Now let's move to the lumen (apical) side of the epithelial cells. As stated above, a glucose molecule can only enter the cell together with a Na∗. (Since the two solutes are transported in the same direction, this type of co-transport is a symport system.) Draw the transport protein that transports glucose and sodium ions into a cell from the gut (label it SGlut-1). This Sodium/Glucose co-transporter allows the glucose to be transported into the cell against its concentration gradient (remember, the solution in the gut is dilute compared to the cell).
Questions:
a. What is the direction of the sodium gradient across the lumenal membrane? That is, on which side of the membrane is the concentration higher, and on which side is it lower? Indicate this on your drawing. (i.e. show where sodium is higher and where it is lower and add an arrow that follows the gradient from high to low - label it "sodium gradient")
