photorespiration

Photorespiration is a metabolic process in many aerobic plants, algae, and some bacteria that reduces photosynthetic efficiency by metabolizing 2-phosphoglycolate, the oxygenation product of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) when it fixes oxygen instead of carbon dioxide during the light reactions of photosynthesis.

The pathway involves coordination among chloroplasts, peroxisomes, and mitochondria. In the chloroplast, 2-phosphoglycolate is dephosphorylated to glycolate. Glycolate is transported to peroxisomes, where it is oxidized to glyoxylate, producing hydrogen peroxide. Glyoxylate is transaminated to glycine, which moves to mitochondria. There, two glycine molecules are converted to serine, releasing CO2 and NH3 in the process. Serine is returned to peroxisomes and converted through subsequent steps to glycerate, which re-enters the chloroplast and is converted to 3-phosphoglycerate to re-enter the Calvin cycle.

Net effects of photorespiration are the regeneration of some Calvin cycle intermediates but at an energetic and carbon cost: the cycle leads to the release of CO2 and NH3 and consumes energy in the form of ATP and reducing equivalents. The process is therefore a drain on fixed carbon, particularly under conditions that favor RuBisCO’s oxygenation activity.

Environmental factors such as high temperature and low CO2 relative to O2 increase photorespiration, as stomatal closure raises internal O2:CO2 ratios. C4 and CAM plants mitigate photorespiration by concentrating CO2 near RuBisCO, illustrating the pathway’s influence on plant physiology and global carbon balance.