Photorespiration

Photorespiration is a process in plants and other photosynthetic organisms in which the enzyme rubisco reacts with oxygen instead of carbon dioxide. It wastes energy and releases some of the carbon already fixed, so it reduces the efficiency of photosynthesis.

Rubisco's active site cannot completely distinguish CO₂ from O₂, which are both small molecules. When it adds oxygen to ribulose bisphosphate, it makes one molecule of 3-phosphoglycerate and one of 2-phosphoglycolate, a compound that is useless and toxic to the cell. The enzyme evolved when the atmosphere was rich in CO₂ and held almost no oxygen, so the mistake was of little consequence then.

The glycolate is recycled through a pathway that crosses three organelles, the chloroplast, the peroxisome and the mitochondrion, where two glycolates are converted to one phosphoglycerate, releasing CO₂ and ammonia. The pathway costs ATP and reducing power and loses about a quarter of the carbon that would have been fixed under warm conditions.

Warmer temperatures raise the proportion of oxygenation, and closing the stomata in dry weather lowers CO₂ inside the leaf, so photorespiration is worst in hot, dry conditions. Plants using C₄ photosynthesis and CAM concentrate CO₂ around rubisco to avoid it, and researchers have engineered shortcuts in crops to raise yield. It also affects how plants respond to rising CO₂ in the carbon cycle and links to the Calvin cycle.

• Rubisco — the enzyme responsible
• Calvin Cycle — the pathway it competes with
• Photosynthesis — the process it reduces