Explain how the structure of red blood cells makes them well adapted for transporting oxygen in the blood.

Pearson Edexcel A-Level Biology A (9BI0) — 1.3 Mass transport in animals · 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, giving a large surface‑area‑to‑volume ratio so oxygen can diffuse in and out quickly.
They contain haemoglobin, which binds oxygen.
The absence of a nucleus frees up space for haemoglobin.
Their small, flexible shape allows them to squeeze through narrow capillaries and deliver oxygen to tissues.

Examiner tips

Use the exact terms: biconcave disc, surface‑area‑to‑volume ratio, haemoglobin, no nucleus, flexible shape.
Show the link between structure and function – how each feature improves oxygen transport.
Keep the answer concise and to the point, matching the 5 marks.
Use bullet points or short sentences to organise the points clearly.

Common mistakes

Writing ‘oval’ or ‘spherical’ instead of biconcave disc.
Forgetting to mention haemoglobin or the lack of a nucleus.
Using vague phrases like ‘good shape’ without explaining the functional advantage.

Mark scheme (5 marks)

Red blood cells have a biconcave disc shape
This gives a large surface area to volume ratio, allowing more oxygen to diffuse in and out quickly
Red blood cells contain haemoglobin, which binds to oxygen
Red blood cells have no nucleus, leaving more space/volume for haemoglobin
Their small size and flexible shape allows them to squeeze through narrow capillaries to deliver oxygen to tissues

Key terms in this question

red blood cells