In secondary active transport, or coupled transport, the energy needed for the "uphill" movement of a molecule or ion is obtained from the "downhill" transport of Na+ into the cell. Hydrolysis of ATP by the action of the Na+/K+ pumps is required indirectly, in order to maintain low intracellular Na+ concentrations. The diffusion of Na+ down its concentration gradient into the cell can then power the movement of a different ion or molecule against its concentration gradient. If the other molecule or ion is moved in the same direction as Na+ (that is, into the cell), the coupled transport is called either cotransport or symport. If the other molecule or ion is moved in the opposite direction (out of the cell), the process is called either countertransport or antiport.
An example of symport is the cotransport of glucose and Na+ from the extracellular fluid into the epithelial cells of the small intestine and kidney tubules. In these cases, a carrier protein simultaneously binds to glucose and Na+ in the extracellular fluid. The downhill transport of Na+ (from higher to lower concentration) into the cell furnishes the energy for the uphill transport of glucose (fig. 6.18). Notice that, in order for this secondary active transport to work, a steep gradient for Na+ must have already been established by the activity of the Na+/K+ pumps.
An example of countertransport is the uphill extrusion of Ca2+ from a cell by a type of pump that is coupled to the passive diffusion of Na+ into the cell. Cellular energy, obtained from ATP, is not used to move Ca2+ directly out of the cell in this case, but energy is constantly required to maintain the steep Na+ gradient. Another example of countertransport is the exchange of chloride (Cl-) for bicarbonate (HCO3-) across the red blood
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.