For each RuBP involved in dark reaction, the formation of 3-phosphoglyceric acid molecules is decreased to half when there is operation of the following pathway.

During photorespiration, O2 reacts with RuBP to produce one molecule of PGA and one molecule of phosphoglycolate (2C). This phosphogylcolate utilizes ATP and undergoes several reactions in organelles like chloroplast, mitochondria and peroxisomes to finally get converted into a molecule of PGA. In C3, C4 and CAM pathways for each RuBP involved, there is formation of 2 molecules of 3-phosphoglyceric acid.