A gardener notices that a plant kept in a dark cupboard has grown tall and spindly, with pale yellow leaves, compared to a plant of the same species kept on a sunny windowsill, which is short and has dark green leaves. Explain how the unequal distribution of auxin causes the shoot of the plant in the dark cupboard to grow tall and spindly.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Auxin is a plant hormone produced at the shoot tip. It promotes cell elongation in shoot tissue. When light hits a shoot from one direction, auxin moves away from the light. In a plant kept in darkness, auxin is distributed evenly throughout the shoot tip.
Model answer (5 marks)
Auxin is produced at the shoot tip.
In darkness auxin is distributed evenly around the shoot.
Auxin promotes cell elongation.
Because the hormone is spread equally, cell elongation occurs uniformly on all sides of the shoot, so the shoot grows straight up.
The continuous, unopposed action of auxin makes the shoot taller and more elongated – the plant is etiolated.
In darkness auxin is distributed evenly around the shoot.
Auxin promotes cell elongation.
Because the hormone is spread equally, cell elongation occurs uniformly on all sides of the shoot, so the shoot grows straight up.
The continuous, unopposed action of auxin makes the shoot taller and more elongated – the plant is etiolated.
Examiner tips
- State where auxin is produced and how it is distributed in darkness. Explain the effect of auxin on cell elongation. Show the link between even distribution and vertical growth. Mention that the plant is etiolated to justify the tall, spindly appearance.
Common mistakes
- Confusing light direction with auxin distribution. Forgetting to mention that the plant is etiolated. Using vague terms like "growth" without linking to cell elongation.
Mark scheme (5 marks)
- Auxin is produced at the (shoot) tip / apex
- In darkness, auxin is distributed evenly / equally on all sides of the shoot
- Auxin stimulates / promotes cell elongation
- Because auxin is evenly distributed, cell elongation occurs equally on all sides of the shoot, causing the shoot to grow straight upwards / vertically
- The shoot grows taller / more elongated overall because auxin continuously promotes cell elongation throughout the shoot with no competing directional stimulus to limit elongation / the plant is etiolated
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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