Specialized Nutrient Transport Systems Exist in the Apical and Basolateral Membranes
Specialized transport mechanisms exist for the movement of molecules across membranes in the intestinal epithelium. These mechanisms are interactions of events involving specific proteins that lie embedded in the matrix of the cell membranes of the epithelial cells.
These proteins provide the transport pathway for the passage of ions and organic molecules across the plasma membranes of cells. As discussed here, there are many transport pathways. In general, the different pathways are polarized within the enterocytes, meaning that specific transport pathways exist on either the apical or the basolateral membrane, but not both. The transport pathway proteins chemically interact with specific organic nutrients and inorganic ions to effect their transport across the membrane. The transport mechanisms can be classified as active transport, secondary active transport, tertiary active transport, and passive transport.Active transport involves the direct consumption of metabolic energy. During active transport, energy stored as ATP is expended to move ions or molecules across membranes against an electrical or chemical gradient. In the large and small intestine, the active transport pathway of greatest importance is the Na4,K1-ATPase pump. This protein pathway lies on the basolateral membrane and uses energy from the hydrolysis of one molecule of ATP to drive three ions of sodium out of the cell, in exchange for the entry of two potassium ions into the cell. This important transport pathway exists in a wide variety of cells, in addition to enterocytes. The Na+,K4-ATPase pump is the mechanism by which (1) the interior of ceils is kept electrically negative with respect to the extracellular fluid and (2) the concentration of sodium is kept very low in the intracellular fluid (see Chapter 1).