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The question is as following:

Which substance has the greatest contribution to the anabolism of glucose in plants. A- Oxygen (O2). B- Water (H2O). C- carbon dioxide (CO2).

The official answer given in this question was C, however I believe H2O and CO2 are equally important in their contribution, because glucose consists of 6 carbon atoms and 12 hydrogen atoms which respectively come from the 6 water molecules and the 6 carbon dioxide molecules needed for the light reaction and calvin cycle. Which step am I missing to the answer? This was by the way a high school biology national olympiad question in my home country, nl.

Kind regards, I'm looking forward to your response!

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    $\begingroup$ Water is not rate limiting in these reactions and is not actively consumed in these reactions, whereas carbon dioxide is fixed by an enzyme called rubisco. Have a look at the Calvin-Benson cycle. $\endgroup$
    – WYSIWYG
    Jul 18 '16 at 6:40
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    $\begingroup$ But sir how is water not actively consumed, don´t the 12 hydrogen atoms in glucose come from hydrolysis of water? I just came up with the idea that carbon has a far higher mass and thus contributes more to the amount of bio mass, could that be a correct explanation? $\endgroup$
    – Jay
    Jul 20 '16 at 11:01
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    $\begingroup$ I meant that water is in such high concentrations that it is neither considered a reactant nor a product. Its production or consumption does not affect its total concentration or any other rate of reaction where it is involved. $\endgroup$
    – WYSIWYG
    Jul 20 '16 at 12:02
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I think what they mean is that since water is split during the light reaction and oxygen is released, most of the mass of the glucose comes from carbon dioxide, because it contributes to the oxygen and carbon part of glucose whereas water only contributes to the hydrogen part which is only a small fraction of the mass

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