Describe how excess amino acids are processed in the body and how the waste product is removed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The human body cannot store excess amino acids. When more protein is consumed than the body needs, the surplus amino acids must be broken down.
Model answer (4 marks)
Excess amino acids are first deaminated in the liver, where the amino (nitrogen‑containing) part is converted into urea. The urea is then released into the bloodstream and carried to the kidneys. In the kidneys, urea is filtered out of the blood and excreted in the urine.
Examiner tips
- Use the term ‘deaminated’ and ‘urea cycle’ to show understanding of the process.
- Mention the liver as the site of conversion and the kidneys as the excretion organ.
- Show the flow: liver → blood → kidneys → urine.
- Use correct UK spelling (e.g. ‘excretion’).
Common mistakes
- Confusing ammonia with urea as the waste product.
- Saying the kidneys produce urea instead of the liver.
- Omitting the transport step via the bloodstream.
Mark scheme (4 marks)
- Excess amino acids are broken down / deaminated in the liver
- The amino / nitrogen-containing part is converted into urea (in the liver)
- Urea is transported in the blood (from the liver) to the kidneys
- The kidneys filter urea from the blood and it is excreted in urine
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Excretion: humans questions
- Explain how urea is produced in the liver and subsequently removed from the body…
- Describe the roles of the lungs and the skin as organs of excretion in humans.
- Describe the role of the liver and the kidneys in the removal of urea from the b…
- Explain why the kidneys are described as organs of excretion.