What are the steps involved in the transportation of Na+ and glucose?
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Solution
Transportation of sodium and glucose: Secondary active transport makes use of the energy stored in the electrochemical gradient (set up by the active transport) to shift other substances against their own gradients. Hence, there is no immediate requirement for a chemical energy source like ATP.
The concentration of sodium ions is higher outside the cytosol (due to the sodium-potassium pump). As a result, an electrochemical gradient is created because the exterior of the cell is more positive compared to the interior of the cell.
The energy stored in this electrochemical gradient is used by the Sodium-Glucose Transporters (SGLTs). These proteins are located in the membranes of the intestinal and kidney cells that facilitate the absorption of glucose into the bloodstream.
The movement of sodium ions into the cytosol is coupled with the transport of glucose using the carrier proteins. These carrier proteins use the energy of the gradient formed by sodium ions for the transport of glucose against its gradient (because glucose is found in higher concentrations inside the cell).
A glucose molecule and two sodium ions bind to the SGLT. Glucose hence moves uphill against its gradient while sodium ions move down their gradient.
Thus, the energy source is the electrochemical gradient, instead of ATP.
