The second stage of photosynthesis is the Calvin cycle, also called the Calvin-Benson cycle. The cycle is named for its discoverers, Melvin Calvin and Andrew Benson. The Calvin cycle is an anabolic process (a chemical reaction that synthesizes molecules in metabolism) that builds the molecules required to make glucose, a six-carbon sugar that is the product of reactions following the Calvin cycle. The cycle uses energy to build a large molecule from smaller ones. The cycle takes place in the stroma (the fluid inside chloroplasts, but outside the thylakoids) in three main steps and can occur in either the presence or the absence of light. That is why this stage of photosynthesis is often referred to as a series of light-independent reactions, or dark reactions.
The first step is known as carbon fixation because it takes in CO2 from the atmosphere and fixes it into organic molecules that can be used by living things. In this step, the enzyme ribulose bisphosphate carboxylase-oxygenase, rubisco, adds one carbon to a five-carbon sugar called ribulose 1,5-bisphosphate (RuBP) during carbon fixation. This forms a six-carbon sugar that is energetically unstable, meaning it cannot hold its form. It immediately splits into two three-carbon molecules called 3-phosphoglycerate (3-PGA).
In the second step, each molecule of 3-phosphoglycerate receives a phosphate group from ATP, forming 1,3-bisphosphoglycerate. Since the process changes ATP to ADP, it is endergonic (uses energy). Then NADPH donates a pair of electrons to 1,3-bisphosphoglycerate, which reduces the molecule. Recall that both ATP and NADPH are molecules produced during the light reactions. NADPH reduces, or donates electrons, to 1,3-bisphosphoglycerate forming glyceraldehyde 3-phosphate (G3P). G3P is a three-carbon sugar formed in the Calvin cycle that is a precursor to glucose. Importantly, for every three molecules of CO2 that enter the cycle, six molecules of G3P are formed. However, five of these G3P molecules will be recycled into RuBP in the next step. Only one molecule of G3P leaves the cycle, resulting in a net production of one G3P for every three CO2 fixed.
The final step is the regeneration of RuBP from the five molecules of G3P in the previous step. This final step involves a complex series of reactions that rearrange the carbon atoms into the five-carbon sugar, consuming an additional three ATP in the process. At the end of this step, RuBP is again ready to receive CO2, and the cycle begins again. Since only one net G3P is produced from a single turn of the cycle, it takes six turns of the cycle to produce a single molecule of glucose.