A plant is kept in darkness for 48 hours and then moved into bright light. Explain why the plant must be kept in darkness before being placed in bright light, and describe what would happen to the starch content of the leaves during each of these two stages.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The plant is kept in darkness to deplete the starch already present in the leaves, ensuring that any subsequent starch measured is produced only after the light is supplied.
During the 48 h in darkness the stored starch is hydrolysed to glucose and the glucose is used in cellular respiration for energy, so the starch content falls to almost zero.
When the plant is then exposed to bright light, photosynthesis begins. The light‑dependent reactions produce ATP and NADPH, which are used in the Calvin cycle to fix CO₂ into glucose. The glucose is then polymerised into starch for storage, so the starch content of the leaves rises sharply.
Thus, darkness removes existing starch, and light restores it by photosynthesis.
During the 48 h in darkness the stored starch is hydrolysed to glucose and the glucose is used in cellular respiration for energy, so the starch content falls to almost zero.
When the plant is then exposed to bright light, photosynthesis begins. The light‑dependent reactions produce ATP and NADPH, which are used in the Calvin cycle to fix CO₂ into glucose. The glucose is then polymerised into starch for storage, so the starch content of the leaves rises sharply.
Thus, darkness removes existing starch, and light restores it by photosynthesis.
Examiner tips
- Mention ‘destarching’ to show understanding of the purpose of darkness
- Explain the two stages in order: starch depletion in darkness, starch synthesis in light
- Use terms ‘photosynthesis’, ‘respiration’, ‘starch’, ‘glucose’
Common mistakes
- Failing to state that starch is broken down in darkness
- Confusing the direction of glucose flow (e.g. saying glucose is produced in darkness)
- Using vague wording such as ‘energy’ without linking it to respiration
Mark scheme (4 marks)
- The plant is kept in darkness to deplete / remove all existing starch from the leaves (destarch the plant).
- In darkness, starch is broken down (into glucose) and used in respiration / for energy.
- In bright light, photosynthesis occurs, producing glucose.
- The glucose produced in photosynthesis is converted into starch for storage, so the starch content of the leaves increases in bright light.
Key terms in this question
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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