At the first concentration containing no sucrose (Molarity 0.0) the potato increased in mass by 53.84% percent. The carrot also gains mass, however not as much and gains 15.40%. The water containing a concentration of sucrose at 0.0 molar had a higher water potential that of the potato and carrot cells, thus causing them to gain water and therefore mass.
When the molarity of the sucrose solution was 0.1, the potato lost a very small amount of mass, only 1.47%. This indicates that it had a similar water potential to the sucrose solution, however not exactly.
At 0.2 molar, the potato again lost mass, this time 8.57% of it, whereas the carrot gained a 12.68% in its mass.
When the molarity of the solution was 0.3, the potato lost mass again, this time 24.59%, indicating that it had a much higher water potential than the surrounding solution.
At 0.4 molar the potato lost 22.95% of its mass, whereas the carrot had gained around 5.43% in mass.
At 0.5 molar, the potato lost mass again. It lost 19.23% of its mass
At 0.6 molar the carrot started losing weight, it lost 4.07%.
From these observations alone it is clear to see that the carrot has a much lower water potential than that of the potato. However it is important to calculate the water potential of the two.
Water Potential
Water potential of a cell is calculated using the following equation:
Water potential = Solute potential + Pressure potential
Pure water has a water potential of 0. The solutes found in water that is not pure reduce how many water molecules there are in a certain area. The results from our graph can show us the molarity of our potato and carrot cells. This is when a dynamic equilibrium is reached and no weight is lost or gained by the cells.
From my graph I can see that the molarity of my potato cells was 0.192 mol dm-3
My carrot cells had a molarity of 0.501 mol dm-3
Using the following table I can calculate the solute and therefore water potential.
Using this table I can see that my potato cell had a water potential of around 540Kpa and my carrots had a water potential of around 1450 Kpa. We can take the solute and water potentials to be the same because the cells are said to be at
The carrot has a much lower water potential than that of the potato. This is because the carrot contains more soluble sugar, such as monosaccharides or disaccharides, which are reducing sugars. We know that it is a reducing sugar, due to the fact when Benedicts is added to it and brought to the boil; a yellow/brown precipitate is form, which shows presence of reducing sugar (e.g. monosaccharides). This means that the carrot has a relatively lower water concentration than the potato. This is because carrots naturally have more soluble sugars and this affects the water potential. Because water molecules have kinetic energy, they are constantly moving around in gaseous or liquid form, moving randomly from one place or another. The sucrose in the solution lowers the total kinetic energy of the free diffusing water molecules will decrease. This means that the water molecules are not able to move around freely and their tendency to diffuse out of a solution will decrease.
However potatoes do not contain as much soluble sugar as the carrot does, it mainly has insoluble macromolecules. It contains starch molecules. Starch is stored very compactly as coils and is very easily stored. It is a very effective store of energy as it can be easily hydrolyzed to give glucose and is made up of thousands of glucose molecules linked up together by hydrogen bonds in a compact spiral or coil shape. It is used as a storage polymer of glucose in plants. It is a non- reducing sugar we know this because when it is boiled with HCl and Benedicts, it again has a Yellow / Brown precipitate.
To test for a starch place a drop of the starch solution is placed in a spotting tile and a drop of iodine in potassium iodide solution. It then turns the orange / yellow iodine in the potassium iodide solution blue / black if starch is present.
The higher water potential of a solution, the greater the concentration of water molecules and the greater the total kinetic energy. This means that as the concentration of sucrose increases, the water potential decreases.
Evaluation
I had found that most of my results as I would have expected the, apart from one, the result for 0.5 molar. The result was that the cell had lost 2.8% when in fact it was expected to lose between 27% and 28%. This could have been due to the fact that maybe the potato discs had not been all the same size or not enough of them have been placed into the test tube.
The best way to do this experiment was the whole class to do it. This way we get the experiment repeated 16 times and we can take an average of the results to help ensure that it is a fair test.
There was quite a large range between the different groups for the class set of results. For 0.0 mol dm-3 the range the largest gain of mass was 53.84% and the lowest was a gain in mass of 11.11%. There was a 42.73% range, which is quite significant difference and a lot of variation in the results.
For 0.1 mol dm-3 the range the largest gain of mass was 9.86% and the lowest was a loss in mass of 5.56%. Here there was a 4.3% range which is not really that big a variation.
For 0.2 mol dm-3 the range the largest gain of mass was 7.32% and the lowest was a loss in mass of 10.71%. Again, the range was not very large at 3.39%.
For 0.3 mol dm-3 the range the lowest loss of mass was 2.70% and the largest loss in mass was 25.00%. Here there was quite a big variation with a range of 22.3%.
For 0.4 mol dm-3 the range the lowest loss of mass was 11.11% and the largest loss in mass was 26.08%. There was a range of 14.97%.
For 0.5 mol dm-3 the range the lowest loss of mass was 2.80% and the largest loss in mass was 36.84%. This result had the most variation in range with the range being 34.4%.
By looking at these results I think that it is fair enough to say that these results are not really reliable with the majority of results have a large range within them.
It seems for these results like 0.0, 0.3, 0.4, 0.5, the sample most have been contaminated or some procedure gone wrong, which led to such high rate of variation. For the other results, 0.1 and 0.2 there is a small variation is fine as it is almost expected when doing such a large experiment with so many people.
Procedures were put in place to try and prevent variation in the results. We try to do everything the same for each of the experiment. All the potato chips were cut the same and the same amount added to each specimen tube. All the chips were left in for the same amount of time and when drying them the same technique was used for all of them.
When it came to accuracy in this experiment, it was extremely difficult to maintain. To keep the test fair, we have to cut all the potato discs the same size, surface area etc. But putting this theory into action wasn’t easy and mistakes were made. But this was down to Human Error.
When it came to weighing the potato tissue after the experiment, we dabbed the discs with a paper towel. However, in doing this we could have removed water that was not excess, and altered our results.
Another way of improving the results would have been to leave the experiment running longer, this would have enabled me to find the saturation point (when the potato can no longer take in any more water) and dehydration point (when the potato cannot lose any more water) and therefore get a more accurate result.
Finally, I could extend the experiment to a more exact level by looking at the potato cylinders under a microscope e.g. incipient plasmolysis. An example of this is immersing onion cells in a range of sucrose solutions of different molarities and watch them till at least 50% of the cells have been plasolytised. Incipient plasmolysis is the point when plasmolysis is about to happen and at this point the pressure potential is equal to zero so the water potential and the solute potential are equal and the water potential of both the cell and the surrounding solution are the same.. At incipient plasmolysis the protoplast has just ceased to exert any pressure against the cell wall, so becomes flaccid. With incipient plasmolysis I would be able to see the cells in greater detail and draw some more observational results.
