Fig. 7: The Calvin Cycle

1. To begin the Calvin cycle, a molecule of CO2 reacts with a five-carbon compound called ribulose bisphosphate (RuBP) producing an unstable six-carbon intermediate which immediately breaks down into two molecules of the three-carbon compound phosphoglycerate (PGA). The carbon that was a part of inorganic CO2 is now part of the carbon skeleton of an organic molecule. The enzyme for this reaction is ribulose bisphosphate carboxylase or Rubisco. A total of six molecules of CO2 must be fixed this way in order to produce one molecule of the six-carbon sugar glucose.

2. The energy from ATP and the reducing power of NADPH (both produced during the light-dependent reactions) is now used to convert the molecules of PGA to glyceraldehyde-3-phosphate (G3P), another three-carbon compound. For every six molecules of CO2 that enter the Calvin cycle, two molecules of G3P are produced. Most of the G3P produced during the Calvin cycle - 10 of every 12 G3P produced - are used to regenerate the RuBP in order for the cycle to continue. Some of the molecules of G3P, however, are used to synthesize glucose and other organic molecules. Two molecules of the three-carbon G3P can be used to synthesize one molecule of the six-carbon sugar glucose. The G3P is also used to synthesize the other organic molecules required by photoautotrophs.

3. As mentioned in the previous step, most of the G3P produced during the Calvin cycle - 10 of every 12 G3P produced - are used to regenerate the RuBP so that the cycle may continue. Ten molecules of the three-carbon compound G3P eventually eventually form six molecules of the four-carbon compound ribulose phosphate (RP). Each molecule of RP then becomes phosphorylated by the hydrolysis of ATP to produce ribulose bisphosphate (RuBP), the starting compound for the Calvin cycle.

 


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