Rate of respiration is more in light than in dark. Photorespiration is the excess respiration taking place in photosynthetic cells due to absence of CO2 and increase of O2 . This condition changes the carboxylase role of RUBISCO into oxygenase. C2 Cycle takes place in chloroplast, peroxisome and mitochondria. RUBP is converted into PGA and a 2C – compound phosphoglycolate by Rubisco enzyme in chloroplast. Since the first product is a 2C – compound, this cycle is known as C2 Cycle. Phosphoglycolate by loss of phosphate becomes glycolate.
Glycolate formed in chloroplast enters into peroxisome to form glyoxylate and hydrogen peroxide. Glyoxylate is converted into glycine and transferred into mitochondria. In mitochondria, two molecules of glycine combine to form serine. Serine enters into peroxisome to form hydroxy pyruvate. Hydroxy pyruvate with help of NADH + H+ becomes glyceric acid. Glyceric acid is cycled back to chloroplast utilising ATP and becomes Phosphoglyceric acid (PGA) and v enters into the Calvin cycle (PCR cycle). Photorespiration does not yield any free energy in the form of ATP. Under certain conditions 50% of the photosynthetic potential is lost because of Photorespiration
