The Cell Membrane is also called the plasma membrane. It's the external limiting membrane of a cell. It is composed of lipids (fat molecules) that have a hydrophobic (insoluble in water) end and a hydrophilic (water-soluble) end. Cell membranes are made of lipid bilayers in which the lipids line up in two layers with the hydrophobic ends facing each other and the hydrophilic ends facing the outside and the inside of the cell.

Lipids: is another word for "fats.") Lipids can be defined as substances such as a fat, oil or wax that dissolves in alcohol but not in water. Lipids contain carbon, hydrogen and oxygen but have far less oxygen proportionally than carbohydrates. Lipids are an important part of living cells. Together with carbohydrates and proteins, lipids are the main constituents of plant and animal cells.The cell membrane is made up of a lipid bilayer. Cell membranes are also described as lipid bilayers. There are two layers of phospholipids with proteins embedded in the layers. The membrane is able to move as needed by the cell. The head end of a phospholipid molecule is hydrophilic. The heads like to be near water and arrange themselves so that tails meet tails and the heads face the cellular fluids and exterior of the cell. Phospholipids:The tail end of a phospholipid molecule is hydrophobic. The tails do not like to be near water and arrange themselves so that tails meet tails and the heads face the cellular fluids and exterior of the cell.

Hydrophobic: Having a strong aversion for water. Hydrophobic molecules are relatively insoluble in water and tend to sorb to soil particles.

Hydrophillic: a water lover; like a sponge. Hydrophilic molecules tend to interact with water and with each other. Hydrophobic molecules avoid interaction with water and tend to interact with other hydrophobic molecules.