A variegated plant has leaves with green regions and white regions. The white regions contain no chlorophyll. Explain why the white regions of a variegated leaf cannot carry out photosynthesis, and describe what would happen to the overall dry mass of a variegated plant kept in the dark for several weeks.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Variegated plants have leaves with patches of different colours. Green regions contain chloroplasts with chlorophyll, while white regions lack chlorophyll entirely.
Model answer (5 marks)
Chlorophyll is required to absorb light energy for photosynthesis.
Without chlorophyll the white areas cannot capture light and therefore cannot convert CO₂ and H₂O into glucose and O₂.
In the dark the whole plant lacks light energy, so no photosynthesis occurs anywhere.
The plant continues to respire, breaking down stored glucose and other organics for energy.
Thus the dry mass of the plant falls because respiration consumes more organic material than is produced.
Without chlorophyll the white areas cannot capture light and therefore cannot convert CO₂ and H₂O into glucose and O₂.
In the dark the whole plant lacks light energy, so no photosynthesis occurs anywhere.
The plant continues to respire, breaking down stored glucose and other organics for energy.
Thus the dry mass of the plant falls because respiration consumes more organic material than is produced.
Examiner tips
- Use the word ‘chlorophyll’ and ‘light energy’ to show understanding of the role of chlorophyll. Explain that photosynthesis requires light, CO₂ and H₂O. State that in darkness photosynthesis stops. Mention respiration and loss of dry mass. Keep the answer concise and point‑wise to match the 5 marks.
Common mistakes
- Saying the white areas can still photosynthesise because they have other pigments. Claiming that the plant will grow in the dark. Forgetting to mention respiration or dry‑mass loss.
Mark scheme (5 marks)
- Chlorophyll is needed to absorb light energy for photosynthesis
- Without light energy, the white regions cannot convert carbon dioxide and water into glucose (and oxygen)
- In the dark, the whole plant cannot photosynthesise (because there is no light energy available)
- The plant will continue to respire in the dark, breaking down glucose / organic molecules
- The dry mass of the plant will decrease because more organic molecules are broken down in respiration than are produced by photosynthesis
Key terms in this question
chlorophyll · photosynthesis · dry mass
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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