Transpiration
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
What is Transpiration?
Transpiration is the loss of water vapour from the leaves of a plant.
Water evaporates from the surfaces of the mesophyll cells into the air spaces inside the leaf. This water vapour then diffuses out of the leaf through the stomata into the surrounding air, down a concentration gradient.
Water evaporates from mesophyll cell surfaces, then diffuses out through the stomata as water vapour
Investigating the Effect of Temperature and Wind Speed on Transpiration Rate
A potometer (or simple bubble method) can be used to measure transpiration rate, by measuring the rate of water uptake by a cut shoot:
- Cut a leafy shoot under water, to prevent air entering the xylem
- Attach it to a capillary tube filled with water, and introduce an air bubble at the cut end
- Measure the distance the air bubble moves along the capillary tube per unit time - this indicates the rate of water uptake, assumed equal to the rate of transpiration
- Test at different temperatures, or different wind speeds using a fan, and compare the bubble's rate of movement
- Result: both higher temperature and higher wind speed cause the air bubble to move faster, showing a higher rate of transpiration
Common Mistakes
- ❌ Assuming a potometer measures transpiration directly
- ✓ a potometer measures the rate of water uptake by the shoot, not transpiration itself. It assumes that all the water taken up is lost by transpiration - a valid assumption under normal conditions, but not a direct measurement
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