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Potato sap and salt solution experiment.

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AIM: An investigation to determine the concentration of a salt solution which is equal to the concentration of the cell cytoplasm and cell sap of potato cells. PLAN: Put potato pieces into a range of concentrations of salt solutions. Record the mass of each piece before and after the experiment. Draw a graph to show how the mass changes in different concentrations. PREDICTION: I believe that the mass of the potato pieces will change during the experiment. The pieces in the low concentration will increase in mass, whilst the pieces in the high concentration will decrease in mass. This is because of osmosis; the pattern will be a bit like the sketch graph opposite. EXPLAIN: A solution is made up of two things, a solute and a solvent. In the salt solution that we are using, the salt is the solute, and the water is the solvent. Inside the potato sap and cell cytoplasm, there is also a solution, consisting of salts and sugars (solutes) and water (solvent). When there is a high water potential in the salt solution (i.e. it is less concentrated than inside the potato cells), the water will move down its concentration gradient into the potato cells. This process is known as osmosis. The water molecules will diffuse through the partially permeable cell membrane of the potato cell to get to the cell sap and cytoplasm and reach equilibrium. ...read more.


Light Can create heat Keep the specimens away from the windows and sources of bright light. Type of potato Some may have different saps and cytoplasm Use the same species of potato Volume Could affect rate of osmosis Keep all volumes of salt solution consistent: 20cm3 Preliminary Work: The following preliminary work helping me in my plan by telling me 2 things: 1 In distilled water, the potato is gaining mass, while in the 0.5 molar salt solution it is losing mass. This means that the point at which no mass change occurs, must be between these two, making this a good range of concentration to use. 2 Mass change continues for some hours but slows down after around 3 hours and in the case of the salt solution actually reverses. This means that the optimum time to leave the potato before reweighing is 3 hours. Potato cylinders were placed in distilled water and in 0.5 molar salt solution and reweighed at intervals: Time (minutes) Mass of cylinder in water (g) Mass of cylinder in 0.5 molar NaCI 0 2.46 2.53 12 2.52 2.45 31 2.56 2.32 57 2.63 2.17 72 2.67 2.10 90 2.70 2.05 102 2.72 2.01 120 2.74 1.96 147 2.75 1.91 170 2.76 1.90 204 2.75 1.89 217 2.76 1.87 1186 2.92 1.92 1906 2.92 1.92 2206 2.96 1.94 2268 2.98 1.95 PROCESS: Using the ...read more.


is 0.18. These results mostly support my predictions. The masses of the potato pieces definitely change in different concentrations. The ones in higher concentrations did decrease in mass, whilst the ones in lower concentrations did increase in mass. My graph is also quite similar to the one I made in my predictions. As I predicted the graph is inversely proportional, although it is not a straight line graph but is curved. EVALUATION: I believe that my experiment was quite successful, as I only had three anomalous results. This means that my experiment must have been relatively accurate. The procedure used was suitable and successful. One thing that could be done to improve results would be to take repeats yourself, so as everything is done the same for all of the results. The anomalous results are probably due to either inaccurate measuring or weighing, but could also be down to the fact that each set was taken by a different person, at different times. However as I only found three anomalous results, I believe that my evidence is strong enough to support my conclusion. Further work that would provide additional evidence to support or disprove my conclusion would be to use more concentrations of salt solution. By this I mean use concentrations between 0.1 and 0.3, but at more regular intervals (e.g. 0.12, 0.13, 0.14 etc.). One problem with this, however, is that it is not possible to ensure that the concentrations are measured to this accuracy. ...read more.

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