Oxygen is released when water molecules are split. The hydrogens are used to ultimately replace those ejected from chlorophyll; the leftover oxygen is a "waste product." This was discovered during the 1940s, using water tagged with a radioisotope of oxygen.
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Yes, it does come from water as opposed to the most common answer which would be CO2. This is with regards to photosynthesis, right?
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Water, NOT CARBON DIOXIDE.