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Biochemistry - Calvin Cycle Question

  1. Mar 15, 2010 #1
    1. The problem statement, all variables and given/known data

    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?

    2. Relevant equations

    3. 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!
  2. jcsd
  3. Mar 15, 2010 #2


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    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?
  4. Mar 15, 2010 #3
    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!
  5. Mar 16, 2010 #4


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    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?
  6. Mar 20, 2010 #5
    had a question like this on bio exam. Remember that Carbon fixation is still occuring but the rest of the cycle is not.
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