5 The length of the potato tubes were also constantly measured throughout the experiment.
Planned method:
A range of sucrose sugar solutions will be prepared with concentrations 0 molar, 0.1 molar, 0.25 molar, 0.5 molar, 0.75 molar, 1 molar, 1.5 molar and 2 molar. This will be done by adding varying amounts of distilled water to varying amounts of sucrose solution. Sections of potato will be cut using a scalpel and will be measured using a ruler. 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 chip will be measured as well so that more results can be obtained. I will use 10 ml of each concentration of sugar solution and once in the dishes they each will be labelled. Then the potato pieces will be removed, the surface solution removed using paper towels and then they will be re-weighed. And measured with a ruler. If I then have time afterwards I will repeat this experiment again as to obtain a second set of results. This will hopefully produce more accurate results from which I will be able to draw a more accurate conclusion.
Method:
1. I took two average sized ground potatoes and checked that they were both healthy and hard.
2 Using a standard cork borer I cut out 8 potato blocks (cylinders).
3. Using a scalpel and ruler I cut the potato into chips, which were 40 mm long. I had to be very careful whilst cutting the potato, as the scalpel is exceptionally sharp.
4. I then placed the chips into the dishes, labelling the dishes 0, 0.1, 0.25, 0.5, 0.75, 1, 1.5 and 2 molars.
5. Using a measuring cylinder I measured out different amounts of sucrose solution and distilled water, which I then poured into the test tubes in a percentage ratio giving me the various molar concentrations.
6. I then weighed every potato chip on an electronic balance and recorded the weights.
7. I put the potato chips into each dish and then started my experiment.
8. Whilst waiting I drew up a basic table for my results. (Look at results)
9. After 1 week I placed all the chips on the paper towel in the order I had put them in the dishes as to not confuse myself as to which chip came from which solution.
10. I dried each chip with the paper towel and then placed each one on the scales so that I could weigh them.
11. Each potato was measured accurately on the electronic scales and then the weights were recorded.
12. As I had time after doing the first set of results I redid the experiment under exactly the same conditions. I also repeated the experiment once again; this gave me the most accurate results I could have hoped for.
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Conclusion:
When the concentration reaches , there appears to be no further water loss or gain, suggesting that the cell is fully plasmolysed. From the graph an estimate to the concentration of the potato cell can be made as 0.28M, as this is the point where the potato is not increasing or decreasing in mass, this is known as the isotonic point. This is where no osmosis is taking place; both the potato and the solution have an identical concentration.
The time I used to test the potato chips was plenty of time to allow sufficient osmosis to occur as. However if I was to repeat this investigation I might decrease the time of the experiment because I don’t think it would have taken 1 week, as I believe osmosis would occur within a couple of hours. Therefore I would test the potato chips after a few hours as I think that the cells would be fully plasmolysed fairly quickly (the rate of plasmolysis varies depending on the type of solution).
From my results we can see that the more water in the solution, in comparison to the sugar, the potato puts on the more weight. This is caused by osmosis. Osmosis is the movement of water from a high concentration to a low concentration through a semi-permeable membrane. When there is more sugar, in comparison to the water, weight is lost. This is because the water that is already present in the potato is passed through to the solution.
I took eight samples of each solution. I think this was a satisfactory amount of samples for the amount of concentrations that I was using. Although, if I was to do the experiment again I would probably take even more samples. This would hopefully ensure more reliable results. And with a gap of less than 0.2 molar
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, i.e. 0.1M, 0.2M, 0.3M, 0.4, 0.5, 0.6 etc. This way would have allowed me to also find the isotonic point far more accurately as the one that I estimated is very approximate.
The evidence obtained from this investigation supports the prediction made. It shows that the potato cells increase in mass, when they are in solutions with a high water concentration, and decrease in mass in solutions with a low water concentration.
From the graph an estimate to the concentration of the potato cell can be made as 0.28M. As this is the point where the potato is not increasing or decreasing in mass. However, it is important to realise that this is only an estimate as the potato cells will not be uniform in their concentration.
Evaluation:
Overall I think this experiment was successful. The accuracy of the investigation was adequate. I obtained a large quantity of accurate results from which I was able to create a hand drawn graph. I was also extremely satisfied with my prediction as it was very successful and accurate.
My prediction was almost completely successful. Because my predictions came true I was not surprised with the results of my experiment as these results were what I had anticipated before hand.
The cutting of the potatoes was a very difficult part of the experiment as I was recording my results by length, it could have well affected the surface area and so the overall rate of osmosis. Osmosis is the passage of water molecules from where there is a high concentration to where there is a low concentration through a partially permeable membrane. If I were to repeat the experiment I would have perhaps found a machine to cut the potato, as it would ensure that each potato cylinder would be the same dimension and mass. I could have also found a more accurate way to measure out solutions and to determine the molar concentrations. Perhaps I could have used a burette. This would ensure that I have an accurate amount of fluid in each test tube. I could also weigh each chip on a more accurate scale. E.g. 0.000g not 0.00g.
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, i.e. 0.1M, 0.2M, 0.3M, 0.4, 0.5, 0.6 etc. This way would have allowed me to also find out the isotonic point far more accurately as the one that I estimated is very approximate.
I think I took just enough results but if I were to do this investigation again I would certainly take more so that I would have a greater chance of obtaining more reliable ones. There were no out of the ordinary results, but some were not as close to the lines as others. This may have been caused by human mistake. When the potato chips were removed from the flat dishes and dried I may well have dried some more thoroughly than others and so some would have more excess water, which would add mass. If the experiment was repeated I could find another way to dry the potatoes, which would ensure that all were dried in the same way for the same amount of time.
