Calvin Cycle Q

[ddasung]

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Can anyone explain to me why it takes 6 calvin cycles to produce 1 molecule of glucose?

Don't you feed in 3 molecules of CO2 into 1st cycle to produce 1 molecule of G3P and 2 cycles will produce 2 moleclues of G3P which will make up 1 molecule of glucose?

I can't quite understand how you feed each molecule of CO2 and at the end of 3rd cycle you get 1 molecule of G3P.

Thanks guys,

Ki-Yoon

 

[UBTom]

I hate the way some texts describe the Calvin cycle. They tend to lead one to believe that somehow 3 carbon dioxides enter 1 cycle, which is absolutely phooey.

The best way to look at it is stoichiometrically:

One CO2 at a time enters a cycle. After 3 Calvin cycles during which 3 CO2s are consumed, one *net* G3P is generated. This makes sense, since you put in 3 carbons (3 CO2s) and get one net 3-carbon molecule (1 G3P).

After 6 Calvin cycles during which 6 CO2s are consumed, two *net* G3Ps are generated and can be used to make one 6-carbon Glucose.

HTH!

 

[savvysearch]

Hmm. I understand it differtly from both of you.
here's how I made sense of it.

1 CO2 combine with 1 (5 carbon) RBP to produce 1 (6 carbon) 3PG. 3PG is very unstable so it splits into 2 (3carbon molecule) of G3P.

To sum it up, 1 cycle uses 1 CO2 to produce 2 G3P.

So, in 6 cycles, 12 G3P are formed.

Now the cycle can do either of two things.

The 12 G3P can combine to reform 6 RBP and in doing so, form 1 glucose molecule as a byproduct. Remember that glucose is a 6 carbon molecule.

G3P is a 3 carbon molecule.
3 x 12 = 36 total carbons
RBP is a 5 carbon molecule
6 x 5 = 30 total carbons
36 - 30 = 6 carbons which is your 6 carbon glucose molecule.
(this explains your 6 cycles for 1 glucose)

OR

if the plant is in immediate need of glucose, every two G3P can combine and rearrange itself to form glucose.
(notice this is a faster way of obtaining glucose, but no RBP is made and RBP is needed to continue the Calvin cycle)