Biological membranes are those membranes enclosing the cell or any of the organelles within it. Membranes serve not only to enclose and define the cell or organelle, but to regulate the flow of materials passing in and out of it. The membrane surrounding a cell is known as the plasma membrane, while those inside it are called internal membranes.

Biological membranes contain approximately equal weights of two different types of molecules: phospholipids and proteins. Phospholipids give the membrane its basic structure, while proteins perform most of the regulatory functions.

The structure of the membrane is best understood by looking at the structure of the phospholipid molecule. In a phospholipid, a glycerol molecule is bonded to two fatty acid chains and one phosphate group. The fatty acids are long and nonpolar, while the phosphate group is relatively short and polar. This gives the molecule as a whole...