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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