2.5.5Unit 2 — Multicellular Organisms
Transpiration
5 min read 5 parts Lesson 24 of 39
Section 1 of 5The Process of Transpiration
1
Water evaporates from the surface of cells lining the air spaces inside the leaf, forming water vapour in those spaces.
2
The water vapour then diffuses out of the leaf through the open stomata into the drier air outside.
3
Water lost in this way is replaced by more water drawn up the xylem from the roots.
4
The result is a continuous upward flow of water called the transpiration stream.
5
This stream brings dissolved mineral ions from the soil and supplies water to leaf cells for photosynthesis.
Section 2 of 5Factors Affecting Transpiration
Wind speed
- Moving air removes water vapour from near the leaf, so the concentration gradient is steeper and transpiration speeds up.
Humidity
- Humid air already holds much water vapour, so the gradient is shallower and transpiration rate falls.
Temperature
- Warmer conditions give water molecules more kinetic energy, so more evaporate and diffuse out and the rate increases.
Surface area
- A larger leaf surface has more stomata through which water vapour can escape, so the rate increases.
| Factor | Change | Effect on rate |
|---|---|---|
| Wind speed | Increases | Increases |
| Humidity | Increases | Decreases |
| Temperature | Increases | Increases |
| Leaf surface area | Increases | Increases |
Section 3 of 5Why It Matters
Why It Matters
Water supply
- Transpiration helps move water to the leaves, where it is used as a raw material in photosynthesis.
Mineral ions
- Minerals dissolved in the soil water are carried upwards in the stream and distributed to cells that need them.
Cost to the plant
- Open stomata lose water, so in dry conditions the plant risks wilting if loss exceeds the uptake of water.
Control
- Guard cells can close the stomata in dry conditions and so reduce the rate of transpiration.
Section 4 of 5Linking Ideas
Linking Ideas
Concentration gradient
- Water vapour diffuses from the high concentration inside the leaf to the lower concentration outside.
Diffusion rate
- Any factor that steepens the gradient, such as wind, increases the rate of diffusion and so transpiration.
Leaf adaptations
- A waxy cuticle limits evaporation from the leaf surface so that most water loss is through the stomata.
Xylem role
- Xylem forms an unbroken pathway, which allows replacement water to reach the leaf as soon as water is lost.
Section 5 of 5Exam Checklist & Tips
2.5.5 – Key Points for the Exam
- Transpiration The loss of water vapour by evaporation from the leaves of a plant.
- Transpiration stream The upward movement of water in the xylem that replaces water lost from the leaves.
- Concentration gradient The difference in concentration between two regions.
- Humidity The amount of water vapour in the air.
- Evaporation The change of liquid water to water vapour.
Common Mistakes
- Saying that transpiration is loss of liquid water; it is loss of water vapour.
- Stating that higher humidity increases the rate of transpiration;
- it reduces the gradient and so lowers the rate.
- Forgetting to mention the concentration gradient when explaining the effect of wind.
- Confusing transpiration, the loss of water vapour, with the uptake of water by the roots.