Explain how the structure of a red blood cell is adapted to carry oxygen efficiently around the body.

AQA A-Level Biology (7402) — 3.3.4 Mass Transport · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Red blood cells are biconcave discs, which gives them a very large surface‑area‑to‑volume ratio, allowing oxygen to diffuse into the cell quickly.
They contain haemoglobin, the protein that binds oxygen; the more haemoglobin, the more oxygen can be carried.
The cells lack a nucleus and most organelles, so the interior is largely free space for haemoglobin.
Their small, flexible shape lets them squeeze through capillaries that are only a few microns wide, ensuring that oxygen reaches every tissue cell.

Examiner tips

  • Use the exact terms: biconcave disc, surface‑area‑to‑volume ratio, haemoglobin, lack of nucleus, flexibility.
  • Show the causal link: shape → diffusion, lack of nucleus → more haemoglobin, flexibility → capillary passage.
  • Keep the answer concise and directly tied to the five points.

Common mistakes

  • Writing that red blood cells are ‘round’ instead of biconcave; missing the surface‑area‑to‑volume point.
  • Forgetting that the lack of a nucleus frees space for haemoglobin.
  • Including irrelevant details such as the role of the spleen or the lifespan of the cell.

Mark scheme (5 marks)

  1. Red blood cells have a biconcave disc shape
  2. Biconcave shape increases the surface area (to volume ratio) for oxygen diffusion
  3. Red blood cells contain haemoglobin, which carries/binds to oxygen
  4. Red blood cells have no nucleus, allowing more space for haemoglobin
  5. Red blood cells are small and flexible, allowing them to pass through narrow capillaries to reach all body cells

Key terms in this question

red blood cell

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