Photorespiration is the light dependent process of oxygenation of ribulose biphosphate (RuBP) and release of carbon dioxide by the photosynthetic organs of a plant. Normally photosynthetic organs do the reverse in the light, i.e., uptake of CO2 and release of O2. At high temperature, RuBP carboxylase functions as oxygenase and instead of fixing carbon dioxide (C3 cycle), oxidises ribulose 1, 5-biphosphate to produce a 3-carbon phosphoglyceric acid and a 2-carbon phosphoglycolate.
Photorespiration does not produce energy or reducing power. Rather, it consumes energy. Further, it undoes the work of photosynthesis. There is 25% loss of fixed CO2. Therefore, photorespiration is a highly wasteful process. This happens only in case of C3 plants. C4 plants have overcome the problem of photorespiration.