Transpiration at Higher Level: The same statement as SL. HL papers ask you to pick the conditions that change the rate, or to state a factor before going on to water potential.
Practise this as you read
- State one environmental factor that affects transpiration rate.
- Pick the condition that increases the rate from four.
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The big idea: Transpiration is the loss of water vapour from a plant's leaves. It happens because the stomata must be open for gas exchange.
The points to remember
- Stomata open so carbon dioxide can diffuse in for photosynthesis.
- The walls of the mesophyll cells are moist; water evaporates from them into the air spaces.
- The air spaces fill with water vapour, so it diffuses out through the open stomata to drier air.
- Water lost is replaced by water brought in the xylem.
- Transpiration is the cost of gas exchange: a leaf cannot take in carbon dioxide without losing water.
Remember it as: Open for carbon dioxide, so open for water too.
Two traps: Transpiration is evaporation then diffusion of water vapour, not liquid water dripping out.
It is not a process the plant chooses for its own sake: it is a side effect of open stomata.
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The big idea: Anything that opens the stomata wider, speeds evaporation or makes the air outside drier increases the rate of transpiration.
| Factor | Effect on the rate | Why |
|---|---|---|
| Light intensity up | Increases | Stomata open wider in the light |
| Temperature up | Increases | Faster evaporation and diffusion |
| Humidity up | Decreases | Less difference in water vapour inside and outside |
| Wind speed up | Increases | Humid air by the stomata is blown away |
| Dry soil | Decreases | The plant closes its stomata |
The points to remember
- Humidity is the only one of these that lowers the rate when it goes up.
- Each graph changes one factor; the others are kept the same.
- Carbon dioxide concentration is not one of the factors to learn.
Trap: More humid air means less transpiration, not more: the water vapour gradient is smaller.
Real example: marram grass. Marram grass grows on windy sand dunes. In dry weather its leaves roll up with the stomata on the inside, trapping humid air next to them, so less water vapour diffuses out.
How it is asked: Expect a list of conditions to pick from, a factor to state, or a short outline of how the factors work.
IB-style questionState[1 mark]
State one environmental factor that affects transpiration rate.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.