And what that means is certain molecules which are permeable to the membrane are able to pass across the membrane without much difficulty. But other molecules which are impermeable to the cell membrane cannot pass across the cell membrane without assistance of some type of protein molecule. Now, inside our body, we have all these different types of molecules. 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?