Explain how the structure of a red blood cell is adapted to its function in transporting oxygen.

IB DP Biology Standard Level (2023 syllabus) — B3.2 Transport · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. The biconcave disc shape increases the surface area to volume ratio.
2. A larger surface area to volume ratio allows oxygen to diffuse into and out of the cell more rapidly.
3. The cell lacks a nucleus and most organelles, giving more space for haemoglobin.
4. Haemoglobin binds oxygen to form oxyhaemoglobin, enabling efficient oxygen transport in the blood.

Examiner tips

  • Use the exact terms: biconcave disc, surface area to volume ratio, haemoglobin, oxyhaemoglobin.
  • Show the causal link between structure and function in each point.
  • Keep each point concise – one sentence per point.
  • Avoid extra words like "in this answer" or "the question asks".

Common mistakes

  • Writing the shape as a sphere instead of biconcave disc.
  • Forgetting to mention the absence of a nucleus or organelles.
  • Using vague phrases like "good shape" instead of specific terms.

Mark scheme (4 marks)

  1. Biconcave disc shape increases the surface area to volume ratio
  2. Large surface area to volume ratio allows faster diffusion of oxygen into and out of the cell
  3. Absence of a nucleus (and most organelles) provides more space for haemoglobin
  4. Haemoglobin binds oxygen / forms oxyhaemoglobin, enabling efficient oxygen transport in the blood

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