Question

Which statement regarding cycling flow of electrons during light reactions is false

• A. PS II is not involved in the process.
• B. ATP synthesis takes place
• C. $NADP+H^{+}$ is synthesised
• D. Takes place only when light of wavelength beyond 680 nm is available for excitation

Answer 1

The correct option is D Takes place only when light of wavelength beyond 680 nm is available for excitation

Cycling flow of electrons during light reactions is known as cyclic photophosphorylation. It is performed by photosystem I independently. PS II is not involved in the process. In PS I, the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called $P_{700}$, while in PS II, reaction centre has an absorption maxima at 680 nm and is called $P_{680}$. As cyclic photophosphorylation requires only PS I, so it takes place only when light of wavelength beyond 700 nm is available for excitation. This process takes place in stroma lamellae membrane. An external source of electrons is not required. There is no evolution of oxygen takes place because it is not connected with photolysis of water. Cyclic photophosphorylation produces ATP only. 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 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.