Photosystem II

Photosystem II is the photosystem responsible for splitting water. Light absorbed by PS-II ejects electrons from its reaction centre P680, leaving P680 oxidised; the electrons released ultimately end up at P700 in PS-I. To regain the electrons it lost, oxidised P680 pulls them from water, through the four-step photolysis reaction 4H₂O → 4H⁺ + 4OH⁻, 4OH⁻ → 4(OH) + 4e⁻, 4OH → 2H₂O + O₂ — net: 4H₂O → 2H₂O + O₂↑ + 4H⁺ + 4e⁻. This is the single reaction that releases the oxygen plants give off during photosynthesis. The protons produced accumulate inside the thylakoid lumen, building a proton gradient across the thylakoid membrane; it is the en …