And the ability of these molecules to actually pass across the cell membrane varies over a wide range of value. So the question that I'd like to discuss in this lecture is what exactly are the factors that determine the permeability of a given molecule? What determines the ability of a given molecule to actually pass across the cell membrane? So perhaps the most important factor is polarity. So what does that actually mean? Well, we know that the core, the inside of the phospholipid bilayer membrane is predominantly nonpolar.