Cell membranes are surprisingly dynamic structures. The diverse functions of a cell's membrane make the membrane as dynamic as the cell's interior.

Contrary to the earliest assumptions about cell membrane structure, we now know that the cell membrane must be more than just a fortress to protect the inside of the cell from the outside. It must contain biological doors, windows, gates, and drawbridges — intricate structures that function together to give the membrane a selectively permeable nature. The health and vitality of the cell depend on the ability of the cell membrane to keep certain substances out while allowing the free passage of others.

Earlier in this unit, we learned that cells are surrounded by plasma membranes composed of a double layer, or bilayer, of phospholipid molecules oriented with their polar groups to the outside. But since a membrane has to perform a wide range of functions, there's a lot more to its structure than a simple phospholipid bilayer!

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