ABSTRACT
Many of the regulatory activities of the human body are carried out at the cellular level. Cells replicate themselves as part of the normal process of degeneration and repair of organs and tissues; specialized cells synthesize receptors, proteins, hormones, and peptides, which may be released into the blood stream to regulate the function of other cells and also act as local feedback inhibitors to the overproduction of the regulatory molecule. All cells are surrounded by a plasma membrane that keeps the cell contents localized, allows for the spatial separation of charge, and acts as a barrier to the external environment. The cell membrane is a phospholipid bilayer about 50 A˚ thick.* Specialized protein structures that span the membrane connect the exterior environment with the cell interior and function as pumps, channels, and receptors. These pumps, channels, and receptors help to maintain the cells’ resting membrane potential, allow cell-to-cell signaling (e.g., propagation of the action potential in nerves and muscle), and provide mechanisms by which molecules and ions may enter and leave the cell. Remember, the internal part of the phospholipid bilayer that forms the cell membrane contains the hydrophobic (water-insoluble) portion of the lipid molecules. Most ions and small molecules such as glucose that are essential for the functioning of the cell are hydrophilic (water-soluble). Without the specialized transporters and fluid-filled protein channels, hydrophilic molecules could enter and leave the cell only very, very slowly (if at all) through the highly hydrophobic interior of the bilayer.
