In the first step, we want to take the ribose phosphosate molecule and transform it into an isomer version, ribulose phosphosphate. And this is actually the same reaction that we discussed in the previous lecture, except it's in reverse. And so we have the same enzyme as we discussed previously, phosphopentose isomerase that catalyzes this reaction. 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.