Active Transport Pumps
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Active transport is the process whereby protein transport pumps move solutes across the membrane against an electrochemical or concentration gradient. Primary active transport requires metabolic energy, which is supplied by ATP hydrolysis. There are three families of ATP-driven pumps: the F-type ATPases that move H+; the P-type adenosine tri- phosphatase (ATPase) that pump ions across membranes; and the ATP-binding cassette (ABC) transporters that transport a wide range of solutes. The ATP synthase located on the inner mitochondrial membrane is an example of an F-type pump; however, in that location it runs backward, allowing H+ to run down its electrochemical gradient, and uses the energy to form a bond between ADP and Pi (see Fig. 3.18). As a general principle, pumps, carriers, and channels can transport in either direction depending on the concentration of substrates on either side of the membrane.
Sodium–potassium ion pump. The sodium–potassium (Na+–K+) pump is a P-type ATPase present in the plasma membranes of virtually
42 UNIT II Cellular Function
ABC transporters. Another important class of ATP-driven transport- ers is the ABC transporter family. These transporters all have a common ATP-binding domain, called the ATP binding cassette (ABC), which hydrolyzes ATP to provide energy for the transport process (Fig. 3.23). This family of membrane transporters is the largest of the transporter families. A clinically important member of this family is a chloride channel in the plasma membrane of epithelial cells. A defect in this transporter is responsible for cystic fibrosis, a common genetic disorder that affects the lungs and pancreas (see Chapter 22). Bacteria use ABC transporters to pump antibiotics out of the cell, resulting in drug resistance (see Chapter 8).
Membrane Transport Carriers Na+-driven carriers. In animal cells, the Na+ gradient created by
the Na+–K+ pump is used to power a variety of transporters by secondary active transport. An important Ca2+ transporter located in the plasma membrane of cardiac muscle cells uses the electrochemical gradient of Na+ to power the transport of Ca2+ out of the cell (see Fig. 3.22, right). The dependence of this calcium transporter on the sodium gradient helps explain the inotropic effects of the commonly prescribed drug digitalis. Digitalis is a cardiac glycoside that inhibits the Na+–K+ pump and allows the accumulation of intracellular Na+. The Na+ concentration gradient across the membrane is thus decreased, leading to less efficient calcium removal by the Na+-dependent Ca2+ pump. A more forceful cardiac muscle contraction results from the increased intracellular Ca2+ concentration. Another example of a transporter that uses secondary active transport is the Na+–H+ exchange carrier, which uses the Na+ gradient to pump out excess hydrogen ions to help maintain intracellular pH balance. The Na+ gradient also can be used to bring substances into the cell. For example, glucose and amino acid transport into epithelial cells is coupled to Na+ entry. As Na+ moves through the transporter down its electrochemical gradient, the sugar or amino acid is “dragged” along. Entry of the nutrient will not occur unless Na+ also enters the cell. The epithelial cells that line the gut and kidney tubules have large numbers of these nutrient transporters present in the luminal (apical) surfaces of their cell membranes. In this way, large amounts of glucose and amino acids can be effectively absorbed. The reuptake of numerous types of neurotransmitters from synapses also occurs via Na+-driven
all animal cells. It serves to maintain low sodium and high potassium concentrations in the cell. The Na+–K+ transporter must pump ions against a steep electrochemical gradient. Almost one-third of the energy of a typical cell is consumed by the Na+–K+ pump. ATP hydrolysis provides the energy to drive the Na+–K+ transporter. The Na+–K+ pump behaves as an enzyme in its ability to split ATP to form ADP and Pi, leading to the protein being termed Na+–K+ ATPase.
Transport of sodium and potassium ions through the Na+–K+ carrier protein is coupled; that is, the transfer of one ion must be accompanied by the simultaneous transport of the other ion. The transporter moves three sodium ions out of the cell for every two potassium ions moved into the cell (Fig. 3.21). The Na+–K+ pump is important in maintaining cell volume. It controls the solute concentration inside the cell, which in turn affects the osmotic forces across the membrane. If Na+ is allowed to accumulate within the cell, the cell will swell and could burst. The role of the Na+–K+ pump can be demonstrated by treating cells with digitalis, a drug that inhibits Na+–K+ ATPase. Cells thus treated will indeed swell and often rupture. The Na+–K+ pump is responsible for maintaining a steep concentration gradient for Na+ across the plasma membrane. This gradient can be harnessed to transport small molecules across the membrane in a process called secondary active transport. Carriers that use ATP directly are engaged in primary active transport.