Transpiration (Edexcel IGCSE Biology)
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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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