The cell membrane's primary characteristics is its particular permeability, which means that it permits some substances to cross it effortlessly, but not others. Small nonpolar molecules,which have no charge, can cross the membrane with ease by means of diffusion, but ions, which are charged molecules as well as larger molecules basically cannot. Moreover, water diffusion is known as osmosis. Oxygen which is a nonpolar and a small molecule, can easily pass through a cell membrane. Carbon dioxide, which is the byproduct of cell respiration, is so small to effortlessly diffuse out of a cell. Furthermore, small and uncharged lipid molecules can pass into the lipid entrails of the membrane.