Example of Naglucose cotransport Figure

a. The carrier for Na'-glucose cotransport is located in the luminal membrane of intestinal mucosal and renal proximal tubule cells.

b. Glucose is transported "uphill"; Na+ is transported "downhill."

c. Energy is derived from the "downhill" movement of Na+. The inwardly directed Na+ gradient is maintained by the Na+-K+ pump on the basolat-eral (blood side) membrane. Poisoning the Na+-K+ pump decreases the transmembrane Na+ gradient and consequently inhibits Na+-glucose cotransport.

3. Example of Na+-Ca2+ countertransport or exchange (Figure 1-2) a. Many cell membranes contain a Na+-Ca2+ exchanger that transports

Ca2+ "uphill" from low intracellular [Ca2+] to high extracellular [Ca2+]. Ca2+ and Na+ move in opposite directions across the cell membrane.

Lumen Intestinal or proximal tubule cell

Secondary active

Blood

Lumen Intestinal or proximal tubule cell

Secondary active

Figure 1-1. Na'-glucose cotransport (symport) in intestinal or proximal tubule epithelial cell.

Primary active

Figure 1-1. Na'-glucose cotransport (symport) in intestinal or proximal tubule epithelial cell.

Secondary active

Ca2+

Secondary active

Ca2+

Primary active

Figure 1-2. Na'-Ca2' countertransport (antiport).

b. The energy is derived from the "downhill" movement of Na+. As with cotransport, the inwardly directed Na+ gradient is maintained by the Na+-K+ pump. Poisoning the Na+-K+ pump therefore inhibits Na+-Ca2+ exchange.

III. Osmosis

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