Characteristics of secondary active transport

a. The transport of two or more solutes is coupled.

b. One of the solutes (usually Na+) is transported "downhill" and provides energy for the "uphill" transport of the other solute(s).

c. Metabolic energy is not provided directly, but indirectly from the Na+ gradient, which is maintained across cell membranes. Thus, inhibition of Na+,K+-ATPase will decrease transport of Na+ out of the cell, decrease the transmembrane Na+ gradient, and eventually inhibit secondary active transport.

d. If the solutes move in the same direction across the cell membrane, it is called cotransport, or symport.

-Examples are Na+-glucose cotransport in the small intestine and Na+-K+-2C1" cotransport in the renal thick ascending limb.

e. If the solutes move in opposite directions across the cell membranes, it is called countertransport, exchange, or antiport.

- Examples are Na+-Ca2+ exchange and Na*-H+ exchange.

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