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A-Level Biology October/November 2024 Q5(c): A red blood cell that is infected with Plasmodium cannot carry out its function as effe…
A-Level Biology · Paper 9700/22 · October/November 2024 · Question 5(c) · [4 marks]
A red blood cell that is infected with Plasmodium cannot carry out its function as effectively as a normal red blood cell. Describe how the size and structure of a red blood cell is related to its function, other than the fact that it contains a very large number of haemoglobin molecules.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
A red blood cell has a distinctive biconcave shape. This shape increases the surface area to volume ratio, which allows for a faster rate of diffusion for the uptake of oxygen.
Additionally, mature red blood cells lack organelles such as a nucleus and mitochondria. The absence of mitochondria is significant because it means the cell does not carry out aerobic respiration. This ensures that the oxygen being transported is not consumed by the red blood cell itself.
How the marks are awarded
- M1 — Correctly identifying the shape as 'biconcave'. (Mark point 7)
- M1 — Linking the biconcave shape to an increased surface area to volume ratio, which facilitates faster oxygen uptake. (Mark point 8)
- M1 — Stating that the cell has no nucleus or mitochondria. (Mark point 3)
- M1 — Explaining that the absence of mitochondria prevents the cell from using the transported oxygen for its own aerobic respiration. (Mark point 5)
Common mistakes
- Describing the cell as 'small' without quantifying its size (6-8 µm) or explaining that this size allows it to pass through narrow capillaries.
- Stating that the biconcave shape increases surface area but failing to link this to the surface area to volume ratio or to a faster rate of diffusion.
- Incorrectly explaining the consequence of lacking a nucleus, for example, stating it makes the cell flexible, when flexibility is primarily a property of the cell surface membrane.
- Mentioning that the lack of a nucleus provides more space for haemoglobin, which, while correct (and a valid mark point), can be a risky answer if not phrased carefully, given the question's prompt to avoid discussing the large number of haemoglobin molecules.
Examiner tip: For questions on specialised cells, always explicitly link each structural feature you mention to a specific functional advantage it provides, using precise biological terminology.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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