Question

(a) Give a schematic outline of photorespiration.
(b)Differentiate between photophosphorylation and photorespiration.

Answer 1

(a) 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 $C{O}_2$.

(b) Difference between photophosphorylation and photorespiration

• Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light. On the other hand, photorespiration is a process which involves loss of fixed carbon as $C{O}_2$ in plants in the presence of light.
• Photophosphorylation occurs in chloroplasts. Photorespiration is initiated in chloroplasts, but it also involves peroxisomes and mitochondria.
• Photophosphorylation is a useful process as it involves the formation of ATP, whereas photorespiration is a wasteful process as it does not produce ATP. Photorespiration consumes energy.
• Photophosphorylation is of two types- cyclic and non-cyclic. But, photorespiration is only cyclic.
• Photophosphorylation occurs in bacteria well as in plants. Cyclic photophosphorylation occurs in bacteria and non-cyclic photophosphorylation occurs in green plants. On the other hand, photorespiration occurs only in $C_3$ plants.
• Photophosphorylation is an essential process for photosynthesis, but photorespiration is not essential.