The liver is an organ involved in excretion. One of its roles is to break down excess amino acids through a process called deamination. The nitrogen-containing waste product produced is eventually excreted by the kidneys in urine. Explain how the liver and kidneys work together to remove the waste products of excess amino acids from the body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The human body cannot store excess amino acids. Instead, the liver converts them into a less harmful substance before they are removed from the body in urine.
Model answer (5 marks)
1. Excess amino acids are broken down in the liver by deamination.
2. Deamination releases ammonia from the nitrogen part of the amino acid.
3. Ammonia is toxic, so the liver converts it into urea via the urea cycle.
4. Urea enters the bloodstream and is transported to the kidneys.
5. The kidneys filter urea from the blood and excrete it in urine.
2. Deamination releases ammonia from the nitrogen part of the amino acid.
3. Ammonia is toxic, so the liver converts it into urea via the urea cycle.
4. Urea enters the bloodstream and is transported to the kidneys.
5. The kidneys filter urea from the blood and excrete it in urine.
Examiner tips
- Use the exact terms: deamination, ammonia, urea cycle, kidneys filter urea.
- Show the flow: liver → blood → kidneys → urine.
- Keep each point short and separate to match the 5 marks.
Common mistakes
- Confusing ammonia with urea as the final waste product.
- Failing to mention the transport via blood to the kidneys.
- Using vague language like ‘breaks down’ without specifying deamination.
Mark scheme (5 marks)
- Excess amino acids are broken down in the liver by deamination
- Deamination produces ammonia (from the nitrogen-containing part of the amino acid)
- Ammonia is toxic / harmful, so it is converted into urea in the liver
- Urea passes from the liver into the blood and is transported to the kidneys
- The kidneys filter urea from the blood and it is excreted in urine
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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