Linear electron flow in chloroplasts involves PSII and PSI in an obligate series in which electrons are transferred from H2O to NADP+. The process begins with absorption of a photon by PSII, causing an electron to move from a P680 chlorophyll a to an acceptor plastoquinone (QB) on the stromal surface (Figure 8-37). The resulting oxidized P680+ strips one electron from the highly unwilling donor H2O, forming an intermediate in O2 formation and a proton, which remains in the thylakoid lumen and contributes to the protonmotive force. After P680 absorbs a second photon, the semiquinone Q_- accepts a second electron and picks up two protons from the stromal space, generating QH2. After diffusing in the membrane, QH2 binds to the Qo site on a cy-tochrome bf complex, which is analogous in structure and function to the cytochrome bcj complex in purple bacteria and the CoQH2-cytochrome c reductase complex in mitochondria. As in these systems a Q cycle operates in the cy-tochrome bf complex in association with the PSII reaction center, thereby increasing the proton-motive force generated by electron transport.
Absorption of a photon by PSI leads to removal of an electron from the reaction-center chlorophyll a, P700 (see Figure 8-37). The resulting oxidized P700+ is reduced by an electron passed from the PSII reaction center via the cytochrome bf complex and plastocyanin, a soluble electron carrier that contains a single copper (Cu) atom. After the cytochrome bf complex accepts electrons from QH2, it transfers them, one at a time, to the Cu2+ form of plastocyanin, reducing it to the Cu+ form. Reduced plastocyanin then diffuses in the thylakoid lumen, carrying the electron to P700+ in PSI. The electron taken up at the luminal surface by P700 moves via several carriers to the stromal surface of the thylakoid membrane, where it is accepted by ferredoxin, an iron-sulfur (Fe-S) protein. Electrons excited in PSI can be transferred from ferredoxin via the electron carrier FAD to NADP+, forming, together with one proton picked up from the stroma, the reduced molecule NADPH.
F0Fj complexes in the thylakoid membrane use the proton-motive force generated during linear electron flow to synthesize ATP on the stromal side of membrane. Thus this pathway yields both NADPH and ATP in the stroma of the chloroplast, where they are utilized for CO2 fixation.
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