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A metabolic pathway that occurs in plants in the presence of light, in which ribulose bisphosphate carboxylase/oxygenase (rubisco), the enzyme involved in carbon dioxide fixation with ribulose bisphosphate, accepts oxygen, in place of carbon dioxide, resulting in the formation of a two-carbon compound, glycolate. Most of the fixed carbon represented by the glycolate can be salvaged by a series of reactions – the glycolate pathway – involving the peroxisomes and mitochondria, and returned to the chloroplasts. However, some of the carbon is lost as carbon dioxide. Unlike respiration, there is no production of ATP. In C3 plants (see C3 pathway) photorespiration has the effect of reducing the rate of photosynthesis, as atmospheric oxygen can combine with rubisco. In C4 plants (see C4 pathway) the effect of photorespiration is negligible as the affinity of phosphoenolpyruvate carboxylase for carbon dioxide is extremely high. As oxygen is a competitive inhibitor of rubisco, photorespiration will increase as oxygen concentration increases or as carbon dioxide concentration decreases.

Subjects: Biological Sciences.

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