11. Membrane Transport of Small Molecules and the Electrical Properties of Membranes

Because of its hydrophobic interior, the lipid bilayer of cell membranes serves as a barrier to the passage of most polar molecules. This barrier function is crucially important because it allows the cell to maintain concentrations of solutes in its cytosol that are different from those in the extracellular fluid and in each of the intracellular membraneenclosed compartments. To make use of this barrier, however, cells have had to evolve ways of transferring specific water-soluble molecules across their membranes in order to ingest essential nutrients, excrete metabolic waste products, and regulate intracellular ion concentrations. The transport of inorganic ions and small water-soluble organic molecules across the lipid bilayer is achieved by specialized transmembrane proteins, each of which is responsible for the transfer of a specific ion, molecule, or group of closely related ions or molecules. Cells can also transfer macromolecules and even large particles across their membranes, but the mechanisms involved in most of these cases are different from those used for transferring small molecules, and they are discussed in Chapters 12 and 13. The importance of membrane transport is indicated by the large number of genes in all organisms that code for transport proteins, which make up between 15 and 30% of the membrane proteins in all cells. Some specialized mammalian cells devote up to two-thirds of their total metabolic energy consumption to membrane transport processes.