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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
The liver removes the amino group from excess amino acids in a process called deamination. The amino group is converted to ammonia (toxic), which is then converted to urea (less toxic). Urea dissolves in the blood, travels to the kidneys, and is excreted in urine.
Deamination is a Supplement topic that explains why urea exists. The chain is: excess amino acids → deamination in liver → ammonia → converted to urea → blood → kidneys → urine. Each step is a potential mark point in an extended-response question.
Why deamination happens
The body cannot store excess amino acids. After digestion, more amino acids may be absorbed than the body needs for protein synthesis and growth. These excess amino acids must be broken down because they cannot simply be stored like glucose (as glycogen) or fatty acids (as fat).
The liver carries out deamination: the removal of the nitrogen-containing amino group (–NH₂) from each excess amino acid. The remaining carbon-containing part can be converted to glucose or fat and used in respiration or stored as an energy source.
From ammonia to urea
The amino groups removed during deamination form ammonia (NH₃). Ammonia is highly toxic and must be dealt with immediately. The liver converts ammonia to urea (CO(NH₂)₂), which is much less toxic and can safely dissolve in the blood at normal concentrations.
Urea dissolves in blood plasma and is transported to the kidneys, where it is filtered from the blood and excreted as part of urine. The conversion of ammonia to urea is essential because ammonia would damage tissues if it accumulated.
Other liver functions
The liver has many metabolic functions beyond deamination:
Detoxification: breaking down harmful substances including alcohol and drugs
Bile production: bile emulsifies lipids in the small intestine
Glycogen storage: converting excess glucose to glycogen for storage, and back to glucose when blood glucose drops
Breakdown of old red blood cells: iron is recycled and bile pigments are excreted
Breakdown of lactic acid: after exercise, the liver breaks down lactic acid using oxygen
The liver receives blood from the hepatic artery (oxygenated) and the hepatic portal vein (carrying absorbed nutrients from the small intestine).
Try a focused question (4 marks)
Describe the process by which excess amino acids are broken down in the liver and explain why this is necessary.
Show the answer and marking guidance
Excess amino acids cannot be stored by the body.
The liver carries out deamination: removing the amino group from excess amino acids.
The amino group is converted to ammonia and then to the less toxic urea.
Urea is transported in the blood to the kidneys and excreted in urine.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying the liver excretes urea: the liver produces urea; the kidneys excrete it.
Confusing urea with urine: urea is a specific molecule; urine is the liquid that contains it.
Forgetting to explain why deamination is necessary (amino acids cannot be stored).
Saying ammonia is directly excreted: it is first converted to urea because ammonia is too toxic.