Photosystem II (image) DOE/Lawrence Berkeley National Laboratory Caption 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 So is generated. S4 is a transient state on the way to So. Credit SLAC Linear Accelerator Center Usage Restrictions None Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.