What is photorespiration ? Explain the mechanism and its significance.

Photorespiration is a process which involves loss of fixed carbon as $C{O}_2$ in plants in the presence of light. It is initiated in chloroplasts. This process does not produce ATP or NADPH and is a wasteful process.

Photorespiration occurs usually when there is the high concentration of oxygen. Under such circumstances, RuBisCO, the enzyme that catalyses the carboxylation of RuBP during the first step of Calvin cycle, functions as an oxygenase. Some $O_2$ does bind to RuBisCO and hence $C{O}_2$ fixation is decreased. The RuBP binds with $O_2$ to form one molecule of PGA (3C compound) and phosphoglycolate (2C compound ) in the pathway of photorespiration. There is neither the synthesis sugar nor of ATP. Rather, it results in the release of $C{O}_2$ with the utilisation of ATP. It leads to a 25 percent loss of the fixed $C{O}_2$.

Significance of photorespiration:

1. Photorespiration helps in dissipation of energy where stomata get closed during daytime because of water stress.
2. Photorespiration protects the plant from photoxidative damage by dissipating excess of excitation energy.