5.24 Auxin and tropisms
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What is the name of the response of a shoot growing towards light?
Choose one answer.
Correct Not quite: the answer is Positive phototropism
Mark scheme
C (positive phototropism)
Model answer
C: positive phototropism.
A plant on a windowsill grows towards the window.
Explain how auxin causes this response.
Mark scheme
- auxin is made in the shoot tip (1)
- auxin moves to the shaded side (1)
- (auxin) causes cells (on the shaded side) to elongate / grow faster (1)
- so the shaded side grows more / unequal growth, so the shoot bends towards the light (1)
Model answer
Auxin is made in the shoot tip. Light from the window makes auxin move to the shaded side of the shoot, away from the window. The extra auxin makes the cells on the shaded side elongate faster, so that side grows longer and the shoot bends towards the light.
A seedling is placed on its side. After two days its root has bent downwards.
Explain why.
Mark scheme
- (gravity makes) auxin collect on the lower side of the root (1)
- in roots auxin inhibits / slows cell elongation (1)
- so the upper side grows faster (than the lower side) (1)
- so the root bends downwards (positive gravitropism) (1)
Model answer
Gravity causes auxin to collect on the lower side of the root. In roots, auxin inhibits cell elongation, so the cells on the lower side grow more slowly than those on the upper side. The upper side grows longer, so the root bends downwards.
Explain why it is useful for a root to grow towards gravity.
Mark scheme
- grows down into the soil (1)
- to absorb water / mineral ions / anchor the plant (1)
Model answer
The root grows down into the soil, where it can absorb water and mineral ions and anchor the plant.
Higher tier (AQA). Give the effect of gibberellins and of ethene on plants.
Mark scheme
- gibberellins: initiate / start seed germination (1)
- ethene: controls fruit ripening / cell division (1)
Model answer
Gibberellins start seed germination. Ethene controls cell division and the ripening of fruit.
Compare the effect of auxin on cells in shoots and in roots.
Mark scheme
- in shoots auxin stimulates / increases cell elongation (1)
- in roots auxin inhibits / reduces cell elongation (1)
Model answer
In shoots auxin stimulates cell elongation, but in roots it inhibits cell elongation.
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