5.4.2 The Calvin Cycle: RuBP Regeneration

Question 1

note

Answer 1

• In the Calvin cycle, carbon dioxide is fixed onto ribulose bisphosphate (RuBP) by the enzyme rubisco to form 3-phosphoglycerate. 3-phosphoglycerate is then phosphorylated and reduced to eventually give rise to
  glyceraldehyde 3-phosphate (G3P). RuBP is then regenerated through a complex series of reactions in order to continue the cycle.
  • For the synthesis of one glyceraldehyde 3-phosphate the Calvin cycle must make three turns.
• In order to produce one glyceraldehyde 3-phosphate, the cycle must make three turns. Let’s follow the numbers of carbons throughout the cycle.
• Starting in phase one of the diagram, if three carbon dioxide (CO2) molecules enter the cycle, they must be fixed to three five-carbon ribulose bisphosphate molecules (RuBP). The three new six-carbon molecules that result from this addition then split apart to make six three-carbon molecules called 3-phosphoglycerate.
• In phase two of the Calvin cycle, the 3-phosphoglycerate molecules get reduced to form
  glyceraldehyde 3-phosphate (G3P). One G3P molecule is released from the cycle for use by the plant cell, leaving five behind. Through a complex series of reactions, in phase three of the cycle, five three-carbon G3P molecules are used to regenerate three five-carbon RuBP molecules. And the cycle continues….

Question 2

The reactions of the Calvin cycles are referred to as light-independent reactions, yet in most plants they occur during daylight hours. Why?

Answer 2

• The Calvin cycle depends on ATP and NADPH from the light-dependent reactions

Question 3

In the Calvin cycle, _______ ATP’s and ________ NADPH’s are required for a net output of two molecules of glyceraldehyde 3-phosphate.

Answer 3

• 18; 12

Question 4

Which of the following molecules represents the end-product of the Calvin cycle?

Answer 4

• glyceraldehyde 3-phosphate

Question 5

In the regeneration phase of the Calvin cycle 10 glyceraldehyde 3-phosphates will be regenerated into __________ RuBP.

Answer 5

• 6

Question 6

Which of the following is not a stage in the Calvin cycle?

Answer 6

• carbon regeneration