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. Property number four, the cell membrane is held together by non covalent interactions. Now, if we examine any one of these individual molecules inside the cell membrane, for instance, we examine the protein molecule, the protein molecule itself. The atoms in the protein molecule are held together by covalent bonds, but the individual molecules.