Investigation to determine the water potential of potato cells.

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Investigation to determine the water potential of potato cells.

Planning

Aim

Carry out an experiment to determine the water potential of potato cells.

Variables

Independent Variable

The variable that will be changed in this experiment is concentration of sucrose solution. I am using this variable because preliminary results show that this method is the best to use to determine the water potential of the cell contents.

Dependent Variable

The variable that will be measured is the mass change of the potato chips I can use these results to determine the average mass change per gram of the potatoes and I can then find out what the isotonic solution is for the potato chips, the concentration for the isotonic solution for the potato chips is the molarity of the potato cell contents and using this I can find out the water potential of potato cells.

Control Variables

There are many factors which control osmosis and the water potential of the cell contents these must be kept constant to ensure the experiment is a fair test.

When the temperature increases, the rate of osmosis increases because the water particles have more energy and can move faster, so they will leave or enter the potato cells more quickly. At cold temperatures the water molecules will have less energy so less molecules will able to diffuse in the allotted time. If the temperature fluctuates significantly the results may be inaccurate, because osmosis will occur faster and 30 minutes at a cold temperature may give different results to 30 minutes at a warmer temperature. If time was extended this would not make a difference because equilibrium between the concentrations of the potato contents and the surrounding solution will still be reached. The temperature will therefore be regulated to room temperature.

The size of the potato chip will affect the final mass, if I use a thin chip and a fat chip the average mass change per gram will be different because if they are placed in distilled water then the thin chip will become turgid long before the fat chip does because the water has to diffuse from the outside of the chip to the centre of the chip.

The volume of the solution used will affect the concentration of the solution so I must keep the concentration of the solution constant, it will be kept at 20cm3.

Different potatoes have different properties, some have differing cell contents, so the same type of potato must be used for each experiment.

The balance used must be the same for every chip because different balances may have different standards of accuracy.

All potato chips will be kept in their respective solutions for 30 minutes this is to ensure that the test is fair because if they are left in their solutions for differing times the cells in the very centre of the potato chip may not have undergone osmosis.

The surface area for diffusion into or out of the potato cells must be kept constant this is achieved by keeping all dimension of the chips constant by cutting them all to 50mm x 10mm x10mm with a ruler and a knife.

Prediction

I believe that the higher the sucrose concentration of the solution surrounding the chips the larger the average percentage of mass lost by the chips.

For experiment 1 there will be a large mass increase because the potato chips are being placed in distilled water and the water potential of distilled water is 0 so the water will diffuse into the potato cells until they are turgid.

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Experiment 2 will also have a mass increase but preliminary experiments show that the mass increase is fairly low so I believe that the point of isotonicity will be close to this point, so my prediction is that the molarity of the potato cells is between 0.2 and 0.4 moles.

Experiment 3 and experiment 4 will show decreases in mass but the results for experiment 5 and experiment 6 will show large mass decreases because the concentration of the sucrose solution will be 0.8 and 1 Mole respectively and preliminary experiments show that this far exceeds the ...

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