Maintaining water and nitrogen balance in the body (biology only) — AQA GCSE Biology
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Maintaining water and nitrogen balance in the body (biology only) explained
Ammonia formed by deamination is highly toxic, so it cannot be allowed to accumulate in the body.
Read the full explanation
The liver immediately converts ammonia into urea, a less toxic nitrogenous compound that can be transported safely in the blood. Urea is then carried to the kidneys, filtered from the blood and excreted in urine. For example, a person eating a high-protein diet produces more ammonia and therefore more urea, which is why urine contains urea. This two-step sequence, ammonia then urea, protects cells while still removing surplus nitrogen. The word immediately matters because rapid conversion prevents toxic ammonia from building up.
Your focus
- Explain why ammonia must be converted to urea.
- State where and how urea is formed.
- Describe the route by which urea leaves the body.
Maintaining water and nitrogen balance in the body (biology only) exam tips
Marking Points
- Ammonia is toxic to cells and must not accumulate.
- The liver converts ammonia to urea.
- Urea is less toxic than ammonia and can be transported safely in the blood.
- Urea is excreted by the kidneys in urine.
- The conversion happens immediately after deamination.
- A high-protein diet increases urea production because more amino acids are deaminated.
Examiner Tips
- 💡Use the sequence deamination → ammonia → urea → urine.
- 💡Compare toxicity explicitly: ammonia toxic, urea less toxic.
- 💡Mention the liver as the conversion site and the kidneys as the excretion route.
Common Mistakes
- Stating that ammonia itself is excreted in urine; correct this by stating ammonia is converted to urea first.
- Saying urea is more toxic than ammonia; correct this by stating urea is less toxic.
- Naming the kidneys as the site of conversion; correct this by naming the liver.