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. So for instance, the lipids and the proteins inside the cell membrane are held together by non covalent interactions, intermolecular bonds. And what that means is, although these intermolecular bonds are weaker than the covalent bonds, because we have so many of these intermolecular bonds, the collective aggregate of all these bonds basically makes that membrane an effective barrier. So the individual lipid and protein molecules are held together by intermolecular bonds.