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Figure 9.10. Electron transport in Rhodobacter sphaeroides. Light is collected through the light-harvesting complexes and excites a special pair of BChl that sits near the periplasm. As this excites, the special pair relaxes and an electron is ejected, reducing the nearby bacteriopheophytin. From here the electron travels toward the cytoplasm where it eventually reduces quinone B near the cytoplasmic side of the membrane. The reduction of quinone B also consumes one proton from the cytoplasm. A second round of excitation of the special pair brings a second electron to quinone B that picks up another proton from the cytoplasm and diffuses away from the reaction center and into the quinone pool of the membrane. This reduced quinone then is oxidized at the cytochrome b/c1 complex in a similar fashion as to what is observed in oxidative phosphorylation. The reduction of quinone B and its oxidation at the cytochrome b/c1 complex results in the generation of a proton motive force. That is used to generate ATP using the ever-familiar ATP synthase we discussed earlier. The low energy electrons from the b/c1 complex are then donated to cytochrome c2, and finally end up reducing the Mg atom in the special pair to complete the cycle. This process is termed cyclic photophosphorylation because the electrons travel a close circuit. Contrast this with oxidative phosphorylation where the electrons are eventually donated to oxygen.