What Factors Affect Osmosis in Potato Cylinders?
. What Factors Affect Osmosis in Potato Cylinders?
The aim of this experiment is to investigate the volume of sucrose solution and the weight and volume of other variables which affect osmosis in potato cylinders. In this experiment I will weigh and blot rings of potato and test them in different concentrations of sucrose solution. I hope to find sufficient results as to investigate this.
In this experiment I will be investigating, and putting to test, the theory of osmosis.
The scientific theory of osmosis is the diffusion of water through a selectively permeable membrane into a more concentrated solution of sucrose. Because the more concentrated solution contains a lower concentration of water molecules, the water flows by diffusion to dilute it until concentrations of solvent are equal on both sides of the membrane. In simple terms, it may also be thought of as the passage of water molecules from a low solution to a high one, across a selectively permeable membrane. (This is shown in the diagram below).
Sugar solution Distilled water
Sugar molecule water molecule
Low High
Water Water
Potential Potential
Selectively permeable membrane
Another experiment, published by the author B.S Becket in "Biology-a modern introduction", gives a clear understanding of osmosis. In this experiment, water can pass through the selectively permeable membrane, but not the sugar molecules, this is because the semi permeable membrane "imprisons" the sugar molecules, Because of the higher concentration of water molecules in the beaker, more water passes into the thistle funnel than out of it, and the level of sugar solution in the funnel rises as water diffuses through into it. (This is shown in the diagram below).
Thistle Funnel 2nd level of sugar solution after several hrs
Water in beaker
1st level of sugar solution
Selectively Strong sugar solution
permeable membrane
The sugar solution is said to have a low water potential. The water could be replaced by a weak sugar solution, and water would still enter the thistle by osmosis, as long as the sugar solution it contained was higher.
There are many factors in this experiment, and I hope to investigate all of them. I will investigate the concentration of sucrose solution, as this will have a huge effect on my final results. Other variables in this test will be;
Size of potato ring (1cm),
Weight of potato ring (g),
Thickness of wire (copper),
Volume of solution (20cm(),
No. of potato rings on wire (3),
Time test takes place (24hrs).
These will all have an effect on the experiment. As to keep this test fair, I must keep my measurements ...
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There are many factors in this experiment, and I hope to investigate all of them. I will investigate the concentration of sucrose solution, as this will have a huge effect on my final results. Other variables in this test will be;
Size of potato ring (1cm),
Weight of potato ring (g),
Thickness of wire (copper),
Volume of solution (20cm(),
No. of potato rings on wire (3),
Time test takes place (24hrs).
These will all have an effect on the experiment. As to keep this test fair, I must keep my measurements equal and unbiased, to keep my measurements accurate. The size and weight of the potato rings are essential to the test, and must be kept the same. I am investigating how the concentration of sucrose solution will affect the potato rings. If the rings are different sizes and weights, then the results will be determined by these changes rather than the sucrose solution, the effect then that this would have on my results, and the osmosis would be great. However, if kept the same weight and size, then I would be sure that it is not the potato rings affecting the experiment in any way. This will allow me to concentrate more on the other factors that may affect the osmosis. The amount of potato pieces on the wire must be kept the same, for the same reasons as before. It is important that we keep the time for every test at 24 hours. This is because the diffusion of the water molecules may occur at different times, and to increase or decrease the time given to the experiments to work would affect the test dramatically. The volume of the solution must be kept the same in all of the test tubes. This is for the reasons that if I want to see how the concentration of the sucrose solution affects osmosis, then I will not be able to measure this accurately if the volume is different for the different concentrations. It is a problem that I could do without in this experiment. The thickness of the copper wire must be kept the same, as to not reduce the mass inside the potato where the wire is put. I am aware that that the amount of sucrose, and the weight of the potato rings will not always be kept the same. To deal with this problem I will include the weights of my potato rings in my results table. This will give us a clearer indication of the weight change. The same wire will be used throughout to keep the thickness the same, and a straight rule will be used to keep the rings equal. I will not the time on a clock at the start of the experiment, and give me the chance to calculate when it needs to end at the end of the next 24 hours. In all there will be seven tests. The concentration of sucrose will be increased in every test to test what affect this will have on the weight of the potato rings. The first test will have distilled water. This will be the control test.
Here are the concentrations of sucrose;
0.0M (distilled),
0.1M,
0.2M,
0.3M,
0.4M,
0.5M,
0.6M.
In order to do this experiment I must first find and setup the equipment that I am going to use;
Copper wire,
Test tube (7),
Ruler,
Scalpel,
Cork borer,
Potato,
Test tube rack,
20cm( of sucrose solution (concentrations of sucrose above).
I must first bore a tube out of the middle of the potato using the cork borer. I will then measure the tube into three rings of 1cm using the ruler, cutting with the scalpel to make sure they are all the same size. I cut three rings so that when I weigh them again once the experiment is over, I can compare the three rings and make sure that there are no anomalies in the experiment. After the rings are cut I weigh each of the 3 potato rings on the weighing scales and write the weights of the rings into my table. I will then put those pieces onto a 6cm long piece of copper wire. They will be placed into a test tube filled with 20cm cubed of sucrose solution. I will place this on the rack, and leave it for 24 hours. After 24 hours, the rings will be weighed again. I will repeat this 6 more times with different concentrations of sucrose solution to see how the weight of the potato rings are affected. (See below for step by step instructions).
). Use a cork borer to bore a tube of potato out of the potato.
2).Using the scalpel, cut the potato tube into three equal size rings.
3). Using weighing scales, weigh each separate ring of potato and record the weight into the table, (make more than one copy in case the original goes missing).
4). Put the potato rings on a 6cm long piece of copper wire, leaving space between each one.
5). Put the wire with the potato rings on it in a test tube filled with 20cm( of the first concentration of sucrose solution.
6). Place on the test tube rack, and leave it for 24 hours.
7). After 24 hours, take the potato rings out of the test tube, off the copper wire, and weigh them again.
8). As before, record the new weight of the potato rings, and make another copy.
9). Repeat this process five more times with all the other concentrations of sucrose solution.
My prediction is that the size and weight of the potato ring will affect osmosis. I believe that if 3 potato rings are in a test tube of an equal concentration of sucrose, then they will all gain the same weight as their selectively permeable membrane is still the same. On this basis, I must investigate the concentrations of sucrose affecting the potato cylinders. The mass of the potato its self and its wire, I do not believe to be important. I predict that the potato rings with 0M sucrose will increase in weight the most. I also believe that as I add higher concentrations of sucrose solution, the less the potato rings will weigh. Due to the theory of osmosis, because there is a higher concentration of water molecules outside the potato than there is inside it at the beginning of the experiment, water molecules will diffuse through the selectively permeable membrane into the lower concentration of water inside the potato, until both sides are equal. I believe that this will cause the potato cylinder to increase in weight. In simple terms, the less sucrose in the solution, the more water particles there are. Because of the selectively permeable membrane, no sucrose can pass through. There is also the possibility that because no sucrose can pass throught the selectively permeable membrane, this would slow the process down dramatically. The high concentration of water, which is in the 0.1M concentration of sucrose, on the outside of the rings, and the low concentration of water on the inside, means that water molecules will contantly be trying to diffuse through to the low concentration behind the selectively permeable membrane. This will cause an increase in the potato's weight. However, the higher the concentration of sucrose molecules there are, the less water molecules there are, meaning less can diffuse and affect the weight of the potato ring. Although the weight of the rings should increase as long as there are water molecules in the solution, the more sucrose molecules added, the less water molecules can get through.
My results show me that through a selectively permeable membrane, the concentration of sucrose in the solution can have a much larger affect on osmosis than I had first predicted. As the sucrose concentration increases my results certainly compliment my first basic predictions. My results show me that the general pattern was, the less sucrose molecules in the solution, the more water molecules were able to diffuse through the selectively permeable membrane, because of the high concentration of water on the outside of the potato. This resulted in more osmosis, and an increase of weight in the potato. One factor in the results surprised me. As the sucrose solution reached high levels of concentration, the weight of the potato rings actually decreased to negative numbers. I knew for a fact, on a basis of scientific theory, that the sucrose would certainly slow down the process of osmosis. I even knew that there may be a chance of the concentration of sucrose stopping the flow of water molecules through almost completely. However, I never predicted a complete reversal. Overall, I received some positive results, and some very interesting upsets.
If time was available I would have repeated this experiment at least once more. I conducted the experiment to the best of my ability, and I am reasonably happy with my results. However, I feel that there are certain anomalies that would make more sense if I had the chance to repeat the experiment. Despite this, I feel that I did everything I could do, and received the most accurate measurements possible in the time that was available to me.
I believe that my experiment went quite well, though I believe my averages were affected by some odd anomalies. For example, the 3 potato rings in the 0.5 molar concentration of sucrose solution were different in weight at the very start of the experiment. This would prove a problem because although I do not believe that the weight of the potato rings affects osmosis, it will still make uneasy reading in my results. Ring 1 and ring 3 were 0.90 and 0.89, whereas the second ring was1.34. I believe this, and some other anomalies had a large affect on the weight of the rings after 24 hours, and the average. The second ring was 0.43g heavier than the other two potato rings in the same test tube. The fact that it weighed more before meant that ultimately although it gained exactly the same weight as the other two, it's final weight was still larger. I had to change it's original weight so that my average would make sense. This tells me that the weight of the potato ring does not directly affect the osmosis in the potato ring. This confirms my prediction earlier that it is the concentration of the sucrose solution not the size or weight of the potato ring that affects osmosis.
Overall my results seemed accurate enough. They backed up and developed my understanding of osmosis, and seemed to fit in with my theory and scientific knowledge. I could of perhaps taken more time over the cutting of the potato cylinders into rings, and measured my sucrose solution as accurately as I could manage. Although I do not think this would have affected my experiment directly, I t would have made my results clearer and easier to interpret. Of course I could have repeated my experiment for more accurate results, but this was just not possible . I believe I have successfully investigated all the factors I intended to.
Thomas Godwin-Coursework
