The reactions leading to the synthesis of glucose from C0₂ are shown on the right. The most remarkable feature about this scheme is that the steps from 3PG (3-phosphoglycerate) to glucose have been lifted bodily from the gluconeogenesis pathway (Page 22), with the same intermediates, same enzymes, and same input of carrier molecules and ejection of phosphate. A pre-existing set of reactions and enzymes has been "borrowed" and put to use at another place (inside chloroplasts) for another purpose.

The object of gluconeogenesis is only to make glucose from pyruvate, whereas photosynthesis must begin with a much less reduced compound, C0₂. How can a set of reactions designed to commence with a three-carbon molecule be adapted to work with a one-carbon molecule? The answer is simple and elegant: Combine the C0₂ with a five-carbon sugar, then cleave the product in half to obtain two three-carbon starting molecules. This plan will work forever if some of the intermediates are shunted off the glucose-synthesis track and used to make enough five-carbon sugar to start the process over again with more C0₂.

This is exactly what has been done in the dark reactions. A portion of a linear process has been turned into one leg of a cycle, known as the Calvin cycle (right) after its discoverer, Melvin Calvin. The five-carbon sugar that keeps the cycle turning is ribulose-1',5’-diphosphate (RuDP). Adding C0₂ and H₂0 to RuDP and cleaving the result in half leads to two molecules of 3PG, an intermediate in gluconeogenesis.