- The Visking Tubing ‘Sausage’ Experiment – I put one visking tubing sausage, which acts as a partially permeable membrane, filled with one mole of sucrose in a beaker of water and one visking tubing sausage filled with water in a beaker of one mole of sucrose. The mass of the sausage filled with one mole of sucrose was 19.16 grams and after it had been left for three days its mass was 23.02 grams. The mass of the sausage filled with water went from 19.1 grams at the start of the experiment to 6.36 grams at the end. This again shows how water moves from a high to low concentration in osmosis.
- Plasmolysis/Turgidity in Onion Cells – This experiment revises cell structure in plants and the principles of osmosis. Some onion cells were put in distilled water and others were put in one mole of sucrose. I then looked at these cells under a microscope to see if there was any difference in the cell structure. The onion cells in distilled water were turgid and the cell membrane was pressed against the cell wall. This indicates that the water has moved from a low to high concentration by osmosis. The onion cells in one mole of sucrose were plasmolysised (made flaccid) and the cell membrane was not near the cell wall. Again this indicates that the water has moved from a high to low concentration by osmosis.
EQUIPMENT:
- Potatoes
- Beakers
- Boiling tubes
- Sucrose
- Distilled water
- Ruler
- Scales
- Knife
- Cork borer
PLANNING:
To perform the measurements necessary to calculate the concentration of cell sap in the vacuoles of potato cells, I will cut cylinders of potato and put them in different concentrations of sucrose. I chose to use a cylinder shape as I found out that cutting cubes or cuboids was very hard and it took a long time make them accurate sizes. The cylinders are easy to cut accurately as, using a cork borer, the diameter is always the same and it is then simple to cut the pieces to the right length to the right length. The potato cylinders will all be 80 mm long. The length of the potato cylinders will be measured at the start of the experiment and at the end, as will the mass. The potato cylinders will be left in the sucrose for two days.
I will test the potatoes with six different concentrations of sucrose. These concentrations are 0% (distilled water), 20%, 40%, 60%, 80% and 100% (one mole of sucrose). These concentrations were made using water and sucrose in the required ratio, for example 40% concentration was made by mixing 40% (by volume) sucrose and 60% water. For a fair test, it will also be necessary to keep the volume of solution that the potato is in constant in each case (20 ml was used), and also the amount of time that the potato cylinders are left in the concentration should be the same. Additionally, the temperature of the room and hence the concentration must be kept the same, as variations will affect the results.
I will repeat the experiment three times at each concentration and take an average of the results. This will increase the accuracy of the results. I will calculate the percentage difference in the length and mass of the potato cylinders at the beginning and the end and then calculate the average percentage differences at each concentration. These results will then be plotted on a graph and the line of best fir drawn. This will enable me to determine the concentration of cell sap that there is in the vacuole of potato cells. This will be where the line crosses the x-axis as at that point there is no change in the mass of the potato cylinders indicating that the concentration inside and outside the potato is the same and no water has transferred in either direction.
PREDICTION:
I predict that the concentration of cell sap in the vacuoles of potato cells will be below 0.5 moles. I think that this will be the case because potatoes are not very sweet and most of the glucose is turned to starch and stored.
For a more precise prediction, I predict that the concentration of cell sap in the vacuoles of plant cells will be 0.4 moles. I believe this to be the case because potatoes are storage organs; this means that most of the sugars in the potatoes are turned to starch i.e. glucose is converted to sucrose and then to starch. Potatoes are also polysaccharide i.e. they are insoluble so as not to cause osmotic problems to the cells. I also know that potatoes are not very sweet which would indicate that they do not have much glucose or sucrose in them. Also peeled potatoes are quite moist to touch when peeled.
DIAGRAM:
RESULTS:
The following table shows the mass of each potato chip before and after they were left in their separate concentrations. It also shows the average mass of the three potato chips at each concentration after the experiment and the calculation and answer for the percentage change.
CONCLUSION:
From the numerical results that I have tabulated and the graph that I have drawn, it can be seen that the concentration of cell sap inside the vacuoles of potato cells is 0.2 moles. I know that this is the concentration of cell sap because there was no percentage change in the mass of the potato cylinders placed in a solution of this concentration. This indicates that the concentration of water was the same inside and outside the potato, meaning that the water would not be able to pass either way by osmosis as water only goes by osmosis from a higher to a lower concentration. Another thing that I can learn from the graph and tabulated results is that the higher the concentration of sucrose that the potato cylinders were put into, the greater the percentage change was.
At the concentration of 0 moles i.e. distilled water, there was an increase in the mass of the potato cylinders because there was a higher concentration of water outside the potato in the distilled water than in the potato. This water then passed by osmosis from the distilled water to the potato cells, down a concentration gradient, through the partially permeable membrane of the cell membrane of the potato cells. As explained above, the mass of the potatoes at the concentration of 0.2 moles remained constant, indicating that the concentrations of water were equal inside and outside the cell. At the concentrations 0.4, 0.6, 0.8 and 1.0 moles, the average masses decreased because there was a lower concentration of water outside the potatoes than inside. The reason why the average masses, at the end of the experiment, became lower as the concentration of sucrose increased was that the greater the sucrose concentration above that in the potato, the more water will leave the potato, in an attempt to balance the concentration inside the potato and outside. Between the concentrations of 0.8 moles and 1.0 mole, the percentage change differed only slightly because there is a certain point where the potato cells cannot loose anymore water by osmosis as the cells are flaccid and there would be little or no more water inside the cells. On the graph, this can be seen in the way that the best-fit line starts to level off at the bottom. The graph also has a decreasing gradient at the top as it becomes harder for potato cells, which are already turgid to gain more water through osmosis.
I know that potatoes are storage organs and are mainly insoluble, hence my original hypothesis that the concentration would be lower than 0.5 moles. Although this part of my hypothesis is correct the second part was not as I said that the exact concentration of cell sap would be 0.4 moles when it was actually 0.2 moles. The results that I have obtained therefore do not support my precise prediction but support my general hypothesis in that the concentration of cell sap inside potato cells is very low because potatoes are mainly made of starch.
EVALUATION:
I believe all my results to be accurate, as they all fit onto the line of best and there are no anomalous results. This shows that the experiment that I have done was performed correctly in an accurate manner and that all the readings were taken with sufficient accuracy. This means that the results that I have obtained are reliable and therefore sufficient to support a firm conclusion. I also believe that I designed a safe experiment, as there were no particular risks involved.
As with most experiments, the results that I obtained can be improved upon by taking a larger number of readings at each concentration and then averaging the results. This would involve using more potato cylinders at each concentration. Results could also be obtained from different concentrations of sucrose e.g. 0.1, 0.3, 0.5, 0.7, and 0.9 moles to achieve greater accuracy. To obtain a more precise
answer for the concentration of cell sap in potato cells, I could have performed the experiment with finer molarities of sucrose such as 0.18, 0.19, 0.21, 0.22 moles etc. to work out the exact answer. This would also provide additional evidence for my conclusion that the concentration of cell sap is 0.2 moles.
As the experiment was performed in a school laboratory where the temperature is not always kept constant, my results may not be entirely accurate. To obtain better results, the potato cylinders and the solutions would have to be kept at a constant temperature throughout the experiment.
To extend the investigation further I could see whether it is necessary for the membranes to be living when the experiment is performed. To do this I would have to destroy the living membranes by boiling the potatoes for one or two minutes. I would then repeat the same experiment again to see if the results are different. The potatoes would not be boiled for any longer than two minutes because they would become soft making the experiment harder to do and also because to kill the membranes it would only take this amount of time. I would expect the results of this additional experiment to be very similar to those of the original experiment as I have observed the same effect with visking tubing, which is not living. The experiment could also be extended to use different size pieces of potato and different volumes of solution, although I would not expect this to alter the conclusion.
