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Is it carbon dioxide or water? I'm talking about the oxygen present in glucose and not the oxygen that is released after photolysis of water.

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RuBisCO attaches one molecule of CO2 onto Ribulose-1,5-bisphosphate (RuBP). RuBP gets split to two molecules of 3-Phosphoglyceric acid (3-PGA). Via reduction to Glyceraldehyde-3-phosphate (GAP) the link to glycolysis is established. By doing the Glycolysis reactions backwards, you reach Glucose.

This is the reaction of RuBisCO (from humboldt.edu):

RuBisCO reaction

EDITED to correct: One molecule of RuBP, one molecule of CO2 and one molecule of water are converted to two molecules of 3-PGA. This compound then is either used to refill the Calvin cycle by production of new RuBP, or is inserted into glycolysis, where it can be converted to Glucose. So, as David correctly points out, the oxygen comes from water as well as from CO2.

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    $\begingroup$ Thank you @David, my bad, I missed that. I hope the edited answer is better. $\endgroup$
    – akraf
    Commented Jun 6, 2018 at 9:34
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    $\begingroup$ Fine. I'll remove my original comment. $\endgroup$
    – David
    Commented Jun 6, 2018 at 11:34

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