Now, once we form the ribulose phyphosphate, it then undergoes a second reaction which is catalyzed by phosphopentose ephemerase. And so what this does is it transforms the ribulose phyphosphate into the zelulose phosphate. And the only difference between these two molecules is the stereochemistry of the third carbon. So this carbon here and this carbon here, they have a different stereo chemistry. Now, why do we need this specific stereo chemistry? Why can't we use this?