You are studying the transport of amino acids across the intestinal epithelium. You set up an in-vitro system (see diagram below) that has a layer of intestinal epithelium separating two chambers. You can alter the contents of the two chambers and you can also remove and test samples of their contents.

a) For your first experiment, the upper and lower chambers contain salt solutions with physiologically normal amounts of Na+, K+, other solutes, but no amino acids. You add the amino acid alanine to the upper chamber, and discover that it rapidly accumulates in the lower chamber. Describe 3 possible mechanisms that could explain this observation.

b) You have the same conditions as above, but this time you ad the chemical ouabain to the solutions. Ouabain is known to inhibit the action of the Na+/K+ pump. This time when you add the amino acid alanine to the upper chamber, it does not eneter the lower chamber at all. What does this experiment tell you about the likely mechanism of amino acid transport across the intestinal epithelium? Why would blocking the Na+/K+ pump, prevent amino acid transport?

c) You perform one final experiment. This time you fill the chambers with a Na+ -free solutions (the solutions are otherwise the same as in the first experiment). What do you predict will happen in this case? Explain