Water molecules can actually pass across the cell membrane. In fact, water molecules are 1 billion times more likely to actually pass across the cell membrane than potassium or than potassium or sodium ions. Now, water, as we know, is in fact a polar molecule. 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?