Plan an experiment to compare the water potentials (Ø) of beetroot, potato and turnip.

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Biology – Planning Exercise

Task

Plan an experiment to compare the water potentials (Ψ) of beetroot, potato and turnip.

Preliminary experiments

Two preliminary experiments were carried out. One into the water potential of beetroot and one into the water potential of potato. Beetroot and potato tissue both store food substances for the plants from which they grow. The cells therefore have large vacuoles, filled with cell sap, which is a solution of ions, sugars and enzymes capable of digesting proteins. The composition of cell sap is different in potato and beetroot cells and this will affect their water potentials.

        Although there are differences between the two tissues, their structures are sufficiently similar that they will respond in the same way to being immersed in hypertonic and hypotonic solutions.

Preliminary experiment 1 (20th November 2003)

6 specimen tubes were labelled and equal volumes of various concentrations (molarities) of sucrose solution were added to the tubes. The first tube contained 10ml of distilled water, the second contained 10ml of 0.1 molar solution, the third had 10ml of 0.2 molar solution, the fourth had 10ml of 0.3 molar solution, the fifth had 10ml of 0.4 molar solution and the sixth had 10ml of 0.5 molar solution.

        Then 6 pieces of potato were borne, and each was cut down to 12mm in length. Each 12mm strip was then cut into 6 discs of 2mm in length. Then, each group was placed on a separate sheet of filter paper.

        Each sheet of filter paper was weighed prior to the discs being placed on them, and then weighed again with the discs on them. The mass of the discs was then calculated from these figures.

        Each group was then put into the labelled tubes and rubber stoppers were placed firmly into them and they were left for 24hrs. Each group, its mass and the tube that the discs were in were recorded in the table below.

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After 24hrs the discs were removed from the tubes. Any excess fluid was quickly and gently removed with the filter paper, using a standardised procedure for all of them. They were then reweighed and the new masses were recorded (see table on previous page).

        Using this table to plot a graph of molarity against % change, the molarity at which no change occurs, was plotted on a standard molarity against osmotic potential graph. I found that the water potential of the potato tuber is an estimated –860 Kpa.

Preliminary experiment 2  (30th March 2003)

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