Final answer:
The glucose-Na+ symport is an active-coupled transporter that uses energy from the sodium ion gradient created by Na+/K+ ATPase pumps to transport glucose into cells against its concentration gradient, a process called secondary active transport.
Step-by-step explanation:
The glucose-Na+ symport is an active-coupled transporter that utilizes the electrochemical gradient of sodium ions to facilitate the uptake of glucose into the cell against its concentration gradient. This process is a form of secondary active transport because it derives its energy from the primary active transport of sodium and potassium ions by the Na+/K+ ATPase pump. Sodium ions, which are actively pumped out of the cell to maintain a low intracellular concentration, move back into the cell through the symporter, thereby providing the energy needed to transport glucose. Consequently, glucose absorption is achieved even when the glucose concentration is higher inside the cell than in the extracellular fluid.
In the kidneys, symporters are crucial for glucose reabsorption, where they operate on the apical membrane of renal tubular cells. The Na+/glucose symport protein assists both Na+ and glucose to move into the cell, with subsequent glucose diffusion into peritubular capillaries. Thus, the transport of glucose is effectively linked to the transport of sodium ions, making it an efficient cellular mechanism.