Linear electron flow in chloroplasts involves PSII and PSI in an obligate series in which electrons are transferred from H2O to NADP+ (see Figure 12-44). 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. The resulting oxidized P680 + strips one electron from the relatively unwilling donor H2O, forming an intermediate in O2 formation, as we shall see shortly, and a proton, which remains in the thylakoid lumen and contributes to the proton-motive 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 cytochrome bf complex that is analogous to the bacterial cytochrome bc1 complex and to the mitochondrial complex III. As in those systems, a Q cycle operates, thereby increasing the proton-motive force generated by electron trans port. After the cytochrome bf complex accepts electrons from QH2, it transfers them, one at a time, to the Cu2+ form of the soluble electron carrier plastocyanin (analogous to cytochrome c), reducing it to the Cu1+ form. Reduced plastocyanin then diffuses in the thylakoid lumen, carrying the electron to PSI.
Absorption of a photon by PSI leads to removal of an electron from the reaction-center chlorophyll a, P700 (see Figure 1). The resulting oxidized P700 + is reduced by an electron from plastocyanin that originated in PSII. Again, this process is analogous to the electron-transport chain in mitochondria. The electron taken up at the luminal surface by the P700 and energized by photon ab sorption moves within PSI via several carriers to the stromal surface of the thylakoid membrane, where it is accepted by ferredoxin, an iron-sulfur (Fe-S) protein. In linear electron flow, electrons excited in PSI are transferred from ferredoxin via the enzyme ferredoxin-NADP+ reductase (FNR). This enzyme uses the prosthetic group FAD as an electron carrier to reduce NADP+, forming, together with one proton picked up from the stroma, the reduced molecule NADPH. The linear electron flow pathway is now completed.
Fig1. Linear electron flow in plants, which requires both chloroplast photosystems, PSI and PSII. Blue arrows indicate flow of electrons; red arrows indicate proton movement. LHCs are not shown. (Left) In the PSII reaction center, two sequential light-induced excitations of the same special-pair P680 chlorophyll result in a two-step reduction of the primary electron acceptor QB to QH2. On the luminal side of PSII, electrons removed from H2O by the oxygen-evolving complex are transferred to P680 +, restoring the reaction-center chlorophylls to the ground state after each excitation. The oxygen-evolving complex contains a cluster of four manganese ions (Mn, violet), a Ca2+ ion (green), and a Cl− ion (teal). These bound ions function in the split ting of H2O and maintain the environment essential for high rates of O2 evolution. Four sequential light-induced excitations of the P680 (double the number illustrated here) are required to oxidize two water molecules and release four protons and one molecule of molecular oxygen (O2). A tyrosine on the protein helps conducts electrons from the Mn ions to the oxidized reaction-center chlorophyll (P680 +), reducing it to the ground state (P680) after excitation by each photon. (Center) The cytochrome bf complex then accepts electrons from QH2 and transports two protons into the lumen. Operation of a Q cycle in the cytochrome bf complex translocates additional protons across the membrane to the thylakoid lumen, increasing the proton-motive force. (Right) In the PSI reaction center, each electron released from light-excited P700 chlorophylls moves via a series of carriers in the reaction center to the stromal surface, where soluble ferredoxin (an Fe-S protein) transfers the electron to ferredoxin-NADP+ reductase (FNR). This enzyme uses the prosthetic group flavin adenine dinucleotide (FAD) and a proton to reduce NADP+, forming NADPH. P700 + is restored to its ground state by addition of an electron carried from PSII via the cytochrome bf complex and plastocyanin, a soluble electron carrier.
F0F1 complexes in the thylakoid membrane use the pro ton-motive force generated during linear electron flow to synthesize ATP on the stromal side of the membrane. Thus this pathway exploits the energy from multiple photons ab sorbed by both PSII and PSI and their antennas to generate both NADPH and ATP in the stroma of the chloroplast, where they are used for CO2 fixation.
