Well, because the enzyme in the second process of this pathway basically uses only this type of stereochemistry and not this type of stereo chemistry. So we need to form the zelulose as a result of this trans ketilase that we'll talk about in just a moment. So ultimately, in process one, we want to transform ribose phyphosphate into a zelulos phosphosphate. And notice we begin with two of them. So we have two of these intermediates and we form two of these zelulose phyphosphate products. Now, one of these xyulose molecules is used in step two and the other one is used in step four, as we'll see in just a moment.