The Z-Scheme of Photosynthesis — NEET Biology
📚 Practice ConceptThis movement of electrons is downhill, in terms of an oxidation-reduction or redox potential scale. The electrons are not used up as they pass through the electron transport chain, but are passed on to the pigments of photosystem PS I. Simultaneously, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm and are transferred to another acceptor molecule that has an even greater redox potential. These electrons then are moved downhill again, this time to a molecule of energy-rich NADP+. The addition of these electrons reduces NADP+ to NADPH + H+. This whole scheme of transfer of electrons, starting from the PS II, uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor, and finally down hill to NADP+ reducing it to NADPH + H+ is called the Z scheme, due to its characteristic shape. This shape is formed when all the carriers are placed in a sequence on a redox potential scale.
Arrange the following events in the correct chronological sequence as they occur during the non-cyclic electron transport (Z-scheme) and chemiosmosis in the light reaction of photosynthesis: (i) Photoexcitation of P680 and electron ejection from Photosystem II. (ii) Photolysis of water, supplying electrons to P680 and releasing protons into the thylakoid lumen. (iii) Electron transport through cytochromes, actively pumping protons from stroma into the thylakoid lumen. (iv) Photoexcitation of P700 and electron ejection from Photosystem I, followed by electron transfer to NADP+. (v) Reduction of NADP+ to NADPH + H+ by NADP reductase. (vi) ATP synthesis as protons diffuse from the thylakoid lumen to the stroma through ATP synthase. Which of the following options represents the correct sequence?
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Correct answer: A — (i) → (ii) → (iii) → (iv) → (v) → (vi)
This question requires arranging the steps of non-cyclic photophosphorylation (Z-scheme) and chemiosmosis in their correct chronological order. (i) Photoexcitation of P680 and electron ejection from Photosystem II: This is the initial step where light energy excites electrons in PSII. (Page 138, Section 11.6) (ii) Photolysis of water, supplying electrons to P680 and releasing protons into the thylakoid lumen: Water splitting immediately follows electron ejection from P680 to replace the lost electrons and contributes to the proton gradient. (Page 139, Section 11.6.1; Page 140, Section 11.6.3) (iii) Electron transport through cytochromes, actively pumping protons from stroma into the thylakoid lumen: As electrons move 'downhill' through the electron transport chain between PSII and PSI, protons are transported across the membrane into the lumen. (Page 139, Section 11.6; Page 141, Figure 11.7) (iv) Photoexcitation of P700 and electron ejection from Photosystem I, followed by electron transfer to NADP+: Electrons then reach PSI, are re-excited by light, and transferred to an acceptor. (Page 139, Section 11.6) (v) Reduction of NADP+ to NADPH + H+ by NADP reductase: The electrons from PSI, along with protons, reduce NADP+ to NADPH + H+. (Page 139, Section 11.6) (vi) ATP synthesis as protons diffuse from the thylakoid lumen to the stroma through ATP synthase: The accumulated proton gradient is then utilized by ATP synthase to produce ATP. This occurs concurrently with the electron flow providing the gradient. (Page 141-142, Section 11.6.3) Therefore, the correct chronological sequence is (i) → (ii) → (iii) → (iv) → (v) → (vi).
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