The gut epithelial cells transport glucose from the gut lumen across the gut epithelium and, ultimately, into the blood. T he gut epithelial cells possess a glucose– Na + symport, which they can use to take up glucose from the gut lumen, even when the concentration of glucose is higher in the epithelial cell’s cytosol than it is inside the gut.

As the electrochemical gradient for Na + is so steep, when Na + moves into the cell down its gradient, glucose is, in a sense, “dragged” into the cell along with it. Because the binding of Na + and glucose is cooperative—the binding of one enhances the binding of the other—if one of the two solutes is missing, the other fails to bind .

The gut epithelial cells have two types of glucose transporters located at opposite ends of the cell. In the apical domain of the plasma membrane, which faces the gut lumen, they have the glucose–Na + symports. In the basal and lateral domains of the plasma membrane, the cells have passive glucose uniports, which release the glucose down its concentration gradient for use by other tissues.

The two types of glucose transporters are kept segregated in their proper domains of the plasma membrane by a diffusion barrier formed by a tight junction around the apex of the cell. This prevents mixing of membrane components between the two domains.