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.

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        Pipette 3cm3 of sucrose solution with each sucrose concentration into other test tubes and label them. Place one drop of methylene blue in each of these test tubes. This will colour the solution but not affect its water potential. These are to be shared among the class.

        Next, using a cork borer and a razor blade, prepare a solid cylinder into thirty discs of approximately equal (2mm) thickness.

        Note the time and place four of the potato discs into each of the seven 3 cm3 test tubes of sucrose solutions. Manipulate the discs so that they are all covered ...

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