Paper 1 Q22 — Change tracks the limiting factor

Photosynthesis — the rate changes only with whichever factor is currently limiting

What the examiners said

“Candidates selecting these options had not realised that changes in the rate of photosynthesis must be related to changes in whichever factor is limiting the rate of photosynthesis at the time.”

Try the question (reconstruction)

On a bright, warm day a plant’s photosynthesis is limited by CO₂ concentration. Which change would increase the rate?

Examiner report: “Candidates did not relate rate changes to the currently limiting factor.”

Explore — find the limiting factor

Set light, CO₂ and temperature. The rate is capped by the lowest one. Raising a non-limiting factor does nothing — only the bottleneck moves the rate.

Light: 8 CO₂: 3 Temperature: 7

Raise the two non-limiting bars as high as you like — the rate does not budge until you lift the limiting one.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H2 Biology 9744 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

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