The electron on QA-is then transferred to QB-site. As already stated, plastoquinone at the QB-site differs from plastoquinone at the QA-site in that it works as a two-electron acceptor and becomes fully reduced and protonated after two photochemical turnovers of the reaction center. The full reduction of plastoquinone at the QB- site requires the addition of two electrons and two protons. The reduced plastoquinone (plastoquinol, QBH2) then unbinds from the reaction center and diffuses in the hydrophobic core of the membrane, after which an oxidized plastoquinone molecule finds its way to the QB-binding site and the process is repeated. Because the QB-site is near the outer aqueous phase, the protons added to plastoquinone during its reduction are taken from the outside of the membrane. Electrons are passed from QBH2 to a membrane-bound cytochrome b6f, concomitant with the release of two protons to the luminal side of the membrane. The cytochrome b6f then transfers one electron to a mobile carrier in the thylakoid lumen, either plastocyanin or cytochrome c6. This mobile carrier serves an electron donor to PSI reaction center, the P700. Upon photon absorption by PSI a charge separation occurs with the electron fed into a bound chain of redox sites; a chlorophyll a (A0), a quinone acceptor (A1) and then a bound Fe–S cluster, and then two Fe–S cluster in ferredoxin, a soluble mobile carrier on the stromal side. Two ferredoxin molecules can reduce NADP+ to NADPH, via the flavoprotein ferredoxin-NADP+ oxidoreductase. NADPH is used as redox currency for many biosynthesis reactions such as CO2 fixation. The energy conserved in a mole of NADPH is about 52.5 kcal/mol, whereas in an ATP hole is 7.3 kcal/mol.
From this scheme it is evident that only approximately one third of the energy absorbed by the two primary electron donors P680 and P700 is turned into chemical form. A 680 nm photon has an energy of 1.82 eV, a 700 nm photon has an energy of 1.77 eV (total = 3.59 eV) that is three times more than sufficient to change the potential of an electron by 1.24 eV, from the redox potential of the water (0.82 eV) to that of ferredoxin-NADP+ oxidoreductase (-0.42 eV).
It is worthwhile to emphasize that any photon that is absorbed by any chlorophyll molecule is energetically equivalent to a red photon because the extra energy of an absorbed photon of shorter wavelength (<680 nm) is lost during the quick fall to the red energy level that represents the lowest excited level.
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