Ribulose biphosphate (RuBP), the 5 carbon compound is the primary acceptor of carbon dioxide in photosynthesis. The 6 molecules of RuBP reacts with 6 molecules carbon dioxide to produce 6 molecules of a 6 carbon compound, this is called carboxylation. This reaction is catalyzed by the enzyme Rubisco. The 6carbon compound produced as a result of carboxylation is highly unstable. This molecules then breakdown immediately to produce 12 molecules of 3-phosphoglyceric acid. The 3-phosphoglyceric acid (PGA) is the first stable compound formed in photosynthesis. It is a three carbon compound, so this cycle is known as the C3 cycle.