Qvault Bio Question #3 - Calvin Cycle

[Member 8586]

How many molecules of ATP are needed during the Calvin Benson Cycle to produce a disaccharide such as sucrose?
Explanation 36 molecules of ATP are needed to produce a single disaccharide, such as sucrose, during the Carbon Benson Cycle. To produce a single 3-carbon G3P molecule a net total of 9 ATP, 3 CO2, and 6 NADPH must be consumed during the Calvin Benson Cycle. However, since the disaccharide sucrose contains 12 carbon atoms it should not be surprising that four molecules of G3P are needed for its synthesis. Therefore, 4 G3P/sucrose X 9 ATP/G3P = 36 ATP/sucrose.


I cant get the explanation to click....as i understood it...12 ATP and 12 NADPH are needed to make 12 G3P (according to Cliffs)....so hows it saying 9 ATP are needed to make 1 G3P?

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[Ari Rezaei]

18 ATP are needed to produce 1 glucose. 12 ATP and 12 NADH are needed to produce 12 G3P. 2 of these G3P go on to produce a monosaccaride, like glucose. However, we need to regenerate the rubisco (RuBp) we used at the beginning of the Calvin cycle (6 CO2 + 6 RuBp). To regenerate the RuBp, we use another 6 ATP to convert the remaining 10 G3P into 6 RuBp, ready for another cycle of carbon fixing into glucose.

So for a disaccaride, we need 2 moles of glucose, and hence 18 * 2 = 36 ATP. Let me know if you have any questions!

I should also add it takes 6 turns to produce 1 glucose. I just wrote all six turns as if it was one cycle. You'll see on the internet sometimes they have 3 CO2, or 1 CO2. It's all the same, just gotta multiply by the correct number to make six total turns.

 

[Member 8586]

18 ATP are needed to produce 1 glucose. 12 ATP and 12 NADH are needed to produce 12 G3P. 2 of these G3P go on to produce a monosaccaride, like glucose. However, we need to regenerate the rubisco (RuBp) we used at the beginning of the Calvin cycle (6 CO2 + 6 RuBp). To regenerate the RuBp, we use another 6 ATP to convert the remaining 10 G3P into 6 RuBp, ready for another cycle of carbon fixing into glucose.

So for a disaccaride, we need 2 moles of glucose, and hence 18 * 2 = 36 ATP. Let me know if you have any questions!

I should also add it takes 6 turns to produce 1 glucose. I just wrote all six turns as if it was one cycle. You'll see on the internet sometimes they have 3 CO2, or 1 CO2. It's all the same, just gotta multiply by the correct number to make six total turns.

thanks that makes a lot more sense...i think their explanation was confusing me...i ended up getting it right basically just with the rationale that it takes 18 ATP for glucose which is a monosaccharide so it must be 36 ATP for a disaccharide like sucrose....that works too right? lol

 

[Ari Rezaei]

That's all it is, 18 ATP for one glucose so times 2 for disaccharide. Their way is a more in depth understanding but still the same explanation. It's quite possible to follow it with the diagram in your cliffs book in front of you, try it.

18 ATP are needed to make 2 G3P, the precursor to glucose. Dividing you get 9 ATP for one G3P.