The release of CO
2
during photosynthesis that is due to the production and metabolism of glycollic acid is usually regarded as outward evidence for the wasteful process of photorespiration in plants. In the light, glycollic acid is produced almost entirely as a result of the oxygenase activity of ribulose bisphosphate carboxylaseoxygenase (Rubisco). Metabolism of the glycollic acid not only releases recently assimilated carbon back into the atmosphere but also uses a considerable amount of energy to recycle remaining carbon from the glycollate to intermediates of the photosynthetic carbon reduction cycle. Furthermore, nitrogen from amino acids is released as ammonia during the metabolism of glycollate; some further energy is needed for this ammonia to be reassimilated. The oxygenation of ribulose bisphosphate is competitive with carboxylation and it appears to be the relative concentrations of oxygen and carbon dioxide present in cells containing the enzyme that mainly determine the relative rates of the two reactions in leaves. Systems which concentrate carbon dioxide in photosynthetic cells decrease the extent of photorespiration in C
4
species, certain algae and cyanobacteria. However, carboxylases from different species also vary considerably in their relative capacities to catalyse carboxylation and oxygenation of ribulose bisphosphate under standard conditions. This variation allows some hope that photorespiration might be decreased without recourse to energydependent systems for increasing cellular CO
2
concentrations.
Regulation of Photosynthetic Carbon Reduction Cycle by Ribulose Bisphosphate and Phosphoglyceric Acid
