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)
In the second preliminary experiment, an identical method to the one in preliminary experiment 1 was used, using beetroot tissue in place of potato tissue. It was found that beetroot had a water potential of –3500Kpa. These results imply that potato has a higher water potential than beetroot as the potato was shown to have a water potential of -860Kpa.
Prediction
If the results of the preliminary experiments are accurate, then it is to be believed that the water potential of potato tissue is approximately –860Kpa and the water potential of beetroot tissue is approximately –3500Kpa. It is also known to me, via a secondary source (), that turnip tissue has an approximate water potential of –730Kpa.
The plasma membrane that bounds a cell is partially permeable, therefore allowing small molecules, like water, to pass through it. All plant cells, like turnip, potato and beetroot, have a call wall. This call wall is fully permeable. This means that water molecules can enter and leave the call, depending upon the difference in the molarities of solutions inside and outside of the cell.
Therefore, it is possible to find out the concentration, and thus the water potential, of a tissue, by placing it in differing molarity solutions and calculating the weight changes that occur. When there is no weight change, it means that the molarity of the solution is equal to the molarity inside the cells of the tissue.
When water moves down a water potential gradient, and out of the cell, the vacuole will shrink and move away from the edges of the cell wall. This reduces turgor pressure and the cell is referred to as being flaccid. When water moves down a water potential gradient and into the cell, the vacuole swells and presses against the inside of the cell wall. This increases turgor pressure inside the cell and the cell is referred to as being turgid.
I would expect that the greatest change in all tissues would be in the samples present in the 1.0 mol.dm-3 solution. I believe that the masses of these tissues will decrease because water will pass down the water potential gradient and out of the cell, into the greater molarity of the surrounding solution. If the results of the preliminary experiments and the secondary source are reliable, beetroot will have the lowest water potential, potato will have the next lowest and turnip will have the highest water potential.
Method
A similar method to the one in the preliminary experiments will be used to find the point of equilibrium, hence, the water potential of beetroot, potato and turnip.
- Wash all apparatus with deionised water
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Label 11 specimen tubes ‘P’ (potato), and the molarity between 0.0 – 1.0 mol.dm-3, which will be placed in the tube. E.g. ‘P 0.0 mol.dm-3’, ‘P 0.1 mol.dm-3’, and so on. Do the same with 11 specimen tubes labelled ‘T’ (turnip) and ‘B’ (beetroot).
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Fill each tube with 10cm3 of the corresponding molarity sucrose solution.
- Use a cork borer and cutting tile to take a cylinder from the potato and using the scalpel, cut 11 discs of approximately equal size. Do the same for turnip and beetroot.
- Using the top pan balance, weigh and record 11 pieces of filter paper, writing the number on the filter paper (1-11). Place each potato disc onto a piece of filter paper. Weigh each piece of filter paper with the disc on, and record the mass. Taking the initial weights from the second will give the masses of the discs. Record these masses.
- Carry out the previous step for turnip and beetroot.
- Place one disc of each of the sample tissues into a specimen tube labelled for the corresponding tissue. I.e. place each of the potato discs into each of the specimen tubes labelled ‘P’, and the same for turnip and beetroot. Place rubber stoppers into the tops of the tubes and leave for at least 24 hours.
- After at least 24 hours remove the sample tissues from the specimen tubes and remove any excess solution from the tissues using tissue paper.
- Again, weigh the discs using the same procedure as before, and record the masses.
Once the results have been recorded, they will be used to plot a graph of molarity against percentage change. These results will then be plotted on a standard molarity against osmotic potential graph to find the water potential of all 3 tissues.
However, the following table can be used as opposed to a graph showing molarity against osmotic potential.
Apparatus
Safety
Safety in all experiments is very important. In this experiment, care and consideration should be taken when using the cork borer and scalpel, as they are sharp and can cause injury. Always cut away from the body when using the scalpel and store the scalpel with its protective cover when not being used. Also, when using the borer and scalpel, make use of the ceramic tile and do not hold the vegetables.
Variables and Fair testing
In all experiments certain variables must be kept the same in order to make the experiment fair.
In this experiment, the independent variable will be the molarity of the solution in which the sample tissues are kept.
In order to ensure that the experiment is fair these variables must be kept the same:
- The same beetroot, potato and turnip should be used to take all samples of the corresponding tissue. This ensures that all samples of a vegetable have the same water potential.
- The same cork borer should be used to ensure that all samples have an equal diameter.
- The length of time that each tissue sample is kept in the solution should be the same. This is to allow and equal amount of time for osmosis to take place.
In this experiment, there are a number of factors that must be considered, in order to obtain accurate and valid results:
- An accurate top pan balance should be used to give an accurate mass of the sample tissues.
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A wide range of results should be taken so that anomalous results can be easily spotted. This also shows the reliability of the results collected. E.g. 0.1 – 1.0 mol.dm-3.
- The experiment should be carried out a minimum of 3 times and mean results calculated. This will increase the overall reliability of the results.