The Calvin cycle is a light independent ("dark-reaction") process of carbon fixation that takes place in the chloroplast. The Calvin cycle is also called the ribulose bisphosphate pathway (RuBP) for the initial enzyme in CO2 fixation, ribulose bisphosphate carboxylase/oxygenase, which is the most abundant and arguable the most important enzyme on earth. The near ubiquity of the RuBP makes it one of the most important biosynthetic cycles on earth because it is used by most photosynthetic organisms (plants, cyanobacteria, purple and green bacteria) to incorporate CO2 into cell carbon. Although all higher organisms eventually obtain their carbon from the products of this cycle, RuBP is not solely the providence of photosynthetic organisms since many other autotrophs use RuBP, making it by far the most commonly found method for CO2 fixation in nature. In eukaryotes, the TCA cycle links the oxidative breakdown of carbon compounds with biosynthesis and energy metabolism. In prokaryotes, the process is reversed, with the oxidation of inorganic compounds (such as CO2) providing the means for carbon assimilation.
The Calvin cycle utilizes light energy stored as ATP and NADPH to convert CO 2 and H2O to organic compounds. (C-3) Intermediates generated within the Calvin cycle enter central metabolic pathways as substrates in the synthesis of carbohydrates including glucose. The Calvin cycle occurs in all photosynthetic eukaryotes and most photosynthetic prokaryotes. The mechanism is most likely more than 2 billion years old and it pre-dates the appearance of oxygenic photosynthesis in the first Cyanobacteria.
Enzymes of the Calvin cycle are functionally equivalent to many enzymes involved in other metabolic pathways such as glycolysis and gluconeogenesis, but they are located in the chloroplast stroma rather than in the cytoplasm, thus functionally separating the reactions.
The enzymes are activated by light (hence light independent rather than "dark reaction") and by products of the light-dependent photosynthetic reactions. These regulatory mechanisms prevent the Calvin cycle from operating in reverse to respiration, thus preventing a continuous cycle of CO2 reduction to carbohydrates occurring simultaneously with carbohydrate oxidation to CO2 (respiration). This regulation prevents the waste of energy (as ATP) in simultaneous reverse reactions that would have no net productivity.