So to begin, let's focus on nonpolar and small molecules. So remember, the cell membrane consists predominantly of a hydrophobic region. So the entire core of the membrane, this entire rest section, is hydrophobic nonpolar because of the presence of these hydrocarbon tails, part of the phospholipid molecules. Now, because a non polar molecule can easily dissolve in a hydrophobic non polar solution, what that means is if a small non polar molecule wants to make its way across the cell membrane, all it has to do is dissolve inside that cell membrane. And this process, by which a small non polar molecule will move down its concentration gradient from a high till low potential through the core of that membrane by dissolving in that cell membrane. This is known as simple diffusion.