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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Transpiration rate increases with higher temperature, lower humidity, greater wind speed and higher light intensity. Each factor works by steepening the water-vapour concentration gradient between the leaf interior and the surrounding air, or by opening stomata wider.
Questions on transpiration factors are common in both Paper 2 and Paper 4. The most frequent error is listing the factors without explaining the mechanism. Every answer must connect the factor to the concentration gradient of water vapour or to stomatal aperture.
How each factor affects the rate
Factor
Effect on transpiration
Why
Temperature increase
Rate increases
Water molecules have more kinetic energy, evaporate faster from mesophyll surfaces; the concentration gradient steepens
Humidity decrease
Rate increases
Drier external air steepens the water-vapour concentration gradient between the leaf interior and the atmosphere
Wind speed increase
Rate increases
Wind removes water vapour from around the leaf surface, maintaining a steep concentration gradient
Light intensity increase
Rate increases
Stomata open wider to admit CO₂ for photosynthesis, allowing more water vapour to escape
Explaining the gradient mechanism
The common thread is the water-vapour concentration gradient between the saturated air spaces inside the leaf and the less saturated external air. Any factor that makes the external air drier relative to the leaf interior steepens this gradient and increases diffusion rate.
Light intensity works differently: rather than changing the gradient directly, it causes guard cells to open stomata more widely, increasing the area through which water vapour can diffuse. In darkness, stomata close and transpiration drops regardless of temperature or wind.
Applying the factors in context
In a tropical climate like Malaysia, high daytime temperatures and strong sunlight drive rapid transpiration. Plants adapted to these conditions often have thick waxy cuticles, fewer stomata on upper surfaces, or the ability to roll their leaves to reduce exposed surface area.
When describing an investigation using a potometer, specify which factor is the independent variable, how you change it (for example, using a fan for wind speed or a lamp for light intensity), and which variables you control. Changing two factors at once is a fair-test violation.
Try a focused question (3 marks)
A student places a leafy shoot in a potometer near a heater. The bubble moves faster. Explain this result.
Show the answer and marking guidance
Higher temperature gives water molecules more kinetic energy, so they evaporate faster from mesophyll cell surfaces.
This increases the water-vapour concentration gradient between the inside of the leaf and the outside air.
Water vapour diffuses out of stomata more rapidly, and water is pulled through the xylem faster, moving the bubble.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying wind pushes water out of the leaf rather than removing the humid layer.
Listing factors without the mechanism: just "temperature increases transpiration" is not enough.
Confusing the potometer reading with transpiration itself; the potometer measures water uptake, not vapour loss directly.
Ignoring light intensity as a factor because it works via stomatal opening rather than the gradient.
Practical connection
Investigating the Rate of Transpiration
The rate of transpiration can be measured using a potometer: a device that tracks how fast water is taken up by a cut shoot. Factors such as wind, humidity, light, and temperature can be tested.