1. The active site of rubisco cannot fully discriminate between carbon dioxide and oxygen, and so it catalyses reactions of carboxylation (producing 2 PGA molecules) and wasteful oxygenation (producing only one PGA to enter the Calvin cycle, plus one phosphoglycolate). With the two substrates at the relative concentrations found in normal air, carboxylation is favoured by about a 3:1 ratio. In C4 plants, and plants with other carbon dioxide concentration mechanisms, the balance is shifted in favour of carboxylation by releasing quantities of CO2 > those of normal air in the neighbourhood of rubisco (e.g. in the bundle sheath cell chloroplasts in C4 plants).

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