text: The process starts when light is absorbed by two BChl molecules that lie near the periplasmic side of the membrane. This pair of chlorophyll molecules, often called the "special pair", absorbs photons at 870 nm or 960 nm, depending on the species and, thus, is called P870 (for Rhodobacter sphaeroides) or P960 (for Blastochloris viridis), with P standing for "pigment"). Once P absorbs a photon, it ejects an electron, which is transferred through another molecule of Bchl to the BPh in the L subunit. This initial charge separation yields a positive charge on P and a negative charge on the BPh. This process takes place in 10 picoseconds (10 seconds).
  The charges on the P and the BPh could undergo charge recombination in this state, which would waste the energy and convert it into heat. Several factors of the reaction center structure serve to prevent this. First, the transfer of an electron from BPh to P960 is relatively slow compared to two other redox reactions in the reaction center. The faster reactions involve the transfer of an electron from BPh (BPh is oxidized to BPh) to the electron acceptor quinone (Q), and the transfer of an electron to P960 (P960 is reduced to P960) from a heme in the cytochrome subunit above the reaction center.
  The high-energy electron that resides on the tightly bound quinone molecule Q is transferred to an exchangeable quinone molecule Q. This molecule is loosely associated with the protein and is fairly easy to detach. Two electrons are required to fully reduce Q to QH, taking up two protons from the cytoplasm in the process. The reduced quinone QH diffuses through the membrane to another protein complex (cytochrome bc-complex) where it is oxidized.  In the process the reducing power of the QH is used to pump protons across the membrane to the periplasmic space. The electrons from the cytochrome bc-complex are then transferred through a soluble cytochrome c intermediate, called cytochrome c, in the periplasm to the cytochrome subunit.