706_F06pset4_ans - 7.06 Fall 2006 Problem Set #4 1. How is...

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7.06 Spring 2006 PS4 KEY – revised 1 of 5 7.06 Fall 2006 Problem Set #4 1. How is glucose transported from the lumen of the intestine across the intestinal epithelium into the blood? Explain the proteins and concentration gradients involved. As glucose is moved across the intestinal epithelium, it is transported up a concentration gradient to enter epithelial cells and then down a concentration gradient to the blood. Transport of glucose into epithelial cells from the intestinal lumen is mediated by a two Na + /glucose symport protein. The energetically favorable cotransport of Na + powers glucose import. The GLUT 2 uniport protein facilitates the movement into the blood stream of glucose at the basolateral surface of epithelial cells. The Na + /K + ATPase establishes the Na + across the epithelial cell membrane. 2. Cholera toxin, produced by Vibrio cholerae , a pathogenic intestinal bacterium, causes an indirect reduction in the activity of the Na + /K + ATPase in intestinal epithelial cells. This results in reduced uptake of small sugars and amino acids from the intestine. Why does this reduction in uptake result from impaired Na + /K + function? Sugar and amino acid uptake from the intestine is mediated by symport proteins located in the apical membrane of intestinal epithelial cells. These symporters use the cotransport of Na + down its electrochemical gradient to drive sugar and amino acid movement up a concentration gradient. Impaired functioning of the Na + /K + ATPase in the basolateral membrane results in a decreased Na + gradient because this pumps Na + out of the epithelial cells into the blood. As a result of the decreased Na + gradient, the ability of the epithelial cells to take up sugar and amino acids against a concentration gradient is reduced. 3. When present in the culture medium, a drug called ouabain inhibits the Na + /K + ATPase in cultured cells, but when the drug is microinjected into cells, it exerts no inhibitory effect. Although ouabain itself possesses no negative charge, ouabain treatment causes the alpha subunits of the Na + /K + ATPase to become more negatively charged but does not alter the charge properties of the beta subunits. a. On which side of the plasma membrane is the ouabain- and ATP-binding sites of the ATPase located? The ouabain-binding site of the Na + /K + ATPase must be located on the extracellular face of the plasma membrane, as microinjected ouabain does not inhibit the ATPase. The ATP-binding site is located on the cytoplasmic face of the plasma membrane, where it has access to ATP in the cytosol.
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7.06 Spring 2006 PS4 KEY – revised 2 of 5
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This note was uploaded on 01/23/2012 for the course BIOLOGY lsm1301 taught by Professor Seow during the Spring '11 term at National University of Singapore.

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706_F06pset4_ans - 7.06 Fall 2006 Problem Set #4 1. How is...

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