Question

Cyclic photophosphorylation is carried out by

• A. PS I only
• B. PS II only
• C. Both A and B
• D. Photolysis and PS II

Answer 1

The correct option is A PS I only

Cyclic photophosphorylation is carried out by PS I only. This process takes place in stroma lamellae membrane. An external source of electrons is not required. Photolysis of water does not take place. There is no evolution of oxygen takes place because it is not connected with photolysis of water. Cyclic photophosphorylation produces ATP only. It does not involve the formation of NADPH. It operates under low light intensity, anaerobic conditions or when $C{O}_2$ availability is low. When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs, due to the cyclic flow of electrons. The membrane and lamella of the grana have both PS I and PS II, the stroma lamella membrane lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to $NAD{P}^{+}$ and is cycled back to the PS I complex through the electron transport chain.