A scientist investigated the response of young bean plant shoots to gravity. The shoots were placed horizontally in the dark for 48 hours. After this time, the shoots had grown upward, away from the direction of gravity. Explain how auxin brings about this response in the shoots.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A scientist investigated the response of young bean plant shoots to gravity. The shoots were placed horizontally in the dark for 48 hours. After this time, the shoots had grown upward, away from the direction of gravity.
Model answer (5 marks)
Auxin is synthesised at the tip of the shoot.
When the shoot is placed horizontally, auxin is transported by polar auxin transport and accumulates on the lower side of the shoot.
Auxin stimulates cell elongation by increasing cell wall loosening and water uptake.
Because the lower side has more auxin, its cells elongate more than those on the upper side.
The unequal elongation bends the shoot upward, giving a negative gravitropic response.
When the shoot is placed horizontally, auxin is transported by polar auxin transport and accumulates on the lower side of the shoot.
Auxin stimulates cell elongation by increasing cell wall loosening and water uptake.
Because the lower side has more auxin, its cells elongate more than those on the upper side.
The unequal elongation bends the shoot upward, giving a negative gravitropic response.
Examiner tips
- Use the word ‘auxin’ and ‘polar transport’ to show understanding of hormone movement.
- Explain that auxin causes cell elongation, not just growth, and link this to the bending direction.
- Mention the lower side has higher auxin concentration to justify unequal growth.
- Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Confusing auxin with gibberellin or other hormones.
- Saying the shoot bends downward (positive gravitropism) instead of upward.
- Omitting the step that auxin causes cell elongation or that it accumulates on the lower side.
Mark scheme (5 marks)
- Auxin is produced at the tip of the shoot
- Auxin moves to / accumulates on the lower side of the horizontal shoot
- Auxin causes cells to elongate / grow longer
- Cells on the lower side elongate more than cells on the upper side
- This unequal growth causes the shoot to bend / curve upward, away from gravity (negative gravitropism)
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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