Question

Photorespiration is a characteristic feature of plants having

• A. $C_4$ cycle
• B. $C_3$ cycle
• C. Aerobic respiration
• D. None of the above

Answer 1

Answer: (B)

$C_3$ cycle

Photorespiration is a characteristic feature of plants having $C_3$ cycle. This process does not occur in $C_4$ plants. It 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. Photo-respiratory reactions are operated in chloroplasts, mitochondria, and peroxisomes. Loss of $C{O}_2$ occurs in mitochondria. Photorespiration occurs usually when there is the high concentration of oxygen. Under such circumstances, RuBisCO, the enzyme that catalyzes the carboxylation of RuBP during the first step of Calvin cycle, functions as an oxygenase. Some $O_2$ does not 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. Rather, it results in the release of $C{O}_2$ with the utilization of ATP. It leads to a 25 percent loss of the fixed $C{O}_2$. $O_2$ is first utilized in chloroplasts and then in peroxisomes. It decreases photosynthetic output.