what types of molecules are able to pass through a semipermeable membrane
Figure 1 summarizes the permeability properties of pure lipid bilayers. Lipid-soluble molecules can readily pass through a lipid bilayer. Examples include gas molecules such as oxygen (O 2) and carbon dioxide (CO 2), steroid molecules, and fat-soluble vitamins (A, D, E, and K). Surprisingly, some small polar molecules are capable of permeating the lipid bilayer without the aid of a membrane transport protein.