2.  Think of the cell as a city. If the city had no way of getting people in and out of it, it would soon grind to a halt.  Similarly, the activities of a living cell depend on the ability of its membrane to:  transport raw materials into the cell  transport manufactured products and wastes out of the cell  prevent the entry of unwanted matter into the cell  prevent the escape of the matter needed to perform the cellular functions

3.  Long ago, scientists noticed that the cell membrane was in fact a bilayer, or a structure consisting of two layers of molecules.  Chemical analysis revealed that this layer is composed of phospholipid molecules (a type of lipid).  Phospholipids have 2 fatty acid chains bonded to a glycerol backbone.  The third glycerol binding site is bonded to a chain containing phosphorus and nitrogen.

4.  This makes the phospholipid shape quite different from the triglyceride.  The phosphate chain forms a “head”, while the two fatty acids form two “tails”.  The electrical charge is unevenly distributed, with the head being polar, and the tails, non-polar.  Since the polar head of a phospholipid molecule is attracted to water molecules, (which are also polar), this makes the phosphate head hydrophilic.  The fatty acid chains are are insoluble in water (or afraid of water), and are referred to as hydrophobic

6.  To form the cell membrane, the non-polar lipid tails face each other and cause the polar heads to face the inside and outside of the cell.  This sandwich-like phospholipid structure is called a phospholipid bilayer, and forms the basis of the cell membrane.

7.  Biologists have discovered that the cell membrane contains a mosaic of different components scattered throughout it.  Two major components of the cell membrane include:  Proteins – involved in cell transport  Cholesterol – increases the fluidity (bendibility) of the membrane.

8.  Thus, the description of the membrane is called the fluid-mosaic membrane model.  The cell membrane is considered to be a semi- permeable membrane, allowing some biological molecules to pass through it.