Final answer:
Scientific experiments using oxygen isotopes have determined that the oxygen released during photosynthesis comes from water, indicated by a matching isotopic composition.
Step-by-step explanation:
We know that the O2 released during photosynthesis comes from water and not carbon dioxide through the use of oxygen isotopes in experiments. The oxygen atoms in water molecules have a different isotopic composition compared to those in carbon dioxide. During photosynthesis, scientists have observed that the oxygen gas released into the atmosphere matches the isotopic composition of the oxygen atoms found in water, not carbon dioxide. This discovery helped confirm that the source of the oxygen produced during photosynthesis is indeed water. Furthermore, the overall chemical equation for photosynthesis is 6CO2 + 6H2O → C6H12O6 + 6O2, clearly showing the involvement of water in the production of oxygen.