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

OCR GCSE Biology A: Gateway Science (J247) — B2.2 The challenges of size · Explain · 4 marks · View as Markdown

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

Red blood cells are specialised cells found in the blood. They are produced in the bone marrow and travel through the circulatory system delivering oxygen to respiring tissues.

Model answer (4 marks)

Red blood cells are biconcave, which increases their surface‑area‑to‑volume ratio and allows oxygen to diffuse into and out of the cell more rapidly.
They lack a nucleus, freeing up internal space for haemoglobin, the protein that binds oxygen. This combination maximises the amount of oxygen that can be carried and delivered efficiently throughout the body.

Examiner tips

  • Use the term "biconcave" and link it to increased surface area. Mention the absence of a nucleus and its effect on haemoglobin capacity. Keep the answer concise and directly address the question.
  • common_mistakes
  • :
  • Forgetting to mention the biconcave shape. Failing to explain why the lack of a nucleus matters. Using vague phrases like "good shape" without specifying the surface‑area benefit.

Mark scheme (4 marks)

  1. Red blood cells have a biconcave shape
  2. This increases the surface area (for gaseous exchange / diffusion of oxygen)
  3. Red blood cells have no nucleus
  4. Having no nucleus leaves more space / room for haemoglobin (to carry oxygen)

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