Experiment One: Raisins in Water
In the experiment (which was completed at home), I placed 8 raisins into a glass of water for 24 hours, to see whether osmosis would occur between the strong sugar solution inside the raisin, and the water in the glass. At the start of the experiment all of the raisins were shrivelled and sank to the bottom of the glass, as shown on the left side of the diagram.
I left the glass in a cool dark cupboard for 24 hours, and after this retrieved the glass to record results. After 24 hours, the raisins had swelled up, and most had floated towards the surface. There was also a discoloration of the water at the bottom of the glass. The swelling of the raisins is due to water passing through the membranes of the raisin’s cells to dilute the sugar solution. The extra water adds extra turgor pressure, pushing the plant cell walls, and making them turgid, creating the swelled effect of the raisin. This is, as I have mentioned, similar to the effect that allows plants to stay straight and upright rather than wilting.
Experiment Two: Potato Slices in Table Salt
This experiment was also performed at home, and consisted of 8 slices of raw potato coated in regular table salt (Sodium Chloride), and left for 24 hours. This would hopefully allow me to see the osmotic process reversed, with water diffusing from the cells to the salt. At the beginning of the experiment, the potatoes were pale yellow and fairly stiff, typical of raw potato. No liquid was visible on the plate. The plate, like the glass of the previous experiment, was left in similar conditions: a cool dark cupboard, for the required 24 hours. When the potatoes were retrieved from the cupboard, they had mostly turned black, and had become very limp. Water was left in the bottom of the plate, into which some salt had dissolved. The potato slices were also slightly smaller than at the start of the experiment.
This is what I had expected, thanks to my background information. The cells have plasmolysed; the water from the weaker sugar solution inside the potato has diffused through the semi-permeable membranes of the potato’s cells, to dilute the pure and therefore more concentrate salt coating. This causes the cells to become flaccid, causing the limp texture and flexibility. This is similar to plants, which wilt when their cells become flaccid and plasmolysed.
My preliminary experiments tell me that osmosis has a very noticeable effect on potatoes and raisins, one that could probably be measured. Because the change in the potato could be measured with equal ease as the raisin, I have to decide that my indicator will be potato, as these can be cut to a standard size for an experiment more easily and more precisely using cork borers and knives. My experiments proved that my background information is correct, and that water exchange has a very simple effect on any cell.
➔ Planning
Fair Testing
In this experiment, to gain some results, I must analyse the possible variables in the experiment, and select one to measure, my measured variable, one to change, the independent variable. The remaining variables will be kept as constant as is possible in the laboratory.
My Independent variable will be the concentration of the liquid that I will place my potatoes into. This is because it is the effect of this on osmosis that I am studying. I will be using solutions of sugar and water, as sugar acts in a similar way to salt, as was seen in the raisin experiment. The solutions will be of varying proportions, including one of 100% water, to act as a control. I want to test a wide range of solutions, and will test 5 different proportions, set at equal increments from each other. This will make them easier to compare. Creating the solutions will be a process of measuring the two components, and then mixing them into a solution. Both sugar and water will be measured in measuring cylinders, as accurately as is possible (to the nearest cm3), to a total volume of 20cm3. The solutions will be of the proportions:
- 100% Water : 0% Sugar
- 85% Water : 15% Sugar
- 70% Water : 30% Sugar
- 55% Water : 45% Sugar
- 40% Water : 60% Sugar
There will be two measured variables; this will allow for a more accurate analysis of the experiment. The potatoes will be prepared by cutting from cylindrical cork borers, so they are all of equal width, and will then be cut with kitchen knives to a length if 1.5 cm. The constant length will make comparison between the potatoes in each solution easier. This length will be measured after the experiment, and will as such be the first measured variable. The second measured variable will be mass, measured on a set of digital scales, accurate to 0.01 of a gram. This again will be measured after the experiment, and compared to see how osmosis differs between the solutions. The most important part of keeping the test fair will be to reset the scales after each weighing, so that there is no effect on my results from other factors.
In this experiment, the other variables, which could have an effect on the experiment, are temperature, volume of the solutions, the time for which the potatoes are left in the solutions, and finally, the number of potato pieces in each test tube.
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Temperature will be kept constant at the average room temperature of 21°C.
- Volume of the solutions will be measured and kept constant across all 5 solutions.
- The potatoes will be left in their solutions for 24 hours, staggered so as to allow time for each solution to be completed and then emptied at the end of the experiment.
- There will be 3 potato pieces in each solution, meaning that there will be 3 results for each solution. These results will be averaged for each solution, so the effect of any anomalous result on my results would be reduced.
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The potatoes will all have to be underneath the surface, so as to ensure that the full surface area of the potato is in contact with the solution.
Prediction
In this experiment, I would expect the different concentrations to show fairly noticeable effects on the lengths and masses of the potatoes, with the water diffusing into the potatoes at a high rate in the pure water solution, and therefore making their size bigger than at the start. As the solutions become stronger, I expect the rate of osmosis to decrease (and, as a result, make the potatoes themselves comparatively smaller at the end). Also, the potatoes will be quite turgid at the start of the experiment, there will be already some water inside, and because of this I expect, in the stronger solutions, for there to be a decrease in the mass and length of the potato, due to a reversal in the effect of osmosis. I know that water can pass in both directions, thanks to my preliminary experiments, and that direction depends on the relative concentrations inside and outside the potato.
Using this information, I can create a predictive graph of results, which shows the basic pattern that I would expect. As you can see, the graph is downhill from left to right, and goes from above the original size to underneath it, reflecting the fact that I expect the osmotic process to change direction. The graph above is my prediction graph.
This is how I think my final results will fit into a graph. The y-axis shows the overall length/mass after the experiment, and the line across the graph shows the original length. The x-axis relates to the % of water in the solution. The y-axis has been left without a scale, as I this is only a predictive graph, without the accuracy that my final graphs will have.
Method
Firstly, I will take the potato from which my samples will be taken, peel it (so that the skin does not interfere with the experiment), and use a size 6 cork borer to take 15 cylindrical potato samples. These are then to be cut to 1.5 cm in length. They are then to be weighed on the scales. After this, the solutions will be prepared, with each of the 5 test tubes being cleaned before the measurements of the sugar and water. The sugar will be measured first, so that the water does not make the sugar ‘stick’ to the measuring cylinder. Then the water will be measured and added to the test tube, making each test tube up to the required 20 cm3. These solutions must then be mixed so that the sugar is thoroughly dissolved in the water, and no grains are left. Finally 3 potato samples are added to each solution, ensuring that the potatoes are all completely underneath the solution. The experiment can now be begun. After 24 hours, the potatoes will be removed from their solutions, and will be measured with a ruler, to find any change in length, and they will each individually be placed on the electric scales to find any gain or loss in mass. These results will be then taken into a table, and the potatoes and solutions will be disposed of.
Apparatus and Diagram
- 5 Test Tubes
- Cork Borers (Size 6)
- Electric Scales (accurate to 2 decimal places)
- Glass Stirrer
- Knife
- Measuring Cylinder
- Potatoes
- Ruler
- Stopwatch (to time the 24 hours)
- Sugar
- Test Tube Rack
- Water
➔ Observations
Results
These are my results. As you can see, each row relates to one particular potato sample, each one with a starting length of 1.5cm and a starting mass of 1.15g. Each solution has 3 samples, and the results for each solution are averaged, and this average is used to calculate the change, both numerically and as a percentage. In the above table, the percentages are in italics, whilst the numerical data is non-italic.
I have also drawn two graphs from my data, which show the pattern of the results more clearly than in this table, and allow me to see how close my prediction and predictive graph were to the actual results.
There is one anomalous result, in the 60% Sugar solution, where the mass has dropped significantly more than the two other samples in the solution.
The first graph is showing the masses and lengths after the experiment, according to solution. As you can see, the effect is similar to that of my prediction, with the lengths and masses falling steadily from solution to solution.
The differing outcomes of the experiment are shown better in my second graph, which, rather than showing the total masses and lengths, shows the change, in grams and cm respectively. In this graph it is clearly shown that the final 3 solutions have lost water, and therefore mass and length, as a result of osmosis, unlike the first 2 solutions. This second graph makes the change much clearer because the defining line of ‘zero change’ becomes the x-axis, a line that is easier to read from than the lines drawn across my graph
Conclusion
My results show me that there is an obvious trend in my results; osmosis has worked at an increasingly slower rate going into the potatoes, before reversing, and steadily increasing in rate diffusing out of the potatoes.
In the first and second solutions, the potatoes have increased in length, and in mass. This is because they have gained water. The only way they could have gained water is by osmosis, as my background information states. Osmosis has occurred, and water has diffused from the solution into the potato, in an effort to dilute the solution in the potato. This is because the solution is of a weaker concentration than the solution inside the potato.
This has caused the potatoes to become turgid, with the inner turgor pressure pushing the cell membranes up against the cell walls. This makes them stiffer and much like the stems of plants, which, as I mentioned in my background information, use their turgid state to hold the plant up. This is different to the third, fourth, and fifth solutions. In these solutions the potatoes have become limp, losing mass and length, again due to water loss. Again, the only way that water can be gained or lost from a plant cell is by osmosis. However, instead of there being osmosis into the potatoes, the water is diffusing out of the potatoes. This is because the solution inside the potatoes is now of a weaker solution than the solution in the test tube, and as a result, the water in the potatoes’ solutions has diffused through the semi-permeable membranes of the potato cells, and into the outer solutions. As a result, the inner turgor pressure has reduced, a little in the third solution, but a progressively more in the fourth and fifth solutions. This reduction in pressure has made the cells’ membranes pull away from the cell’s walls, as there is no turgor pressure, and the cell becomes plasmolysed. This makes the potato limp, and weak, much like a plant’s stem which, without water, begins to wilt because there is too little turgor pressure to hold it upright.
The lines of best fit on my graphs also throw up some interesting patterns. In the first graph, there seems to be one anomalous average, which is apparent in both mass and length. This is the 30% Sugar solution. There is almost perfect correlation, providing an almost straight line for the length. The length line also provides us with an estimate for the osmotic potential, at almost exactly 30% Sugar. This is shown by the fact that the line of best fit crosses the original length close to where the 30% Sugar bar is. This tells me that isotonic status would be reached at 30% concentration, this being the approximate concentration inside the potatoes.
My experiment shows me that the concentration of a liquid has a very noticeable effect on osmosis, as the bigger the difference between the solutions on either side of the semi-permeable membrane, the higher the rate of osmosis down the concentration gradient. I know this because my second graph shows a strong negative correlation between concentration and the rate of osmosis.
➔Evaluation
Experiment Evaluation
This experiment went very well, and provided us with results that proved that the concentrations of liquids have an effect on osmosis. However, the experiment was not perfect, and could have been improved. This is most obvious where I have an anomalous result, which does not tie in with the other two results for that solution. This anomalous result could have been the result of human error, or may have been due to the potato surfacing in the solution (meaning that the upper part was not submerged in the solution).
However, there are many factors that could have been improved upon. Firstly, my measurements could have been more accurate; I might have used a micrometer instead of a ruler, and more accurate electric scales to weigh the potato pieces. I could also have measured the width of the potato, adding an extra dimension to my results and therefore more accuracy in my findings. Using the extra information I could then calculate the volume of the potatoes, using the formula lπr2, the formula for cylinders. This would give a completely accurate result, which would justify for any change in size, be it in any direction. Therefore, if one potato increased in width but not length, and another increased in length but not width, both changes would be accounted for. The potatoes could also have been cut with a scalpel instead of a knife, making the original measurements even more accurate.
I could also have changed the structure of the experiment to ensure more accuracy. I could have tested more solutions, which would have given me a more accurate picture of how the concentrations effect osmosis. I could also have used more samples to test each solution, which would iron out any anomalous results even more efficiently, and produce a more accurate result for each solution. Finally, the experiment could have taken place over a longer period of time, to accentuate my results, as osmosis would continue until the solutions were all of equal concentration, and the effects of the original solutions would be highly visible. I did not change my plan during the experiment, as I used my preliminary experiment to iron out possibilities for the experiment to falter, which fortunately it only did once. The fact that it did however must be taken into account; so more caution must be used for future experiments.