Which of the following is an example of secondary active counter-transport?

To understand why the choice of the Na/H+ exchanger in the renal tubule is considered an example of secondary active counter-transport, it's important to consider the mechanisms of active transport in the context of how these transporters function.

Secondary active transport involves the movement of one substance across a membrane against its concentration gradient using the energy derived from the movement of another substance down its concentration gradient. In this case, the Na/H+ exchanger utilizes the sodium gradient, which is maintained by primary active transport mechanisms such as the Na/K pump. As sodium enters the cell passively—driven by its gradient—this energy is utilized to extrude hydrogen ions (H+) from the cell against their concentration gradient.

In the renal tubules, the Na/H+ exchanger plays a crucial role in regulating acid-base balance by allowing sodium to enter the cell while simultaneously transporting hydrogen ions out. This process highlights the counter-transport mechanism, as one ion is moved into the cell while another is moved out.

Focusing on the other choices:

• The Na/K pump is a primary active transporter, which directly utilizes ATP to move sodium out of the cell and potassium into the cell, thus not fitting the definition of secondary active transport.

• The Na/Ca exchanger is a