Photorespiration is an alternate pathway for rubisco, the main enzyme of photosynthesis (specifically, the Calvin cycle). Although rubisco favors carbon, it can also use oxygen, producing glycolate . This usually occurs when oxygen levels are high; for example, when the stomata (tiny pores on the underside of the leaf) are closed to prevent water loss.
The reaction is:
The glycolate is then sent to peroxisomes.
Photorespiration is a wasteful process because no PGAL is created. PGAL is used to create nearly all of the food and structures in the plant. Plants cope with photorespiration via CAM and C4 metabolism.