Putting It Together: Cell Membranes


Over the course of this lesson you learned some key features of the cell membrane and how it controls entry and exit from cells.

  • The plasma membrane is often described as a fluid-mosaic model, hinting at its permeability, changeability, and the multitude of components that make up its structure.
  • A key responsibility of cell membranes is to control entry and exit. Transport can occur with or without an input of energy and with or without facilitating proteins
  • Larger molecules enter and leave cells via endocytosis and exocytosis


Let’s examine the disease cystic fibrosis in more detail. CF is caused by a defect in a single transmembrane protein: cystic fibrosis transmembrane conductance regulator. That’s a long way of saying it is a chloride ion channel that crosses through the plasma membrane. This channel is specifically active in epithelial tissues where it normally helps produce thin mucus via the movement of water. When mutated, the channel does not work properly and mucus becomes thick and viscous. This in turn leads directly to many of the symptoms of CF: thick, sticky mucus, frequent chest infections, and coughing or shortness of breath.

To learn more about CF, visit these sites: