When we study calvin cycle, we study it with 6 molecules so as to form 1 glucose in a single cycle as:

6 RuBP + 6 CO2 => 12 3-PGAL => 1 Glucose + 6 RuMP

But if we look deep, then we know that the molecules are synthesized one-by-one & 6 cycles give 1 glucose, but:

1 RuBP + 1 CO2 => ... => 1/6 Glucose + 1 RuMP

My question: What is actually this 1/6 glucose (organic carbon fixed from inorganic carbon and how are such 6 molecules joined together?


It's not really possible to break it down this way. The CO2 fixated by the RUBISCO enzyme generates two phosphoglycerate (2 x 3C) molecules from one ribulose bisphophate molecule (5C), while the rest of the Calvin cycle serves to regenerate 3 ribulose bisphophate from 5 phosphoglycerate. Hence, one 3-carbon sugar (glyceraldehyde phosphate or dihydroxyacetone phosphate) can exit the cycle for every 3 CO3 captured. The regeneration of ribulose bisphosphate involves several enzymes, and the stoichiometry is rather complicated; it would take an entire essay to describe it all. I suggest you look the calvin cycle in the metacyc database, it's a nice interactive resource where you can explore all the details of the reactions. Wikipedia also provides some details on these reactions.

The glyceraldehyde phosphate produced by the Calvin cycle is then converted to glucose and other sugars by the enzymes of gluconeogenesis. The key enzyme is fructose diphosphate aldolase (or simply "aldolase") which condenses glyceraldehyde phosphate and dihydroxyacetone phosphate to one fructose-1,6-diphosphate. A chart of the pathway leading to other sugars is found here.

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  • $\begingroup$ Glucose (and other hexoses) are not products of the Calvin cycle. Glyceraldehyde-phosphate (a triose) is the immediate product the cycle; 2 of these can be condensed to fructose-2,6-biphophate by aldolase, and from there on to other hexoses, like glucose. $\endgroup$ – Roland Feb 15 '16 at 17:20
  • $\begingroup$ You were asking about the "1/6 of glucose" and how it derives from CO2; this is via glyceraldehyde-phosphate. No, I did not imply that 2 CO2 makes 1 glucose, that's obviously not correct; but 2 glyceraldehyde-phosphate makes 1 glucose. And for net production of 1 triose, 3 CO2 must be captured. I don't think it makes much sense to talk about "turns" of the cycle, I hear this often about metabolic cycles, but I think it's better to just look at the stoichiometry of the reactions. (It's not actually a wheel :) I will extend the answer with the aldolase part. $\endgroup$ – Roland Feb 16 '16 at 7:12

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