The reactants in the Calvin cycle are carbon dioxide, water and a sugar with five carbon atoms known as ribulose. The enzyme catalyst RuBisCo and energy donators aAdenosine triphosphate, or ATP, and nicotinamide adenine dinucleotide phosphate, or NADPH, are necessary for the reaction but aren't reactants in the technical sense.
The Calvin cycle is one of the major ways photosynthetic organisms make carbon dioxide and water into sugars, such as glucose and fructose. They do this by combining water and carbon dioxide with ribulose, which produces an intermediate molecule with six carbon atoms. This immediately splits into two molecules with three carbon atoms each, known as phosphoglyceric acid. Energy from ATP is used to strip the phosphate from the acid, and some of the molecules that result are used to make fructose. The remainder is recycled back into ribulose to continue the cycle.
The Calvin cycle takes place in the chloroplast, the same organelle that generates chemical energy from sunlight, but the cycle is not directly involved in that reaction. Instead, the energy is transferred from the parts of the organelle that process the sunlight to the site of the Calvin cycle. The Calvin cycle is one of the most important chemical reactions on earth, and the enzyme catalyst RuBisCo is probably the most common protein in existence.