Structures and Functions in Living Organisms · 5 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 19% of your exam marks.
Transpiration and the roles of xylem and phloem are tested on almost every paper in recent years.
The rate of transpiration depends on how quickly water vapour can leave the leaf and how steep the water-vapour gradient is between the inside of the leaf and the air outside. Four environmental factors matter most:
| Factor | Effect when increased | Why |
|---|---|---|
| Light intensity | Transpiration increases | Brighter light makes the guard cells open the stomata wider, so more water vapour can escape |
| Temperature | Transpiration increases | Warmer water molecules have more kinetic energy; they evaporate faster and diffuse out more quickly |
| Wind speed (air movement) | Transpiration increases | Moving air sweeps water vapour away from the leaf as soon as it leaves the stomata, keeping the concentration gradient steep |
| Humidity | Transpiration decreases | High humidity means lots of water vapour is already in the air, so the gradient between leaf and air is shallow and diffusion slows down |
A dry, hot, sunny, windy day is therefore the worst combination for water loss. A still, cool, humid, dark night is the best for keeping water in the plant.