A student investigates how light intensity affects the rate of photosynthesis in a pondweed plant. As light intensity increases, the rate of photosynthesis increases up to a certain point, after which it levels off. Explain why the rate of photosynthesis increases as light intensity increases, and why the rate eventually stops increasing even when light intensity continues to rise.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Photosynthesis is an endothermic reaction that takes place in the chloroplasts, converting carbon dioxide and water into glucose and oxygen using light energy. The overall equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Model answer (5 marks)
Light energy is required for photosynthesis to occur.
As light intensity increases, more photons reach the chloroplasts, so the light‑dependent reactions produce ATP and NADPH more rapidly, allowing the Calvin cycle to fix CO₂ into glucose at a higher rate.
However, once the light reactions are saturated, further increases in light intensity do not increase the rate because another factor becomes limiting.
The limiting factor is usually the availability of CO₂ (or water), which cannot be fixed faster than the supply.
Thus, even with excess light energy, the rate of photosynthesis cannot rise because the supply of CO₂ (or water) is insufficient to sustain a faster reaction.
As light intensity increases, more photons reach the chloroplasts, so the light‑dependent reactions produce ATP and NADPH more rapidly, allowing the Calvin cycle to fix CO₂ into glucose at a higher rate.
However, once the light reactions are saturated, further increases in light intensity do not increase the rate because another factor becomes limiting.
The limiting factor is usually the availability of CO₂ (or water), which cannot be fixed faster than the supply.
Thus, even with excess light energy, the rate of photosynthesis cannot rise because the supply of CO₂ (or water) is insufficient to sustain a faster reaction.
Examiner tips
- Use the word "light energy" and link it to the light‑dependent reactions. Show the cause–effect chain: more light → more ATP/NADPH → faster CO₂ fixation. Mention saturation and a limiting factor. State the limiting factor explicitly (CO₂ or water).
Common mistakes
- Confusing light intensity with light quality. Forgetting to mention the limiting factor. Using vague terms like "more energy" without linking to ATP/NADPH or CO₂ fixation.
Mark scheme (5 marks)
- Light energy is required/used for photosynthesis to occur
- As light intensity increases, more light energy is available so photosynthesis can occur at a faster rate, producing more glucose (and oxygen)
- At high light intensities the rate levels off because another factor is limiting / becomes a limiting factor
- The limiting factor could be carbon dioxide concentration (or water availability)
- Even with more light energy available, the rate cannot increase further because the supply of carbon dioxide (or water) is insufficient to sustain a faster rate of reaction
Key terms in this question
photosynthesis · light intensity
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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