In the first step of Calvin Cycle of Photosynthesis, RUBP combines with CO2 to form 2 molecules of 3PGA in the presence of the enzyme RUBISCO.
However, when it is too hot, in some C3 plants some O2 binds to RUBisCO. CO2 fixation decreases. The RUBP binds with O2 to form one molecule of 3PGA and one molecule of phosphoglycolate in a pathway called photorespiration. In this pathway, there is neither synthesis of sugars nor release of ATP or NADPH. Rather it releases CO2 with utilisation of ATP. Thus, Photorespiration is a wasteful process.
Photorespiration does not occur in C4 plants as they have a mechanism to increase CO2 at the enzyme site ensuring that RUBisCO functions as carboxylase minimising the oxygenase activity.
Thus the C4 plants are more productive than C3 plants and alao that C4 plants show greater tolerence to high temperatures