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The Calvin Cycle: Converting CO2 to Sugar through ATP and NADPH, Slides of Biochemistry

The calvin cycle is a biochemical process that converts carbon dioxide (co2) into sugar using atp and nadph produced during the light reactions of photosynthesis. This process involves three main phases: carbon fixation, reduction, and regeneration of the starting material, rubp. For every three co2 molecules that enter the cycle, six three-carbon sugar molecules (glyceraldehyde 3-phosphate, or g3p) are produced, with a net gain of three carbons.

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The Calvin cycle uses ATP and NADPH to convert CO2 to sugar:
ATP and NADPH produced by the light reactions are used in the Calvin cycle
to reduce carbon dioxide to sugar.
• The Calvin cycle is similar to the Krebs cycle in that the starting material is
regenerated by the end of the cycle.
• Carbon enters the Calvin cycle as CO2 and leaves as sugar.
• ATP is the energy source, while NADPH is the reducing agent that adds
high-energy electrons to form sugar.
• The Calvin cycle actually produces a three-carbon sugar glyceraldehyde
3-phosphate (G3P).
For the Calvin cycle to synthesize one molecule of sugar (G3P), three
molecules of CO2 Must enter the cycle. The cycle may be divided into three
phases:
Phase 1: Carbon Fixation. The Calvin cycle begins when each molecule of
CO2 is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).
• This reaction is catalyzed by the enzyme RuBP carboxylase (rubisco) - the
most abundant protein on Earth!
• The product of this reaction is an unstable six-carbon intermediate that
immediately splits into two molecules of 3-phosphoglycerate.
• For every three CO2 molecules that enter the Calvin cycle via rubisco, three
RuBP molecules are carboxylated forming six molecules of
3-phosphoglycerate.
Phase 2: Reduction. This endergonic reduction phase is a two-step process
that couples ATP hydrolysis with the reduction of 3-phosphoglycerate to
glyceraldehyde phosphate.
• An enzyme phosphorylates 3-phosphoglycerate by transferring a phosphate
group from ATP. This reaction:
- produces 1, 3-bisphosphoglycerate.
- uses six ATP molecules to produce six molecules of
1,3-bisphosphoglycerate.
- primes 1,3-bisphosphoglycerate for the addition of high-energy
electrons from NADPH.
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The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar:

ATP and NADPH produced by the light reactions are used in the Calvin cycle to reduce carbon dioxide to sugar.

  • The Calvin cycle is similar to the Krebs cycle in that the starting material is regenerated by the end of the cycle.
  • Carbon enters the Calvin cycle as CO 2 and leaves as sugar.
  • ATP is the energy source, while NADPH is the reducing agent that adds high-energy electrons to form sugar.
  • The Calvin cycle actually produces a three-carbon sugar glyceraldehyde 3-phosphate (G3P).

For the Calvin cycle to synthesize one molecule of sugar (G3P), three molecules of CO 2 Must enter the cycle. The cycle may be divided into three phases:

Phase 1: Carbon Fixation. The Calvin cycle begins when each molecule of CO 2 is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).

  • This reaction is catalyzed by the enzyme RuBP carboxylase (rubisco ) - the most abundant protein on Earth!
  • The product of this reaction is an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate.
  • For every three CO 2 molecules that enter the Calvin cycle via rubisco, three RuBP molecules are carboxylated forming six molecules of 3-phosphoglycerate.

Phase 2: Reduction. This endergonic reduction phase is a two-step process that couples ATP hydrolysis with the reduction of 3-phosphoglycerate to glyceraldehyde phosphate.

  • An enzyme phosphorylates 3-phosphoglycerate by transferring a phosphate group from ATP. This reaction:
    • produces 1, 3-bisphosphoglycerate.
    • uses six ATP molecules to produce six molecules of 1,3-bisphosphoglycerate.
    • primes 1,3-bisphosphoglycerate for the addition of high-energy electrons from NADPH.

Page 2 of 2

  • Electrons from NADPH reduce the carboxyl group of 1,3-bisphosphoglycerate to the aldehyde group of glyceraldehyde 3-phosphate (G3P).
    • The product, G3P, stores more potential energy than the initial reactant, 3-phosphoglycerate.
    • G3P is the same three-carbon sugar produced when glycolysis splits glucose.
  • For every three CO 2 molecules that enter the Calvin cycle, six G3P molecules are produced, only one of which can be counted as net gain.
    • The cycle begins with three five-carbon RuBP molecules – a total of 15 carbons.
    • The six G3P molecules produced contain 18 carbons, a net gain of three carbons from CO 2.
    • One G3P molecule exits the cycle; the other five are recycled to regenerate three molecules of RuBP.

Phase 3: Regeneration of Starting Material (RuBP). A complex series of reactions rearranges the carbon skeletons of five G3P molecules into three RuBP molecules.

  • These reactions require three ATP molecules.
  • RuBP is thus regenerated to begin the cycle again.

For the net synthesis of one G3P molecule, the Calvin cycle uses the products of the light reactions:

  • 9 ATP molecules
  • 6 NADPH molecules

G3P produced by the Calvin cycle is the raw material used to synthesize glucose and other carbohydrates.

  • The Calvin cycle uses 18 ATP and 12 NADPH molecules to produce one glucose molecule.