So compared to this unmodified glucose, the phosphorylated glucose contains a higher charge and so it is more polar. And because this molecule is more polar, it is much less likely to actually spontaneously leave the cell. Why? 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.