Biochemistry - Calvin Cycle Question

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Discussion Overview

The discussion revolves around the effects of light cessation on the levels of 3-phosphoglycerate and Ribulose 1,5-Bisphosphate in the Calvin Cycle during photosynthesis. It explores the biochemical processes involved and the dependencies on light-driven reactions.

Discussion Character

  • Homework-related
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant suggests that the levels of both 3-phosphoglycerate and Ribulose 1,5-Bisphosphate would decrease due to the reliance on CO2 from light-dependent reactions.
  • Another participant challenges this view, stating that 3-phosphoglycerate levels might increase instead and asks for clarification on this point.
  • A participant corrects an earlier statement about the light-dependent reactions, identifying ATP and NADPH as the key products and asserting that Ribulose 1,5-Bisphosphate levels would decrease without light.
  • It is noted that ATP and NADPH are not required to produce 3-phosphoglycerate, leading to uncertainty about how light cessation affects its levels.
  • Another participant prompts consideration of the reactions that produce and consume 3-phosphoglycerate, questioning which are dependent on ATP and NADPH.
  • A final contribution references a similar question encountered in a biology exam, indicating that carbon fixation may still occur even if other parts of the cycle are inactive.

Areas of Agreement / Disagreement

Participants express differing views on the effects of light cessation on 3-phosphoglycerate and Ribulose 1,5-Bisphosphate levels, with no consensus reached on the outcomes.

Contextual Notes

There are unresolved aspects regarding the specific reactions that utilize or produce 3-phosphoglycerate and how they relate to the presence or absence of ATP and NADPH.

persephone
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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!
 
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persephone said:
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?
 
Ygggdrasil said:
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!
 
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?
 
had a question like this on bio exam. Remember that Carbon fixation is still occurring but the rest of the cycle is not.
 

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