Photorespiration

Photorespiration(or "photo-respiration")is the alternate pathway for production of glyceraldehyde 3-phosphate (G3P) by RuBisCO, the main enzyme of the light-independent reactions of photosynthesis (also known as the Calvin cycle or the Primary Carbon Reduction (PCR) cycle). Although RuBisCO favors carbon dioxide to oxygen,(approximately 3 carboxylations per oxygenation), oxygenation of RuBisCO occurs frequently, producing a glycolate and a glycerate. This usually occurs when oxygen levels are high; for example, when the stomata (tiny pores on the leaf) are closed to prevent water loss on dry days. It involves three cellular organelles: chloroplasts, peroxisomes, and mitochondria