Cell Membrane

The simple facts students normally study about the cell membrane is that it´s made of fat and used for protection. If we were to see from a molecules´ point of view, what would happen inside the cell membrane? Actually, if we get through the cell membrane since it is selectively permeable, we could go through by either passive or active transport.   Through passive transport, substance can cross the cell membrane without using cellular energy. In other words, the cell doesn´t need to use its own energy.  On the other hand, active transport requires the cell to use its own energy in order to get through the cell membrane. Below, there´s an informative video I found which explains more clearly what I roughly explained.