Determination of the water potential of root/tuber cells by the weighing method.

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Determination of the water potential of root/tuber cells by the weighing method

Introduction

When a plant cell is bathed in a solution of the same water potential, its mass and volume remain the same, because water enters ad leaves at the same rate.

If samples of tissue are immersed in a range of solutions of different concentrations (molarities), the cells will gain water by a method known as osmosis and therefore, mass, in solutions of higher water potential and lose water and mass in solutions of lower water potential. The water potential of the tissue is equal to that of the solution in which it neither gains nor loses mass. The purpose of this practical is to estimate the water potential of the potato tuber cells by this method.

In practice, none of the experimental solutions is likely to have exactly the same water potential as the cells but the solution in which there would have been no gain or loss in mass can be estimated from the graph.

Root vegetables and potato tubers are widely available in bulk, but in principle this technique could be applied to any plant tissue.

Apparatus

* Boiling tubes x6

* Boiling tube rack

* Wax pencil

* Cork borer

* Razor blade or sharp knife

* White tile

* Root/tuber

* Filter papers

* Forceps

* Balance to 0.001g

* Distilled water

* 1.0 mol sucrose solution

* 2x 25cm3 measuring cylinders

* Stop clock

Procedure

. Label 6 boiling tubes "DW" (distilled water), 0.2, 0.4, 0.6, 0.8 and 1.0 mol dm3. Place 20ml of distilled water in the first tube and the appropriate amounts of sucrose solution and water in each of the other tubes to make up the correct concentrations.

2. Using a cork borer and a razor blade or sharp knife, prepare 6 root/potato cylinders, each about 10mm diameter and 50mm long. Place each on a separate piece of filter paper on which you have labelled with the figures 0, 0.2, 0.4, 0.6, 0.8 and 1.0 respectively.

3. Take the cylinders and the boiling tube rack to a balance. For each cylinder, record its mass on the filter paper, transfer it to the boiling tube with forceps, make a note of which tube it is in and record the mass of the filter paper on its own. Calculate the initial mass of each cylinder once you have transferred them all to the tubes. Note the time.

4. After at least 30minutes, remove the cylinders from the tubes in turn and in the same order that you inserted them. Remove any surplus fluid quickly and gently with the filter paper. Then reweigh each cylinder and record its mass.

5. Work out the percentage change in mass of each cylinder (change in mass x 100 ÷ original mass) Plot this against the molarity of the sucrose solutions. Your vertical axis will have increased mass at the top, no change in mass in the middle and decrease in mass at the bottom.
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6. Calculate the water potential of the potato cells as follows. Find where your line crosses the place on the vertical axis corresponding to no change in mass. Read off the horizontal axis the molarity of sucrose at this point. From the table below, find the water potential of a sucrose solution of that molarity. That is the water potential of your sample of potato cells.

Table: relationship between molarity and solute potential of sucrose solutions

Molarity (mol dm-3)

Solute potential (Kpa)

0.05

-130

0.10

-260

0.15

-410

...

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