What Is the Composition of the Plasma Membrane?

Each cell is enveloped by a very thin membrane measuring only about 10 nanometers (nm) thick. A nanometer is one-billionth of a meter— pretty thin indeed. The makeup of the plasma membrane is a very clever

combination of lipids and proteins with just a touch of carbohydrate and other molecules. Interestingly, plasma membranes use the basic principle of water solubility to allow for its barrier properties and it is the lipid that provides this character. Molecules that are somewhat similar to triglycerides (fat) called phospholipids are arranged to provide a water-insoluble capsule surrounding cells. What that means is that water-soluble substances such as sodium, potassium, and chloride, carbohydrates, proteins, and amino acids are not able to move freely through the membrane whereas some lipid substances and gases move more freely. The plasma membrane will also contain the lipid substance cholesterol. Cholesterol appears to increase the stability of the plasma membranes.
 

Since the plasma membrane functions as a barrier between the outside and inside of the cell, there must be a means (or doorways) whereby many water-soluble substances can either enter or exit a cell. One of the roles of proteins in the plasma membrane is to function as doors, thereby allowing substances such as sodium, potassium, chloride, glucose, and amino acids to enter or exit a cell. This is shown in Figures 2.1 and 2.2.