A gardener places a potted plant on its side so that the shoot is horizontal. After several days, the shoot has curved upwards. Explain how auxins bring about this upward curvature of the shoot.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Auxins are redistributed by the plant’s gravity-sensing system so that they accumulate on the lower side of the horizontal shoot. The higher auxin concentration on the lower side stimulates greater cell elongation there, whereas the upper side, with a lower auxin level, elongates less. The unequal elongation – faster growth on the lower side – causes the shoot to bend upwards, giving a positive gravitropic response (the shoot curves away from the direction of gravity).
Examiner tips
- Use the word ‘accumulate’ or ‘redistribute’ to show auxin movement; mention ‘lower side’ and ‘higher concentration’.
- Explain that higher auxin causes greater cell elongation and lower auxin causes less.
- State that unequal elongation makes the shoot curve upwards – a positive gravitropic response.
Common mistakes
- Confusing positive with negative gravitropism; saying the shoot bends downwards.
- Forgetting to mention that auxins move to the lower side or that the upper side has lower auxin.
Mark scheme (4 marks)
- Auxins accumulate / move to the lower side of the horizontal shoot due to gravity
- The higher concentration of auxins on the lower side causes greater cell elongation on the lower side
- The upper side of the shoot has a lower concentration of auxins and so elongates less / grows more slowly
- Unequal elongation / faster growth on the lower side causes the shoot to curve upwards — this is a positive gravitropism / the shoot shows negative gravitropism away from gravity
Key terms in this question
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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