In previous sections, we saw how ATP-powered pumps generate ion concentration gradients across cellular membranes and how K⁺ channels use the K⁺ concentration gradient to establish an electric potential across the plasma membrane. In this section, we see how cotransporters use the energy stored in electric potentials and concentration gradients of Na⁺ or H⁺ ions to power the uphill movement of another substance, which may be a small organic molecule such as glucose, or an amino acid, or a different ion. An important feature of such cotransport is that neither substance can move alone; movement of both substances together is obligatory, or coupled.

Cotransporters share features with uniporters such as the GLUT proteins. The two types of transporters exhibit certain structural similarities, operate at equivalent rates, and undergo cyclical conformational changes during transport of their substrates. They differ in that uniporters can only accelerate thermodynamically favorable transport down a concentration gradient, whereas cotransporters can harness the energy released when one substance moves down its concentration gradient to drive the movement of another substance against its concentration gradient.

When the transported molecule and cotransported ion move in the same direction, the process is called symport; when they move in opposite directions, the process is called antiport (see Figure 11-2). Some cotransporters transport only positive ions (cations), while others transport only negative ions (anions). Yet other cotransporters mediate movement of both cations and anions together. Cotransporters are present in all organisms, including bacteria, plants, and animals. In this section, we describe the operation and function of several physiologically important symporters and antiporters.