PSII also relies on light to drive the formation of proton gradients in chloroplasts, however, PSII utilizes vectorial redox chemistry to achieve this goal. Rather than physically transporting protons through the protein, reactions requiring the binding of protons will occur on the extracellular side while reactions requiring the release of protons will occur on the intracellular side. Absorption of photons of 680nm wavelength is used to excite two electrons in P to a higher energy level. These higher energy electrons are transferred to protein-bound plastoquinone (PQ)  and then to unbound plastoquinone (PQ). This reduces plastoquinone (PQ) to plastoquinol (PQH) which is released from PSII after gaining two protons from the stroma. The electrons in P are replenished by oxidizing water through the oxygen-evolving complex (OEC). This results in release of O and H into the lumen, for a total reaction of
After being released from PSII, PQH travels to the cytochrome bf complex, which then transfers two electrons from PQH to plastocyanin in two separate reactions. The process that occurs is similar to the Q-cycle in Complex III of the electron transport chain. In the first reaction, PQH binds to the complex on the lumen side and one electron is transferred to the iron-sulfur center which then transfers it to cytochrome f which then transfers it to plastocyanin. The second electron is transferred to heme b which then transfers it to heme b which then transfers it to PQ. In the second reaction, a second PQH gets oxidized, adding an electron to another plastocyanin and PQ. Both reactions together transfer four protons into the lumen.