Why not a pentose? Or a tetrose? Or a deoxy sugar? Or just some other hexose, like fructose?

Is there some chemical reason life should have settled on glucose as the standard photosynthetic output, or is it just random? It seems very suspicious that this ubiquitous molecule nicely break down into exactly 3CO2 + 3CH4, or 6CO2 + 6H2O - 6O2, or 6H2CO, when such convenient formulas are not true of the actual immediate products of carbon fixation.

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    $\begingroup$ Or why a sugar at all? CO(2) is the most oxidized form of a single carbon atom, and in photosynthesis carbon is reduced (where water is the source of electrons), but not fully reduced: the most reduced form of a C-6 compound is hexane, not glucose (and the most reduced form of a single carbon atom is methane) $\endgroup$
    – user338907
    Commented Jun 6, 2022 at 20:20
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    $\begingroup$ @user338907 The plants use the glucose too. It's not (just) a waste product. It isn't particularly helpful to extract the most amount of energy out of photosynthesis just to then waste way more than that when trying to build e.g. cellulose. Evolution doesn't select the "best" process in isolation, it's always part of a specific environment. Mind, that's not knowing where glucose came from - it's very likely that was long before photosynthesis developed. And of course, photosynthesis doesn't actually produce glucose - it produces GAP/G3P. That's quickly turned into glucose, usually. $\endgroup$
    – Luaan
    Commented Jun 7, 2022 at 7:13

1 Answer 1


Glucose is such a ubiquitous energy storage molecule, that we show the simple photosynthetic chemical equation to include the capture of six CO2 molecules to create one glucose molecule. However, the steps specific to photosynthesis, the light-dependent and light-independent reactions, don't produce glucose directly. Instead, they produce an intermediary molecule, the 3-carbon D-glyceraldehyde 3-phosphate (G3P), which can directly enter the general metabolic cycle, which includes gluconeogenesis. Gluconeogenesis includes a pathway from G3P to glucose.

So, photosynthesis itself doesn't produce glucose, but can be considered to add to the production of glucose because it is integrated into ubiquitous metabolic cycles.

There are existing hypotheses as to why glucose is so ubiquitous. One hypothesis is that glucose is less likely to nonspecifically interact with proteins than other molecules of its kind (aldohexoses) because it is more stable in its cyclic form (instead of the open-chain form) compared to other aldohexoses. Another hypothesis is that β-D-glucose has all five of its hydroxy groups in the equatorial position, making them more accessible to chemical modification, such as esterification, acetal formation, and linkage into polysaccharides.

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    $\begingroup$ Excellent answer. I would just emphasize that cellular life existed on Earth existed for at least a billion years before the evolution of photosynthesis. That's a long time for organisms to have refined the storage and utilization of organic molecules for energy. It makes a lot of sense that photosynthesis would evolve in a way that most efficiently feeds into these existing metabolic cycles. $\endgroup$
    – MikeyC
    Commented Jun 7, 2022 at 17:05
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    $\begingroup$ I'd always assumed that being a precursor for handy molecules like starch and cellulose would have made it quite an attractive choice for plants as well $\endgroup$
    – uhoh
    Commented Jun 7, 2022 at 21:07

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