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
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!