Dialysis tubing is a selectively permeable membrane that can be filled with a solution to simulate a cell. A piece of dialysis tubing has been filled with a cloudy white solution of 5 percent starch, 5 percent egg albumin (protein), and 5 percent glucose (a small sugar) dissolved in water to simulate a cell. This cell is weighed and then placed in a beaker of 3 percent iodine dissolved in water. When iodine and starch are mixed together, they form a bluish-black compound that is easily seen. After soaking in the yellow iodine-water solution for one hour, the cell is once again weighed. You also notice that the solution inside of the cell is now a dark blue color while the solution in the beaker is still a light yellow color from the iodine. What do you expect to happen to the weight of
the cell over time and what can you deduce from the color change?
a. The cell should lose weight over time because it is hypotonic to the beaker environment and water will enter the cell through osmosis. The color change indicates that the iodine molecules were small enough to diffuse across the membrane and react with the starch inside the cell.
b. The cell should lose weight over time because it is hypertonic to the beaker environment and water will leave the cell through osmosis. There is nothing that the color change can tell us.
c. The cell should gain weight over time because it is hypertonic to the beaker environment and water will enter the cell through osmosis. The color change indicates that the iodine molecules were small enough to diffuse across the membrane and react with the starch inside the cell. No color change in the beaker tells us that the starch was too large to cross the membrane from the cell and into the beaker.
d. The cell should gain weight over time because it is isotonic to the beaker environment and water will be drawn into the cell because of the albumin. The color change does not really tell us anything about the experiment.
Answer: c. The cell should gain weight over time because it is hypertonic to the beaker environment and water will enter the cell through osmosis. The color change indicates that the iodine molecules were small enough to diffuse across the membrane and react with the starch inside the cell. No color change in the beaker tells us that the starch was too large to cross the membrane from the cell and into the beaker.
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