Question

Cyclic phosphorylation cannot sustain photosynthesis

• A. PS I does not function beyond 680 nm
• B. No evolution of oxygen
• C. Unidirectional cyclic movement of electrons
• D. Only ATP is formed, NADPH is not formed

Answer 1

Answer: (D)

Only ATP is formed, NADPH is not formed

Cyclic photophosphorylation is performed by photosystem I independently. 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 phosphorylation cannot sustain photosynthesis because it 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.