What is an example of secondary active transport?

Secondary active transport refers to the movement of molecules across a cell membrane using energy derived from the electrochemical gradient established by primary active transport processes, rather than direct ATP hydrolysis. In this context, the sodium-calcium antiporter is a prominent example of secondary active transport.

The sodium-calcium antiporter utilizes the sodium gradient, which is established by a sodium-potassium pump (primary active transport), to facilitate the influx of sodium ions into the cell while extruding calcium ions. In this mechanism, sodium ions enter the cell down their concentration gradient, and this movement drives the opposite transport of calcium ions out of the cell against its own concentration gradient.

This process exemplifies secondary active transport as it relies on the energy stored in the sodium gradient rather than directly consuming ATP for the transport of calcium ions.