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!


  • List several functions of cell membranes.
  • Explain how the structure of cell membranes is related to its key functions.
  • Define the term "membrane fluidity" and explain its role in the function of cell membranes.
  • Compare and contrast passive and active transport mechanisms.
  • Explain how osmosis works and describe its role in the function of cells.
  • List several functions of membrane proteins.
  • Explain how the enzymatic activity of a membrane protein can be affected by a substance outside the cell.
Last modified: Friday, October 1, 2010, 10:00 AM