rubisco catalyzes the reaction that fixes a carbon dioxide molecule into a ribulose 1,5-bisphosophate backbone, which immediately splits into two 3-carbon intermediates. as the cycle progresses, another 3-carbon product is formed, g3p. only one out of six g3ps will be used to make glucose or other biosynthetic products. why? choose one: a. the ribulose 1,5-bisphosphate must be regenerated. b. the g3p immediately splits into carbon dioxide and a 2-carbon product. c. making glucose takes all six g3p. d. the g3p serves as a catabolic substrate.