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. And what that means is the cell actually picks and chooses what to allow into the cell and what to not allow into the cell. Function number three is that in signal transduction, what that means is certain molecules, for instance, hormones, can actually interact with the outside portion of the cell membrane and that will create many different types of signals and processes inside the cell, as we'll see in future lectures. And function number four, energy storage.