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In photosynthesis, the oxygen released into the atmosphere comes from

Awater (H₂O), split during light reactions
BCO₂ split during dark reactions
Cglucose, the final product
Datmospheric N₂ recycled by the chloroplast
Answer & Solution
Correct answer: A. water (H₂O), split during light reactions
Isotope tracing with ¹⁸O-labelled water confirms: the oxygen released comes from H₂O. The light reaction's photolysis splits water into H⁺, electrons, and O₂. CO₂ becomes the carbon backbone of glucose, but its oxygen ends up in the water products, not in the released gas.
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