In some cases, ferredoxin does not transfer electrons to NADP +-oxidoreductase, but to plastoquinone via a cytochrome hence the name cyclic transfer. two protons for each electron transferred by the high potential chain of the cytochrome b 6f complex to PS I (§ 1.2.).ġ.2.one proton for each electron released by water oxidation by PS II (§ 1.1.).In total, for each electron transferred from H 2O to the reducing side of PS I, three protons accumulate within the lumen : The other electron is used to regenerate PQH 2 (thus completing the Q cycle).ĭuring this process, the cytochrome b 6f complex pumps protons from the stroma that accumulate into the lumen through the thylakoid membrane.īalance sheet.One of them reduce P700 (from PSI) via plastocyanin.PQH 2 diffuses towards the cytochrome b 6f complex and transmits two electrons to it (Figure 1) : Both PQ/PQH 2 forms are mobile within the membrane and participate in the Q cycle. When receiving two electrons, PQ absorbs two protons from the stroma, thus giving PQH 2. PSII transmits electrons to plastoquinone (PQ) at the stromal face of the thylakoid membrane. Oxygen production is thus accompanied by the release of protons into the lumen: one proton for each electron released by the oxidation of H 2O.įrom PSII to PSI. It does this by oxidizing water, according to the following reaction (Figure 1) :ĢH 2O (water) → O 2 (oxygen) + 4H + (protons) + 4 e – (electrons) For the system to work, it is essential that P680 + recovers each individual electron that was given. P680 then goes into a positively charged oxidized state (P680 +). Each photon reaching the P680 (at the PSII) induces a charge separation that initiates the transfer of electrons to the PSI and NADP+. The proton gradient established on either side of the membrane will allow ATP synthesis by ATP synthase. Overview of proton and electron transfers across the thylacoid membrane. Protons and linear electron transfer Figure 1. Establishing a transmembrane pH gradient 1.1. This gradient is used by ATP synthase for the phosphorylation of ADP to ATP. The flow of electrons along this membrane chain generates a proton concentration gradient, and thus an electrochemical gradient, across the thylakoid membrane. Within the thylacoid membrane (see Shedding light on photosynthesis), light induces electron transfer from water to NADP + through photosystems II and I, the cytochrome b 6f complex, and through various mobile molecules: plastoquinones, plastocyanin and ferredoxin (Figure 1). It is this last example that is taken here. In eukaryotes, its synthesis takes place in the mitochondria – during respiration – and in the chloroplasts – during photosynthesis. It is the most important donor of free energy in any biological system. ATP plays a central role in cellular metabolism.
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