Explain why a plant seedling placed on its side will, over time, have its shoot growing upwards and its root growing downwards.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A gardener places a newly germinated seedling horizontally in a pot of soil and leaves it for several days in the dark.
Model answer (4 marks)
Auxin is redistributed by gravity so that it accumulates on the lower side of both the shoot and the root. In the shoot the higher concentration of auxin on the lower side stimulates greater or faster cell elongation there, so the cells on the lower side grow longer than those on the upper side. This differential growth bends the shoot tip upwards, giving negative gravitropism. In the root the higher concentration of auxin on the lower side inhibits cell elongation on that side, so the cells on the upper side elongate more. The differential growth bends the root tip downwards, giving positive gravitropism.
Examiner tips
- Mention auxin redistribution by gravity first; link higher auxin to cell elongation in shoot and inhibition in root; explain bending direction as negative/positive gravitropism.
- Use precise terms: auxin, cell elongation, lower side, upper side, negative/positive gravitropism.
- Keep answer concise – 4 marks can be earned with 4 clear points.
Mark scheme (4 marks)
- Auxin moves to / accumulates on the lower side of both the shoot and the root
- In the shoot, the higher concentration of auxin on the lower side causes greater / faster cell elongation on the lower side
- Greater elongation on the lower side causes the shoot tip to bend / curve upwards (away from gravity / negative gravitropism)
- In the root, the higher concentration of auxin on the lower side inhibits cell elongation on that side, so the upper side elongates more, causing the root to bend / curve downwards (towards gravity / positive gravitropism)
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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