The plasma membrane, or plasmalemma, of a cell consists of a phospholipid bilayer with associated integral and peripheral proteins, glycoproteins and glycolipids, and cholesterol. The plasma membrane is only 7–10 nm thick, which is below the resolution of the light microscope.

The plasma membrane has several major functions and serves to:

• Provide a physical barrier that protects, supports, and separates the cytoplasm from the extracellular environment
• Regulate the traffic of ions, gases, nutrients, and wastes in and out of the cell
• Establish and maintain electrochemical gradients between the intracellular and extracellular environment
• Provide attachment and communication with other cells and the extracellular matrix
• Participate in cell signaling events that direct cell growth, division, survival, motility, and differentiation
   

In very high magnification, osmium-fixed TEM specimens, the structure of the plasma membrane can be seen as a dark or electron dense line. This specimen shows a desmosome where two adjacent plasma membranes connect the intermediate filament network of these neighboring cells. One of these plasma membranes can be seen between the tips of the arrows.

A thick external coat of glycoproteins and glycolipids on the outside of the plasma membrane is called a glycocalyx. Many cells have a glycocalyx, but it is especially well developed on the microvilli of absorptive cells lining the small intestine (arrows).

Although the plasma membrane is generally not visible in the light microscope, occasionally its presence can be discerned, such as in the epithelial cells lining the collecting ducts of the kidney medulla, due to an abundance of associated proteins that stain with eosin (arrows).

 

Let's now take a look at membrane bound organelles within the cell.