The first reaction of photorespiration is oxygenation — RuBisCO catalyses the addition of $\text{O}_2$ (instead of $\text{CO}_2$) to ribulose-1,5-bisphosphate (RuBP), producing one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate. This competitive oxygenase activity of RuBisCO initiates the photorespiratory carbon oxidation cycle. The phosphoglycolate is then dephosphorylated to glycolate, which enters the peroxisome. Photorespiration reduces photosynthetic efficiency in $\text{C}_3$ plants by up to 25–30%.