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Exam questions · Biology · Plant Hormones

Auxins, Phototropism and Gravitropism

  • 6 exam questions
  • 19 marks
  • 9 quick checks
  1. 1 State [1 mark]

    State what is meant by gravitropism.

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    Model answer

    A growth response of a plant to gravity.

    Mark scheme

    • Growth response to gravity — 1 mark
  2. 2 Name [2 marks]

    Name the hormone that causes phototropism, and state where it is made in a shoot.

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    Model answer

    Auxin, made in the tip of the shoot.

    Mark scheme

    • Auxin — 1 mark
    • Tip — 1 mark
  3. 3 Explain [3 marks]

    A seedling is lit from the left. Explain why the shoot bends towards the left.

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    Model answer

    Auxin moves to the shaded, right-hand side of the shoot. The cells on the shaded side elongate faster. The shoot bends towards the left, which is towards the light.

    Mark scheme

    • Auxin moves to the shaded side — 1 mark
    • Cells on the shaded side elongate faster — 1 mark
    • Shoot bends towards the light — 1 mark
  4. 4 Explain [4 marks]

    A seedling is placed on its side. Explain why the root grows downwards and the shoot grows upwards.

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    Model answer

    Auxin collects on the lower side of both. In the shoot, the extra auxin makes the lower side grow faster, so the shoot bends up. In the root, the extra auxin slows the growth on the lower side, so the upper side grows faster, so the root bends down.

    Mark scheme

    • Auxin collects on the lower side — 1 mark
    • Lower side of the shoot grows faster — 1 mark
    • Shoot bends up — 1 mark
    • Lower side of the root grows slower, so it bends down — 1 mark
  5. 5 Calculate [3 marks]

    The mean cell length on the lit side of a shoot was 30 micrometres, and on the shaded side it was 42 micrometres. Calculate the percentage by which the cells on the shaded side were longer.

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    Model answer

    42 − 30 = 12. 12 ÷ 30 × 100 = 40%.

    Mark scheme

    • Difference of 12 — 1 mark
    • Divided by 30 — 1 mark
    • 40% — 1 mark
  6. 6 Evaluate [6 marks]

    Describe how the responses of a plant to light and to gravity help it to survive, and explain how they are caused.

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    Model answer

    A shoot shows positive phototropism, so it grows towards light. This puts its leaves in the best position to absorb light for photosynthesis, which makes the glucose that the plant needs. The shoot also shows negative gravitropism, so it grows upwards, away from the soil. A root shows positive gravitropism, so it grows downwards into the soil, where it can take up water and mineral ions and anchor the plant. All of these are caused by unequal distributions of auxin. In a shoot, auxin makes the cells on the shaded or lower side elongate faster. In a root, auxin on the lower side slows the growth, so the root bends downwards. Without these responses, the plant would not get enough light or water.

    Mark scheme

    • Shoot grows towards light for photosynthesis — 1 mark
    • Shoot grows upwards, away from gravity — 1 mark
    • Root grows down to take up water and minerals — 1 mark
    • Root anchors the plant — 1 mark
    • Unequal auxin distribution causes unequal growth — 1 mark
    • Opposite effects of auxin on shoot and root — 1 mark

Quick check

  1. 1

    What is the response of a plant to the direction of light called?

    1. AGravitropism
    2. BPhototropism
    3. CTranspiration
    4. DTranslocation
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    B: Phototropism

    It is called phototropism.

  2. 2

    Where is auxin made?

    1. AIn the leaves
    2. BIn the flowers
    3. CIn the tips
    4. DIn the xylem
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    C: In the tips

    In the tips of shoots and roots.

  3. 3

    In a shoot lit from one side, where does the auxin move to?

    1. AThe shaded side
    2. BThe lit side
    3. CThe roots only
    4. DThe leaves only
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    A: The shaded side

    It moves to the shaded side.

  4. 4

    What does auxin do to the cells on the shaded side of a shoot?

    1. AMakes them die
    2. BMakes them stop growing
    3. CMakes them divide in two
    4. DMakes them elongate faster
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    D: Makes them elongate faster

    It makes them elongate faster.

  5. 5

    What is the growth of a root in the direction of gravity called?

    1. ANegative gravitropism
    2. BPositive gravitropism
    3. CNegative phototropism
    4. DPositive phototropism
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    B: Positive gravitropism

    Positive gravitropism.

  6. 6

    What does extra auxin on the lower side of a root do?

    1. AIt speeds up the growth
    2. BIt kills the root
    3. CIt slows the growth
    4. DIt stops gravity
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    C: It slows the growth

    It slows the growth there, so the root bends down.

  7. 7

    Why are shoots with their tips covered in foil unable to bend towards light?

    1. AThe tip cannot detect the light
    2. BThe foil is too heavy
    3. CThe stem is too long
    4. DThey have no auxin in the leaves
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    A: The tip cannot detect the light

    The tip detects the light, so the shoot cannot respond if the tip is covered.

  8. 8

    Why is gravitropism useful in a root?

    1. AIt takes the root to the light
    2. BIt makes the leaves bigger
    3. CIt makes flowers
    4. DIt takes the root to water and anchors the plant
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    D: It takes the root to water and anchors the plant

    The root grows down to water and mineral ions and anchors the plant.

  9. 9

    The cells on the lit side of a shoot are 20 micrometres long, and those on the shaded side are 30 micrometres long. What is the percentage difference, based on the lit side?

    1. A10%
    2. B33%
    3. C50%
    4. D150%
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    C: 50%

    \((30 - 20) \div 20 \times 100 = 50\%\).