Explain why the rate of transpiration from a leafy shoot is higher on a hot, dry, windy day than on a cool, humid, still day.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Higher temperature increases the evaporation of water from the mesophyll into the leaf air spaces.
Dry air (low humidity) keeps a steep concentration gradient for water vapour between the leaf air spaces and the surrounding air.
Wind removes water vapour from around the leaf surface, maintaining or increasing that gradient.
Water vapour diffuses out of the leaf through the stomata down the concentration gradient, so a steeper gradient and faster evaporation give a higher rate of transpiration.
Dry air (low humidity) keeps a steep concentration gradient for water vapour between the leaf air spaces and the surrounding air.
Wind removes water vapour from around the leaf surface, maintaining or increasing that gradient.
Water vapour diffuses out of the leaf through the stomata down the concentration gradient, so a steeper gradient and faster evaporation give a higher rate of transpiration.
Examiner tips
- Use the word ‘higher’ or ‘steeper’ to link temperature, humidity and wind to the concentration gradient. Mention that water vapour moves through the stomata by diffusion. Show the cause–effect chain: temperature → evaporation → gradient → diffusion → transpiration.
Common mistakes
- Failing to mention the role of wind in removing water vapour. Using vague terms like ‘more water’ instead of ‘steeper concentration gradient’. Omitting the diffusion through stomata step.
Mark scheme (4 marks)
- Higher temperature increases the rate of evaporation of water from the mesophyll cells (into the air spaces)
- Dry air / low humidity maintains a steep concentration gradient for water vapour between the leaf air spaces and the surrounding air
- Wind removes water vapour from around the leaf surface, maintaining or increasing the concentration gradient
- Water vapour diffuses out of the leaf through the stomata (down the concentration gradient), so a steeper gradient / faster evaporation leads to a higher rate of transpiration
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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