Well, because around the cell we have the cell membrane which consists predominantly of non polar lipid molecules. And so this highly polar glucose phosphate cannot spontaneously leave the cell because it cannot pass across the non polar bilayer membrane surrounding the cell. So once again, the first step in glucose metabolism involves modifying the glucose by adding phosphoryl group onto carbon number six. Now, phosphorylation of glucose gives it a net negative charge and transforms it into an anion. And this is done for two reasons. The first reason is to prevent the glucose from spontaneously exiting the cell.