The oxygen produced in photosynthesis originates from which molecule?

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Multiple Choice

The oxygen produced in photosynthesis originates from which molecule?

Explanation:
In the light-dependent reactions, water is split to supply electrons for the electron transport chain. The oxygen gas released comes from this water, not from carbon dioxide or the sugars later formed. The key step is 2 H2O -> O2 + 4 H+ + 4 e-. Isotope labeling experiments show that the oxygen in the evolved O2 stems from water. Meanwhile, carbon dioxide is fixed into organic sugars, so its oxygen ends up in those molecules rather than in the free oxygen gas.

In the light-dependent reactions, water is split to supply electrons for the electron transport chain. The oxygen gas released comes from this water, not from carbon dioxide or the sugars later formed. The key step is 2 H2O -> O2 + 4 H+ + 4 e-. Isotope labeling experiments show that the oxygen in the evolved O2 stems from water. Meanwhile, carbon dioxide is fixed into organic sugars, so its oxygen ends up in those molecules rather than in the free oxygen gas.

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