Question

What is photorespiration ? How are photorespiratory losses overcome by $C_4$ plants ?

Answer 1

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 $C{O}_2$.

$C_4$ plants overcome photorespiratory losses by having the mechanism that increases the concentration of $C{O}_2$ at the enzyme site. During the $C_4$ pathway, when the $C_4$ acid from the mesophyll cells is broken down in the bundle sheath cells, it releases $C{O}_2$ this results in increasing the intracellular concentration of $C{O}_2$. This, in turn, ensures that the RuBisCO functions as a carboxylase minimizing the oxygenase activity.