Final answer:
Sodium increases the affinity of the carrier molecule for glucose on the extracellular side during the cotransport of glucose with sodium. This process uses a symporter mechanism facilitated by sodium ions moving down their electrochemical gradient to aid in glucose transport against its gradient.
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 side. This occurs in a process known as secondary active transport, specifically via a symporter mechanism. Here, sodium ions entering the cell down their electrochemical gradient assist in the transport of glucose against its gradient into the cell. On the extracellular side, when glucose and sodium ions both bind to the carrier protein, this cooperative binding increases the carrier's affinity for glucose, thus facilitating its transport into the cell.
Notably, in the kidneys and small intestine, the sodium-glucose symporters (SGLT) operate by moving both sodium and glucose from the apical side (the side facing the lumen or exterior of the organ) of the cell into the cytoplasm. The sodium-potassium ATPase pump then actively transports sodium out of the cell on the basal side, maintaining the sodium gradient required for the symporter to function effectively.