deamination produces ammonia. This links protein metabolism to nitrogen balance: the body must dispose of nitrogen safely, and ammonia is the first nitrogenous product formed. The liver is therefore central to homeostasis, processing amino acids rather than excreting them unchanged.

What earns the marks

• Deamination occurs in the liver, not in the kidneys or blood plasma.
• Excess amino acids, not all dietary amino acids, are deaminated.
• The amino group is removed from the amino acid during deamination.
• Ammonia (NH₃) is the nitrogen-containing product formed.
• The remaining part of the amino acid can be respired or converted to carbohydrate or fat.
• Deamination helps maintain nitrogen balance by removing surplus nitrogen from the body.

Common mistakes and how to put them right

• Saying deamination happens in the kidneys.

  Put it right by stating it occurs in the liver.

• Writing that amino acids are converted directly to urea.

  Put it right by stating ammonia is formed first.

• Stating that all amino acids are deaminated.

  Put it right by specifying excess amino acids only.

Exam tips

• Name the organ and the product: liver and ammonia.
• Use the word deamination accurately rather than describing it as digestion.
• Link the process to nitrogen balance so the answer addresses the section context.