- Firstly, the surface area of the potato tubes has an effect on the rate of osmosis taking place. A larger surface area would take up more water, likewise a smaller surface area would have an opposite effect.
- The size of the water and sugar particles also comes under consideration. If the sugar molecules are smaller than the breaks in the selectively permeable membrane, they will diffuse through into the potato and osmosis would not take place. Also, using a smaller water molecule would have an adverse effect; more molecules would be allowed to pass through the membrane of the potato, speeding up the process of osmosis.
- The volume of the sugar solution surrounding the potato tissue is another important variable. A larger volume of sugar solution would mean that there are more water molecules to pass into the tissue.
- In addition to considering the concentration of the sucrose/water mixture, the concentration of the potato cells must also be taken into account. A tube of potato with many cells in it will expand less rapidly than a tube of tissue with not as many cells in. This is because the cells have more space to expand in the tube with not so many cells.
As only one of the above variables will be tested, the concentration of the sugar solution, the other variables will need to be kept at constant levels. The same mass of potato must be used each time the experiment is repeated, and the same volume of sucrose solution must be applied. To ensure that the test is as fair as possible, the same pieces of apparatus will be utilised, and the temperature of the room will be kept at an unwavering value.
The following items of apparatus will be used throughout the course of the experimental process:
- 6 x boiling tubes
- 2 x 50ml measuring cylinders
- 1 x boiling tube rack
- 1 x white tile
- 1 x scalpel
- 1 x potato
- 1 x electronic scales
- 6 x rubber bungs (to fit boiling tubes)
- 1 x cork borer
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60cm3 of 1 molar stock sugar solution
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60cm3 of water
The experiment will be split into two halves: the potato will be prepared and the tubes weighed. After 24 hours the potato tubes are taken out of the sugar solution and re-weighed. Any changes are noted.
The experiment will be initiated in the following order:
- The six boiling tubes will be numbered from 1-6 using a non-permanent pen. In each of the boiling tubes there will be 20ml of sugar solution, each tube holding a solution with a differing molarity (see table 1).
- The potato will be peeled then six tubes will be bored from the potato, using the potato borer. These tubes will be cut to make twelve similar smaller tubes. This is to make sure that two similar results are achieved, so an average can be taken. These tubes will be of roughly the same mass, but will not be identical as an average of there weights will be taken to cancel out any extreme differences.
- Two potato tubes will be placed into each boiling tube and will be left to stand for twenty-four hours. The potato chips will be removed, dried and re-weighed. Any change in mass will be noted, using the readings taken before and after the tubes’ immersion in sugar solution.
As mentioned earlier, the concentration of the sugar solution will vary from one boiling tube to another. The first boiling tube will contain a solution of ratio 20:0 (sugar : water) and progress through six steps to 0:20, altering each substance by 4ml each step. It is important that the total volume of water and sucrose in each boiling tube equals 20ml. The change in mass noted after 24 hours will be dependent on the molarity (or ratio) of the sugar solution in each particular boiling tube.
Table 1
N.B – The molarity of the sugar solution tells you how strong the solution.
Eg. A solution of molarity 1.0 is comprised of 100% sugar.
A solution of molarity 0.4 is comprised of 40% sugar, 60% water etc.
RESULTS:
Two sets of results will be taken: the mass of the potato tubes before the immersion in the sugar solution, and the mass of the tubes after the experiment. The results will be displayed in two forms, tabular and graphical. The table headings that will be used are shown below:-
Using my background knowledge on osmotic processes, I can predict that the potato tubes in boiling tube No. 1 will lose mass over the twenty-four hour period. Because the solution around the tissue contains no water, the water inside the potato will diffuse out of the cells, decreasing the mass of the potato chip. I can also say that the potato tissue in the sixth boiling tube will have swollen considerably. This is due to high concentration of water around the potato tube, which causes water to diffuse into the potato cells.
After the experiment was completed in a safe and careful method, the results were organised into several different formats. Firstly, a table, using the headings outlined in “Planning”, was set up and the results entered. This table can be seen in Annex 1.
To enable us to manipulate and effectively analyse the results that had been formulated, the values in the table were converted into points on a line graph.
The graph can be seen in Annex 2. On the graph, the point where the line intersects with the x axis, point M, represents the molarity at which the potato tissue neither gains nor loses mass. It can also be said that at this point, the potato tissue is isotonic with the sucrose solution: I.e. This point is the osmotic pressure of the tissue.
As a result of undertaking this experiment, the following things can be concluded:
Firstly, from observing the results in the form of both the graph and the table, it is obvious that a pattern exists within the result values. The line shown on the graph can be described as being mirrored in both axes. This symmetry occurs because the amount of water the potato tissue loses through osmosis is almost proportional to the amount of water it gains through osmosis. Each side of the point M, the line gradually becomes shallower as it moves away from the x-axis, suggesting that if the line were continued, it would level out. I believe that this behaviour would be mirrored in other experiments of this nature, as one specific amount of potato tissue can only lose or gain a certain amount of weight under osmosis, relative to its mass. For example, a quantity of potato tissue cannot lose an amount of water larger than its actual mass, meaning that the line on the graph must level out at some stage. In this way, the potato tissue will never reach zero mass as it consists of other elements than just water.
The results that were attained as a consequence of undertaking this experiment reinforced the predictions that were made at the end of the “Planning” section. As the table of results shows, the potato tubes in the first boiling tube did decrease in weight over the twenty-four hour period. The potato tissue in boiling tube No. 6 also followed the pattern I expected. The samples of potato tissue in the boiling tubes that lie between Nos. 1 and 6 all followed the same pattern, they became gradually larger until they had reached their original mass and gone past this point.
When both potato tubes within a boiling-tube were weighed, it was obvious that they both a lost a similar amount of weight. The only plotted point on the graph that didn’t lie on the line of best fit was the reading for the boiling-tube containing sugar solution with a molarity of 0.4. However, this point does not lie far off the line, and does not have any effect on the outcome of the experiment. An appropriate amount of readings were taken, as there is enough to be able to analyse the results, but not so many as that the analysis becomes harder. The reason why two tubes were tested in each tube is that the experiment is effectively being repeated.
The method that was used in this experiment was the best possible. It kept all the variables but one the same, and dealt only with changing the concentration of the sugar solution. If the experiment were to be repeated, I would not test two tubes of potato in the same boiling tube, I would test them in separate boiling tubes. This would make the test fairer, as it proves that the presence of the second tube is not affecting the osmotic actions of the first. This modification will also ensure that each potato tube is immersed in the same amount of water. Placing two tubes in the same receptacle could mean that one of the tubes takes up water faster than the other.
The evidence collected was sufficient enough for me to make a conclusion. Apart from the modification mentioned earlier, there are no other tests that could be applied to enhance the experiment. The readings that were taken were to a sufficient degree of accuracy, because a pair of electronic scales was used to pinpoint the tubes’ weights to two decimal places.
Overall, I believe that the experiment was planned and completed to an adequate standard. The results table is clearly set out and diagrams help to explain the principles of osmosis. The graph is also accurate and shows the pattern well.