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I think this question can be broken down into two sub-questions:

  1. Why do plants produce $C_6H_{12}O_6$ as opposed to another molecule following the formula of $C_nH_{2n}O_n$? Why not $C_8H_{16}O_8$, $C_4H_{8}O_4$, or any other of the many possible variations? Does it likely have something to do with stability or some other property that glucose has that many other variations on the formula do not?

  2. Why have they evolved to produce a molecule following the formula $C_nH_{2n}O_n$ at all? In short...why carbohydrates as opposed to another family of compounds?

My assumption is these are questions that have been researched extensively in biology and evolutionary biology and have at least tentative answers.

I'm looking for either your best hypothesis or a currently popular hypothesis within the relevant scientific disciplines. I understand there is likely no "definitive" answer, but probably at best pretty good hypotheses.

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  • $\begingroup$ Evolution had already worked out how to break down glucose for energy...why reinvent the wheel? $\endgroup$ – swbarnes2 Jul 26 at 18:04
  • $\begingroup$ I hope you get an interesting answer, but be aware that while "why" questions are easy to ask they are very difficult if not impossible to answer. Maybe there were multiple metabolisms and the glucose based one was better. Maybe glucose based metabolism happened to occur first and now we are stuck with it because it is too central to change. Assume you came up with a reasonable hypothesis — how would you test it? $\endgroup$ – tyersome Jul 26 at 23:33
  • $\begingroup$ @tyersome Perhaps you couldn't test it. I've amended the post to ask for "your best hypothesis or a currently popular hypothesis within the relevant scientific disciplines." I understand there is no "definitive" answer to this and I hope this addition to my question makes that clear. $\endgroup$ – n_bandit Jul 27 at 2:55
  • $\begingroup$ Merely a comment to add some figures to your question. CO2 is the most oxidized form of a single C atom. To fully reduce CO2 to methane requires 8 electrons, and the two O atoms are eliminated as H2O. To reduce six carbon dioxide to one glucose requires 12 reducing equivalents (24 electrons). If nature had decided to make hexane (CH3CH2CH2CH2CH2CH3) the photosynthetic storage product, this would have require 19 reducing equivalent (38 electrons), and if methane was the final photosynthetic product, then 4 reducing equivalents (eight electrons) per CO2 oxidized would have been required... $\endgroup$ – user1136 Jul 27 at 13:26
  • $\begingroup$ ... the question then becomes, why did nature not choose to to fully reduce CO2 especially as electrons 'held' in the C-O and O-H bonds have no 'calorific' value? (All the reducing equivalents passed to oxygen in aerobic respiration come from electrons 'held' in C-C and C-H bonds). Presumably solubility has something to do with it. --More (from me) here and here $\endgroup$ – user1136 Jul 27 at 13:37
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This is a brief answer (for me) to point out that:

Your question is not really concerned with photosynthesis but is actually about the choice of the carbon constituents of living cells.

First you talk about photosynthesis as if it is a single process, however it is two separable processes:

  1. The Light Reactions convert solar energy into chemical energy (in the form of ATP and NAD(P)H).

  2. The Dark Reactions use the chemical energy to reduce carbon dioxide and elaborate it to more complex molecules.

Photosynthesis is so elaborate chemically that it clearly was not the first process by which living organisms managed to convert environmental energy into a usable form. So the question becomes:

Q. Why is chemical energy trapped by living organisms used to make compounds such as glucose 6-P?

and giving what may at first sight seem to be a frivolous answer

A. Because glucose 6-P and its precursors and metabolites were suitable molecules for building the structural components of cells, that had arisen early in evolution.

So the question becomes:

Q. Why did the structural components, energy storage, genetic information etc. of cells evolve to employ the particular molecules that we find today, most of which can be formed from glucose metabolites?

And that is a very broad question about a very speculative area of chemical evolution, which cannot be summarized here.

In general one would imagine that the answer is related to the carbon components available in the environment and the possibilities of chemical transformations. However the trap is that one ends up arguing that things are the way they are because that was best. Which is all very well until one discovers dirty little secrets such as the fact that archaea evolved different chemical constituents for their cell membranes.

Footnote

My first degree was in chemistry. Since becoming a biochemist/molecular biologist I have almost never talked in terms of overall chemical formulae of molecules in the manner of the question. It has seldom been a useful way to approach biochemical reactions or intermediates. What has been more important is to look at their structures and ask how they lend themselves to their functions or interconversions.

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