2.19 Measuring transpiration with a potometer
6 exam questions worth 13 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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In a potometer, the bubble moved 45 mm in 3 minutes.
Calculate the rate of water uptake in mm per minute.
Mark scheme
- 45 ÷ 3 (1)
- = 15 (mm per minute) (1)
Model answer
rate = 45 ÷ 3 = 15 mm per minute.
The capillary tube has a radius of 0.4 mm. The bubble moved 50 mm.
Calculate the volume of water taken up. Use π = 3.14 and volume = π r2 × distance. Give your answer to 3 significant figures.
Mark scheme
- 0.42 = 0.16 (1)
- 3.14 × 0.16 × 50 (1)
- = 25.1 (mm3) (1)
Model answer
volume = 3.14 × 0.42 × 50 = 3.14 × 0.16 × 50 = 25.1 mm3.
Explain why the shoot is cut under water.
Mark scheme
- to stop air entering the xylem (1)
- (air) would block the flow of water / stop water uptake (1)
Model answer
Cutting under water stops air bubbles getting into the xylem, which would block the movement of water up the stem.
A potometer measures water uptake, not transpiration.
Give one reason why the two are not exactly the same.
Mark scheme
some water is used in photosynthesis / to keep cells turgid / retained by the plant
Model answer
Some of the water taken up is used by the plant in photosynthesis, so it is not all lost by transpiration.
A student wants to find out how air movement affects the rate of transpiration using a potometer.
Describe how the student should change the independent variable, and give two variables to control.
Mark scheme
- use a fan at different distances / speeds (1)
- control: light intensity (1)
- temperature / humidity / same shoot / same time interval (1)
Model answer
Put a fan at different distances from the shoot, or use different fan speeds. Keep the light intensity and the temperature the same, and use the same shoot.
Predict and explain what happens to the movement of the bubble when a lamp is switched off.
Mark scheme
- bubble moves more slowly (1)
- stomata close (in the dark) so less transpiration / less water lost (1)
Model answer
The bubble moves more slowly, because in the dark the stomata close and less water vapour is lost from the leaves.
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