Question

Which of the following is correct regarding non-cyclic photophosphorylation?

• A. The excited electron starts its journey from PS-II and goes on to PS-I and does not return to PS-II
• B. It involves both PS-I and PS-II
• C. ATP and NADPH are synthesised during non-cyclic photophosphorylation
• D. All of the above

Answer 1

The correct option is D All of the above

When ATP is synthesized from ADP and Pi (inorganic phosphate), in the presence of light (photons), it is called photophosphorylation.

During non-cyclic photophosphorylation, the reaction centre of PS-II absorbs light energy and releases energy rich electrons. These electrons are picked up by electron acceptors and the electrons pass through the electron transport chain. Simultaneously, the reaction centre of PS-I absorbs light of wavelength 700 nm. The electrons released by P700 are accepted by primary acceptor and are finally passed on to NADP+. Here, the electrons along with the protons (H+) reduce NADP to NADPH. The electrons lost by PS-I are replaced by electrons from PS-II. The electrons lost by PS-II are filled by electrons released due to splitting of water. Hence, during non-cyclic photophosphorylation, both PS-I and PS-II are involved.

During photosynthetic electron transport and also due to splitting of water, protons accumulate at high concentration inside the thylakoid space. The concentration gradient between the inside and outside of the thylakoids is the energy source used by the ATP synthase and this gradient is called proton gradient. Protons pass through a channel of the CF${}_0$-CF${}_1$ of ATP synthase enzyme. This movement of protons from a region of high concentration (inner thylakoid space) to low concentration (stroma) activates the enzyme to synthesise ATP.