A student investigates the effect of light intensity on the rate of photosynthesis in pond weed. As light intensity increases from low to high, the rate of photosynthesis increases, then levels off at a maximum rate. Explain why the rate of photosynthesis increases as light intensity increases, and suggest one reason why the rate levels off at a maximum even when light intensity continues to increase.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Pond weed is submerged in water and exposed to different light intensities. The number of oxygen bubbles produced per minute is used as a measure of the rate of photosynthesis. As light intensity increases, the bubble count increases, but eventually stays constant despite further increases in light intensity.
Model answer (5 marks)
Light supplies the energy that drives the light‑dependent reactions of photosynthesis.
As light intensity rises, more photons reach the chloroplasts, so more energy is available for the photochemical reactions. This increases the rate at which ATP and NADPH are produced, which in turn drives the Calvin cycle to fix more CO₂ and produce more glucose and O₂.
However, after a certain point the rate no longer rises. At high light intensity another factor becomes limiting – for example the concentration of CO₂ or the temperature of the cells. When that factor is exhausted, the photosynthetic machinery cannot use the extra light energy, so the rate plateaus at a maximum.
As light intensity rises, more photons reach the chloroplasts, so more energy is available for the photochemical reactions. This increases the rate at which ATP and NADPH are produced, which in turn drives the Calvin cycle to fix more CO₂ and produce more glucose and O₂.
However, after a certain point the rate no longer rises. At high light intensity another factor becomes limiting – for example the concentration of CO₂ or the temperature of the cells. When that factor is exhausted, the photosynthetic machinery cannot use the extra light energy, so the rate plateaus at a maximum.
Examiner tips
- Use the term ‘light intensity’ and explain its role as the energy source. Show the link between more photons, more ATP/NADPH and more glucose/O₂. Mention a specific limiting factor (CO₂ or temperature) to justify the plateau.
- Common mistakes: 1) Saying light is ‘just a stimulus’ without linking it to energy. 2) Forgetting to name a limiting factor or giving a vague answer like ‘other conditions’. 3) Mixing up the stages of photosynthesis (light‑dependent vs. Calvin cycle) in the explanation.
Mark scheme (5 marks)
- Light provides the energy for photosynthesis
- More light means more energy available, so the rate of photosynthesis increases
- More glucose and/or oxygen is produced as light intensity increases
- The rate levels off because another factor becomes limiting
- A named limiting factor — carbon dioxide concentration or temperature (or amount of chlorophyll)
Key terms in this question
photosynthesis · light intensity
Related
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