Explain how the structure of a red blood cell (erythrocyte) is adapted to maximise oxygen transport in mammals.
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 oxygen to diffuse in and out quickly.
The cell lacks a nucleus and other organelles, providing more space for haemoglobin, so the cell can carry more oxygen.
Haemoglobin is present at a high concentration; each molecule can bind up to four O₂ molecules and shows cooperative binding, so oxygen is loaded efficiently in the lungs.
The plasma membrane is highly flexible, enabling the cell to deform and pass through narrow capillaries, ensuring continuous oxygen delivery to tissues.
The cell lacks a nucleus and other organelles, providing more space for haemoglobin, so the cell can carry more oxygen.
Haemoglobin is present at a high concentration; each molecule can bind up to four O₂ molecules and shows cooperative binding, so oxygen is loaded efficiently in the lungs.
The plasma membrane is highly flexible, enabling the cell to deform and pass through narrow capillaries, ensuring continuous oxygen delivery to tissues.
Examiner tips
- Mention the biconcave shape first to score the surface‑area point. Highlight the absence of a nucleus to gain the organelle point. Explain haemoglobin’s capacity and cooperative binding for the loading point. State membrane flexibility for the deformation point.
Common mistakes
- Forgetting to mention the biconcave shape. Describing the cell as rigid instead of flexible. Omitting the cooperative binding detail of haemoglobin.
Mark scheme (4 marks)
- Biconcave disc shape increases surface area to volume ratio, allowing faster diffusion of oxygen into and out of the cell
- Absence of a nucleus (and other organelles) creates more space for haemoglobin, increasing oxygen-carrying capacity
- High haemoglobin concentration enables binding of up to four oxygen molecules per haemoglobin molecule (cooperative binding), loading oxygen efficiently at the lungs
- Flexible plasma membrane allows the cell to deform and squeeze through narrow capillaries, maintaining continuous oxygen delivery to tissues
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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