What did the later form of photosynthesis use as its source of electrons and hydrogen?

1. A later form of photosynthesis used water as a source of electrons and hydrogen. 2. That form generated free oxygen for the first time. 3. Over time the atmosphere became oxygenated as a result. 4. Cells could then use that oxygen to extract far more energy from sugar through the citric acid cycle and oxidative phosphorylation. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 7 "Cellular Respiration", section 7.6 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways_