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I‘ve read that 6 turns of the Calvin cycle are required to make 1 glucose molecule.

But, 3CO2 and 3RuBP are used in one cycle and 6 triose phosphate is produced. Only 5 triose phosphate molecules are required for RuBP regeneration. The remaining 1 triose phosphate has 3 carbon atoms in it. Only 3 more carbon atoms are needed, which can be produced again in the next cycle. So, it would seem that, in total, only 2 cycles are needed to produce 1 glucose molecule. Why do most authors write that six are needed 6?

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  • $\begingroup$ I still think you should state exactly what you mean by a "turn of the cycle", because that is what your (semantic) argument is about. However, as I don't find the answers so far address that point, I have done so myself. But in future, always, always, always clarify your terms. So much time is wasted by people not being clear what they are arguing about. $\endgroup$
    – David
    Commented Oct 11, 2021 at 15:00

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You're looking at a version of the Calvin cycle that has been simplified to consider 3 "turns" all together as the steps necessary to produce one 3-carbon product. A hint is that you see three CO2 molecules entering rather than one.

You can look for another diagram that has a one-carbon version, like at https://en.m.wikipedia.org/wiki/Calvin_cycle

Calvin cycle

(image credit: Mike Jones, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons)

However, arguably this diagram doesn't make clear that 2/3rds of the 3-phosphoglycerate gets recycled back into the Calvin cycle in the abstracted "phase 3".

So yes, you need 2*3 = 6 "turns" of the cycle to make a 6-carbon product. Importantly, though, it's not like there is some singular machine performing this cycle - all the steps are free to proceed on their own independently and simultaneously. The Calvin cycle (and the Krebs cycle in complement) are just the simplification down to the minimum individual reactions.

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    $\begingroup$ Please include a diagram in your answer, otherwise only those that already understand will be able to follow. Alternatively, comment and vote to close. The question is similarly incomprehensible. $\endgroup$
    – David
    Commented Oct 6, 2021 at 19:29
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    $\begingroup$ @David Added; I think the question is pretty clear. Textbooks often say you need "6 runs" through the Calvin cycle to make glucose; this makes a lot of sense because you're taking in one carbon through CO2, so to make a 6-carbon sugar you need to do something 6 times. OP also is looking at a picture showing "3CO2 and 3RuBP is used in one cycle and 6 triose phosphate is produced" - this is equivalent to three runs. That's a handy condensed form because you can consider one "free" triose phosphate to be made per 3 CO2; "free" as in it isn't used up in regenerating the reaction products. $\endgroup$
    – Bryan Krause
    Commented Oct 6, 2021 at 19:47
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Question and Scope of Answer

The question is about metaphors and semantics relating to the carbon stoichiometry of the reactions producing dihydroxyacetone phosphate from carbon dioxide. I have decided to provide a clarification of the carbon stoichiometry, ignoring hydrogen and oxygen stoichiometry (even though I find the reduction and energetics more interesting). The answer depends on what you consider the metaphor “turn of the cycle” means in relation to this sequence of reactions, and is of little interest or importance.

‘Answer’

The so-called dark reactions of photosynthesis produce one molecule of glucose (hexose) from six molecules of carbon dioxide in a two step process.

6 CO2 → 1 C6H12O6

In the first step — the so-called Calvin Cycle — three molecules of CO2 are converted to the triose, dihydroxyacetone phosphate (DHAP).

3 CO2 → 1 C3H7O3PO32–

In the second step, similar to the final part of gluconeogenesis, two DHAP molecules are converted to glucose.

2 C3H7O3PO32– → 1 C6H12O6

The reason the reactions of the Calvin Cycle are considered a metabolic cycle is that one of the intermediates, the pentose ribulose 1,5-bisphosphate (RuBP), is consumed by reaction with CO2 to give two molecules of triose:

1 ribulose 1,5-bisphosphate + 1 CO2 → 2 3-phosphoglycerate

The problem with the epithet cycle in this case is that it differs from the tricarboxylic and urea cycles where a single molecule enters the cycle and is converted to a molecule(s) leaving the cycle in a single series of reactions regenerating the molecule with which the input molecule first reacted. In the Calvin Cycle, the initial reaction between RuBP and CO2 has to occur three times for the stoichiometric production of one molecule of the triose, DHAP.

Many diagrams that represent the series of reactions as a cycle tend to be stoichiometrically incorrect (or more charitably, ignore carbon stoichiometry to emphasize energetics etc.), rendering the metaphorical expression “turn” of the cycle almost meanless. This is unfortunately the case for the diagrams in two other answers — one molecule of RuBP and one molecule of carbon dioxide clearly cannot generate one molecule of triose DHAP and regenerate the RuBP.

The diagram below from Berg et al. is stoichiometric as it considers the total number of molecules to produce one of DHAP. (The three arrows to the product, Ribulose 5-P, imply three molecules are produced.) I have modified the diagram by de-emphasizing other aspects of the reactions than inputs and outputs.

Calvin Cycle — stoichiometric

Conclusion

If you consider (as the anonymous source in the question and many others do) that each time one molecule of RuBP reacts with one molecule of CO2 a “turn of a cycle” is initiated, then six “turns” (or “rounds” à la Berg) are required to produce one molecule of glucose; if you consider each series of reactions that produces one molecule of the triose, DHAP, represents one “turn of a cycle” then two “turns” are required. I personally would avoid the metaphor, state what happens, and concentrate on the more interesting aspects of the process, which are the energetics and reductions using the products of the light reaction.

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  • $\begingroup$ @user338907 — Thanks. Corrected. $\endgroup$
    – David
    Commented Oct 11, 2021 at 18:06
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I think you didn't got Calvin cycle properly For ease of understanding, calvin cycle can be described under three stages; Carboxylation, Reduction, and Regeneration

  1. 1.CARBOXYLATION- It is the fixation of CO2 into stable organic intermediate. it is most crucial step of the calvin cycle where CO2 is utilised for the carboxylation of RuBP. This reation is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA.

  2. 2.REDUCTION- These are a series of reactions that lead to formqation of glucose. the steps involve utilisation of 2 molecules of ATP for phosphorylation and 2 of NADPH for reduction per CO2 molecule fixed.The fixation of 6 molecules of CO2 and 6 turns of the cycles are required for the formation of one molecule of glucose.

  3. 3.REGENERATION- Regenration of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. the regeneration steps require one ATP for phosphorylation to form RuBP... HERE you see only one molecule of CO2 is being used ... and our product i.e, Glucose is a hexose sugar(6-Carbon)... Therefore, we have to carryout calvin cycle six times... you can check for cycle on link https://en.wikipedia.org/wiki/Calvin_cycle .... HOPE IT HELPS enter image description here

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  • $\begingroup$ David, I hope now my answer is complete. Thankyou for your suggestion. $\endgroup$
    – factfile
    Commented Oct 10, 2021 at 14:07
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    $\begingroup$ Sorry, but I don't think the diagram addresses the question of carbon stoichiometry, which is what I think the question is about. I have added my own answer now, so I have deleted my previous comments. $\endgroup$
    – David
    Commented Oct 11, 2021 at 15:02

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