So we have a partial negative charge on the oxygen and partial positive charges on the hydrogen atoms. It is a polar molecule, yet somehow it's able to actually dissolve across and pass across that cell membrane. The question is why? Well, not only does the polarity determine the ability, the permeability of that molecule, but also the size and the concentration also determine its ability to actually pass across the cell membrane. So water molecules are actually relatively small molecules and they can squeeze across the cell membrane with ease. On top of that, if we discuss the polarity of the water molecule and we compare it to, let's say, the tryptophan or this potassium, the tryptophan and potassium actually have full charges.