A farmer grows two varieties of tomato plant in the same field. Variety A has broad, flat leaves and Variety B has narrow, upright leaves. The farmer notices that Variety A produces a greater mass of tomatoes per plant than Variety B when grown in full sunlight, but Variety B produces a similar mass to Variety A when grown in partial shade. Explain why Variety A produces a greater mass of tomatoes than Variety B in full sunlight, and suggest why the difference between the two varieties is much smaller in partial shade.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Photosynthesis is the process by which plants convert light energy, carbon dioxide and water into glucose and oxygen. The glucose produced can be used in respiration or converted into other organic molecules, including those used to build new cells and tissues. The rate of photosynthesis depends on the availability of raw materials and light energy.
Model answer (5 marks)
Variety A has broad, flat leaves which give a larger surface area than the narrow, upright leaves of Variety B.
The larger surface area allows Variety A to absorb more light energy per unit time.
With more light energy available, the rate of photosynthesis in Variety A is higher, producing more glucose.
The extra glucose can be used for growth, synthesis of new cells and tissues and ultimately for fruit development, so Variety A yields a greater mass of tomatoes.
In partial shade both varieties receive less light. Light becomes the limiting factor for photosynthesis in both, so the advantage of a larger leaf area is reduced. Consequently the difference in tomato mass between the two varieties is much smaller.
The larger surface area allows Variety A to absorb more light energy per unit time.
With more light energy available, the rate of photosynthesis in Variety A is higher, producing more glucose.
The extra glucose can be used for growth, synthesis of new cells and tissues and ultimately for fruit development, so Variety A yields a greater mass of tomatoes.
In partial shade both varieties receive less light. Light becomes the limiting factor for photosynthesis in both, so the advantage of a larger leaf area is reduced. Consequently the difference in tomato mass between the two varieties is much smaller.
Examiner tips
- Mention leaf surface area and light absorption first; link to photosynthetic rate and glucose production; explain the effect of light limitation in shade; keep answer concise and use correct terminology
Common mistakes
- Confusing leaf shape with chlorophyll content; not linking light absorption to glucose production; ignoring the role of light as the limiting factor in shade
Mark scheme (5 marks)
- Broad, flat leaves have a greater surface area (than narrow leaves)
- Greater surface area allows more light energy to be absorbed / captured for photosynthesis
- Higher rate of photosynthesis in Variety A (in full sunlight) so more glucose is produced
- More glucose is available to be used for growth / building new cells / making organic molecules, leading to greater mass of tomatoes
- In partial shade, light is the limiting factor for both varieties so the advantage of broad leaves is reduced / both varieties are limited by the same lack of light, not leaf shape
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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