B

© g> Compartment

'NAJ

FIGURE 17 Na+-coupled co-transport (A) and countertransport (B). S, solute; C, carrier molecule.

'NAJ

FIGURE 17 Na+-coupled co-transport (A) and countertransport (B). S, solute; C, carrier molecule.

us further assume that the carrier can rotate only when both sites are either empty or filled but not when only one site is filled. Clearly, this mechanism can bring about a one-to-one exchange of Na+ for S across the membrane. If there is little or no Na+ in compartment i, the system will only be able to exchange Na+ in compartment o for S in compartment i and not vice versa. Thus, the downhill movement of Na+ from o to i can bring about uphill flow of S from i to o; this mechanism is referred to as secondary active counter-transport. And, once more, the trick is the removal of Na+ from compartment i, which is accomplished by the primary active Na+ transport mechanism that is energized by ATP hydrolysis. Two examples of such a countertransport system are (l)Na+-H+ exchange and (2) Na+-Ca2+ exchange; both of these mechanisms have been found in a wide variety of cell types. In both instances, H+ and Ca2+ are extruded from the cell coupled to the downhill influx of Na+ into the cell.

Cellular models of these co- and countertransport processes are illustrated in Fig. 18. The essential common feature of these transport processes is that metabolic energy is directly invested into the operation of the (Na+-K+) pump, a primary active transport mechanism. The operation of this pump results in a cell Na+ concentration that is much lower than that in the extracellular fluid. This transmembrane Na+

compartment i will exceed that in compartment o so that the system will have actively transported S without a direct linkup to metabolic energy. This system is referred to as secondary active cotransport.

How is this possible? We have stipulated that every Na+ that enters compartment i is removed. In animal cells, which is accomplished by the (Na+-K+) pump that is directly linked to (energized by) ATP hydrolysis. Thus, in essence, energy is directly invested into a primary active transport mechanism that is responsible for extruding Na+ from the cell (in exchange for K+) and thereby maintaining a low intracellular Na+ concentration. The Na-S cotransport mechanism can then bring about the uphill movement of S energized by the downhill flow of Na+. There are many Na+-coupled secondary active cotransport processes in animal cell membranes. They include sugar and amino acid uptake across the apical membranes of small intestinal and renal proximal tubule cells; the uptake of many L-amino acids by virtually all nonepithelial cells; and Cl" uptake by a variety of epithelial and nonepithelial cells.

Figure 17B illustrates another mechanism for secondary active transport that operates along similar principles. Let us assume that the carrier C has two sites, one facing compartment o and the other compartment i. Let

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