Transpiration
Transpiration is the loss of water vapour from the leaves,
mainly through the stomata , by evaporation and diffusionWater evaporates from the mesophyll cells into the air spaces and diffuses
out through the stomata. This pulls more water up the xylem from the roots:
the transpiration stream . It delivers water for photosynthesis and
support, and mineral ions for growth
Factor
Increase in it
Effect on transpiration
Why
Temperature warmer
faster water evaporates faster and molecules diffuse faster
Humidity more humid
slower smaller concentration gradient of water vapour between leaf and air
Air movement windier
faster
water vapour is blown away, keeping the gradient steep
Light intensity brighter
faster
stomata open in the light for photosynthesis, so more water escapes
So transpiration is fastest on a warm, dry, windy, bright day
Rate of transpiration can be measured as water uptake with a
potometer : rate = = = ÷ \div ÷
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A potometer — measuring the rate of water uptake leafy shoot, cut under water airtight seal — petroleum jelly round the bung air bubble the bubble moves this way as water is taken up scale in mm — measure how far the bubble moves in a fixed time reservoir with a tap — used to push the bubble back to the start so the reading can be repeated The shoot pulls water up as it transpires, so the bubble moves along the scale. The rate of water uptake is taken as the rate of transpiration. Transpiration is faster in higher temperature, brighter light, windier conditions and drier air. Change one of those at a time and keep the rest the same. A potometer: a leafy shoot sealed into a capillary tube with an air bubble, a reservoir and a scale, with the bubble's movement showing water uptake
The rest of this topic is in the full Exam Guide The complete AQA GCSE Biology guide — every topic, exam checklist and common mistake — is coming soon. Until then, the first topic in every unit is free to read.
In the full topic
• Transpiration• Stomata and guard cells• TranslocationRead the free biology topics →