A researcher supplies ¹³C-labelled CO₂ for 60 s to an illuminated C3 leaf and then analyses the metabolites. The earliest radiolabelled compound is 3-phosphoglycerate. This supports:

Correct answer: (d) The Calvin–Benson cycle as the primary carboxylation pathway

Explanation

In C3 plants, CO₂ is fixed by the enzyme RuBisCO in the Calvin–Benson cycle. RuBisCO joins CO₂ to ribulose-1,5-bisphosphate, and the product splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound.

So if 3-PGA is the first compound to carry the label after a short exposure to labelled CO₂, the plant is fixing carbon through the Calvin cycle. In C4 and CAM plants, PEP carboxylase fixes CO₂ first, so the first labelled compounds are four-carbon acids (oxaloacetate, malate, aspartate). Melvin Calvin used this kind of experiment to work out the cycle.