Plant Hormones and Tropisms

1. 1 State [1 mark]

   State what is meant by phototropism.

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   Model answer

   The growth of a plant in response to the direction of light.

   Mark scheme

   • Growth in response to light direction — 1 mark
2. 2 State [2 marks]

   State the name of the hormone that causes phototropism, and the part of a shoot where it is made.

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   Model answer

   Auxin, which is made in the tip of the shoot.

   Mark scheme

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

   A shoot is lit from the right-hand side only. Explain why the shoot bends towards the light.

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   Model answer

   The auxin moves to the shaded, left-hand side of the shoot. This makes the cells on that side elongate more than those on the lit side. The shoot therefore bends towards the light.

   Mark scheme

   • Auxin moves to the shaded side — 1 mark
   • Cells on that side elongate more — 1 mark
   • Shoot bends towards the light — 1 mark
4. 4 Explain [3 marks]

   Explain why a seedling placed on its side has a shoot that grows upwards and a root that grows downwards.

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   Model answer

   Auxin collects on the lower side of both. In the shoot, it makes the lower cells elongate, so that the shoot bends up. In the root, it slows the growth of the lower side, so that the root bends down.

   Mark scheme

   • Auxin collects on the lower side — 1 mark
   • Shoot: lower side grows faster, bends up — 1 mark
   • Root: lower side grows slower, bends down — 1 mark
5. 5 Explain [3 marks]

   A scientist covered the tips of some shoots with foil, and found that they did not bend towards a lamp. Explain what this shows.

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   Model answer

   It shows that the tip is the part that detects the light. If the tip is covered, the auxin is not moved to the shaded side, so the shoot does not bend.

   Mark scheme

   • Tip detects the light — 1 mark
   • Auxin not moved to the shaded side — 1 mark
   • Therefore no bending — 1 mark
6. 6 Evaluate [6 marks]

   Evaluate the importance of tropisms to the survival of a young plant.

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   Model answer

   A young shoot that grows towards the light is able to absorb more light for photosynthesis, which provides the glucose that the plant needs to grow. Gravitropism makes the shoot grow upwards, away from the soil, and out into the light. The root grows downwards in response to gravity, and so reaches water and mineral ions in the soil, and anchors the plant so that it is not blown over. Without these responses, the shoot might grow into the soil, and the roots into the air, and the plant would die. They are controlled by auxin, which is made in the tips, so they happen automatically, without the plant needing a nervous system. Because the responses are slow, they could not help a plant to avoid sudden danger, but they are very important for growth.

   Mark scheme

   • Shoot grows to the light for photosynthesis — 1 mark
   • Gravitropism takes the shoot upwards — 1 mark
   • Roots grow down to water and minerals — 1 mark
   • Roots anchor the plant — 1 mark
   • Controlled automatically by auxin — 1 mark
   • A reasoned judgement — 1 mark

Quick check

1. 1

   Where is auxin made in a shoot?

   1. AIn the roots
   2. BIn the tip
   3. CIn the flowers
   4. DIn the xylem
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   B: In the tip

   In the tip of the shoot.

2. 2

   What does auxin do in a shoot?

   1. AIt makes the cells grow longer
   2. BIt makes cells stop growing
   3. CIt makes photosynthesis faster
   4. DIt makes the leaves fall
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   A: It makes the cells grow longer

   It makes the cells elongate.

3. 3

   In phototropism, where does auxin move to?

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

   It moves to the shaded side of the shoot.

4. 4

   What is the growth of a root downwards called?

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

   It is positive gravitropism, as the root grows towards gravity.

5. 5

   What happens to the growth of root cells on the side with more auxin?

   1. AThey grow faster
   2. BThey grow slower
   3. CThey stop being cells
   4. DThey divide in two
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   B: They grow slower

   Auxin slows the growth of cells in a root.

6. 6

   A shoot is covered with foil at the tip, and lit from one side. What happens?

   1. AThe shoot does not bend
   2. BThe shoot bends towards the light
   3. CThe shoot bends away from the light
   4. DThe shoot dies
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   A: The shoot does not bend

   The tip cannot detect the light, so the shoot does not bend.

7. 7

   Which scientist put the tips on agar blocks and showed that a chemical moved?

   1. AMendel
   2. BDarwin
   3. CWent
   4. DHooke
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   C: Went

   Frits Went in 1928.

8. 8

   Why is negative gravitropism useful in a shoot?

   1. AIt takes the shoot down
   2. BIt stops photosynthesis
   3. CIt makes the leaves smaller
   4. DIt brings the shoot to the light
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   D: It brings the shoot to the light

   The shoot grows up towards the light, away from the soil.

9. 9

   A seedling is placed on its side in the dark. Which statement is correct?

   1. ANeither the shoot nor the root bends
   2. BThe shoot bends up and the root bends down
   3. CThe shoot bends down and the root bends up
   4. DBoth bend up
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   B: The shoot bends up and the root bends down

   Gravity causes auxin to collect on the lower side, so the shoot bends up and the root bends down, even in the dark.