TranspirationCambridge IGCSE Biology: Revision notes
Section 1
What Is Transpiration?
Transpiration is the loss of water vapour from the leaves of a plant.
- Water evaporates from the surfaces of mesophyll cells inside the leaf, into the air spaces between them
- This water vapour then diffuses out of the leaf through the stomata (tiny pores, mostly on the underside of the leaf)
- Transpiration is a side effect of gas exchange - stomata must open for carbon dioxide to enter for photosynthesis, and water vapour escapes at the same time
Section 2
Investigating Transpiration Rate
The rate of transpiration can be investigated experimentally, for example using a potometer, and the effects of environmental factors measured:
- Increasing temperature generally increases the rate of transpiration
- Increasing wind speed generally increases the rate of transpiration
When describing an investigation, always mention keeping other variables constant (e.g. light, humidity) so the effect of the one variable being tested can be measured fairly.
Section 3
Factors Affecting Transpiration: Structure
The rate of water vapour loss is related to leaf structure:
- The large internal surface area provided by interconnecting air spaces between mesophyll cells allows more water to evaporate
- The size and number of stomata affects how easily water vapour can diffuse out of the leaf - more or larger stomata increase water loss
Section 4
Factors Affecting Transpiration: Environment
Several environmental factors change the rate of transpiration:
| Factor | Effect on transpiration rate |
|---|---|
| Higher temperature | Increases rate (more kinetic energy, faster evaporation and diffusion) |
| Higher wind speed | Increases rate (removes water vapour from around stomata, maintaining a steep diffusion gradient) |
| Higher humidity | Decreases rate (less of a concentration gradient between the leaf and the air, so diffusion of water vapour out slows) |
Higher humidity decreases transpiration rate, not increases it - a common error is assuming all environmental factors increase the rate.
Section 5
Wilting
Wilting occurs when a plant loses water faster than it can absorb it from the soil.
- Cells lose water and turgor pressure, so they become flaccid
- Without turgor pressure supporting the plant's soft tissues, stems and leaves droop
- Wilting can be reversed if the plant is given enough water before permanent damage occurs
Must Know
- Transpiration is the loss of water vapour from the leaves, via evaporation from mesophyll cells and diffusion out through stomata
- Rate of transpiration increases with higher temperature and wind speed, but decreases with higher humidity
- Large internal surface area of mesophyll and number/size of stomata affect water loss
- Wilting happens when water loss exceeds water uptake, causing loss of turgor pressure
That's the notes covered.
Carry on to the next subtopic.