What is one turn of the Calvin Cycle?

[deeps005]

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What's considered one turn of the Calvin Cycle? Most sources I've seen say it takes 6 turns to make one molecule of glucose. From this wouldn't it mean that every three turns gives you one molecule of PGAL since it takes 2 PGAL to get 1 glucose?

I just took the kaplan final that i got from a friend and one of the questions asks: what will 10 turns of the Calvin Cycle produce? they give the answer as 20 PGAL.

From kaplan's answer it makes me think that 1 turn of the calvin cycle gives 1 molecule of glucose. What do you guys think is right???? thanks!

 

[grettlin2]

1 PGAL. So, 1 molecule of glucose needs 6 turns.

 

[rsweeney]

One turn or the Calvin cycle produces 2 molecules of PGAL, a three carbon molecule. One molecule of CO2 enters the calvin cycle and reacts with RuBP (a five carbon molecule) to produce a six carbon molecule that quickly cleaves to become two molecules of PGAL (a three carbon molecule). In one turn, one of the PGALs is the net product that is turned into various things, for example a three carbon sugar. The other PGAL continues in the reaction to recycle the one RuBP that was already present at the beginning when one CO2 was added to keep the cycle going. For six turns of the Calvin cycle, 12 PGAL are formed---thats 36 carbons in all. Therefore, one molecule of glucose is formed (6 carbons), and the 30 remaining carbons, or the 10 other PGAL, are recycled into 6 RuBP molecules---b/c one RuBP was already present at the beginning of each cycle---so 6 cycles equals 6 RuBPs. Do not get confused when a book says "sugar" is formed versus when "glucose" is formed. Remember from orgo, you can have 3-carbon surgars, too, etc. Does this help? Good luck to you!

-Richard

 

[deeps005]

thanks! that clears things up...i was spending forever trying to figure it out...i'm taking the DAT in a week so I'm sure i'll be hittin' you guys up with more questions between now and then. thanks again!