THE CELL MEMBRANE



History: Up until now you knew that the function of the cell membrane was to separate the inside of the cell from its outside. In some pictures you have seen, the cell membrane may have looked like this: 



Scientists thought that this is what it looked like for a long time.  Then the electron microscope came along and they were able to see the cell membrane.  Below is a picture of how the cell membrane looked to them.  



Scientists were then able to break apart the cell membrane and find out what chemicals were in it.  The current best model of what a cell membrane looks like is in the figure below.  As you can see it is made up of two parts.  They are the phospholipids (magenta and green), the proteins (orange and red), and the carbohydrates (black).  Carbohydrates can be attached to either the phospholipids or the proteins in the cell membrane.


The cell membrane is just like the other organelles of cells in that it serves the cell by having its own specialized jobs.  One of its jobs is to control what enters and exits the cell and thereby to protect the cell.  In the figure above we see a magnified model of a cell membrane.
 
        Location: The cell membrane surrounds the cell.

        Structure: Cell membranes are made up of phospholipid molecules (fats) with various large globular protein       molecules suspended in them.  In the figure above, you can see the lipid bi-layer in purple and the proteins in orange. The lipid bi-layer (two layers of phospholipids) is formed because of the chemical structure of a phospholipid.  Since cells are constantly in water, the phospholipids form a double layer, with the heads towards the water and the tails inside so that they can stay away from the water.  These bi-layers have proteins scattered about in them.  Sometimes carbohydrates (sugars) are attached to cell membrane phospholipids and to cell membrane proteins.  A selectively permeable (sometimes called semi-permeable) membrane allows some molecules across but not others.


JOB:  The cell membrane controls what enters and exits the cell and thus protects the cell. 



Q: So how do things enter and exit the cell?  We did say that was one of the jobs of the cell membrane.

A: Well, molecules can only enter if they can go through the phospholipid bilayer (fat) or they have a special protein in the cell membrane that they can use.



So now you know that there is much more to the cell membrane than meets the eye.

    1.  Functionally the cell membrane serves as both a gateway and a barrier for the cell.
    2.  The cell membrane is composed of phospholipids and proteins.
    3.  The heads of the phospholipids are polar so they like to be with water which is inside and outside of the cell.
    4.  The tails of the phospholipids are not polar.
    5.  The tails do not like to be with water and thus a lipid bi-layer may be formed when phospholipids are exposed to water.       

Click here to see a real picture of a cell membrane.

Created March 22, 2013
Center for Teaching and Learning