The lipid bilayer, one of the major components of a cell membrane, is composed of fats that are arranged in order to carry out the barrier-like functions of the bilayer. The bilayer, which is composed of a hydrophobic area, which is called the polar head region, and a hydrophobic area, called the nonpolar tail region. Being highly impermeable, it does not allow molecules to freely pass across it; however, water and small gas molecules can easily pass through the bilayer. This is because water, although a polar molecule, has a relatively small molecular diameter which can easily pass through the spaces of the hydrophobic interior of the bilayer without the assistance of other structures of the cell membrane.