Transport in Plants: Question 10
Syllabus 8.3
A student sets up a potometer, an instrument that measures the rate at which a leafy shoot takes up water by tracking how far a small air bubble moves along a graduated capillary tube in a fixed time. She assumes that, over the course of each short trial, the rate of water uptake gives a reasonable estimate of the shoot's rate of transpiration. The same shoot is used throughout, and she records the distance the bubble moves in five minutes under three different conditions.
Set-up P: high light intensity, 20°C, bubble moved 45 mm in 5 minutes. Set-up Q: low light intensity (shoot covered with a black cloth), 20°C, bubble moved 18 mm in 5 minutes. Set-up R: high light intensity, 30°C, bubble moved 63 mm in 5 minutes.
(a) Using set-ups P and Q, describe and explain the effect of light intensity on the rate of water uptake by the shoot. [3]
(b) Using set-ups P and R, describe the effect of temperature on the rate of water uptake by the shoot. [2]
(c) Explain why the rate at which the bubble moves along the potometer tube gives only an estimate of the shoot's rate of transpiration, rather than an exact, direct measurement of it. [2]
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Worked solution
Part (a): Effect of light intensity
Set-ups P and Q are both kept at 20°C, differing only in light intensity. Set-up P, in high light, moved the bubble 45 mm in five minutes, compared with only 18 mm for set-up Q in low light. Since light intensity is the only factor that differs between them, this shows that higher light intensity increases the rate of water uptake.
The explanation lies in the stomata: greater light intensity causes the stomata in the leaf to open wider. A wider stomatal opening allows water vapour to diffuse out of the leaf’s internal air spaces more quickly, increasing the rate of transpiration. As more water is lost from the leaf, more water is drawn up through the shoot and into the potometer tube to replace it, which is why the bubble moves faster.
Part (b): Effect of temperature
Set-ups P and R are both kept in high light, differing only in temperature. Set-up R, at 30°C, moved the bubble 63 mm in five minutes, more than the 45 mm moved in set-up P at 20°C. Since temperature is the only factor that differs between them, this shows that a higher temperature increases the rate of water uptake.
Part (c): Why the potometer only estimates transpiration rate
The potometer directly measures the volume of water drawn into the cut end of the shoot to replace what has been used, shown by the movement of the bubble along the tube. However, not every drop of water taken up by the shoot is immediately lost as water vapour through the stomata. A small proportion of it is used inside the plant for other purposes, such as being used in photosynthesis or kept within cells to maintain their turgidity. Because of this, the rate of water uptake recorded by the potometer is a very close estimate of the rate of transpiration, but it is not an exact, direct measurement of the water actually lost from the leaves.
Final answers
- (a) Higher light intensity increases the rate of water uptake (P moved further than Q), because more light causes stomata to open wider, increasing the rate of transpiration.
- (b) Higher temperature increases the rate of water uptake (R moved further than P).
- (c) The potometer measures water uptake into the shoot, not water loss from the leaves directly, and some of the water taken up is used within the plant rather than transpired, so the reading is only an estimate of the transpiration rate.