To make sure the potato cylinders are the same size I will use a cork borer, as they will then all have the same diameter, and by using a scalpel and cutting the ends straight so that the cylinders are the same length, they will all be the same volume. Vernier callipers, which measure to 0.1mm, can give me a precise length. I will use mass to compare all the cylinders, so I will weigh the cylinders before and after, and I will look at the percentage difference. I will also have to remove the potato skin, if there is any, from the cylinders, as the skin is less permeable and leaving it on could affect the net movement. For safety, and to protect the work surface, I will do all the cutting of potatoes on a white tile.
Preliminary work
I am going to place 2 potato cylinders in each boiling tube, but each of these two cylinders will be left in for a different amount of time. By doing this I can see how time affects osmosis and I can then compare these results to the results I get from other boiling tubes, and from this I will also see how concentration affects osmosis. Another advantage of having the two cylinders in the same boiling tube is that it cuts down on the amount of equipment I need to use in this experiment.
Concentration table
I plan to use 40cm³ sucrose solution each time, and the table below shows how I will make up this solution for each molar of sucrose.
I will set my equipment up as above.
Equipment:
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Boiling tubes to hold the sucrose solutions and potato cylinders
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Boiling tube rack to hold the tubes
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Tweezers to pick up the potato cylinders
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Scalpel to cut the cylinders to the correct size
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Cork borer to cut the cylinders from the potato
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Tile to cut on to avoid marking the work surface
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Stopwatches to time the experiment
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Bungs to stop the solution evaporating from the boiling tubes
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Vernier callipers to measure an accurate length of potato cylinders
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Filter paper to absorb excess water from the cylinders
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Measuring cylinder to measure solution- measures to 100cm³
Method:
- Cut 8 potato cylinders using the cork borer. On the tile, use the scalpel to cut the cylinders to the desired length, using vernier callipers to ensure maximum accuracy.
- Use a measuring tube or burettes to measure the desired volume of solution, and pour the same volume of each concentration into the four boiling tubes, and label each boiling tube with the concentration that it contains. Place a bung in the top of each boiling tube to prevent any of the solution evaporating.
- Start the stopwatch and put 2 potato cylinders in each boiling tube, replacing the bungs after, and leaving one minute intervals between each pair.
- After 20 minutes, remove one cylinder from the four boiling tubes at one minute intervals to make sure that each of them is in for exactly 20 minutes.
- In these one minute intervals, measure the cylinders and record the net increase, or decrease, for each cylinder. Before measuring each cylinder, roll once quickly on a piece of filter paper to remove excess solution.
- After this initial procedure, leave the remaining cylinders in the boiling tubes for another 20 minutes (making 40 minutes in total) then once again remove the one last cylinder from each boiling tube, following steps as described in points 4 and 5.
Predictions
Where there is a high concentration of sucrose I predict that there will be a net decrease in the length of the potato cylinder , as the water in the cylinder will move to the high concentration, which is, in this case, outside the potato cylinder.
I predict that the net length will increase in the potato cylinder in distilled water, as there will be a higher sucrose solution inside the potato than in the pure water, so water will move into the potato cylinder.
I also predict that there will be more of a change in mass if the potato cylinder is left in longer, as there will be more time for the water to move and for osmosis to take place.
Graph prediction
If my predictions above are correct, then my graph should resemble the sketch graph below.
Preliminary experiment results
I found that there was little mass difference in the cylinders that were in the solution for 20 minutes, and looking at my results I can see that I found more percentage difference in the cylinders that were left in the solution for 40 minutes, so 40 minutes would be a more useful time to use than 20 minutes.
I also found that I had difficulty getting cylinders 79mm long and I wasted a lot of time trying to get cylinders of this length, so when I next do the experiment I will use smaller cylinders- about 55mm long, which should be a more suitable length.
In order to ensure that my results are reliable, I will repeat the experiment 3 times, and then take the average difference of the 3 results for each concentration. By repeating the experiment, any anomalous results will stand out more clearly, so I can then decide whether to repeat that particular cylinder and concentration.
Raw Data
Analysis
Looking at my results, I can see that with a high concentration of sucrose there was a net decrease in the length of the potato cylinders, and with a low concentration of Molar sucrose there was a net increase in the length of the potato cylinders. There is a clear trend shown in the graph- the graph shows that as the molarity increases, the length of the potato cylinder decreases.
In the distilled water solutions (0.0 Mol/dm³), there were no sucrose molecules in the solutions, but there were sucrose molecules in the potato cylinders. This means that there was a higher concentration of water outside the cylinders. To even out the concentration of water both inside and outside of the cylinders, water moved into the cylinders- out of the solutions. This means that the potato cylinders in the distilled water solutions gained water, as water moved out of the solutions and into the cylinders, and the cylinders grew in length- looking at the graph I can see that the average change for the potato cylinders placed in the distilled water solution was 3.3%.
In the 1 Mol/dm³ sucrose solutions, there was a higher concentration of sucrose molecules in the solution than in the potato cylinders. This means that in order to level out the concentrations, water had to move out of the potato cylinders, and into the solutions, so the cylinders suffered a net loss of length. The graph shows that the average loss of the potato cylinders in the 1M/dm³ was 6.6%.
By looking at the graph, I can also see the isosmotic, or equilibrium, point. This is the point at which the concentrations both inside the potato cylinder, and outside in the solution, are equal, or in equilibrium. On the graph, it is the point at which the curve of best fit crosses the X axis. On my graph, the isosmotic point is between 0.2 M/dm³ and 0.4 M/dm³, at approximately 0.3M/dm³. If this experiment was to be carried out using the concentration of Molar sucrose that the curve of best fit crosses the line at, then there would be no net movement of water in either direction.
Partially permeable membranes allow only particles of a certain size through. In this experiment, sucrose molecules did not pass through as they are too large, but water molecules are small enough to pass through, which is why the concentration of water in the potato cylinder increases or decreases.
The error bars on my graph show that for some sucrose concentrations (e.g. 0.6 M/dm³) there was very little error in the results of the experiment. However the error bars also show that for some sucrose concentrations, for example, 0.4M/dm³, the results were not very accurate. This could be because of the time between the cylinders being cut and being put into the solution, or because of the potato these cylinders came from- not all potatoes are the same, so the potato these cylinders came from could have had a different water concentration to the other potatoes.
In my plan, I predicted that when there was a high concentration of sucrose, there would be a net decrease in the length of the potato, and my results and the graph support my prediction. For example, when there was a 1M/dm³ concentration of sucrose, there was an average net decrease of 6.6% in length, and at 0.6M/dm³ concentration of sucrose solution, there was an average net decrease in length of 3.8%. This supports my prediction, because there was a larger net decrease in length for the higher concentration of sucrose solution.
Evaluation
I think that generally my method gave me reliable results as they fitted in with my prediction and expected graph.
My method allowed plenty of time for me to clear up as I went along which meant I could focus on what I needed to do, and I think that rolling the potato cylinder once quickly on a piece of filter paper after it had been taken out of the sucrose solution did remove just the excess water. However, I feel that having one minute to remove the cylinders from the boiling tubes, roll and measure them did mean I was a bit rushed.
The vernier callipers helped in being accurate with measuring the length of the potato cylinders, and using the burettes meant I could have a precise concentration. I was very careful with the equipment and tried to be as accurate as possible but I did find the vernier callipers complicated to use at the beginning.
In the first set of cylinders in my experiment, I got an anomalous result (a result that stood out as being odd, and did not fit in with the other results) for the potato cylinder in the 1M/dm³ sucrose solution, where I found it had a larger percentage decrease than the cylinder in the 0.8M/dm³ solution, which was less concentrated. This result could have been anomalous because not every potato is exactly the same, they all have a different solute concentration, and so if this potato cylinder had a different concentration, it would have been a higher or lower concentration to the other cylinders, so the movement of water would have been different. Also, while waiting to be put in the sucrose solution it may have lost more or less water by evaporation, or had to wait a different amount of time to the other potato cylinders. I do not think I could change the method to reduce anomalies, other than to make the time gap between cutting the cylinders and putting them in the boiling tubes with the solutions in smaller, so long as I have sufficient time to repeat that part of the experiment, which I did. I decided to include the anomaly in with the other results to show the difference between the anomaly and the sort of result that I would have expected. When I repeated the experiment I did get the result that I would have expected.
I think if I was to repeat the whole experiment I would probably get similar results. I drew error bars on my graph and found that some of the bars showed little variation, and this means that those results have quite high levels of accuracy and reliability. However, some of the error bars showed more variation- the 0.4M/dm³ in particular- which means that those results are unfortunately less accurate and reliable. However, the average points allowed a line of best fit to be drawn.
To extend and go into more detail on this experiment, I would repeat it exactly using one or more other types of vegetable, such as turnip or carrot. To do this I would have to cut exactly the same size cylinders of each vegetable tissue, and make different concentrations of sucrose solutions, as I did in this experiment. I could then use the same timings I used for the potato cylinders (40 minutes), and leave the other vegetable cylinders in for that same amount of time, before taking them out, removing excess solution, and recording the difference in mass. By trying the experiment with other tissues, I could compare the difference the concentration of sucrose makes on the net movement of water molecules in, or out, of the vegetable tissue.
Risk Assessment
- While the sucrose solution is not corrosive, any spillages will become slippery and potentially hazardous, so they should be wiped up immediately.
- To protect clothing, wear laboratory coats, and use goggles to protect eyes.
- Also, the sucrose solution is not particularly irritant, but as there is the possible risk of an allergy, it should be wiped off if it comes in contact with skin, and if it comes in contact with the eyes the eyes should be washed out.
- Cork borers can be potentially hazardous, so when using to cut potato cylinders, ensure that the hand holding the potato is not covering the exit route of the cork borer.
- If any glass is broken, a teacher should be alerted to that fact, and the glass should be cleaned up using a dustpan and brush in a safe manner. Be careful that all the pieces of glass are cleared up.
- Be careful with sharp pieces of equipment (such as scalpels), so as not to cut yourself, and when cutting use a tile to protect the surface you are working on.
Bibliography
- Mastering Biology - O F G Kilgour, P D Riley
- Coordinated Science (Biology) - Mary Jones, Geoff Jones
- Essential Biology- Glenn and Susan Toole
- The Usborne Dictionary of Science – C Stockley, C Oxdale, J Wertheim
- Biology for You – Gareth Williams
- Biology the Revision Guide – CGP, edited by Richard Parsons
- www.bbc.co.uk/schools/gcsebitesize
Joanna Steel Biology Coursework Page