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Lecture 6- photosynthesis 2

Membrane the proton motive force established across

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Unformatted text preview: the lumen by the cyclic reduction and oxidation of PQ as it moves from photosystem II to the cytochrome complex - 2nd, the gradient is enhanced by the addition of 2 protons to the lumen from the oxidation of water, which occurs on the luminal side of photosystem II - 3rd, the removal of one proton from the stroma for each NADPH molecule synthesized further decreases the H+ concentration in the stroma, thereby enhancing the gradient across the thylakoid membrane - the proton motive force established across the thylakoid membrane is used to synthesize ATP by chemiosmosis using the chloroplast ATP synthase • The light reactions use the solar power of photons absorbed by both photosystem I and photosystem II to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH. - photons are providing energy to move an electron to a higher position so it has more potential energy and can do more work 10 In noncyclic electron flow (1) 1 H2O split 1 ATP 1 NADPH (2) 1 photon to drive 1 electron through 1 photosystem • So 1 photon drives 1 electron through 1 photosystem • H2O split 2 electrons • 1 electron through 2 photosystems requires 2 photons • 2 electrons through 2 photosystems requires 4 photons • To produce 1 O2 would require 8 photons • Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow. - photophosphorylation is the process of using light to generate ATP – Excited electrons cycle from their reaction center to a primary acceptor, along an electron transport chain, and returns to the oxidized P700 chlorophyll. – As electrons flow along the electron transport chain, they generate ATP by cyclic photophosphorylation. 11 • Noncyclic electron flow produces ATP and NADPH in roughly equal quantities. • However, the Calvin cycle consumes more ATP than NADPH. • Cyclic electron flow allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle. Fig 7.13 - light reactions byproducts can work in dark 12 13...
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