A biology teacher explains to a class that carbon dioxide and urea are both waste products that must be removed from the body, but they are excreted by different organs. Explain how carbon dioxide and urea are each produced in the body and state which organ is responsible for excreting each waste product.

OCR A-Level Biology B — Advancing Biology (H422) — 5.6 Excretion · Explain · 5 marks · View as Markdown

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

Carbon dioxide and urea are both metabolic waste products found in the blood. If they were allowed to build up, they would become toxic to the body's cells.

Model answer (5 marks)

Carbon dioxide is produced during aerobic respiration, when glucose is oxidised to CO₂ and H₂O in the mitochondria. It is excreted by the lungs, which remove it from the blood during breathing.
Urea is produced in the liver by the urea cycle, where excess amino‑acid nitrogen is removed through deamination and converted into urea. Urea is then excreted by the kidneys in the urine.

Examiner tips

  • Use the exact terms ‘aerobic respiration’, ‘lungs’, ‘urea cycle’, ‘liver’, ‘kidneys’, ‘urine’.
  • Show the two separate pathways clearly – one for CO₂, one for urea.
  • Mention the organ that removes each waste product.
  • Keep the answer concise and to the point.

Common mistakes

  • Confusing CO₂ excretion with the kidneys instead of the lungs.
  • Lumping urea production with the kidneys instead of the liver.
  • Using vague terms like ‘body’ or ‘organ’ without specifying the correct organ.

Mark scheme (5 marks)

  1. Carbon dioxide is produced during (aerobic) respiration
  2. Carbon dioxide is excreted by the lungs
  3. Urea is produced by the breakdown/deamination of excess amino acids
  4. Urea is produced in the liver
  5. Urea is excreted by the kidneys (in urine)

Key terms in this question

urea · carbon dioxide

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