photorespiration 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.

photorespiration A light-activated type of respiration that occurs in the chloroplasts of many plants. It differs biochemically from normal (dark) respiration in that it involves glycolate metabolism (see GLYCOLATE CYCLE). It is a process that is wasteful of both carbon dioxide and energy, and those plants in which it occurs have high carbon dioxide compensation points. It occurs only at very low rates in C4 plants which consequently have lower compensation points and greater photo-synthetic efficiency. Its biological function is not clearly understood.