If polar or charged molecules want to make their way into the cell or outside the cell, they have to use these channels. These pumps we call proteins, as we'll see in just a moment. So the lipid molecules are emphatic. They contain both polar and non polar regions in an aqueous environment, which basically means in a polar environment that consists of water, the polar regions of the lipids interact with the environment while the non polar regions aggregate inside the membrane. This makes the membrane mostly hydrophobic and it serves as a barrier to polar and charged molecules. Now, what that also means, if a non polar molecule such as a cholesterol molecule, for instance, wants to make its way across the cell membrane, it can easily do so because the cell membrane is predominantly nonpolar.