A student investigates the effect of light intensity on the rate of photosynthesis in pondweed. She places the pondweed at different distances from a lamp and counts the number of bubbles produced per minute. Explain why the rate of photosynthesis decreases as the pondweed is moved further away from the lamp.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student investigates the effect of light intensity on the rate of photosynthesis in pondweed. She places the pondweed at different distances from a lamp and counts the number of bubbles produced per minute.
Model answer (4 marks)
As the pondweed is moved further from the lamp, the distance increases and the light intensity reaching the plant decreases. Light is a limiting factor for photosynthesis, so with less light energy available the photosynthetic reactions are less able to convert light into chemical energy. Consequently less glucose (and oxygen) is produced per minute, so the rate of photosynthesis decreases.
Examiner tips
- Use the term ‘light intensity’ and explain it decreases with distance; mention light as a limiting factor; link reduced light to less glucose/oxygen produced; keep answer concise and use correct terminology.
Common mistakes
- Saying the rate decreases because the plant is tired or because of temperature; not linking light intensity to the photosynthetic reaction; using vague phrases like ‘less light’ without explaining the effect on energy conversion.
Mark scheme (4 marks)
- As distance increases, light intensity decreases
- Light is a limiting factor for photosynthesis
- Less light energy is available for photosynthesis / to convert light energy into chemical energy
- So less glucose (and oxygen) is produced per minute / rate of photosynthesis decreases
Key terms in this question
photosynthesis · light intensity
Related
- Revision notes: Photosynthesis →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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