symport

Symport, or cotransport, is a membrane transport process in which two or more substances are moved across a membrane in the same direction by a single transporter protein called a symporter. This is distinct from antiport, where substrates move in opposite directions, and from simple facilitated diffusion, where a single substrate is transported down its gradient without coupling.

Mechanism and energy source: Symport typically uses secondary active transport. The downhill movement of one substrate down its electrochemical gradient provides the energy to move a second substrate against its gradient. The transporter binds both substrates on one side of the membrane, undergoes a conformational change to expose them on the other side, and then releases them before resetting. The process often involves the alternating-access model of transporter function. Depending on the substrates, the net charge transferred can be electroneutral or electrogenic.

Examples: A well-known example is the sodium-glucose cotransporter SGLT1 in intestinal epithelial cells and SGLT2 in kidney proximal tubules, which couple Na+ influx with glucose uptake. The lactose permease LacY of Escherichia coli is another classic example, coupling lactose transport to a proton gradient. In plants and many bacteria, proton or sodium symporters enable uptake of nutrients, such as nitrate or amino acids, along with H+ or Na+.

Significance and regulation: Symport systems are essential for nutrient absorption, renal reabsorption of glucose, and microbial nutrient uptake. Inhibitors of symporters have clinical relevance; for example, phlorizin historically blocked SGLT transport, and modern SGLT2 inhibitors (such as canagliflozin or dapagliflozin) are used to treat type 2 diabetes by reducing renal glucose reabsorption.