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6.3 outline the main steps of the Calvin cycle as: the production of phosphoglycerate from the combining of carbon dioxide with an acceptor molecule the reduction of phosphoglycerate into glyceraldehyde phosphate in two reactions that use ATP and NADPH produced in the light reactions the regeneration of the initial carbon dioxide
outline the main steps of the Calvin cycle as:
- the production of phosphoglycerate from the combining of carbon dioxide with an acceptor molecule
- the reduction of phosphoglycerate into glyceraldehyde phosphate in two reactions that use ATP and NADPH produced in the light reactions
- the regeneration of the initial carbon dioxide acceptor
- The earliest labeled product from Calvin’s experiments was phosphoglycerate (3-phosphoglycerate or 3PG) formed by the addition of CO2 to a 5-carbon acceptor. The resulting 6-carbon compound is broken into two molecules.
CO2 + 5-carbon acceptor → [6-carbon intermediate] → two phosphoglycerate.
- In the next step a phosphate group from ATP is added to each molecule of phosphoglycerate (3PG) to form an intermediate called 1,3-diphosphoglycerate.
- In the next step a pair of electrons donated by NADPH reduces this intermediate to form glyceraldehyde phosphate.
- Glyceraldehyde phosphate is a 3-carbon sugar, not the 6-carbon sugar glucose generally identified as the end product of photosynthesis. Glyceraldehyde phosphate leaves the Calvin cycle and is converted to glucose and stored as starch in the stroma of the chloroplast or used in other reactions in the cytosol to make sucrose for transporting to other parts of the plant.
- The regeneration of the initial 5-carbon acceptor in the Calvin cycle avoids wasteful reactions that use large amounts of ATP and NADPH and allows continuous CO2 fixation. In order to regenerate the 5-carbon acceptor the cycle runs three times.
3C5 + 3C1 > 3C6 (unstable) > 6C3.
One of these C3 molecules leaves the cycle and the other five C3 molecules are regenerated to form 3 molecules of the C5 acceptor.
Counting C atoms: |
3C x 5 + 3C x 1 = 18C
3C x 1 leaves the cycle = 3C
3C x 5 regenerated = 15C
|
3CO2 + 9ATP + 6NADPH + water → glyceraldehyde phosphate + 8Pi + 9ADP + 6NADP+.
The ATP and NADPH come from the light dependent reactions, the inorganic phosphate (Pi), the adenosine diphosphate (ADP) and nicotinimide adenosine diphosphate are recycled to the light reactions