Cotransport by Symporters and Antiporters
■ Cotransporters use the energy released by movement of an ion (usually H+ or Na+) down its electrochemical gradient to power the import or export of a small molecule or different ion against its concentration gradient.
■ The cells lining the small intestine and kidney tubules express symport proteins that couple the energetically favorable entry of Na+ to the import of glucose and amino acids against their concentration gradients (see Figure 7-21).
■ In cardiac muscle cells, the export of Ca2+ is coupled to and powered by the import of Na+ by a cation antiporter, which transports 3 Na+ ions inward for each Ca2+ ion exported.
■ Two cotransporters that are activated at low pH help maintain the cytosolic pH in animal cells very close to 7.4 despite metabolic production of carbonic and lactic acids. One, a Na+/H+ antiporter, exports excess protons. The other, a Na+HCO3~/Cl~ cotransporter, imports HCO3~, which dissociates in the cytosol to yield pH-raising OH~ ions.
■ A Cl~/HCO3~ antiporter that is activated at high pH functions to export HCO3~ when the cytosolic pH rises above normal, and causes a decrease in pH.
■ Uptake of sucrose, Na+, Ca2 + , and other substances into plant vacuoles is carried out by proton antiporters in the vacuolar membrane. Ion channels and proton pumps in the membrane are critical in generating a large enough proton concentration gradient to power accumulation of ions and metabolites in vacuoles by these proton antiporters (see Figure 7-23).
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