The Calvin cycle takes place in the stroma of chloroplasts. Carbon fixation: short version: In carbon fixation, each CO2 molecule is incorporated, one at a time, by attaching it to a five-carbon sugar, ribulose bisphosphate (RuBP), to form a 6-carbon molecule . The 6-carbon molecule splits to form two molecules of 3-phosphoglyceric acid (3-PGA). Long version: In the stroma, in addition to CO2, two other components are present to initiate light-independent reactions: an enzyme called ribulose bisphosphate carboxylase (RuBisCO) and three molecules of ribulose bisphosphate (RuBP). RuBP has five carbon atoms, flanked by two phosphates. RuBisCO catalyzes a reaction between CO2 and RuBP. For every CO2 molecule that reacts with a RuBP, two molecules of 3-phosphoglyceric acid (3-PGA) are formed. 3-PGA has three carbons and one phosphate. Each round of the cycle involves a single RuBP and a single carbon dioxide and forms two molecules of