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H2 Biology 9744 · Nov 2023Biology (H2)Paper 1Challenging

Paper 1 Q21 — One graph, one conclusion

Photosynthesis — a single variable’s graph cannot prove another factor’s effect

What the examiners said

“These candidates recognised the relationship between light absorbance and temperature but had not considered that the rate of the light-dependent reactions will not only be determined by light absorbance. The effect of temperature… cannot therefore be concluded from the graph.”

Try the question (reconstruction)

A graph shows that pigment light-absorbance increases with temperature. What can you correctly conclude about the rate of the light-dependent reactions?
Examiner report: “Candidates concluded a temperature effect on rate from a graph that only showed absorbance.”

Explore — what a single graph can and can’t tell you

The rate depends on several inputs. Adjust absorbance alone and see that the overall light-dependent rate is capped by whatever factor is limiting — so absorbance alone does not fix the rate.

When enzymes are limiting, pushing absorbance higher does nothing — proof that one variable’s graph cannot determine the rate.

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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