ribulose bisphosphate

(RuBP) A phosphorylated form of ribulose that acts as the initial carbon dioxide acceptor in C₃ photosynthesis and as the eventual acceptor in C₄ photosynthesis and crassulacean acid metabolism. Addition of carbon dioxide occurs between the second and third carbon atoms and results in the formation of two molecules of phosphoglyceric acid. These feed into the Calvin cycle, which allows the fixed carbon dioxide to be converted into carbohydrate and regenerates the ribulose bisphosphate.