Final answer:
Sodium increases the affinity of the carrier molecule for glucose when the binding site is facing the extracellular fluid. This occurs during secondary active transport, such as the sodium-glucose symporter mechanism.option E is correct answer.
Step-by-step explanation:
During cotransport of glucose with sodium, sodium increases the affinity of the carrier molecule for glucose when the binding site is facing the extracellular fluid. So, the correct answer is E) the extracellular fluid.
Symport mechanisms, like the sodium-glucose symporter, move two or more substances in the same direction. This process is a type of secondary active transport, where the movement of glucose against its concentration gradient is powered by the movement of sodium down its electrochemical gradient. The sodium ions have a high affinity for the symporter when they are on the exterior side of the cell, and their entry provides the necessary energy to also transport glucose molecules into the cell.
It is the action of the Na+/K+ ATPases on the basal membrane of cells, such as those in the kidney tubules, that maintains a low concentration of sodium inside the cell, creating a gradient that drives the cotransport mechanism. Once sodium and glucose are inside the cell, glucose either gets utilized or moves further into the body's tissues, while sodium gets pumped back out to maintain the gradient.