After being released from PSII, PQH 2 travels to the cytochrome b 6 f complex, which then transfers two electrons from PQH 2 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 2 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 L which then transfers it to heme b H which then transfers it to PQ. In the second reaction, a second PQH 2 gets oxidized, adding an electron to another plastocyanin and PQ. Both reactions together transfer four protons into the lumen.