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An Investigation into the water potential of a white potato

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Biology Coursework An Investigation into the water potential of a white potato Ben Smith, 10S Aim: To discover the water potential of a white potato, by investigating the extent to which osmosis occurs in sucrose solutions of differing concentrations. Prediction: Osmosis is defined as "the spontaneous passage or diffusion of water or other solvents through a semi permeable membrane" (such as a cell membrane). The presence of a cell membrane is important because it allows only particles small enough to pass though it to escape, placing a limit on the extent to which osmosis occurs. During my research into "stem tubers" (the part of the potato plant we eat), I found that the energy contained in them was converted into starch for storage. Starch is an insoluble material, with a fairly large size and therefore cannot pass through the membrane, and also cannot dissolve and then pass through the membrane. These characteristics of starch as a storage medium will limit how much osmosis occurs in the potatoes I am testing. However, I believe some osmosis will occur, as not all glucose will have become starch. I think there will be enough un-converted glucose for osmosis to have an effect. Using my knowledge of osmosis, and the chemical makeup of a potato, I think the water potential will lie below ?0.5, and possibly below ?0.4, depending on whether the potato is particularly moist or green. ...read more.


Originally I had planned to measure all the concentrations between 0 and 1M, in 0.1M increments. However, In my hypothesis I have predicted that the water potential will lie under 0.5M. To save time during the actual experiment, I found what readings were redundant. I did this by carrying out the experiment exactly as planned, but afterwards I saw that above a 0.6mola concentration, no significant change in mass was recorded. This indicates that above this concentration all osmosis change had finished, and these results were redundant. To save time, I will not carry out the experiment with these concentrations. Constants and Variables: In order for the experiment to be fair, I must ensure that each solution and potato pieces are tested under identical circumstances. These are the factors that I will keep the same: * The temperature at which the experiment occurs: this will be room temperature for all of the solutions. * The volume of the solution: there will always be enough to completely cover the potato pieces, and this amount will be kept the same across all the solutions. * Type of potato: As stated before, I will always use the common "White" potato (or "Solanum tuberosum"). This is because different potatoes have a different makeup in the cells (Eg: differing % amounts of starch) * Duration of experiment: All the potato pieces were left in the solutions for 1 hour. ...read more.


During the experiment I received two anomalous results, but fortunately I was able to repeat them and the repeated (and correct) results are included on the graph. I found that these "freak" results were a result of discrepancies between the measurements of the beaker (which were only approximate) and the measuring cylinder (which was far more accurate). Because the amounts of sucrose and water mixed was wrong, this gave a different concentration and could have heavily swayed my results if I hadn't corrected the error. Looking at the graph, I believe I collected sufficient evidence to allow me to find a satisfactory result. The fact the figures obtained follow a line of best fit, and in every case both potato pieces tested the changes match means the evidence is quite reliable. To improve the experiment in future I could try testing multiple potato pieces in different beakers, to prevent any mistakes when making up the solution. However, for my experiment I don't think this would have helped, but it is something I could investigate. I could also improve my experiment by attempting to be more consistent when drying excess solution before weighing. Although this only makes a slight difference, I may have accidentally dried one piece more thoroughly then the other, affecting my results. In future, I could find a different method, or attempt to be consistent. These are the only improvements I can think of, because my experiment was already quite reliable and accurate. ...read more.

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