Question

Cyclic-electron flow in the chloroplast during the light-dependent reactions will produce which of the following compounds?
I. ATP
II. Glucose
III. NADPH

• A. I only
• B. III only
• C. I and II only
• D. I and III only
• E. I, II and III only

Answer 1

The correct option is A I only

In the light reaction of photosynthesis, the formation of ATP from ADP and H₃PO₄ in the presence of light and chlorophyll-a during the cyclic transfer of electrons is called cyclic photophosphorylation. Only PS I is involved in cyclic photophosphorylation. The various steps are:

1. The reaction centre of PS I (chlorophyll-a 700 i.e. P700) is activated on receiving light energy in the form of photons. PS I in its excited state expels an electron and becomes ionised chlorophyll-a.

2. The energy rich electrons from PS I are first accepted by FRS and transferred to Ferredoxin. From Ferredoxin, the electrons are transferred to cytochrome-b6, cytochrome-f and then to Plastocyanin which are then finally accepted by the same ionised chlorophyll-a⁺ of PS I.

On receiving the lost electrons, chlorophyll a⁺ comes back to the ground state and can again participate in the process. In this process, the initial donor and final acceptor of electrons is the same chlorophyll-a of PS I. Hence the movement of electrons is cyclic.

3.During the cyclic transfer of energy rich electrons, there is loss of electron energy at each transfer. It is used for the formation of ATP from ADP and H₃PO₄.

Hence Cyclic-electron flow in the chloroplast during the light-dependent reactions will produce only ATP.

So, the correct answer is 'I only'.