Question

Which of the following statements is true with regard to light reaction of photosynthetic mechanism in plants?

• A. Chlorophyll-a occurs with peak absorption at 680 nm in photosystem -I and at 700 nm in photosystem - II
• B. Magnesium and sodium ions are associated with photolysis of water molecules
• C. $O_2$ is evolved during cyclic photophosphorylation
• D. Photosystem - I and II are both involved in non-cyclic photophosphorylation
• E. Both ATP and $NADP{H}_2$ are formed during cyclic photophosphorylation

Answer 1

The correct option is D Photosystem - I and II are both involved in non-cyclic photophosphorylation

Of the given statements, only the statement "Both PS-I and II are involved in non-cyclic photophosphorylation" is correct.

Chlorophyll-a occurs with peak absorption at 700 nm in photosystem -I and at 680 nm in photosystem - II.

Manganese ions are associated with photolysis of water molecules.

$O_2$ is evolved during non-cyclic photophosphorylation. Only in non-cyclic photophosphorylation you have an external electron donor which is water. Water undergoes photolysis to evolve oxygen, and the electrons produced during the photolysis flows through an electron transport system (ETS) before it is used to create the high energy molecule NADPH. But before the electrons are given away to NADP+, it undergoes a series of redox reaction while flowing down the ETS. And the energy released during these redox reactions are stored in the membrane as a proton gradient and used to synthesize ATP. However in cyclic photophosphorylation there is no external electron donor, the electrons that flow down the electron transport chain arise at photosystem I and are transferred back to photosystem I itself. Therefore there is no photolysis of water, in this case and no evolution of oxygen. However, the electrons flowing down the ETS from PS I back to PS I stores the energy released and utilizes it to synthesize ATP, like seen in non-cyclic photophosphorylation.

Thus both ATP and $NADP{H}_2$ are formed during non-cyclic photophosphorylation where as only ATP is produced during cyclic photophosphorylation.