Find the water potential of potato cells.

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Paul May 11B

GCSE Coursework investigation – Water Potential of Potato Cells

Aim: to find the water potential of potato cells

Introduction

In my investigation, I hope to determine the water potential of potato cells. The plant material I will use in my investigation will be potato chips, which I will cut myself prior to the investigation.

 I hope to find the concentration of a solution where the chip stays the same mass and length, which will tell me that the water potential inside the cells is the same as the water potential outside the cells.

Background

Osmosis is defined as the movement of water through a semi-permeable membrane. Semi – permeable membranes are permeable to water but impermeable to certain dissolved substances. In the case of this experiment, the sucrose molecules cannot diffuse through the cell membrane because the sucrose molecules are much bigger than water molecules.

The water potential of a solution is a measure how dilute the solution is, and therefore of how readily water will diffuse from this solution, through a semi-permeable membrane, into another solution. Pure distilled water has a water potential of zero: the water potential becomes more negative as more substances are dissolved in it. When two solutions are separated by a semi-permeable membrane, water moves from the side with high water potential to the side with low water potential:

If we dissolve substances in water we lower the water potential of the solution:-

High water potential                         Some water potential                     Low Water Potential

                                               

           ‘Turgid’                                Normal                                ‘Plasmolysed'

So, in basic terms, in my experiment:

                                           

Chip grows                        Chip stays the same                        Chip shrinks

Key Variables

  • Mass of the chip – I think that the larger the mass of the chip then the bigger the difference in results will be, therefore I think that to make experiment more easy it would be best to use a large chip.
  • Dimensions (length, cross-sectional area) – I believe that changing the dimensions of the chip would have a similar effect to changing the mass of the chip. I will measure the length and cross-sectional area for each chip.
  • Time the chip is left in the solution – I think that if I were to leave the chips in the solution for only a small amount of time there might not be sufficient time for the osmosis to occur, so that the chips would not differ in size enough for conclusive results. Therefore, I will need to give a good amount of time for the osmosis to occur.
  • Temperature – I believe that all reactions happen quicker when there is a higher temperature, so diffusion across the membrane may be faster at higher temperature.
  • Concentration of sucrose solution – I think that the lower the concentration of the external sucrose solution, the more the chip will expand (because the cells become turgid).
  • Total Volume of sucrose solution – I think that changing the volume of sucrose solution may affect the results of my investigation.
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Variable I will change:-

I will change the concentration of sucrose solution in my experiment, because I believe the results of that experiment will tell me the water potential of the potato cells.

Fair Test

I will carry out all my experiments at the same temperature, 20°C, because that is room temperature, and I will not have to heat up anything. I will cut my chips to the same dimensions every time, using a ruler, and a sharp knife (for accuracy) – to measure the cross-sectional area I will use a calculator. I will also put all ...

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