The relative numbers of the molecules involved are indicated on the flow diagram on the previous page. To make one glucose molecule, six C0₂ are combined with six RuDP to produce twelve molecules of 3PG. These in principle could be used to make six glucose molecules, but then the process would not be cyclic, and would grind to a halt as soon as all the RuDP was used up. Instead, only two of the molecules of 3PG are destined to end as glucose, while the other ten, which contain a total of thirty carbon atoms, continue around the Calvin cycle and eventually are converted into six molecules of five-carbon RuDP, ready for reuse.

The Calvin cycle as an adaptation of gluconeogenesis is a beautiful example of the subtlety that trial-and-error and three billion years of evolution are capable of. The breakdown of glucose to C0₂ requires all of the complex chemistry of the citric acid cycle. The synthesis of glucose from C0₂ avoids the necessity of running a citric acid cycle in reverse by the trick of using RuDP as a working molecule, and turning a linear gluconeogenesis pathway into a cycle. If our current ideas about the order in which various steps in metabolism evolved on Earth are correct, then at the time that gluconeogenesis was adapted for the purposes of the dark reactions of photosynthesis, life was still anaerobic and the citric acid cycle did not yet exist.