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In the light-independent reaction of photosynthesis, one of the products is glyceraldehyde 3-phosphate, and the Wikipedia page on the light-independent reactions states that 6 of these can be used to form glucose. However, the Wikipedia article on gluconeogenesis does not mention this, and the Wikibooks article on gluconeogenesis only mentions the molecule glycerinaldehyde 3-phosphate (which I'm not sure if it's a typo, because I can't find any information on glycerinaldehyde).

So how does G3P actually become glucose (and is there a good reason this information is not available on those Wikipedia pages, plus the page on G3P)?

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Advice to students of biochemistry

This site is concerned with biology, not with biological entries in Wikipedia. Wikipedia is a voluntary effort to which anyone may contribute, and is full of errors and omissions. It’s structure means that it is focussed on individual small topics, rather than presenting an integrated account of various areas of science. The student who wishes a balanced integrated account of a topic that has been subjected to editorial review should consult a text book. Those whose resources do not permit this should try searching on NCBI Bookshelf, which provides free on-line search-only access to old editions of texts. For biochemistry, Berg et al. is recommended.

The generation of glucose from triose produced in the dark reaction is well understood

The answer to the question can be found in e.g. Berg et al. 20.1.3:

The 3-phosphoglycerate product of rubisco is next converted into three forms of hexose phosphate: glucose 1-phosphate, glucose 6-phosphate, and fructose 6-phosphate... The steps in this conversion (Figure 20.9) are like those of the gluconeogenic pathway (Section 16.3.1), except that glyceraldehyde 3-phosphate dehydrogenase in chloroplasts, which generates glyceraldehyde 3-phosphate (GAP), is specific for NADPH rather than NADH. Alternatively, the glyceraldehyde 3-phosphate can be transported to the cytosol for glucose synthesis.

Stryer Fig 20.9

Berg et al. Fig. 20.9

The Section 16.3.1 referred to is a general treatment of gluconeogenesis, which is the same in all organisms or cells that possess the enzymes to catalyse its unique steps. (Some cells may not have — or need to have — the initial steps from pyruvate. In fact, Fig. 20.9 shows the steps to G 6-P — the only step missing is the phosphatase step that generates glucose.) There is therefore no reason to expect an account specific for plants.

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  • $\begingroup$ Nice! That paragraph from Berg was exactly what I was looking for. +1 $\endgroup$ – Roland Oct 26 '17 at 17:34
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Glucose is made from the trioses (3-carbon sugars) in plants according to the usual gluconeogenesis pathway. That is, glyceraldehyde phosphate is converted to fructose-1,6-diphosphate by triose phosphate isomerase and aldolase, and then dephosphorylated to obtain hexose phosphates. Free glucose is not usually the end product in plants though; instead, glucose is coupled to ADP for use in synthesis of starch.

I'm afraid I did not find any good open-access references on plant gluconeogenesis, but it should be covered in most biochemistry textbooks.

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  • $\begingroup$ The tricky part of gluconeogenesis is going from pyruvate to phosphoenolpyruvate. If you're starting with glyceraldehyde 3-phosphate, going backwards to glucose is easy. It's just the normal (reversible) glycolysis enzymes, used in reverse. (So if you're looking for references, look for those which say that that part of glycolysis is reversible.) -- This probably explains why it's not mentioned explicitly. People intimately familiar with central metabolism would think it "obvious" that it can run backwards. $\endgroup$ – R.M. Oct 25 '17 at 20:33
  • $\begingroup$ @R.M. Actually, glycolysis "above" glyceraldehyde phosphate is in fact not reversible, due to the phosphofructokinase step, which must be bypassed by fructose bisphophatase to be energetically favorable. $\endgroup$ – Roland Oct 25 '17 at 20:47
  • $\begingroup$ @RM — The ”tricky step“ is hardly relevant to this question as we are starting from glyceraldehyde 3-phosphate, not pyruvate. Remember that cells such as fat cells that use the pentose shunt to generate NADPH can return G 3-P to glucose via the latter half of the gluconeogenic pathway but do not possess the enzymes for using pyruvate as a gluconeogenic substrate. $\endgroup$ – David Oct 26 '17 at 13:20

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