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Transpiration (Edexcel IGCSE Biology)
Revision Note
Transpiration
- Transpiration is defined as
The loss of water vapour from the parts of the plant that are above ground (leaves, stem, flowers)
- Loss of water occurs through evaporation of water at the surfaces of the spongy mesophyll cells followed by diffusion of water vapour through the stomata
- Transpiration has several functions in plants:
- Transporting mineral ions
- Providing water to keep cells turgid in order to support the structure of the plant
- Providing water to leaf cells for photosynthesis
- Keeping the leaves cool as heat energy is removed from the leaves when water evaporates
Transpiration in plants
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Factors Affecting Transpiration
- There are several environmental conditions which have an impact on the rate of transpiration
- Air movement
- Humidity
- Temperature
- Light intensity
Factors affecting transpiration rate table
| Factor | Relationship to transpiration rate | Explanation |
| Air movement | As wind speed increases, the transpiration rate increases | When it is windy, water molecules that diffuse out of stomata are quickly blown away from the leaf; this creates a concentration gradient and more water vapour diffuses out of the leaf |
| Temperature | As temperature increases, the transpiration rate increases | When temperatures are high water vapour molecules have more kinetic energy and move around faster, so they are more likely to move out of the stomata by diffusion |
| Humidity | As humidity increases, the transpiration rate decreases | Humid air contains a high concentration of water vapour; this reduces the diffusion gradient between the inside of the leaf and the outside air, so less water vapour diffuses out of the leaf |
| Light intensity | As light intensity increases, the transpiration rate increases | Increased light intensity increases the rate of photosynthesis, so the stomata open to allow gas exchange; when this happens, water vapour diffuses out of the stomata |
- The table above explains how these four factors affect the rate of transpiration when they are all high; the opposite effect would be observed if they were low
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