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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.
