Explain how the rate of transpiration in a plant would change on a hot, sunny, windy day compared with a cool, calm, cloudy day.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Transpiration is the loss of water from plant leaves and shoots via diffusion and evaporation. Several environmental factors affect how quickly this process occurs.
Model answer (4 marks)
The rate of transpiration would be higher on a hot, sunny, windy day.
1. Higher temperature increases the kinetic energy of water molecules, raising the rate of evaporation and diffusion from the leaf.
2. Greater light intensity causes guard cells to become turgid, opening the stomata wider and allowing more water vapour to escape.
3. Wind or air flow removes the water‑vapour layer around the leaf, keeping a steep concentration gradient and speeding diffusion of water out of the leaf.
1. Higher temperature increases the kinetic energy of water molecules, raising the rate of evaporation and diffusion from the leaf.
2. Greater light intensity causes guard cells to become turgid, opening the stomata wider and allowing more water vapour to escape.
3. Wind or air flow removes the water‑vapour layer around the leaf, keeping a steep concentration gradient and speeding diffusion of water out of the leaf.
Examiner tips
- Use the word ‘higher’ or ‘faster’ to show the change in rate. Show the three environmental factors and link each to a specific effect on transpiration. Use correct terminology: evaporation, diffusion, stomata, guard cells, concentration gradient.
Common mistakes
- Using vague terms like ‘more water’ instead of ‘higher rate of transpiration’. Omitting the role of wind or light intensity. Mixing up evaporation with transpiration or not mentioning stomatal opening.
Mark scheme (4 marks)
- The rate of transpiration would be higher / faster on the hot, sunny, windy day
- Higher temperature increases the rate of evaporation / diffusion of water molecules from the leaf
- Greater light intensity causes stomata to open wider (guard cells become turgid), allowing more water vapour to escape
- Wind / air flow removes water vapour from around the leaf, maintaining a steep concentration gradient so diffusion of water out of the leaf is faster
Key terms in this question
Related
- Revision notes: Transport in plants →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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