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Exam questions · Biology · Transport in Plants

Measuring Transpiration

  • 7 exam questions
  • 17 marks
  • 8 quick checks
  1. 1 State [1 mark]

    Name the apparatus used to measure the rate of water uptake by a plant shoot.

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    Model answer

    A potometer.

    Mark scheme

    • Potometer — 1 mark
  2. 2 Name [3 marks]

    Figure 1 shows a potometer. Name the parts labelled A, B and C.

    A potometer with three parts marked A, B and C.
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    Model answer

    A is the (leafy) shoot. B is the (air) bubble. C is the (water) reservoir.

    Mark scheme

    • A: shoot — 1 mark
    • B: air bubble — 1 mark
    • C: water reservoir — 1 mark
  3. 3 Calculate [2 marks]

    In a potometer, an air bubble moves 45 mm in 15 minutes. Calculate the rate of water uptake in mm per minute.

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    Model answer

    45 ÷ 15 = 3 mm per minute.

    Mark scheme

    • 45 ÷ 15 — 1 mark
    • 3 mm per minute — 1 mark
  4. 4 Calculate [3 marks]

    The bubble in a potometer moves 24 mm along a capillary tube of radius 0.6 mm. Calculate the volume of water taken up. Use \(\pi = 3.14\). Give your answer to 3 significant figures.

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    Model answer

    Volume = π r² × distance = 3.14 × 0.6² × 24 = 3.14 × 0.36 × 24 = 27.1 mm³.

    Mark scheme

    • Uses π r² × distance — 1 mark
    • 3.14 × 0.36 × 24 — 1 mark
    • 27.1 mm³ — 1 mark
  5. 5 Explain [2 marks]

    Explain why the shoot must be cut under water and the apparatus set up under water.

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    Model answer

    This stops air entering the xylem and the apparatus. Air bubbles would block the flow of water, so the readings would not show the real rate of uptake.

    Mark scheme

    • Prevents air entering the xylem — 1 mark
    • Air would block the water / give false results — 1 mark
  6. 6 Describe [3 marks]

    A student investigates the effect of light intensity on the rate of water uptake. Describe how the student could make the investigation a fair test.

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    Model answer

    Change only the distance of the lamp from the shoot. Keep the same shoot, the same temperature, the same wind conditions and the same length of time for each reading. Repeat each distance several times and calculate a mean.

    Mark scheme

    • Change only one variable (lamp distance) — 1 mark
    • Control other variables, e.g. temperature, shoot, time — 1 mark
    • Repeat and calculate a mean — 1 mark
  7. 7 Calculate [3 marks]

    A student counts the stomata in three fields of view, each of area 0.4 mm². The counts are 22, 18 and 20. Calculate the mean number of stomata per mm².

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    Model answer

    Mean count = (22 + 18 + 20) ÷ 3 = 20. Stomatal density = 20 ÷ 0.4 = 50 stomata per mm².

    Mark scheme

    • Mean count of 20 — 1 mark
    • 20 ÷ 0.4 — 1 mark
    • 50 per mm² — 1 mark

Quick check

  1. 1

    What does the air bubble in a potometer show?

    1. AThe temperature
    2. BThe light intensity
    3. CThe water taken up
    4. DThe oxygen produced
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    C: The water taken up

    The bubble is pulled along the tube as the shoot takes up water, so its movement shows the water uptake.

  2. 2

    Which of these is a control variable when changing the wind speed?

    1. AThe temperature
    2. BThe wind speed
    3. CThe distance the bubble moves
    4. DThe fan
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    A: The temperature

    The temperature must be kept the same so that only the wind speed changes.

  3. 3

    The bubble moves 30 mm in 10 minutes. What is the rate of uptake?

    1. A0.3 mm per minute
    2. B3 mm per minute
    3. C30 mm per minute
    4. D300 mm per minute
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    B: 3 mm per minute

    30 ÷ 10 = 3 mm per minute.

  4. 4

    Why should you repeat the readings?

    1. ATo use up time
    2. BTo change the independent variable
    3. CTo make the bubble move faster
    4. DTo check reliability and find a mean
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    D: To check reliability and find a mean

    Repeating lets you calculate a mean, which is more reliable.

  5. 5

    How can the humidity around a shoot be increased?

    1. ACover it with a clear plastic bag
    2. BPut a fan next to it
    3. CPut it in the sun
    4. DRemove the leaves
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    A: Cover it with a clear plastic bag

    A clear plastic bag traps water vapour around the leaves.

  6. 6

    Why is the volume of water taken up an estimate of transpiration?

    1. ABecause the shoot has no roots
    2. BBecause it is measured in mm
    3. CBecause some water is used by the plant
    4. DBecause the bubble can stick
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    C: Because some water is used by the plant

    Some of the water is used for photosynthesis and growth, so uptake is slightly higher than water lost.

  7. 7

    How are stomata seen under a microscope in this investigation?

    1. ABy staining the xylem
    2. BUsing a nail varnish impression
    3. CBy cutting the roots
    4. DBy weighing the leaf
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    B: Using a nail varnish impression

    A nail varnish impression of the leaf surface shows the stomata and guard cells.

  8. 8

    18 stomata are counted in an area of 0.6 mm². What is the stomatal density?

    1. A3 per mm²
    2. B10 per mm²
    3. C20 per mm²
    4. D30 per mm²
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    D: 30 per mm²

    18 ÷ 0.6 = 30 stomata per mm².