This diagram shows the transcellular (transepithelial) transport of sodium ion a

Question

This diagram shows the transcellular (transepithelial) transport of sodium ion and substance X. a. Draw the graphs for the concentrations of Na^+ and substance X that go below the diagram to the left X axis is distance going from lumen on the left to blood on the right. Y axis are concentrations of Na^+ and substance X. b. Explain how the sodium concentration gradients can be used for transport of substance X from the lumen into the blood. Include the specific type of transport for both Na^+ and substance X on either side of the cell in your answer.

Explanation / Answer

b) In transepithelial transport, the epithelium layer will serve as a barrier amongst some other compartments of the body. In this case, let us consider substance X to be glucose. Trans epithelial transport of glucose across these barriers are brought about by proteins called glucose transporters. The glucose transporters shown in this case are sodium dependent glucose co transporters and belong to the SGLT family. The sodium potassium ATPase membrane protein is considered of prime importance in these transport systems.  The concentration of active the Na-K-ATPase inside the cells is straight away regulated by the intracellular sodium concentration and this along with basolateral potassium permeability regulation by intracellular ATP imparts the ability to the cells so as to maintain the ionic Na+/K+ homeostasis. This sodium dependent glucose transporter is present in the small intestine and is present in the apical plasma membrane of these epithelial cells.

At first, the Na+/K+ ATPase pump will use ATP to move three molecules of Na ions out side in the blood. Also give with this, they also bring in two molecules of potassium ions. This result ps in a downhill gradient of sodium ions inside the cell. This downhill gradient of the sodium ion provides energy to the SGLT proteins, which is generated by the ATPase, and this energy is used to transport the glucose molecule across the apical membrane. Here, since both Na and glucose are on the same side of the membrane and have to move in the same direction across the membrane, the SGLT are known as symporters.

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