Oxygen produced during photosynthesis must diffuse from the mesophyll cells of a leaf to the air outside. Explain how the concentration gradient and the diffusion distance each affect the rate at which oxygen diffuses out of the leaf.

Cambridge International IGCSE Biology (0610) — 3.1 Diffusion · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A greater concentration gradient increases the rate of diffusion of oxygen out of the leaf, because more oxygen molecules move from the high concentration area (mesophyll) to the low concentration area (air) per unit time, giving a greater net movement of particles down the gradient.
A smaller diffusion distance increases the rate of diffusion of oxygen, because oxygen molecules need to travel a shorter distance and therefore take less time to reach the outside air.

Examiner tips

  • Use the words "concentration gradient" and "diffusion distance" explicitly. Show the causal link: higher gradient → more molecules per unit time; shorter distance → less time to travel. Keep the answer concise – 4 marks only.
  • Mention the direction of movement (high to low concentration).

Common mistakes

  • Confusing diffusion distance with thickness of the leaf; not recognising that the distance is the path to the outside air. Using vague phrases like "more oxygen leaves" without linking to gradient or distance. Failing to mention the net movement of particles or the time aspect.

Mark scheme (4 marks)

  1. A greater concentration gradient increases the rate of diffusion of oxygen out of the leaf
  2. Because more oxygen molecules move from the high concentration area to the low concentration area per unit time / there is a greater net movement of particles down the gradient
  3. A smaller diffusion distance increases the rate of diffusion of oxygen
  4. Because oxygen molecules need to travel a shorter distance / take less time to reach the outside air

Key terms in this question

diffusion · concentration gradient · diffusion distance

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