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. It is hydrophobic as a result of those hydrocarbon tails. And what that means is for a molecule to actually be able to pass across the cell membrane, it must be able to dissolve inside the core of that phospholipid Bilay. So that implies that the permeability of a molecule depends on its ability to actually dissolve in a nonpolar hydrophobic solution.