A doctor explains to a patient that red blood cells are highly specialised cells. Describe the features of a red blood cell that make it well adapted for carrying oxygen around the body, and explain how each feature helps it carry out this function.

OCR A-Level Biology B — Advancing Biology (H422) — 2.2 Biological molecules including biochemistry · Describe · 5 marks · View as Markdown

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

Red blood cells are produced in the bone marrow and released into the blood, where they transport oxygen to respiring cells throughout the body.

Model answer (5 marks)

1. Biconcave shape – increases surface area for oxygen diffusion.
2. Lack of nucleus – frees space for haemoglobin.
3. Haemoglobin – binds and carries oxygen.
4. The larger surface area allows rapid O₂ uptake.
5. More haemoglobin means higher oxygen capacity.

Examiner tips

  • Use the exact phrases: biconcave shape, no nucleus, haemoglobin. Explain each feature’s advantage. Keep answer to 5 points, one per mark.

Common mistakes

  • Confusing cell membrane with haemoglobin. Omitting the explanation of how each feature helps. Using vague terms like "big" instead of "more space".

Mark scheme (5 marks)

  1. Red blood cells have a biconcave shape
  2. The biconcave shape increases the surface area (for oxygen absorption / diffusion of oxygen into the cell)
  3. Red blood cells have no nucleus
  4. Having no nucleus leaves more space / room for haemoglobin (so more oxygen can be carried)
  5. Red blood cells contain haemoglobin, which binds to / carries oxygen

Key terms in this question

red blood cells

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