Explain3 marksIGCSE
A plant in a sealed greenhouse is given extra carbon dioxide and its rate of photosynthesis increases. Providing more carbon dioxide then makes no difference, but increasing the light intensity does. Explain these observations.
Show the mark scheme and the examiner note
Answer: Carbon dioxide was the limiting factor at first; once it was supplied in excess, light became the limiting factor, so only more light increased the rate.
- Rate increased, so carbon dioxide was limiting at the start
- Adding more had no effect, so it was no longer limiting
- Increasing light raised the rate, so light had become the limiting factor
Examiner technique: Limiting factor questions are answered by elimination: whatever change produces an increase is the factor that was limiting. Say that logic out loud in your answer.