The gradients established by primary active transport (like the sodium gradient) store potential energy that can be harnessed for other transport mechanisms. This is known as secondary active transport or co-transport. For instance, the high concentration of sodium outside the cell (created by the sodium-potassium pump) drives sodium back into the cell down its gradient. Cells couple this inward rush of sodium to the simultaneous transport of other molecules, such as glucose or amino acids, essentially using the stored energy of one gradient to power the movement of another substance.
This process is carried out by specific transmembrane proteins, often referred to as "carrier proteins" or "pumps." These proteins bind to a specific substrate, change shape using energy derived from ATP, and release the substrate on the other side of the membrane.