Ribulose 1, 5- biphosphate (RuBP) is $ C{{O}_{2}} $ acceptor in $ {{C}_{3}} $ plants. It combines with $ C{{O}_{2}} $ in presence of Rubisco and form an unstable 6- C compound. This compound immediately breaks into two molecules of 3- C, 3- phosphoglyceric acid (PGA), the first stable compound formed in Calvin cycle.