Concept 3 Review

Phospholipids and the Lipid Bilayer

Phospholipids spontaneously form lipid bilayers, which generate biological membranes.

Diglycerides contain two fatty acids linked to glycerol. Many diglycerides contain a phosphate group attached to the third -OH group of glycerol, producing a phospholipid. Phospholipids often contain additional charged groups attached to the phosphate.

In phospholipids, the two fatty acids are hydrophobic, or insoluble in water. But the phosphate group is hydrophilic, or soluble in water. When phospholipids are mixed with water, they spontaneously rearrange themselves to form the lowest free-energy configuration. This means that the hydrophobic regions find ways to remove themselves from water, while the hydrophilic regions interact with water.

The resulting structure is called a lipid bilayer. All biological membranes (except for those found in certain unusual bacteria, members of the Archaea) contain lipid bilayers, as well as proteins, which provide membranes with stability and specialized functions.

The image above is based on original work by H. Heller, M. Schaefer, & K. Schulten, "Molecular dynamics simulation of a bilayer of 200 lipids in the gel and in the liquid-crystal phases", J. Phys. Chem. 97:8343-60, 1993.