Explain why the concentration of urea in the blood plasma leaving the liver is higher than the concentration of urea in the blood plasma entering the liver.

OCR A-Level Biology A (H420) — 5.2 Excretion · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Urea is produced in the liver from the breakdown of excess amino acids. It is transported in the blood to the kidneys, where it is filtered and excreted in urine. A patient with liver disease may show abnormally low blood urea levels despite consuming a high-protein diet.

Model answer (4 marks)

1. Excess amino acids cannot be stored, so they are broken down.
2. The amino group is removed by deamination, producing ammonia.
3. Ammonia is toxic, so the liver converts it to urea via the ornithine cycle.
4. Therefore blood leaving the liver contains a higher urea concentration than blood entering it.

Examiner tips

  • Use the exact terms ‘deamination’ and ‘ornithine cycle’.
  • Show the logical sequence: no storage → deamination → ammonia → urea conversion → higher concentration.
  • Keep each point short and to the point.

Mark scheme (4 marks)

  1. Amino acids cannot be stored, so excess amino acids must be broken down
  2. The amino group is removed from excess amino acids by deamination
  3. Deamination produces ammonia, which is toxic
  4. Ammonia is converted to urea in the liver via the ornithine cycle, so blood leaving the liver contains more urea

Key terms in this question

urea

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