The abstract below was written by Melanie Shadish. It came so close (along with its support documents) to what I wanted that I have added nothing from abstracts by other students. Yes there is some editing that needs to be done, but this one person understood the exercise and communicated that understanding very well.
Osmosis and Water Potential in Potato Tissue
Melanie Shadish, Rob Harris, Patricia Tellekamp
Water appears to cross the differentially permeable membrane of potato cells by a process called osmosis. The measure of the energy involved in osmosis is called water potential. Since water must lose energy as it moves by osmosis, water must move from an area of greater potential to an area of less water potential, until the water potential on either side of the membrane is the same, at which point the cell has reached equilibrium. The two major parameters influencing water potential are solute and pressure potential. In most situations, the addition of these two values equals the water potential of the cell. In this experiment, equilibrium osmotic effects were first examined using potato tissue. Seven cups were set up, one as a control containing water and potato tissue, the other six with differing amounts of 1 M sorbitol stock solution and distilled water, and uniform cylinders of potato, in order to measure the gain or loss of water as an indicator of net osmosis. The cylinders were weighed, placed in their respective solutions, and weighed at 20-minute intervals to test for water loss or gain. Since the weight of the potato in the 0.4 M sorbitol solution did not change, it was determined that this was an approximately isosmotic solution. Finally the osmolarity of our potato sap was measured. To do this, a piece of undamaged potato was ground to a pulp in a mortar, and the sap was squeezed into an Eppendorff centrifuge tube and centrifuged for two minutes. The osmolarity of the sap was measured in a Wescor Vapor Pressure Osmometer. The osmolarity of our potato was 0.420 M, which is very similar to the concentrations of salts present in our potato sample in the isotonic solution. However the sorbitol solutions higher than 0.4 M were determined to be hyperosmotic, since the cells lost water, and those solutions lower than 0.4 M were determined to be hyposmotic, since the cells gained water. Indeed, the cylinders that had been in the 0.0 - 0.2 M solutions were much crispier and rigid than the cylinders that had been in the 0.5 - 0.6 M solutions due to turgor pressure, which increases the water potential of the cell. Next , the time course, or kinetics, of water movement was followed. The cylinders from the 0.0M and 0.5 M solutions, which had reached equilibrium with their respective solutions, were then switched. The data collection was repeated as before but at five-minute intervals. In this experiment, the relative cell volume of the 0.0 to 0.5 M cylinders decreased, as did the gross permeability of the cell. However, the relative cell volume of the cells from the 0.5 to 0.0 M solutions increased, but the gross permeability decreased. The reason why the former cell size decreased is because the cells were in a hypertonic solution, which meant that the solute concentration was greater in the solution than inside the cell. These cells had a high water potential. The latter cell's size increased as a result of being placed in a hypotonic solution in which the cells had a low water potential. In both the 0.0 to 0.5 M and 0.5 to 0.0 M reactions, the rate of water transport decreased over time. This is due to the fact that both reactions are approaching equilibrium. The permeability for both reactions also decreased, although the rate of reaction involving the 0.5 to 0.0 M solution decreased at faster rate than the other. This is most likely due to the fact that the cylinder from the 0.5 M solution was in a solution of greater solubility, where the permeability had been greater because the plasmodesmata of the cells were ripped as the cell shrunk. This lab was important in that it demonstrated the properties of water movement thorough a cell. It also enabled the student to measure this change in water potential.
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