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:

RuBP + rubisco + O₂ → phosphoglycolate

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.