The tight junctions form a seal between absorption cells, preventing any species from leaking directly from the gut to the extracellular fluid, or vice versa. This helps maintain the concentration gradients the the enterocytes use to drive nutrient absorption. In addition, the tight junctions serve as a barrier between the apical and baso-lateral membranes. If the passive and active transporters were free to move between them, there would be no net flow of nutrients into or out of the gut.
