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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.
Lipids and Phospholipids
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.
Figure %: Basic Lipid Structure
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.
Figure %: Phospholipid Structure
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 Bilayer
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.
Figure %: Lipid Bilayer
Properties of the Lipid Bilayer
As we have already mentioned, 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.
