photorespiratory

Photorespiration, also known as the photorespiratory pathway, is a metabolic process in plants, algae, and some bacteria that recycles the two-carbon phosphoglycolate produced when the enzyme Rubisco fixes oxygen instead of carbon dioxide during photosynthesis. The pathway links chloroplasts, peroxisomes, and mitochondria and generally reduces net photosynthetic carbon gain, especially in C3 plants under high oxygen, high temperature, or drought.

The process proceeds in several organelles. In the chloroplast, RuBP oxygenation yields 2-phosphoglycolate, which is dephosphorylated to glycolate. Glycolate is transported to peroxisomes and oxidized to glyoxylate, generating hydrogen peroxide. Glyoxylate then receives an amino group from glutamate to form glycine. In mitochondria, two glycine molecules are converted to serine with release of CO2 and NH3 via the glycine decarboxylase complex. Serine is transported back to the chloroplast and converted to glycerate, which is phosphorylated to 3-phosphoglycerate and re-enters the Calvin cycle.

Physiological and ecological implications of photorespiration are significant. It consumes energy and fixed carbon, acting as a loss under many conditions, yet it may help mitigate photodamage and ROS production and participates in one-carbon metabolism. C4 and CAM plants reduce photorespiration by concentrating CO2 around Rubisco, illustrating a major adaptation to alleviate this pathway's cost.

Research and agricultural relevance focus on bypass strategies and metabolic engineering to improve photosynthetic efficiency, aiming to reduce carbon loss without compromising cellular protection.