The cell membrane acts as a diffusion barrier to the movement of substances into and out of the cell. Because of the hydrophobic core of the lipid bilayer that forms the base of the membrane, only substances that are hydrophobic will be able to pass through the membrane by the process of simple diffusion. In general, all hydrophilic substances will not be able to pass through this hydrophobic core (the one exception is water which is permeable through most membranes).
How do hydrophilic substances get across the membrane? There are two major mechanisms by which this is accomplished.