Describe the role of limiting factors in photosynthesis. In your answer, give two examples of limiting factors and explain how each one limits the rate of photosynthesis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A limiting factor is a factor that limits the rate of photosynthesis when there is not enough of it.
Light intensity is a limiting factor. Low light intensity means less energy is transferred to the chloroplasts, so the rate of photosynthesis decreases.
Carbon dioxide concentration is another limiting factor. Low CO₂ concentration means less raw material is available for the Calvin cycle, so the rate of photosynthesis decreases.
Light intensity is a limiting factor. Low light intensity means less energy is transferred to the chloroplasts, so the rate of photosynthesis decreases.
Carbon dioxide concentration is another limiting factor. Low CO₂ concentration means less raw material is available for the Calvin cycle, so the rate of photosynthesis decreases.
Examiner tips
- Define limiting factor first, then name two factors, each with a brief explanation of how it limits the rate.
- Use the exact wording from the mark scheme (e.g. ‘low light intensity’ and ‘low CO₂ concentration’).
Common mistakes
- Using ‘temperature’ without explaining its effect, or giving a factor that is not a limiting factor (e.g. water availability).
- Failing to explain how each factor limits the rate, just naming them.
Mark scheme (4 marks)
- A limiting factor is a factor that limits the rate of photosynthesis when there is not enough of it
- Names light intensity as a limiting factor
- Explains how the named first factor (e.g. light intensity) limits the rate — e.g. low light intensity means less energy is transferred to the chloroplasts so the rate of photosynthesis decreases
- Names a second limiting factor (e.g. carbon dioxide concentration or temperature) AND explains how it limits the rate — e.g. low CO₂ concentration means less raw material is available so the rate of photosynthesis decreases
Key terms in this question
limiting factor · photosynthesis
Related
- Revision notes: Photosynthesis →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →