Flexi Says: Oxygen is released during the light reactions of photosynthesis. The light reactions capture energy from sunlight, which they change to chemical energy that is stored in molecules of NADPH and ATP. When light strikes chlorophyll (or an accessory pigment) within the chloroplast, it energizes electrons within that molecule. The excited electrons leave chlorophyll to participate in further reactions, leaving the chlorophyll “at a loss”; eventually they must be replaced. That replacement process also requires light, working with an enzyme complex to split water molecules. In this process of photolysis (“splitting by light”), H2O molecules are broken into hydrogen ions, electrons, and oxygen atoms. The electrons replace those originally lost from chlorophyll. Hydrogen ions and the high-energy electrons from chlorophyll will carry on the energy transformation after the light reactions are over. The oxygen atoms combine to form oxygen gas, which exits the leaf through the stomata.
