Further information on potato plant cells:
Plant cells always have a strong cell wall surrounding them. When they take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises and eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what makes the green parts of the plant "stand up" into the sunlight.
When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid." This is the exact opposite of "turgid". The content of the potato cell shrinks and pulls away from the cell wall. These cells are said to be plasmolysed.
When plant cells are placed in a solution that has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient Plasmolysis. "Incipient" means, "about to be".
Variables:
To create a fair test certain aspects of the experiment will have to be kept the same whilst one key variable is changed. I have chosen to vary the concentration of the sugar solution. This will give me a vary varied set of results from which I hope to make a decent conclusion. If any of the non-variables below are not kept constant it would mean it would not be a fair test. For instance if one of the potato chips was 1cm longer the surface area of the chip would be larger and there would therefore be more space for osmosis to occur. Doing all the tests at one temperature will control the temperature.
1. For the purpose of my experiment I am going to do all the experiments in a water bath set at 40oc.
2. To keep the water potential of the potato initially will be kept the same by using the same type of potato, which have been treated in the same way, e.g. have all been cut without being washed and peeled.
3. The mass of the potato is a dependent variable, and this means that it will be measured
throughout the experiment. I will measure the mass in grams. The potato chip will be
measured before it is put in the solution, and after. This will allow us to see whether
osmosis has taken place, and to what extent.
4. The volume of the solution that the potato chips are kept in must be fair. The must be
totally covered in the solution, and the amount of solution will be kept the same because
all the potato chips are the same size.
5. I am also going to use the same balance to weigh my potato chips. This is because the measurements may slightly vary between scales.
Planned method:
A range of sucrose sugar solutions will be prepared with concentrations 0 molar, 0.2 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.
Sections of potato will be cut out using a number 4 cork bore and a scalpel which will used to cut them to a length of 3 cm using a rule. 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. Six chips will be placed in to test tube three times so that I can take an average for each sugar solution. I will use 10 ml of each concentration of sugar solution and once in the test tubes they each will be labelled. The potato pieces will then be placed in the different test tubes and then left for 1 hour. Then the potato pieces will be removed, the surface solution removed using paper towels and then they will be re-weighed.
Obtaining evidence
Method:
1. I took two average sized ground potatoes and checked that they were both healthy and hard.
2. Using a standard number 4 cork bore I cut a length of chip on a white tile.
3. Using a scalpel and ruler I cut the potato to a length of 3 cm long. I had to be very careful whilst cutting the potato, as the scalpel is exceptionally sharp. I then had 18 chips.
4. Taking a test tube rack I placed 6 test tubes and then labelled them 0 molar, 0.2 molar, 0.4 molar, 0.6 molar, 0.8 molar and 1 molar.
5. Using a syringe I measured out different amounts of sucrose solution and distilled water, which I then inserted 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 placed the chips into the test tubes, which I left for 1 hour.
8. After 1 hour I drained out the solutions in the sink and placed all the chips on paper towel in the order I had put them in the test tubes as to not confuse myself as to which chip came from which solution.
9. I dried each chip with the paper towel and then placed each one on the scales so that I could weigh them.
10. Each potato was measured accurately on the electronic scales.
Precautions:
1.As was stated in my planning section of the coursework, I had to keep all of the different non-variables the same, to make sure that none of them affected the results of the experiment in any way.
2. Whilst cutting the potato, extreme care and precision had to be taken with the scalpel, as it is very sharp and could easily cause a serious wound.
3. The measurements for the solutions had to be perfect as to not change the out come of the experiment.
4. I had to ensure that every time I handled the potatoes my hands were clean and dry. This was to stop any kind of contamination and made sure that I did not pass on any extra water onto the potato.
Conclusion / Evaluation:
The experiment was very unsuccessful in my opinion. I obtained a large quantity of very inaccurate results from which I was able to create informative graphs which showed me that the experiment did not work. I think I took easily enough results for the amount of concentrations that I was using, and the time that I used for the experiment to last was enough to allow sufficient osmosis to occur. However if I was to repeat the experiment I might well increase the time of the result to allow more osmosis to happen and possibly find out the saturation point of the chips. 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. 0.10m, 1.15m, 1.20m, and so on. 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.
The cutting of the potatoes was the most difficult part of the experiment as although I was recording my results by mass, it could well have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment I would have possibly found a machine to cut the potato, as it would ensure that all potatoes would be the same weight and dimensions. As well as the potato I could have found a more accurate way to measure out the 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. not to 0.00g but to 0.0000g.
When the potato chips were removed from the test tubes and dried I may well have dried some potatoes more thoroughly than others and so some would have more excess water, which would add to the mass. If the experiment was repeated I could find another way to dry the potatoes that would ensure that all were dried in the same way for the same time. However with all this said I think that the experiment was truly successful and I was very pleased with the complete comparison of my results with my initial prediction.
