Key Concepts of Section 11.5

• The electrochemical gradient across a semipermeable membrane determines the direction of ion movement through transmembrane proteins. The two forces constituting the electrochemical gradient—the membrane electric potential and the ion concentration gradient—may act in the same or opposite directions (see Figure 11-25).

• 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 contain symporters that couple the energetically favorable entry of Na⁺ to the import of glucose against its concentration gradient (see Figure 11-26). Amino acids also enter cells by means of Na⁺-linked symporters.

• The molecular structure of a bacterial Na⁺/amino acid symporter reveals how binding of Na⁺ and leucine are coupled and provides a snapshot of an occluded transport intermediate in which the bound substrates cannot diffuse out of the protein (see Figure 11-27).

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• In cardiac muscle cells, the export of Ca²⁺ is coupled to and powered by the import of Na⁺ by a cation antiporter, which transports three 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 when the cytosolic pH rises above normal decreases pH by exporting HCO₃⁻.

• AE1, a Cl⁻/HCO₃⁻ antiporter in the erythrocyte membrane, increases the ability of blood to transport CO₂ from tissues to the lungs (see Figure 11-28).

• 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 these proton antiporters (see Figure 11-29).