How is the glucose concentration gradient maintained between the inside of the small intestine and the villi?
On the apical surface of the epithelial cells lining the lumen of the gut (the side of the cells lining the gut that faces the inside of the small intestine), there is a glucose-sodium symporter. What that means is that there is a protein transporter that uses active transport to drive the uptake of glucose into the cell (the cell is in the villi). The active transport is driven by the sodium gradient since import of sodium (down its concentration gradient) is coupled with the import of glucose (up its concentration gradient). On the basal and lateral sides of the cells in the villi, glucose can passively transport out through a uniporter, diffusing into other body cells. It is the glucose-Na symporter, though, that maintains the gradient.
