Explain how urea is produced in the liver and subsequently removed from the body by the kidneys.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Excess amino acids are deaminated in the liver, releasing ammonia. The liver converts the toxic ammonia into urea through the urea cycle. Urea then enters the bloodstream and is carried to the kidneys. In the kidneys, urea is filtered out of the blood by the glomerulus and is excreted in the urine.
Examiner tips
- Use the word "deaminated" and "urea cycle" to show knowledge of the process.
- Mention that urea is transported in the blood and filtered by the glomerulus.
- Show the flow from liver to blood to kidneys to urine.
- Use correct UK spelling (e.g., "excreted").
Common mistakes
- Confusing ammonia with urea as the final product.
- Failing to mention the transport of urea in the blood.
- Using the word "excrete" instead of "excreted" or omitting the kidneys entirely.
Mark scheme (4 marks)
- Excess amino acids cannot be stored and are broken down (deaminated) in the liver
- The nitrogen-containing part of the amino acid is converted to urea (in the liver)
- Urea passes from the liver into the blood and is transported to the kidneys
- The kidneys filter urea out of the blood and it is excreted in urine
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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