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 chloroplast is involved in both stages of photosynthesis. The light reactions take place in the thylakoid. There, water (H2O) is oxidized, and oxygen (O2) is released. The electrons that freed from the water are transferred to ATP and NADPh

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throwdolbeau throwdolbeau · Answer 2 · 2021-12-30T12:58:34+01:00

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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