Tuber Cells

DETERMINATION OF THE WATER POTENTIAL OF POTATO TUBER CELLS.

Method.

Five sucrose solutions with varying molarity and one control containing distilled water were prepared and poured into test tubes. The potato discs were dried, weighed and added to the test tubes. The discs were then weighed again after a period of 24 hours. The percentage change in mass was then calculated.

Apparatus.

 Specimen tubes with stoppers x6

 1cm3 diameter cork borer

 razor blade

 filter papers

 balance

 distilled water

 sucrose solutions with varying concentrations

 potato cut into small discs

Results. (Fig 1.0)

Sucrose concentration (M) Initial mass

(kg) Final mass

(kg) Change in mass

(%)

0.10 0.95 1.25 31.57

0.20 0.94 1.13 20.21

0.30 1.03 1.15 11.65

0.40 0.95 1.05 10.52

0.50 0.88 0.86 -2.27

0.60 0.93 0.84 -9.67

Control 0.99 1.40 42.41

Discussion.

Osmosis is the passive diffusion of water molecules across a selectively permeable membrane from a down a concentration gradient. The water potential of a system is the tendency for water to exit the system. In this experiment the aim was to measure the tendency for water to leave the tuber cells. As the water potential of pure water is zero the concentration of sucrose in solution will have an effect on the water potential, this is called the solute potential. The greater the concentration of sucrose the more negative the water potential, because water moves from a high to low water potential.

When the potato is put into water it contains solute molecules which draw water in providing the external solute concentration is lower. The more s...

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...ent was quite limited as the degree of experimental error that could occur gives rise to scepticism over the validity of the results. The cutting up of the potato into pieces of equal surface area and mass was extremely difficult given the equipment available. If error had occurred then this would have an effect on the amount of water moving to and from the cells. However this may have had little influence in this case, as the potato discs were meticulously weighed and cut out. Another area of likely error was the drying out of the discs. There was a degree of difficulty in deciding if the discs were dry enough, and the length of drying time each disc received. If the discs had not been dried sufficiently then the percentage change in mass recorded would have been greater than before. Such a change would result in a more negative water potential, making the results less accurate.

If given the chance to repeat this experiment I would like to use more solutions ranging from 0.30M to 0.60M to give a more accurate graph, thus giving a more accurate determination of the water potential. I would also like to determine with the same degree of accuracy the point of incipient plasmolysis.