The sodium–potassium pump is found only in animal cells.
This protein is thought to have evolved in primitive animals as the primary means to maintain cell volume and as the mechanism to generate the steep Na and K gradients that play such a key role in the formation of impulses in nerve and muscle cells. Plant cells have a H-transporting,
P-type, plasma membrane pump.
In plants, this proton pump plays a key role in the secondary transport of
solutes (discussed later), in the control of cytosolic pH, and possibly in control of cell growth by means of acidification of the plant cell wall.
The epithelial lining of the stomach also contains a P-type pump, the H/K-ATPase, which secretes a solution of concentrated acid (up to 0.16 N HCl) into the
stomach chamber.
In the resting state, these pump molecules are situated in cytoplasmic membranes of the parietal
cells of the stomach lining and are nonfunctional (Figure4.46).
When food enters the stomach, a hormonal message is transmitted to the parietal cells that causes the pumpcontaining membranes to move to the apical cell surface, where they fuse with the plasma membrane and begin secreting acid (Figure 4.46).
In addition to functioning in digestion, stomach acid can also lead to heartburn.
Prilosec is a widely used drug that prevents heartburn by inhibiting the stomach’s H/K-ATPase. Other acid-blocking heartburn medications (e.g., Zantac, Pepcid, and Tagamet) do not inhibit the H/K-ATPase directly, but block a receptor on the surface of the parietal cells, thereby stopping
the cells from becoming activated by the hormone.
Unlike P-type pumps, V-type pumps utilize the energy of ATP without forming a phosphorylated protein intermediate.
V-type pumps actively transport hydrogen ions across the walls of cytoplasmic organelles and vacuoles (hence the designation V-type).
They occur in the membranes that line lysosomes, secretory granules, andplant cell vacuoles.
V-type pumps have also been found in the plasma membranes of a variety of cells.
For example, a V-type pump in the plasma membranes of kidney tubules helps maintain the body’s acid–base balance by secreting protons into the forming urine.
V-type pumps have a structure similar to that of the ATP synthase shown in Figure 5.23.