Explain why a plant kept in darkness for several days will eventually stop growing, even if it is supplied with water and mineral ions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A gardener places a healthy potted plant in a completely dark cupboard. The plant is watered regularly and the soil contains all necessary mineral ions. After several days, the plant's leaves turn yellow and new growth stops.
Model answer (4 marks)
1. In darkness photosynthesis cannot occur because light is required for the light‑dependent reactions.
2. Without photosynthesis no glucose (sugar) is produced.
3. Glucose is required as a reactant for cellular respiration, which releases energy for growth.
4. Glucose and other carbon compounds are also building blocks for new cells, proteins and other organic molecules needed for growth.
2. Without photosynthesis no glucose (sugar) is produced.
3. Glucose is required as a reactant for cellular respiration, which releases energy for growth.
4. Glucose and other carbon compounds are also building blocks for new cells, proteins and other organic molecules needed for growth.
Examiner tips
- Use the exact phrase "photosynthesis cannot occur" and "light is required" to gain the first mark. Show the chain: no photosynthesis → no glucose → no energy for respiration → no growth. Mention both energy and building‑block roles of glucose to cover all marks.
Common mistakes
- Confusing photosynthesis with respiration; stating that respiration produces glucose. Omitting the need for glucose as a building block for new cells. Using vague terms like "lack of light" without linking to photosynthesis.
Mark scheme (4 marks)
- In darkness, photosynthesis cannot occur / light is required for photosynthesis
- Without photosynthesis, glucose / sugar is not produced
- Glucose is needed as a reactant / energy source for respiration to release energy for growth
- Glucose / carbon compounds are also needed as building blocks to make new cells / proteins / other organic molecules required for growth
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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