This shows that the cell and it’s main features we can see the cell wall where the cell gets its main support. The water will then leave this cell this occurs as the cell is placed in a solution of lower water potential.
We can see that Plasmolysis is in progress as we can see the protoplast is starting to shrink away from the cell wall- so the cell is beginning to plasmolyse.
Now the cell has fully plasmolysed as we can see the protoplast has shrunk away from the cell wall and also the external solution (the water and sucrose) has passed through the freely permeable cell wall and is still in contact with the protoplast.
When a graph is plotted of gain or loss in mass of the potato chips (g) against water potential of sucrose solution the line will intercept the X-axis, water potential of sucrose solution. This intercept is the water potential at which there was no gain or loss in mass and is therefore the water potential of the potato. By this method, a quantitative value for the water potential of potatoes is obtained.
Apparatus list
Safety
Measurements
In my experiment, 2 sets of results are going to be taken. However, in my investigation, I am going to use the class results, as it will be more accurate. I am going to calculate the change in mass and the percentage (%) change in mass. In order to record precisely, I would use instruments with higher accuracy to get the most accurate results.
Plan
Take three sets of boiling tubes with the concentrations: 0.0 (distilled water), 0.2, 0.4, 0.6, 0.8, and 1.0 mole dm3 e solution. Place about half a tube- full of different concentration of solution in each test tube.
For the potatoes, cut the potatoes using a potato cutter and scalpel so that they are all the same, 5cm long rectangle with the same width. Place them each on a separate sheet of filter paper with the pencil labelling what the concentrations of each potato are respectively.
Take the cut potatoes and the boiling tube rack to the balance. Take the balance of the filter paper and then record the mass of each of the potatoes with it on the filter paper. Then calculate the mass of each potato by subtracting the mass of filter paper from the total mass.
After at least 2 hours remove the chip of potato from the tubes in turn, in the same order you have inserted them. Drain through a small sieve and place on absorbent paper for one minute. Do not squeeze the clips of potatoes, or they will all lose water. Reweigh each cylinder and record its mass.
Work out the percentage change in mass of each potato chip (change in mass multiply by 100 divided by the original mass). Produce a summary table, displaying the mean percentage change for each group and an overall mean for the whole class. Plot the class means against the morality of the sucrose solution and draw a line of best fit, adding error bars on.
Fair testing
The only variable in this investigation will be the concentrations of sucrose solution. This will be controlled by using different volumes of 1 mol dm-3 sucrose solution, with different volumes of water. This method of dilution is the most accurate, as it allows me to chose a wider range of concentrations and there is less likely to be contamination or inaccuracies in the concentrations of sucrose.
The table below is how I would make up my dilutions to a volumes of 10cm3.
Diagram
Results
Table of results – Class mean, max and min
Table of results – my results
Summary
The graph has shown that it is a curve. By comparing my results, I can see that when the concentration is zero there should have more water outside of the potato then the water inside of the potato however, due to natural characteristics of potatoes in liquids, they become starchy and break. Therefore there would be a certain amount of water lost even water should be from higher concentration to lower concentration by osmosis. Osmosis is the movement of water from a low concentration to a high concentration through a semi-permeable membrane. It then started to increase as the concentration gradient between the outside and inside became closer to each other at about 0.4 in my results and about 0.3 in the class average. Then the percentage change in mass increase as the concentration gradient increases therefore the graph has a larger percentage change in mass due to the cell is fascinating/shrinking.
Conclusion
From the results (class average) we can predict a molarity with some accuracy where the potato would be in equilibrium, from the graph this would be approximately 0.34Moles, which is between my 0.2M and 0.4M prediction, this isn’t very far of the actual concentration of cell sap which is 2.9Moles according to ‘Biology a Functional’. From the evidence above, I can conclude that osmosis does happen and that the water flows from an area of high concentration of water to low concentration of water.
Evaluation
My results are reliable, as they are nearly all on the best-fit line. Ideally, I would have liked to do more experiments, I would do this by having a potato from every half a mole from 0-to1 mole, this would enable me to get more accurate results, and
The main variable with the experiment was the drying of the chips, as some may not have been dried as much as the others may, if they were not dry, this could effect the concentration of the sugar. In addition, different areas of the potato may have different concentrations of cells, which can affect the experiment, although it is a minor point it could have a great impact also not all potatoes in the test are exactly in the same size as there will be measuring errors in both instrument and human. Even if the potatoes are in the same size, may not contain same no. of cells which will affect the rate of osmosis. When you cut the potato, you cannot avoid to "destroy" some of the cells. It also causes leakage of cell contents to the outside which causes shrinkage of the cells. Systematic errors like the measurement error when making the solutions can also cause errors. These are all minor points because they cannot be improved in a more accurate way except having better measuring instruments would help.
