Photosytem II utilizes a water-splitting manganese-calcium enzyme that when energized by sunlight catalyzes a four photon-step cycle of oxidation states (S0-to-S3). When S3 absorbs a photon, molecular oxygen (O2) is released and S0 is generated. S4 is a transient state on the way to S0. (Image courtesy of SLAC National Accelerator Laboratory)
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