Determining the water Potential of Cereliac

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Determining the water Potential of Cereliac

The Celeriac Plant produces a beetroot like stem tuber. This stem tuber is called Celeriac.

Water leaves or enters cell by osmosis. This depends on the water potential inside and outside the cell. Generally water travels from a region of higher water potential to a region of lower water potential. However other factors like pressure can also have an influence on osmosis. ‘Pure water has the highest possible water potential.’

To determine the water potential of celeriac we must carry out some experiments using a range of solutions with different concentrations and viewing how the cereliac samples change in the solutions by using a microscope to see how many have then plasmolysed.

        In the class room an experiment was carried out where the incipient point of some onion cells was found in a very similar way as described above. There is a graph present showing the results of the experiment on a separate page. As shown on the graph it takes a sucrose solution of 0.5867 mol/dm³ to reach the incipient plasmolysis point of the white onion. The white onion therefore has a water potential of -1.667MPa. The water potential can be calculated from the table given on a separate page when one knows the sucrose solution concentration.

‘The point at which pressure potential has just reached zero and plasmolysis is about to occur is referred to as the incipient plasmolysis.’ This means that the water potential of the cell (Ψcell) is equal to the solute potential (Ψs) only as pressure potential (Ψp) is zero at this point:

Ψs+Ψp=Ψcell, since Ψp = 0 the equation simplifies to Ψs = Ψcell

 

The incipient point of the cells being viewed by the microscope can be found by finding the percentage of the cells that have plasmolysed. This can be done by dividing the number of cells that have plasmolysed by the total number of calls then multiplying by one hundred. When the percentage of plasmolysis cells is 50 % the cells are said to be at their incipient plasmolysis point. This is because half of them are plasmolysed and the other half are not. The concentration of the sucrose solution surrounding the cells at this point has the same water potential as the cells. The water potential of the cells can thus be found by finding the water potential of the sucrose solution.

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        Sucrose solution is being used as the solute instead of another water soluble chemical because sucrose is the only non reducing sugar. This means that sucrose is quite unreactive and will give accurate results. Salts could not be used as it breaks down into ions when in solution and is thus unreliable.

Method

  1. First a series of sucrose solutions from 0.2 mol/dm³ to 1.0 mol/dm³. This can easily be done using two burettes. One with 1.0mol/dm³ sucrose solution and the other with distilled water. It is advised to make solutions every 0.05 mol/dm³ apart in concentration ...

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