Explain how the process of diffusion allows oxygen to move from the alveoli into the blood in the lungs, and describe two features of the alveoli that make this process efficient.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration.
Oxygen concentration is higher in the alveoli than in the blood, creating a concentration gradient that drives oxygen molecules into the blood.
Alveoli have a very large surface area, which increases the area available for diffusion.
The alveolar walls are only one cell thick, giving a very short diffusion distance and a thin membrane.
A rich capillary network surrounds the alveoli, maintaining the concentration gradient and allowing rapid uptake of oxygen.
Oxygen concentration is higher in the alveoli than in the blood, creating a concentration gradient that drives oxygen molecules into the blood.
Alveoli have a very large surface area, which increases the area available for diffusion.
The alveolar walls are only one cell thick, giving a very short diffusion distance and a thin membrane.
A rich capillary network surrounds the alveoli, maintaining the concentration gradient and allowing rapid uptake of oxygen.
Examiner tips
- Use the definition of diffusion first, then state the concentration gradient. Mention both large surface area and thin walls as the two features. Show the link between capillary network and maintaining the gradient.
Common mistakes
- Forgetting to state that oxygen concentration is higher in the alveoli. Describing the alveoli as having a thick wall or long diffusion distance. Failing to mention the capillary network as a feature that keeps the gradient.
Mark scheme (5 marks)
- Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration
- Oxygen concentration is higher in the alveoli than in the blood / there is a concentration gradient between the alveoli and the blood
- Alveoli have a large surface area (to allow more diffusion to occur)
- Alveoli walls are one cell thick / have a thin membrane / short diffusion distance
- Alveoli have a good blood supply / rich capillary network that maintains the concentration gradient
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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