Biochemistry - Calvin Cycle Question

[persephone]

Homework Statement

An illuminated suspension of chloroplasts is actively carrying out photosynthesis. What would happen to the levels of 3-phosphoglycerate and Ribulose 1,5-Bisphosphate if the light was suddenly switched off?

Homework Equations

The Attempt at a Solution

I thought that the answer would be that the levels of 3-phosphoglycerate and Ribulose 1,5-Bisphosphate would decrease because the CO2 that is fed into the Calvin Cycle to make these compounds comes from the light-dependent reactions. Also, 3 of the enzymes in the stroma that regulate the Calvin Cycle are light-dependent as well.

However, someone told me that the 3-phosphoglycerate levels would increase instead. I was wondering if someone could please tell me why this would happen?

Thanks in advance!

 

[Ygggdrasil]

the CO2 that is fed into the Calvin Cycle to make these compounds comes from the light-dependent reactions.

This statement is incorrect. There are two important molecules that are produced by the light-dependent reactions. What are they and how might removing them affect the Calvin cycle?

 

[persephone]

This statement is incorrect. There are two important molecules that are produced by the light-dependent reactions. What are they and how might removing them affect the Calvin cycle?

Sorry, I don't know why I said that. The molecules that are produced by the light-dependent reactions are ATP and NADPH. I know that you need both to make Ribulose 1,5-Bisphosphate in the Calvin Cycle, so that means shutting off the light would decrease the levels of Ribulose 1,5-Bisphosphate.

However, ATP and NADPH are NOT needed to make 3-Phosphoglycerate. So I am still not sure how shutting off the lights would effect the levels of this compound...If you could please clarify this for me I would really appreciate it!

 

[Ygggdrasil]

Think of the reactions that produce 3-PG and those that use up 3-PG. Which reactions are dependent on ATP and NADPH and which ones do not require these compounds?

 

[Hockeystar]

had a question like this on bio exam. Remember that Carbon fixation is still occurring but the rest of the cycle is not.