Proton Gradient Formation in Photosynthesis — NEET Biology

Correct answer: A — (A), (B), (C), (D), (E), (F)

The core concept tested is the sequential steps involved in ATP synthesis via the chemiosmotic hypothesis in chloroplasts. (A) Water splitting complex releases protons into the thylakoid lumen: This is one of the initial processes that contribute to proton accumulation in the lumen. The NCERT states, 'splitting of the water molecule takes place on the inner side of the membrane, the protons or hydrogen ions that are produced by the splitting of water accumulate within the lumen of the thylakoids.' (B) Electron carriers transport protons from the stroma to the lumen: As electrons move through photosystems, proton carriers actively pump protons from the stroma into the lumen, further increasing proton concentration inside the thylakoid. The NCERT states, 'As electrons move through the photosystems, protons are transported across the membrane... this molecule removes a proton from the stroma while transporting an electron... the proton is released into the inner side or the lumen side of the membrane.' (C) A proton gradient with higher concentration in the lumen is established: The combined effect of water splitting (A) and proton transport by electron carriers (B), along with proton consumption in the stroma by NADP+ reductase (not listed as a separate step but implied by the gradient formation), leads to a high concentration of protons in the lumen and a lower concentration in the stroma, creating the gradient. The NCERT states, 'Hence, within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons. This creates a proton gradient across the thylakoid membrane'. (D) Protons flow through the CF0 channel to the stroma: The established proton gradient drives protons from the high-concentration lumen back to the low-concentration stroma through the CF0 transmembrane channel of ATP synthase. The NCERT states, 'The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the CF0 of the ATP synthase'. (E) The CF1 portion of ATP synthase undergoes conformational change: The energy released by the movement of protons through CF0 causes a conformational change in the CF1 particle, activating the enzyme. The NCERT states, 'The break down of the gradient provides enough energy to cause a conformational change in the CF1 particle of the ATP synthase'. (F) ATP is synthesized from ADP and inorganic phosphate: This conformational change enables the CF1 portion of ATP synthase to catalyze the synthesis of ATP from ADP and inorganic phosphate. The NCERT states, 'which makes the enzyme synthesise several molecules of energy-packed ATP'. Therefore, the correct chronological sequence of events is (A), (B), (C), (D), (E), (F).