In the overall equation of photosynthesis, carbon dioxide and water become carbohydrate and oxygen gas. From which reactant do the atoms of the released O2 come?

Core Concept

Photosystem II replaces the electrons lost from excited chlorophyll by pulling four electrons out of two water molecules. This splitting reaction occurs at the manganese containing oxygen evolving complex, which liberates O2 as a waste product and deposits protons in the thylakoid lumen. Isotope experiments using heavy oxygen confirmed that every molecule of released O2 traces back to water rather than to CO2.