RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme responsible for carbon fixation in photosynthesis but also exhibits an oxygenase activity. Carboxylase activity (CO2 fixation): RuBP + CO2 → 2 molecules of 3-PGA. This is the productive Calvin cycle reaction. Oxygenase activity (photorespiration): RuBP + O2 → 1 molecule of 3-PGA + 1 molecule of 2-phosphoglycolate. This is the wasteful photorespiratory reaction. The two activities compete — high CO2 promotes carboxylase, high O2 promotes oxygenase. Temperature increases oxygenase activity (O2 becomes more soluble relative to CO2 at higher temperatures). C4 plants concentrate CO2 to favour carboxylase. RuBisCO is the most abundant protein on Earth.

The photorespiratory or C2 oxidative carbon cycle processes the phosphoglycolate produced by RuBisCO oxygenase. In the CHLOROPLAST: phosphoglycolate (2C) → glycolate (by phosphoglycolate phosphatase). In the PEROXISOME: glycolate → glyoxylate (by glycolate oxidase, producing H2O2 which is destroyed by catalase); glyoxylate + glutamate → glycine + 2-oxoglutarate (transamination). In the MITOCHONDRIA: 2 glycine → serine + CO2 + NH3 + NADH (by glycine decarboxylase). CO2 is released here. Back in PEROXISOME: serine → glycerate (transamination). Back in CHLOROPLAST: glycerate → 3-PGA (by glycerate kinase, using ATP) → re-enters Calvin cycle. Net result: for every 2 phosphoglycolate molecules, 1 CO2 is released and resources are consumed without energy gain.

C3 plants: carbon fixation by RuBisCO directly in mesophyll cells. First product = 3-PGA (3 carbons). Photorespiration occurs significantly, especially at high temperatures and light. Examples: wheat, rice, barley, potato, sunflower, soybean, most trees. Net photosynthetic efficiency reduced by 25-50% due to photorespiration. C4 plants: initial CO2 fixation by PEP carboxylase (no oxygenase activity) in mesophyll cells. First product = OAA (4 carbons). CO2 concentrated in bundle sheath cells (around RuBisCO), suppressing oxygenase activity. Hatch-Slack (C4) pathway acts as CO2 pump. Examples: maize, sugarcane, sorghum, millet, Amaranthus. Photorespiration virtually absent. More efficient at high temperatures, high light, and limited water.

Photorespiration is particularly significant in agricultural contexts because it reduces crop productivity in major C3 food crops like wheat and rice. At current atmospheric CO2 levels (~420 ppm) and typical summer growing temperatures (25-35°C), photorespiration can reduce net photosynthesis by 20-40% in C3 crops. Engineering C4-like photosynthesis into C3 crops (the "C4 Rice Project") is a major international research goal that could potentially increase rice yields by 50%, significantly contributing to global food security for a growing population. Elevated CO2 (as is occurring with climate change) partially suppresses photorespiration in C3 plants by shifting the carboxylase/oxygenase balance, partially explaining why elevated CO2 increases C3 crop yields but has less effect on C4 crops.