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The endeavour of this investigation is to ascertain if there is any effect of water potential changes on potato tissue.

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Aim: The endeavour of this investigation is to ascertain if there is any effect of water potential changes on potato tissue. Background knowledge: There are four key aspects of natural science which are pertinent to this investigation and must be discussed prior to conducting the experimentation. These are: ==> Osmosis ==> Water potential ==> Solute potential ==> Pressure potential I will also discuss how the different potentials affect cells in general Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. It is regarded as a special kind of dissemination in which water molecules are the only molecules diffusing owing to the presence of a partially permeable membrane which does not allow the passage of solute particles. Water molecules move from a region of their high water potential (a dilute solution) to a region of their low water potential (a more concentrated solution), through a partially permeable membrane. It is thought that this process may occur more rapidly than can be accounted for by simple diffusion, and that mass flow is involved because the membrane is also partially permeable to some solute molecules. Water potential, w, 'is a measure of the ability of water molecules to move from one region to another.'1 The more water molecules there are per volume of the cell the more likely that by random movement they will collide with the cell's plasma membrane, and travel out of it. Pure water has a w of 0. As all solutions have less water molecules per volume than pure water they have a lower w; therefore all solutions have negative water potentials. The movement of water molecules is not totally random. The net movement of water molecules is always from a region of high water potential to one of lower water potential. They move down a water potential gradient until equilibrium is reached. ...read more.


pushes up against the cell wall, which provides pressure to stop the cell collapsing. When the potato is placed in a higher concentration of solute or a hyper tonic (1.0M) solution the water will move out of the cell as it performs osmosis. This will result in the vacuole shrinking and the protoplast pulling away from the cell wall, which will no longer be under any pressure. The space between the cell wall and shrunken protoplast will be filled with the external solution, as the cell wall is completely permeable. Osmosis will occur until equilibrium is reached and the water potential outside the cell is equal to that in the cell. Therefore the water potential of the potato cells can be found by placing the potato tissue in an external solution, which produces no change in mass or length in the tissue. The osmotic potential can be found by balancing the tissue with an external solution, which produces incipient plasmolysis. Incipient plasmolysis is when pressure potential has just reached zero: ?p = 0 , so ?w = ?s Water potential = Solute potential + Pressure potential ?w ?s ?p Summary: When the potato is placed in a solution with a higher water potential (less negative) the potato cells will gain in mass and when placed in a solution with a lower water potential the potato will lose mass, as the net movement of water molecules is down a water potential gradient. I expect as the sucrose concentration increases the mass of the potato chip will decrease due to the greater water potential being in the sucrose concentration each time until it reaches a point where the equilibrium is reached and the water potential being equal in both the cell and sucrose solution. This will mean there no will be no net movement of water so the mass stays the same and the water potential then is found by reading of the molarity found. ...read more.


Percentage change in mass of 3 chips 0.25 -613 0.30 -735 0.35 -858 0.40 -981 0.45 -1103 0.50 -1225 Range of Change * I will try to use 1.0 grams as the initial mass. It is much easier to then calculate the %change in mass. Comprehensive Analysis of results: I will first record my readings into the table above and then for each result I will find the molarity and find the analogous water potential of the potato cell. From this I could check if the equilibrium point is also correct with molarities which create no change in mass. This will tell how accurate my experiment is as well. The water potentials will be found using a secondary source, a book called 'biological science' which contains a table with sucrose concentration and the corresponding water potential. From the results I will plot a graph of change in mass (g) against the water potential of the sucrose concentrations. And one for percentage change of mass against water potential of the sucrose concentration. The water potential of the potato cells can be calculated from the intercept on the x-axis on the graphs. Because at this point there is no change in mass therefore it is the point of equilibrium where the ? is the same inside the potato cells and in the external solution. This point is incipient plasmolysis. The x-intercept can be calculated from both graphs, however the ? of the potato calculated from the graph showing percentage change would be more accurate as the numbers are larger and they cover a larger range and the larger numbers will reduce the percentage error; also the initial mass is taken into consideration, although in my particular experiment the initial masses were all equal. The values should be very alike. The graphs should look like the specimen below from a past preliminary experiment: Appendix: I have four different sources of secondary data that are required for cross referencing: Table 1 Table 2 Table 3 Table 4 The table in secondary preliminary experiment was courteously taken from a source from the internet legally. ...read more.

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