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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
  • $\begingroup$ Water is consumed in photosynthesis, it is the source of electrons for all of the C-C and C-H bonds in glucose, forming O(2) as a 'waste' product, and it is these reducing equivalents that are 'burned' in aerobic respiration. CO(2) merely provides the C skeleton. Glucose is an incomplete C-6 reduction product of CO(2). Hexane is the fully reduced C-6 compounds. Also, although Rubisco 'fixes' CO(2) no net synthesis of a C-C bond occurs (except fleetingly). In PS, reducing equivalents enter the carbon skeleton in the GAPdh rxn, and C-C bonds are formed by aldol condensation by aldolase. $\endgroup$ – user1136 Oct 21 '18 at 22:34
  • $\begingroup$ The fact that water is present at high concentration (I presume you mean 55M, but as Jencks points out, the effective conc may be much, much higher in an enzyme active site), this does not in any way preclude a step involving water being rate determining. The step may (as is the case in PS) be energetically very unfavourable. In addition, the fact that changes in water concentration may be ignored (eg in a hydrolyse reaction) does not mean that water is not consumed in the reaction: it may be possible (and this has been done many times) to show this isotopically. $\endgroup$ – user1136 Oct 21 '18 at 22:44
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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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