The lipid bilayer is a universal component of all cell membranes. Its role is critical because its
structural components provide the barrier that marks the boundaries of a cell. The structure is
called a "lipid bilayer" because it is composed of two layers of fat cells organized in two sheets.
The lipid bilayer is typically about five nanometers thick and surrounds all cells providing the
cell membrane structure.
The structure of the lipid bilayer explains its function as a barrier. Lipids are fats, like oil, that
are insoluble in water. There are two important regions of a lipid that provide the structure of the
lipid bilayer. Each lipid molecule contains a hydrophilic region, also called a polar head region,
and a hydrophobic, or nonpolar tail region.
The hydrophilic region is attracted to aqueous water conditions while the hydrophobic region is
repelled from such conditions. Since a lipid molecule contains regions that are both polar and
nonpolar, they are called amphipathic molecules.
The most abundant class of lipid molecule found in cell membranes is the phospholipid. The
phospholipid molecule's polar head group contains a phosphate group. It also sports two
nonpolar fatty acid chain groups as its tail.
The fatty acid tail is composed of a string of carbons and hydrogens. It has a kink in one of the
chains because of its double-bond structure.
The phospholipids organize themselves in a bilayer to hide their hydrophobic tail regions and
expose the hydrophilic regions to water. This organization is spontaneous, meaning it is a natural
process and does not require energy. This structure forms the layer that is the wall between the
inside and outside of the cell.
The most important property of the lipid bilayer is that it is a highly impermeable structure.
Impermeable simply means that it does not allow molecules to freely pass across it. Only water
and gases can easily pass through the bilayer. This property means that large molecules and
small polar molecules cannot cross the bilayer, and thus the cell membrane, without the
assistance of other structures.
The lipid bilayer contains lipid molecules, and, as we will discuss later, it also contains proteins.
The bilayer's fluidity allows these structures mobility within the lipid bilayer. This fluidity is
biologically important, influencing membrane transport. Fluidity is dependent on both the
specific structure of the fatty acid chains and temperatur