Simple diffusion is the primary type of passive transport in which substances travel through the cell membrane, passing individually through the lipid bilayer. Facilitated diffusion is performed when molecules cannot pass through the lipid bilayer on their own. In these instances, helpful proteins, called pumps or transporters, provide safe passage through the cell wall and into the cytoplasm.
Moving At The Movies
When you go to the movies, you are acting like a molecule entering a cell.
You, the audience member, are necessary--but there is a limit to how many people can be seated at one time and not everyone is allowed into every show. The ushers and ticket booth act like the cell membrane and helpful proteins, making sure that everyone with a ticket gets in to the right movie theater. As you know, your ticket only allows you to go through certain doors. So if you try to go into the wrong theater, you will be blocked or redirected.
Just like molecules moving into/out of a cell.
If the wrong molecule tries to move through the lipid bilayer, it will be blocked. The cell membrane, pumps and transporter proteins make sure that each molecule goes in and out of the cell a certain way. They keep track of the molecule traffic and adjust other cell activities accordingly.
The ability to allow certain molecules and block others is called selective permeability.