As a result of the transition from daytime to nighttime, how will the relative concentrations of glyceraldehyde 3-phosphate (G3P) and ribulose 1,5-bisphosphate (RuBP) change in C3 plants?

Question

A.  The concentrations of G3P and RuBP would remain the same.
B.  The concentrations of G3P and RuBP would both decrease.
C.  G3P would increase and RuBP would decrease.
D.  G3P would decrease and RuBP would increase.

Clarify Question
What is the key concept addressed by the question?
What type of thinking is required?

  Gather Content
What do you already know about G3P and RuBP? How does C3 plant photosynthesis change from day to night?
   Choose Answer
Do you have all the information needed to determine which molecule would be affected most?

  Reflect on Process Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

C.  G3P would increase and RuBP would decrease.

Clarify Question
What is the key concept addressed by the question?
        · The question asks you to determine how the relative concentrations of G3P and RuBP change as C3 plants transition from day to night.
What type of thinking is required?
        · You are being asked to dissect, or analyze, C3 plant daytime to nighttime transition to determine how G3P and RuBP concentrations will change.

  Gather Content
What do you already know about G3P and RuBP? How does C3 plant photosynthesis change from day to night?
        · Glyceraldehyde-3-phosphate is one of two 3-carbon molecules produced at the beginning of the Calvin cycle. Initially, a single carbon dioxide molecule will be bound to RuBP, a 5-carbon molecule, by the enzyme rubisco; the resulting six-carbon molecule will split into two 3-carbon molecules of G3P.
        · Under daylight conditions, the light reactions of photosynthesis will produce ATP and NADPH, which are then used by the Calvin cycle to fix carbon dioxide. Recall that producing each 3-carbon G3P molecule requires three turns of the Calvin cycle.
   Choose Answer
Do you have all the information needed to determine which molecule would be affected most?
        · First let’s consider what happens to G3P and RuBPin the Calvin cycle during the day when there is plenty of light available. RuBP, a 5-carbon molecule, accepts a single carbon dioxide from the atmosphere,
        · Let’s dissect the Calvin cycle a little bit to see which molecule would be affected more than the others. There are three phases to the Calvin cycle. During the first phase, carbon fixation, RuBP binds to a carbon dioxide molecule and forms a six carbon molecule that is then converted into two molecules of 3-phosphoglycerate. The next phase of the Calvin cycle, reduction, serves to convert 3-phosphoglycerate to glyceraldehyde-3-phosphate. The final phase of the Calvin cycle, regeneration, requires RuBP to be remade; otherwise the mechanism to accept further carbon dioxide does not exist.
        · Regeneration of RuBP is heavily dependent on G3P, since it takes 5 G3P molecules to produce three RuBP molecules. So how can we use this information to answer the question?
        · During the night, neither the light reactions nor the Calvin cycle operate. The ATP and NADPH required to convert G3P into RuBP has decreased; therefore the level of RuBP would have to go down. Since multiple G3P molecules are needed to regenerate RuBP, the concentration of G3P would necessarily have to go up as a result.

  Reflect on Process Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        · Answering this question correctly depended on your ability to analyze C3 plant daytime to nighttime transition for changes in G3P and RuBP concentrations.
        · If you got the correct answer, good job! If you got an incorrect answer, did you recall the different phases of the Calvin cycle? Were you able to identify the key molecules involved? Were you able to determine how G3P and RuBP concentration would change?