In experiment 3, there is a concentration of 50%, which means that there is an equal concentration of water and sucrose solution, equilibrium. This means that the mass should not increase, nor should it decrease. The surface area of the potato and the volume of solution should also stay the same as;
Experiment 4, concentration 75%, has a higher concentration of sucrose solution than water solution; this means that the mass will decrease, as will the surface area of the potato cylinder. However, in this case, the volume solution will increase as will the angle of turgidity. The final experiment, experiment 5, has a concentration of 100% - this means that there is only sucrose solution in the boiling tube and no other solution. As a result of this, the mass of the potato cylinder will decrease sufficiently, as will the surface area of the potato cylinder. The volume of the solution will increase greatly and so will the angle of turgidity. This is because:
Experimental Procedure
The key factor that I plan to investigate is sucrose concentration, and I will vary the sucrose concentration, and calculate the effects of this factor on osmosis in the potato chips by recording the mass before (before the chip is placed into the solution), the mass after (after the chip has been placed into the solution for a certain length of time; every 15 minutes for 30 minutes then after 24 hours), and the difference between these masses. Also, at each concentration I will take two readings and then I will calculate the required averages. I will do this by;
- Cut out a cylinder of potato using an apple corer with a diameter of 1cm.
- Measure a length of 4cm with a ruler and cut off the excess potato.
- Repeat this procedure so you have 10 potato cylinders of the same size (5 for the experiments and 5 for the repeats)
- Sort the potatoes into 5 sets of 2 potato cylinders and tie a piece of cotton around one of the cylinders in each set of 2.
- Measure the mass of the potato cylinders and record the masses onto a results table.
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Measure out 20cm3 of distilled water using a syringe and pour in a boiling tube. Record the temperature of the solution. Add the 2 potato cylinders to each of the 5 boiling tubes and start the stop watch.
- Record the mass of the potato cylinders every 15 minutes for 30 minutes.
- Repeat for the other volumes of solution.
- After 24 hours take the potato cylinders out of the solution and weigh them once more.
- Measure the final volume of the solutions and record them.
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Finally take the potato cylinders and attach them to a piece of cork with a pin and measure the angle:
Apparatus List
- 5 Boiling Tubes
- Boiling Tube Rack
- 1 Potato
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50cm3 Sucrose Solution
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50cm3 Distilled Water
- Apple corer
- Cork and Pins
- Thermometer
- Electronic Balance
- Ruler
- Syringe
- Stop Watch
- Tweezers
Safety Precautions
I must make sure that my hair is tied back at all times and remember wear an overall. Finally I must ensure that I am careful when using the knife and the apple corer as they are both extremely sharp and I could cut myself when using them.
I am going to use a set of 5 values as I feel this will give me a wide range of results. These are the values that I am planning to use;
I will try to keep constant the other factors that effect osmosis, those other than the concentration of sucrose (and water). I will be using potato chips from the same variety of potato as this will ensure that the partially permeable membranes are all similar. I will use the same volume of solution at each concentration of sucrose solution (that being 20 cm3). I will leave the potato chips in solution for the same time at each solution concentration. I will use the same method at each concentration, and for each potato chip and dry the excess water off the chips, and use the same accuracy of the weighing scales, prepare the chips for use in the same way and other aspects. I will keep the surface area of the chips as constant as possible, this being that I will use chips of the same diameter (1cm) and length (4cm). I found in my preliminary work that the chips in the highest concentrations of sucrose solution tended to float resulting in a certain part of the potato chip being exposed from the solution. I will carry out all of my experimental work at room temperature and so temperature will not affect my experiment; however I will measure the starting and end temperature to ensure that there are no drastic changes in temperature. The time will not affect my results either because I plan to leave my chips in solution for 15minutes for 30 minutes and then for 24 hours at each concentration, I discovered from my trial experiments that 10 minutes was a short period of time to leave the potatoes in the solution for and so decided to leaves them in for 15 minutes instead. I also chose to measure the change in mass after 3 periods of time as I felt this would give me a greater range of results. I will also have to try and remove all the potato cylinders from the tubes as quickly as possible whilst ensuring that the cylinders are not mixed up. I will ensure that they are not mixed up by tying a piece of cotton around the repeat potato cylinder, thus ensuring I can tell the difference between the two cylinders. I will also have to make sure that all the cylinders are put back in their own boiling tubes as quickly as possible and ensure that the stop watch is not started until all the cylinders are put back in their own tubes.
I will vary the sucrose concentration, using concentrations of 0.0mol/dm3 (distilled water), 0.25 mol/ dm3, 0.5 mol/ dm3, 0.75 mol/ dm3, and 1.0 mol/ dm3 (sucrose solution). I think that the reason for this prediction is that when the sucrose concentration is low, the concentration of water outside the cells of the potato chips will be greater than that inside, and therefore water will move into the cells of the chip which will gain mass. As the concentration of sucrose increases, the concentration of water outside the cell will eventually become less than inside the cells of the chip, and so the water will travel out of the chip and mass will be lost. In the higher sucrose concentrated solution, the net movement of water (osmosis) is to the outside of the cell, and the chip will lose mass, the cells will become plasmolysed. In the chip in a low concentration of sucrose solution, the water moves into the cells of the chip, mass is gained, through osmosis of water into the plant cells, the cells will become turgid.
I decided on all the above after conducting my trial experiments. When doing my trial experiments I used 3 values; these three values were at the very extremes of my data and also the middle value. The three concentrations that I used are:
I collected the results for my trial experiments and plotted a graph, I thought that these results were accurate and fair and so I decided to use these values and add in concentrations of 25% and 75%.
Obtaining
Analysing
As you can see from my tables and graph, I gathered one set of results at each concentration of sucrose solution, and also one repeat. I then proceeded to take an average between these two values at each concentration, and I constructed a further table of average mass before, average mass after each period of time, Next I plotted a graph of average change in mass against percentage sucrose solution. From the graph after 24 hours, you can see that as sucrose concentration increases, first (at a concentration of 0.0 mol/ dm3 - distilled water) the mass change is a gain of 0.44g. This falls to an increase of 0.18 at a concentration of 0.25 mol/ dm3. The change in mass continues to fall, and at a concentration of 0.5mol/ dm3, the mass decreases by 0.0g. The decrease continues, and at a concentration of 0.75 mol/ dm3, the decrease in mass is 0.18g. When the concentration is 1.0 mol/ dm3, the mass lost is 0.51g.
The reason for these changes in mass is osmosis, and that the rate of osmosis is different at varying concentrations. When the concentration of the sucrose is 0.0 mol/litre (no sucrose is present) the water concentration is higher outside the potato chip. This concentration gradient therefore causes water molecules to travel down the concentration gradient, across the partially permeable membrane and into the plant cells of the potato chips. This was the case at concentrations of 0.0 and 0.25 mol/ dm3, where mass was gained by the chips. This causes the cells to become turgid (the chips that gained mass were stiff, because their cells are filled with water). At a concentration of 0.5 mol/ dm3, the concentration of water inside the chip and outside the chip are the same, therefore the net movement of water is 0 and no mass is gained or lost by the chip, equilibrium is reached. At concentrations of 0.75, and 1.0 mol/ dm3, mass is lost by the chips, this is because the concentration of water inside the cell is greater that that outside the cell, therefore water passes out of the cell. At the concentration of 0.75 and 1.0 mass is lost because of this, and the cells of the chips became plasmolysed, the chips are quite soft because the cells had lost water. The trend of my graph shows that firstly the mass is gained, the mass increase then decreases until it becomes a mass decrease.
More mass is lost at a higher concentration because there is a greater difference between the potato cylinder and the sucrose solution, therefore there is a greater rate of osmosis.
The graph of concentration of sucrose solution against angle of turgidity of potato cylinder shows that as the concentration increases the angle of turgidity decreases. The average angle at a concentration of 0% is 90° and the average angle at a concentration of 100% is 69.5°, a change of 20.5°.
The graph of volume against concentration after 24hours shows that the volume increases by 2cm3 at a concentration of 0% and decreases by 2cm3. At a concentration of 50% however, there is no change in volume, this is because; the normal turgid state of the plant cells is the result of osmosis. This involves the movement of water by diffusion into the cell, from a region of higher water potential outside the cell to the vacuole of the cell, which has a lower water potential. The increasing volume of liquid in the vacuole causes it to expand and press the cell contents harder against the cell wall which causes a decrease in the volume of solution outside the cell. The higher the water potential of a plant cell vacuole the smaller the osmotic force which causes water to be drawn in, this results in water being drawn out and the cell becoming flaccid. Equilibrium is reached when sufficient water has moved to balance out the solute concentration on both sides of the membrane, and at that point, net flow of water ceases, which is probably the reason for their being no change in volume (I would expect equilibrium to be reached after 24hours).
My final calculation shows the change in surface area of the potato cylinder in different concentrations of solution. I did this by using the following formula;
Surface area of a cylinder = 2πr2+2πrh,
where h=4 and r=0.5.
I feel that my graph fits my prediction well; however I feel that this is not the graph that I was expecting. I was expecting the graph to curve at the end which did not happen. The mass did increase at a concentration of 0%, as did the surface area – this is what I predicted. For a concentration of 100% both the mass and the surface area decreased as did the angle of turgidity in comparison to a concentration of 0%.
Evaluating
On a whole I think that my procedure was quite reliable as for both my repeats and real experiments the change in mass was similar, but there were a few certain aspects of my experimental work that I think I could have improved. If I could have prevented the potato chips from floating (at concentrations of 0.75 mol/dm3 and 1.0 mol/ dm3) all of the surface area of the chips would be submerged in water, and so all potato cylinder (all the surface area) would be fully in contact with the solutions like the concentrations of 0.0mol/ dm3, 0.25mol/ dm3 and 0.5mol/ dm3 were, thus resulting in more accurate results.
I believe that my results support my prediction that. On my graph, I am unable to identify any anomalous results, but I feel that my results are slightly strange, as in the graph does not curve as I would expect it to. I believe that the reason for this may be that due to unforeseen circumstances I had to leave my solutions in the fridge for quite some time, which resulted in the temperature of the solutions being less then room temperature, which reduces the rate at which osmosis happens.
The graph of change in volume of solution against concentration is extremely anomalous. I feel that the results that I got were the opposite to that I would have expected. This is because in normal circumstances in the higher sucrose concentrated solution, the net movement of water would be to the outside of the cell, and the volume of the solution should increase. Whereas my graph shows the opposite; it shows that in the higher concentrated solution, the volume of sucrose solution decreases – this is not what I expected and I feel that the reason for this may be; when I removed the chips from the solution some of the water may have been removed, or however, I feel that I my have mixed up my results. I can conclude from this anomalous graph that I need to be more careful either when conducting my results or processing my results.
To change this particular procedure, I would investigate using different sizes and shapes of potato chip (investigating the effect of surface area on osmosis). I would also probably leave the chips in the solutions for a longer period of time (and so I would be able to compare the results with those of this experiment to check whether the results are different over longer periods of time). I would also probably concentrate on just one or two variables instead of all the ones that I did as I feel by concentrating on only two variables rather than several I could have improved the accuracy of all my results resulting in a more ideal graph.
Therefore as further work I would investigate into the effects of the other factors that affect osmosis in potato chips and again compare my results with these and I would investigate the effect of the temperatures at which the osmosis occurred. I would also take more repeat results to improve the accuracy of my experiment. I may also use different plants or types of potato, or maybe even an adapted species of plant (xerophytes). I could also use another type of vegetable; instead of using potatoes I could use carrots and compare the way in which osmosis differs between the two; this would also be appropriate as I would be able to use the same method for this experiment as I did in my experiment. I feel that it would also be interesting to see how the age of the potato affects the rate at which osmosis happens; instead of using only one potato I could experiment using potatoes that are 2 months, 3 or even 4 months old and compare with a fresh new potato.