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 680 to a higher energy level. These higher energy electrons are transferred to protein-bound plastoquinone (PQ A) and then to unbound plastoquinone (PQ B). This reduces plastoquinone (PQ) to plastoquinol (PQH 2) which is released from PSII after gaining two protons from the stroma. The electrons in P 680 are replenished by oxidizing water through the oxygen-evolving complex (OEC). This results in release of O 2 and H into the lumen, for a total reaction of 4 h ν ν + 2 H 2 O + 2 PQ + 4 H + ( stroma ) ⟶ ⟶ O 2 + 2 PQH 2 + 4 H + ( lumen )