Oxygen diffuses from the alveoli in the lungs into the surrounding blood capillaries. Explain why oxygen moves in this direction and describe two features of the alveoli that increase the rate of this diffusion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Oxygen moves from the alveoli into the blood because the concentration of O₂ in the alveolar air (≈21 %) is higher than that in the capillary blood (≈5 %) – a concentration gradient drives diffusion.
Two features of the alveoli that increase the rate of diffusion are:
1. Their very large surface area (≈70 m² in a human lung) provides more space for O₂ molecules to cross.
2. Their walls are extremely thin (≈0.2 µm) so the diffusion distance is short, speeding the transfer.
Two features of the alveoli that increase the rate of diffusion are:
1. Their very large surface area (≈70 m² in a human lung) provides more space for O₂ molecules to cross.
2. Their walls are extremely thin (≈0.2 µm) so the diffusion distance is short, speeding the transfer.
Examiner tips
- State the concentration gradient first to secure the 1 mark. Mention surface area and thin walls as the two features to earn the 3 marks. Use the exact terms ‘concentration gradient’, ‘surface area’ and ‘thin walls’.
Common mistakes
- Confusing the direction of diffusion (writing that O₂ moves from blood to alveoli). Forgetting to mention the thinness of the alveolar wall. Using vague phrases like ‘many alveoli’ instead of ‘large surface area’.
Mark scheme (4 marks)
- Oxygen moves from the alveoli into the blood because there is a higher concentration of oxygen in the alveoli than in the blood capillaries (concentration gradient exists).
- The large surface area of the alveoli increases the rate of diffusion because there is more space available for molecules to diffuse through.
- The thin walls of the alveoli (short diffusion distance) increase the rate of diffusion.
- A steep / maintained concentration gradient is kept by the blood flow (or continuous removal of oxygen by blood / continuous supply of air) which maintains the rate of diffusion.
Key terms in this question
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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