Cotransport proteins (cotransporters or symporters) are secondary active transporters. They utilize the energy stored in the electrochemical gradient of a driving ion (typically, but not always, Na⁺ or H⁺), to drive the transport of another ion or molecule (referred to as the driven ion/molecule) across the plasma membrane. Cotransporters couple the movement of a driving ion across the plasma membrane down its electrochemical gradient to the uphill movement of the driven ion/molecule across the plasma membrane against its electrochemical or concentration gradient. In cotransport, the direction of transport is the same for both the driving ion and the driven ion/molecule. Cotransporters are found in many different cells and tissues and perform a variety of important physiological functions. Six examples are shown in this figure. The Na⁺/glucose cotransporter (SGLT1) is found in the apical membrane of kidney proximal tubule epithelial cells, where it functions along with SGLT2 to reabsorb glucose from the ultrafiltrate, as well as in the apical membrane of small intestine epithelial cells, where it functions to absorb glucose from the lumen of the small intestine. The Na⁺/phosphate cotransporter (NaPi) is found in the apical membrane of epithelial cells of the small intestine, where it serves to absorb dietary phosphate, and renal proximal tubules, where it serves to reabsorb filtered phosphate. The Na⁺/I⁻ symporter (NIS) is located in the basolateral membrane of thyroid follicular cells, and is responsible for the accumulation of iodide into the thyroid gland. The Na⁺/K⁺/2Cl⁻ cotransporter (NKCC), Na⁺/Cl⁻ cotransporter (NCC), and K⁺/Cl⁻ cotransporter (KCC) are found in many different cell types, where they perform essential roles in ion homeostasis. Na⁺, sodium; K⁺, potassium; Cl⁻, chloride; I⁻, iodide; P_i, inorganic phosphate.

See also:
Secondary Active Transport (in Lecture Notes)