Explain how the structure of a red blood cell (erythrocyte) is adapted for the transport of oxygen in mammals.

IB DP Biology Standard Level (2023 syllabus) — B2.3 Cell specialisation · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A red blood cell is a biconcave disc, which gives a high surface area to volume ratio, allowing rapid diffusion of O₂ into and out of the cell.
It is anucleate, so the absence of a nucleus frees up space for haemoglobin, increasing the amount of O₂ that can be carried.
The cytoplasm contains a very high concentration of haemoglobin, which binds O₂ and transports it.
Its plasma membrane is highly flexible, enabling the cell to deform and pass through narrow capillaries, ensuring efficient delivery of O₂ to tissues.

Examiner tips

  • Use the exact terms: biconcave disc, anucleate, haemoglobin, flexible membrane
  • Explain each adaptation in one sentence, linking structure to function
  • Keep answer concise – 4 marks, so one sentence per point

Common mistakes

  • Confusing the disc shape with a sphere, losing the surface area point
  • Mentioning the nucleus instead of the lack of it
  • Failing to state that haemoglobin binds O₂

Mark scheme (4 marks)

  1. Biconcave disc shape increases the surface area to volume ratio, allowing more rapid diffusion of oxygen into and out of the cell.
  2. Lack of a nucleus (anucleate) provides more intracellular space for haemoglobin, maximising oxygen-carrying capacity.
  3. High concentration of haemoglobin in the cytoplasm enables binding and transport of oxygen molecules.
  4. Flexible plasma membrane allows the cell to deform and pass through narrow capillaries, maintaining efficient oxygen delivery to tissues.

Key terms in this question

erythrocyte

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