The Calvin cycle takes place in the stroma of the chloroplast. Carbon dioxide enters the stroma of the chloroplast to combine with the five-carbon compound called ribulose1, 5-Biphosphate to form two molecules of a three-carbon compound called 3-Phosphoglyceric acid. This reaction is catalyzed by an enzyme called Ribulose bi-phosphate Carboxylase.
As the three-carbon compound is the first stable product of photosynthesis, this cycle is called the C3 cycle, or Calvin cycle. The cycle of reactions occurring here is termed the Calvin cycle, named after the discoverer. This process of fixing carbon in carbon dioxide as an organic compound is termed "carbon fixation." The energy molecules, Adenosine Triphosphate and Nicotinamide Adenine Dinucleotide phosphate, generated from the light reactions are used up in the Calvin cycle.