ABSTRACT
The basic structure of the plasma membrane is formed by phospholipids (Figure 2.1). These molecules are one of the more abundant of the membrane components. Phospholipids are amphipathic molecules that have both polar (water-soluble) and nonpolar (water-insoluble) regions. They are composed of a phosphorylated glycerol backbone that forms a polar head group that is hydrophilic; and a nonpolar region that contains two hydrophobic fatty acid chains. In an aqueous environment such as the body, these molecules are arranged in a formation referred to as the lipid bilayer, consisting of two layers of phospholipids. The polar region of the molecule is oriented toward the outer surface of the membrane where it can interact with water; and the nonpolar, hydrophobic fatty acids are in the center of the membrane away from the water. The functional signicance of this lipid bilayer is that it creates a semipermeable barrier. Lipophilic, or nonwater-soluble, substances can readily cross the membrane by simply passing through its lipid core. Important examples of these substances include gases, such as oxygen and carbon dioxide, and fatty acid molecules, which are used to form energy within muscle cells.
