cotransports

Cotransport, also known as secondary active transport, is the process by which two different solutes cross a cell membrane via a single transporter protein. In cotransport, the transport of one solute down its electrochemical gradient provides the energy to move a second solute, often against its gradient. The energy source is typically the gradient established by a primary active transporter, such as the Na+/K+ ATPase, which maintains the necessary ion gradients.

There are two main forms of cotransport. In symport, the two substrates move in the same direction across the membrane. In antiport, they move in opposite directions. The transporter undergoes conformational changes that shuttle the bound substrates from one side of the membrane to the other, coordinating binding and release to accomplish net solute movement without direct ATP consumption.

Key examples illustrate the scope of cotransport. The Na+-glucose cotransporters SGLT1 and SGLT2 import glucose into intestinal epithelial cells and proximal tubule cells of the kidney, using the downhill Na+ gradient. In bacteria and plants, proton-driven symporters couple substrate uptake to a proton gradient. Antiporters include the Na+/H+ exchanger (NHE), which swaps Na+ for H+, and the Na+/Ca2+ exchanger (NCX), which moves Na+ and Ca2+ in opposite directions to regulate intracellular calcium. Other examples are anion exchangers that swap Cl− for HCO3−.

Physiological roles of cotransporters include nutrient absorption, renal reabsorption of glucose, ion and pH homeostasis, and maintenance of membrane potential in excitable tissues. Clinically, cotransporters are targets for therapies such as SGLT inhibitors in diabetes management, and various inhibitors or modulators of cotransport activity influence fluid balance and electrolyte disorders.