A student investigating plant responses placed a young seedling horizontally in a dark box for 48 hours. After 48 hours, the shoot had curved upward and the root had curved downward. Explain how the unequal distribution of auxin caused these two different growth responses.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Auxin is a plant hormone produced at the tip of shoots and roots. When a plant is placed on its side, gravity causes auxin to move to the lower side of both the shoot and the root.
Model answer (5 marks)
Auxin is produced at the tip of the shoot and root and, when the seedling is placed horizontally, it is redistributed by gravity to the lower side of both organs.
In the shoot the lower side receives a higher concentration of auxin. Auxin stimulates cell elongation, so cells on the lower side grow longer than those on the upper side. The differential growth causes the shoot to bend upward – a positive gravitropic response.
In the root the situation is opposite. The higher auxin concentration on the lower side inhibits cell elongation there, while cells on the upper side elongate more. The root therefore bends downward, also a positive gravitropic response.
In the shoot the lower side receives a higher concentration of auxin. Auxin stimulates cell elongation, so cells on the lower side grow longer than those on the upper side. The differential growth causes the shoot to bend upward – a positive gravitropic response.
In the root the situation is opposite. The higher auxin concentration on the lower side inhibits cell elongation there, while cells on the upper side elongate more. The root therefore bends downward, also a positive gravitropic response.
Examiner tips
- State that auxin moves to the lower side of both shoot and root. Explain that in shoots auxin promotes elongation, in roots it inhibits it. Show the differential growth leads to upward shoot bending and downward root bending. Use terms ‘positive gravitropism’ and ‘cell elongation’ as per mark scheme.
Common mistakes
- Confusing the direction of bending (e.g. saying shoot bends downward). Forgetting that auxin inhibits elongation in roots. Using vague phrases like ‘growth’ without specifying cell elongation.
Mark scheme (5 marks)
- Auxin moves to / accumulates on the lower side of both the shoot and root
- In the shoot, the higher concentration of auxin on the lower side causes greater cell elongation on that side
- Because the lower side of the shoot grows more than the upper side, the shoot bends/curves upward (positive gravitropism)
- In the root, auxin inhibits/reduces cell elongation at the concentration present on the lower side
- Because the upper side of the root elongates more than the lower side, the root bends/curves downward (positive gravitropism)
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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