A group of students investigated how auxin controls the growth of plant shoots. They took identical young shoots and treated them as follows: shoot A had auxin applied evenly to both sides of the tip; shoot B had auxin applied only to the right-hand side of the tip; shoot C had its tip removed entirely and no auxin was applied. Explain the pattern of growth you would expect to observe in each of the three shoots after several days.
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 moves down the shoot and promotes cell elongation. When light strikes a shoot from one side, auxin is redistributed to the shaded side.
Model answer (5 marks)
Shoot A: grows straight up – auxin is applied evenly, so both sides elongate equally.
Shoot B: bends to the left – auxin only on the right side causes that side to elongate more, so the shoot curves away from the higher‑auxin side.
Shoot C: little or no growth – the tip (site of auxin synthesis) is removed, so no auxin is produced and cells do not elongate, keeping the shoot upright and short.
Shoot B: bends to the left – auxin only on the right side causes that side to elongate more, so the shoot curves away from the higher‑auxin side.
Shoot C: little or no growth – the tip (site of auxin synthesis) is removed, so no auxin is produced and cells do not elongate, keeping the shoot upright and short.
Examiner tips
- State the effect of equal vs. unequal auxin distribution on cell elongation. Explain the direction of curvature using the side with less auxin. Mention that removal of the tip stops auxin synthesis and growth.
Common mistakes
- Confusing the direction of bending (claiming it bends toward the auxin side). Forgetting that shoot C will not elongate because auxin is absent. Using vague terms like "more growth" instead of "cell elongation".
Mark scheme (5 marks)
- Shoot A grows straight upwards because auxin is distributed equally on both sides, so both sides elongate at the same rate.
- In shoot B, the side with auxin applied (right-hand side) has more auxin, so cells on that side elongate more / faster.
- Shoot B therefore bends / curves to the left (away from the side with more auxin / towards the side with less auxin).
- Shoot C does not produce auxin (because the tip, where auxin is made, has been removed), so little or no cell elongation occurs.
- Shoot C therefore shows little or no growth and remains upright / does not bend in any direction.
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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