Each potato will be prepared identically i.e. they will all be of equal size, washed and peeled in the same way. I will measure the mass of the potato chip in grams. The potato chip will be measured before it is put in the solution, and after. I will therefore be able to detect whether osmosis has taken place, and to what extent.
The sugar solution used must be used consistently (sucrose). The potato must be completely submerged (to ensure that osmosis occurs evenly throughout), and the amount of solution will be kept the same because all the potato pieces are the same size.
I am also going to use the same balance to weigh my potato chips. This is because the measurements may slightly vary between scales and they may also measure to different degrees of accuracy, meaning that the results would be unreliable.
It would be possible to measure the change in the lengths of the potato cylinders but given the relatively low accuracy of the rulers available to me (up to 1mm) and the fact that this would introduce a considerable degree of human error, this would not be desirable.
Planned method:
A range of sucrose sugar solutions will be prepared with concentrations 0 molar, 0.20 molar, 0.4 molar, 0.6 molar 0.8 molar and 1 molar. This will be done by adding varying amounts of distilled water to varying amounts of sucrose solution up to a total volume of 20ml. Sections of potato will be cut using a size 2 cork borer and measured using a ruler and a straight edge. Any remaining pieces of skin will be cut from either end. This part of the preparation must be done very accurately as a change in the surface area may allow more or less osmosis to occur. The mass of each potato cylinder will be measured as well so that more results can be obtained. I will then weigh each piece and record it using a balance. The potato pieces will then be placed in the different test tubes (which will be corked and labelled) and left for thirty minutes. Subsequently, the potato pieces will be removed, the surface solution absorbed by paper towels and they will be re-weighed.
Although my preliminary results were inconclusive due to lack of time, it was evident that those potatoes submerged in 1.0M sugar solution lost weight whereas those in distilled water gained weight.
Equipment required: average sized potatoes, two measuring cylinders, scalpel, white tile, ruler, size 2 cork borer, 1 molar sucrose, distilled water, 5 boiling tubes, test tube rack, electronic balance, paper towels, 5 corks, self-adhesive labels
Method:
1. I took two average sized ground potatoes and checked that they were both healthy and hard.
2. Using a size 2 cork borer, I produced 5 cylindrical pieces of potato.
3. I cut the remaining pieces of skin on a white tile with a scalpel and, using a ruler and straight edge, I measured and cut the potatoes to 3.5cm in length. I then dried them lightly in paper towels and weighed them on the balance, noting their weights.
4. Using 2 measuring cylinders I measured out different amounts of sucrose solution and distilled water which I then poured into the boiling tubes to give 20ml of solution of different concentrations in each boiling tube.
5. I next placed the potatoes into the tubes and sealed them using the corks.
6. I then labelled each test tube with its concentration and the initial weight of the potato piece that it contained.
7. The following day, after 17 hours, I drained out the solutions and placed all the potato pieces on the paper towel in the order I had put them in the test tubes, to avoid confusion as to which potato came from which solution.
8. I dried each potato cylinder with the paper towel and then placed each one on the balance so that I could weigh them.
9.Each potato was measured accurately on the electronic scales and then the weights were recorded.
I carried out this experiment three times, allowing me to take an average of the weight changes each time and increasing the accuracy of my final results.
Results:
Analysis and conclusions:
The graph shown on the previous page gives the line of best fit for the percentage change in mass of the potato chips over the course of the experiment. The graph is a curve that slopes downwards and does not go through the origin. As the line is not straight and does not pass through the origin, it can be seen that the percentage gain and loss in mass and concentration are not directly proportional. However, as the concentration of the solution increases, the percentage change in mass decreases. The graph shows that the percentage gain and loss is inversely proportional to the concentration. From the line of best fit, it can be seen that all of my points were very close to creating a perfectly smooth curve, apart from one anomalous result meaning that my results are reasonably reliable.
From the graph an estimate to the concentration of the potato cell can be made as being between 0.2 and 0.4 molar, as at these points no change in mass was detected on three occasions. The point where a 0% change took place was also between these concentration values and on the graph, this is where the line of best fit crosses the X axis. This is known as the isotonic point and is where no osmosis is taking place and both the potato and the solution have an identical concentration (i.e. they are in equilibrium).
This graph of the change in mass helps prove the point of complete plasmolysis, whereby the potato cannot expand and take in any more water. As you can see, as the molar concentration increases, the change in mass decreases. With a 0M concentration of sugar solution, there is a weight increase of approximately 25% as the sugar solution has a higher water potential than the potato. This decreases as the concentration is increased (the two are inversely proportional) until a weight loss of 30 – 40% occurs and equilibrium is reached across the semi-permeable membrane. A weight loss takes place when the potato has a higher water potential than the solution.
Evaluation:
In my opinion, the investigation was very successful. I obtained a large quantity of accurate results from which I was able to create informative graphs and the time that I used for the experiment to last was enough to allow sufficient osmosis to occur. If I had allowed the experiment to continue for a longer period of time, it is likely that the solution would have begun to ferment, causing potato particles to mix with it and reducing the reliability of the measurements taken. The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have more varied results (for example: 0.10M, 1.20M, 1.40M) This way would have allowed me to calculate the isotonic point far more accurately and the one that I estimated is very approximate.
Due to the fact that each potato piece began at different weights, I have used percentage change in order to allow a clearer comparison to be made between them. This was achieved by subtracting the final weight from the initial weight and subsequently dividing that by the initial mass.
Additionally I carried out all of the experiments at room temperature. This was approximately 20oC each time but is likely to have decreased throughout the night, meaning I did not have complete control over this. If I were to make any improvements, I could use a water bath set at a constant temperature. It would also allow me to measure the effects of different temperatures on the change in the mass of the potatoes.
I could also have used a more accurate means of measuring the amounts of solution being used, such as a burette. This would ensure that I have an accurate amount of fluid in each test tube. A more accurate electronic balance could have been used, measuring to a greater number of decimal places (i.e. a smaller denomination than 0.1g). The lengths of the potato cylinders may not have been completely constant throughout, as it was only possible using my ruler to measure to a maximum of 1 decimal place.
There was only one noticeably abnormal result (highlighted on the graph). This may have been caused by human error, such as failing to reset the balance after it was previously used. When drying the potatoes, both before and after putting them in the boiling tubes, I may have dried some pieces more thoroughly than others and so some would have had excess water, which would have added to the mass. I could maybe have used another method of drying them, such as a fan, but this could also have caused the potatoes to have ‘shrivelled’, reducing their mass.
Ultimately, given the equipment available, I believe that I gained a complete and relatively accurate set of results, fulfilling my initial aims and whilst I have made the above suggestions for improvement, it is unlikely that they would have a significant effect on the overall outcome of my investigation.
