In two experiments, radioactively-labeled oxygen (shown in blue) was used to follow oxygen through the photosynthetic process. The results of the experiment are shown below:

The oxygen formed clearly comes from H₂O and not from CO₂, because photosynthesis in the presence of water labeled with O₁₈, produces oxygen labeled with O₁₈, whereas carbon dioxide labeled with O₁₈, does not give oxygen labeled with O₁₈.During oxygenic photosynthesis, light energy transfers electrons from water (H₂O) taken up by plant roots to CO₂ to produce carbohydrates. In this transfer, the CO₂ is "reduced," or receives electrons, and the water is "oxidized," or loses electrons. Oxygen is produced along with carbohydrates.Oxygen as a high-energy molecule increases ATP production from 4 ATP molecules to about 30 ATP molecules.

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