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: cacti. Many cacti open their stomata at night, when the air is cooler and more humid, and keep them closed in the heat of the day. They take in carbon dioxide while losing far less water.
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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 questionOutline[4 marks]
Outline how environmental factors affect the rate of transpiration in plants.
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See the answer broken into clear points, with guidance on how each point earns marks.