Well, the cell membrane has four important functions. Function number one is it creates a protective barrier between the outside and the inside environment of the cell. It basically prevents toxins and other pathogenic agents from actually entering the cell and it prevents molecules from actually spontaneously exiting the cell in the first place. Now, function number two is that in transport, as we'll see in just a moment, within a cell membrane, we have proteins. And these proteins basically create these channels or pumps that basically allow the selective movement of certain things outside or into that cell. So this is a semipermeable membrane.