H1 Biology 8876 · Nov 2023BiologyPaper 1Commented

Paper 1 Q7 — Reading the one variable that changed

Data handling — attributing an effect only to the variable that actually changed

What the examiners said

“Candidates selecting options including statement 2 had not noted that the only variable changing in the graph was oxygen concentration. Changes in carbon dioxide concentration could not therefore account for changes in oxygen saturation.”

Try the question

A graph plots haemoglobin’s oxygen saturation against oxygen partial pressure, at a single fixed CO₂ level. Which conclusion is valid?

Examiner report: “Candidates invoked CO₂ even though only oxygen concentration changed in the graph.”

Explore — which variable is actually moving?

The oxygen dissociation curve. Move along the oxygen axis (the variable that changes) and confirm CO₂ stays fixed — so only oxygen can explain the saturation change on this curve.

Oxygen partial pressure (kPa): 5 CO\u2082 level (fixed for this curve): normal

A controlled experiment changes one variable at a time. If CO₂ is constant, it is not part of any cause-and-effect you read from this graph.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H1 Biology 8876 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

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