The source of oxygen liberated as a byproduct during photosynthesis is

CONCEPT: Photosynthesis has two linked stages: the light-dependent reactions in the thylakoid membrane, and the light-independent (Calvin cycle) reactions in the stroma. In the light-dependent reactions, water molecules are split by light energy at Photosystem II (photolysis) into electrons, protons (H+), and oxygen gas, which escapes as the process's gaseous byproduct.

APPLICATION: So the oxygen released during photosynthesis comes from splitting water, not from carbon dioxide, glucose, or chlorophyll:

• Glucose (C6H12O6) is the sugar ultimately built from G3P, the Calvin cycle's direct carbon-fixation product — it is a downstream output of photosynthesis, not a source of the released gas.

• Chlorophyll is the light-absorbing pigment: antenna chlorophyll molecules capture light energy and pass that excitation energy (not electrons) to the reaction-centre chlorophyll, which then donates an excited electron into the electron transport chain — chlorophyll itself is not split apart to release oxygen.

• Carbon dioxide supplies the carbon that is fixed into sugars; its own oxygen atoms are incorporated into organic molecules and water during fixation, not released as free O2.

CROSS-CHECK: This is confirmed independently by isotope-tracing experiments (extending C. B. van Niel's hypothesis): when plants were supplied water containing the heavy oxygen isotope (H218O), the O2 they released was isotope-labelled; when instead supplied CO2 containing the same heavy isotope, the released O2 carried no label. This proves the released oxygen originates from water.

Hence, the source of the oxygen liberated as a byproduct of photosynthesis is water.