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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