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Water Potential of Potato Tuber Cells: The Density Method

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Valentina Zunarelli IB Biology Practical Water Potential of Potato Tuber Cells: The Density Method Introduction Aim To find out the water potential of potato tuber cells by testing whether a drop of coloured solution sinks or floats in test tubes containing solutions of different densities. This is called the density method. Hypothesis My hypothesis is that the drop of coloured solution will rise if the tissue gained water from the solution, making it denser. However, if the water potential is the same as the solution's, the drop will neither fall nor rise. Method Firstly label seven test tubes: 0.10, 0.15, 0.20, 0.25, 0.30, 0.50, 1.00 mol dm-3. Then, using a separate graduated syringe in each case, place 5cm3 of the appropriate solution in each test tube. ...read more.


This test tube of decanted solution should be placed in the test tube rack in the position of previously occupied by the tube containing the potato discs immersed in 0.1mol dm-3 sucrose. Do this with all the solutions. *Using a teat pipette to collect a small amount of blue sucrose solution from the 0.1 mol dm-3 sucrose tube. Now with great care, introduce a single drop of this blue fluid into the tube with the same concentration but without the methylene blue that was decanted from the potato slices. The drop should be released carefully into the centre of the liquid about 5 mm below the surface. Watch the drop and note whether it remains in the same place, sinks, or rises. ...read more.


At 0.3 mol dm-3 the speed at which the drop rose was extremely slow. When the solution is more concentrated than drop of blue sucrose solution the sucrose solution rises and vice versa when the sucrose is more concentrated than the solution. We have found the isotonic point (when the drop of blue sucrose solution remains still, does not rise nor fall); this is at about 0.5 Mol dm-3. Evaluation This experiment was rather successful since the isotonic point was found. However, it could have been repeated more times in order to have more fair results. Then an average could have been found and it would have been more accurate. Another factor is that the potato from which the cylinders are taken could be abnormal - this could be prevented by amalgamating sets of results, for example of a whole class, where each experimenter used a different potato. ...read more.

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