On a graph, these changes in percentage gain or loss of mass can be shown as followed:
Point A is where water particles have moved into the cell, as the concentration of sucrose in the beaker is lower than that in the vacuole, and therefore the potato cylinder will increase in mass when water moves into the vacuole.
Point B is where the two solutions are isotonic, so there is not net flow of water particles, and therefore no change in mass.
Point C is where water particles have moved out of the cell, as the concentration of sucrose in the beaker is higher than that in the vacuole, and therefore the mass of the potato cylinder will decrease when water moves out of the vacuole.
The graph will start to level out after Point B as the cells have not lost all of their water, but they have lost all the water that they can, so theoretically, the mass will not decrease below a certain point.
Method
The equipment I will use is:
- Tile
- Scalpel
- Callipers
- Cork Borer
- Weighing Scales
- 15 Boiling Tubes
- 3 Boiling Tube Racks
- Sucrose and Distilled Water
- Paper Towels
- 5 beakers
- Syringe
- Cling Film
- Thermometer
- 5 large potatoes
- Bunsen Burner and Matches
- Gauze
- Heatproof Mat
Before I start making up the sucrose solutions, I will need to do a control experiment. To do the control I will boil a potato cylinder of the same size that I will use in the rest of the experiments, so that the selectively permeable cell membrane will be destroyed. I will then leave the potato cylinder to cool and weigh it, and then place it in a beaker of distilled water for 60 minutes. Hopefully the cell membrane will be destroyed and no osmosis will take place, to show that it is the cell membrane that lets through water particles and causes an increase or decrease in the mass of the potato. After the potato has been in the water for 60 minutes I will weigh it again (on the same scales) and record the results. I will expect there to have been a slight change in its mass, as some of the potato may have broken off into the water, as it may have gone soft after boiling.
First of all I will need to make up the sucrose solutions. I am going to use 5 different concentrations which are 0% sucrose (distilled water), 5% sucrose, 10% sucrose, 15% sucrose and 20% sucrose.
Each boiling tube will contain 30ml solution, so here are the volumes of sucrose and distilled water that I will use:
I will use 5 potatoes, one for each concentration of sucrose solution. I will get the three repeats for each one out of the same potato, so that I can measure percentage change, but the repeats will all be from the same potato, so will have the same concentration of sucrose in the vacuole.
I will put the raw potatoes on the tile and make cylinders using a cork borer. I will cut all of the potato cylinders so that they are all exactly 5cm long, and I will make sure that I don’t cut the ends slanted, so that I keep the surface area the same. I will check the lengths with callipers.
Using the same weighing scales throughout the experiment, I will weigh each of the three potato cylinders that are to go in the 0% sucrose solution individually. I will then weigh all of the other cylinders and record their masses on a chart, making sure I zero the scales before weighing each cylinder.
I will make up the solutions as I described previously, making sure to measure the volumes accurately with a syringe. I will put sticky labels on the boiling tube racks so that I know which one is which, and I will fill the 15 boiling tubes with correct solutions. I will measure the temperature of the solutions using a thermometer, but I will not be able to control the temperature much, just monitor it.
In case I need to do any experiments again, I will not keep the same solutions, because if I leave them out in the lab, some water may evaporate, changing the concentration of the solution.
I will make a note of the time and then put the potato cylinders into the boiling tubes and wait for 1 hour. After an hour I will take each cylinder out, place them on paper towels to soak up any excess water they may have taken form the boiling tube and weigh them all again individually, using the same scales as I used before.
From the results I record, I will be able to work out the percentage change in mass, and should theoretically be able to work out the average concentration of the solutions in the cells’ vacuoles.
I am going to measure the masses of the potato cylinders before and after having been in the sucrose solutions for 1 hour by using weighing scales. The weighing scales are accurate to 2 decimal places, so this should ensure that my results are very accurate.
I will make my results valid, precise and reliable in the following ways:
- I will measure the lengths of the potato cylinders very carefully so that the surface area is not changed, as a change in the surface area may change the rate of osmosis taking place. I will use the same cork borer for each cylinder, as this would also change the surface area, which means that more water is taken into the potato by osmosis.
- I will keep the volume of the solution the solution exactly the same as this would also affect the rate of osmosis. If there was more liquid then there would be a higher number of water particles which would move into the potato and therefore change the rate of osmosis.
- I must keep all of the potato cylinders in the boiling tubes with the solution for the same amount of time, 1 hour, to make sure that the results are valid, as this too could alter the reliability of the results I collect. If they are in the water for different amounts of time then it would be unfair as some potatoes would have more osmosis happening to them.
- I am doing three repeats for each concentration so that I can work out the mean for all of the readings I take for the same concentrations. If there are any odd results they will not change the shape of the graph too much.
- I am using the same equipment throughout, including syringes, boiling tubes, weighing scales and cork borers, so as to keep the results as accurate as they can be. Some cork borers are different sized, which would increase or decrease the surface area of the potatoes, and change the amount of osmosis taking place.
- Although I am not controlling the temperatures at which osmosis takes place, I am going to measure the temperature of all of the solutions so that if there are any anomalies, they may be due to a change in temperature. Higher temperatures would increase the rate of reaction as particles have more energy to move faster, and so lose or gain more water than other potatoes.
- I am going to place the potato cylinders on paper towels after they have been in the boiling tubes to soak up any excess water which may be on the outside of them and add to the mass.
- I will use the same potato for the three repeats of each concentration of sucrose solution, as different potatoes will probably have different concentrations of sucrose in their vacuoles.
Preliminary Experiments
I decided on the range of readings that I will take by doing a trial experiment very similar to the one I will do.
The differences between the trial experiment and the one I plan to do are that I only did 2 repeats, and I only left the cylinders in the boiling tubes for 30 minutes.
Here are the results of the trial experiments:
Although these results do not fit completely with my prediction, they do generally follow the pattern that fits with the scientific theory behind it. The only anomaly is the one for 15%, which has a decrease of 0.3%, and according to the rest of the results it should be an even larger decrease in mass (larger than 0.5%).
The results have shown that the experiment works quite well, and the ranges that I have chosen are appropriate. When I do the final experiments I will leave the potatoes in the solutions for a longer period of time as when I did the preliminary experiments, the masses did not change much so not as much osmosis would have been able to take place. If I leave the cylinders in for longer then there is a higher chance of there being significant changes in the masses of the potato cylinders.
The reason I chose the ranges I did was because 0% sucrose is a control as it is distilled water and contains no sucrose particles. The highest concentration I can do before the solution becomes saturated is 20% sucrose.
I decided that the way in which I would get the most accurate and well spread out results would be to do ranges of equal intervals in between so I chose to do 0% sucrose, 5%, 10%, 15%, and 20%.
The trial experiment also showed me the importance of measuring the potato cylinders accurately, because the scales are so precise that even if the cylinders are 0.5 cm too long or short it will have a dramatic effect on the results.
I will now make sure that when I do the final experiment, I keep all of the other variables except concentration of sucrose solution the same. I will do things such as measuring the cylinders and the volume of liquid very carefully, as these things can have a big impact on the final results.
Despite the fact that the results did not fit exactly with my prediction, they showed me that there is a definite link between the concentration of a sucrose solution that a potato is placed in for an hour, and the rate of osmosis.
Results:
Control Experiment:
Experiment 1:
Experiment 2:
Experiment 3:
Averages:
I decided not to measure the temperature of each of the concentrations of water but I measured the temperature of the science lab hourly to see if an increase or decrease in temperature could explain any anomalies. Here are the hourly temperature measurements:
Analysis and Conclusion
I recorded the results that I obtained from doing the control experiments in a table, and I felt it was unnecessary to present them on a graph, as there were only two repeats. However, they did show me that there is not a very large increase or decrease in mass when the potato has been boiled, due to the cell membrane having been destroyed.
The first potato cylinder lost 3.2% of its mass, which agreed with my prediction as I stated that the mass would decrease slightly, as some potato may break off, but no osmosis would take place. When I boiled that potato it went very soft, and when I placed it into the distilled water it was in several piece, so some small parts of the potato might have become detached and floated off into the water. This could be what caused the decrease in mass.
The second control potato cylinder gained 1.6% of its mass. When I boiled it I decided that I shouldn’t leave it in for as long as the first one, so that it would stay in one piece, but I now realise that it may have increased, even if not by much, as the cell membrane hadn’t been destroyed as it had not been boiling for long enough.
The reason why the two results differ could be because I used different cork borers and therefore the cylinders were of different sizes and masses, and I also used two different potatoes. I thought that if I was working out percentage change, this would not matter, but I now know that the size of the potato can alter the results, as can using a different potato. There is always natural variation, so some potatoes will lose or gain water more quickly than others, so this may be, along with the different boiling times, the reason for the increase and decrease in mass in the two control experiments.
I chose to draw a line graph with four different lines on it to show the percentage change in the three different experiments, and then the averages. I drew the three experiments as this allowed me to see trends in the raw data, and see more clearly if there were any anomalies. It is harder to see anomalies in averages as they are balanced out by the other results, but line 4 (the average line) shows me the overall trends in the data.
The first set of experiments has a line of best fit, which is straight, and there is a negative correlation of data. This tells me that as I increased the concentration, the potato cylinders started off gaining mass and this mass gain got smaller, and eventually they lost mass. Although some of the results for the first experiment are not all exactly on the line, they are not far away from it, meaning that the line of best fit is representative of that particular experiment.
The second set of experiments also showed the general trend that I predicted it would, the line of best fit has a higher gradient, and is quite far away from the first set. It crosses the x-axis at 12%, as opposed to the first line which crosses it 6.5%, implying that overall the second set of potatoes has a higher concentration of sucrose due to natural variation.
However, this disagrees with my method and theory, as I used the same potatoes for the same concentration, so the same five potatoes would have been used for the first and second repeat. This means that for each of the three repeat experiments, the isotonic point should have been roughly the same.
This change in the isotonic point could due to something such as the second set being left further away from the window, or sat in the sun, as an increase in temperature would explain the increase in the rate of osmosis.
The third set of experiments shows the same pattern as the others because the final mass decreased with an increase in concentration, but it was different in that it began to level off at the end, towards the point where I thought the cells would not be able to lose any more water.
There was an anomaly, which was the mass reading for 15% sucrose, when a potato cylinder lost 12.7% of its mass. This could have been due to a number of things, such as there being a limited number of potatoes. Although I tried hard to get the cylinders exactly the same size, I may have cut that cylinder a different size, so that it lost a large amount of water. Before I measured the masses of the potato cylinders I got rid of any excess water onto a paper towel. The anomalous result could have even been down to something as simple as I drying it too much could, and even squeezing out a bit of water from the potato. As there are no other results that are that anomalous, I accept that it was probably a freak result that was down to unfair testing, as the other results all fit a pattern.
Although the first and second set gradually decreased in mass, they did not level off at the end as I predicted they would. However, the third set of experiments seemed to level off at the end, but this may be due to the anomaly, although even if it had fit the pattern more, the line would still curve at the end.
On the same line graph I also drew a line which represented the average percentage change in mass for the five different concentrations, to help me see any overall trends in the results I collected. This line of best fit, along with the line for set 3 of experiments curved at the end, as I had predicted it would in the plan.
The difference between the graph I drew in my prediction and the results graph is that the results graph does not level off at the end, but is still in a curve rather than a horizontal line. This implies that it could lose some more water before it reached the point where it was unable to allow any more water to leave the cells.
From these lines and points I plotted on the graph, I am now able to estimate the concentrations of the potatoes used in the three experiments, and the overall average concentrations of the potatoes. To do this I must look at where the lines of best fit cross the horizontal axis, as this shows me the point at which the two solutions inside and outside of the potato were the same, i.e. Isotonic.
Potato Cell Beaker
These two solutions are isotonic, as they have the same ratio of sucrose molecules to water molecules. This means that there is no natural tendency for osmosis in one particular direction, and even though some water particles will pass through the selectively permeable membrane, there will be no net flow of particles.
This is what happens when the solutions inside the potato cells and in the beaker are the same, so theoretically the mass of the potato should not change in mass. If I read from the graph when the line of best fit crosses the horizontal axis, I should be able to estimate the concentrations of sucrose in the vacuoles in the potatoes. Where the line crosses the horizontal axis is where there has been no percentage increase or decrease in the potato cylinders, so therefore no net flow of particles.
This table shows me that the average concentration of the sucrose solutions in the potatoes used in the experiment is 6%, as that is where the ‘average line’ crosses the horizontal axis. Set 2 of experiments is an anomaly as far as the isotonic points go, as it had an estimated concentration of 12% sucrose, even though the same potatoes were used. This anomalous reading could be down to things such as the potatoes being cut slightly different sizes, or that experiment accidentally being left in a slightly warmer environment.
I also drew a bar chart which showed me that as the day went on, the temperature in the science laboratory steadily increased. I decided that instead of the measuring the solutions it would be less time-consuming to measure the room temperature hourly. However, I have concluded that this slight rise in temperature did not have an affect on the results, as I did all of the experiments at the same time and so it wasn’t like different experiments were being done under very different conditions.
All of the results that I collected have helped me come to the conclusion that there is a definite relationship between the concentration of a sucrose solution and the rate of osmosis in potato cells. When the potato cylinders were placed in a hypertonic solution, water particles moved out of the cells, and when they were placed in a hypotonic solution, water particles moved into the cells.
This is due to osmosis, which involves the movement of water particles from an area of high concentration to an area of low concentration through a selectively permeable membrane (the cell membrane). When potato cylinders were placed in distilled water, there was a very high concentration of water particles in the beaker, so some of these moved through the holes in the cell membrane into the cells to cause an increase in mass. When there was a very strong solution of sucrose, there were fewer water molecules in the beaker than in the potato cells, and therefore water moved out of the cells, causing an increase in mass.
After a while in some of the experiments, the amount of osmosis decreased, and the straight line curved and started to level out, although never became horizontal. This implied that there is a limit as to how much water the cells can lose, although I did not appear to find this limit in these experiments.
My conclusions that I have obtained from my results mainly support my prediction, but they do not in that the lines on the graphs did not level out, showing that the cells did not lose as much water as they could have done.
My results support my prediction, as the graphs that I drew from them are similar to the graph that I drew in my prediction, and overall they reflect the theory behind osmosis.
Evaluation
I feel that the procedure I used to obtain the results and evidence from these experiments worked sufficiently to prove and explain my prediction.
The tables and graphs that I constructed fit well with the patterns that I expected to emerge, and although the graphs did not level off at the end, they did begin to, telling me that there was a limit as to how much water they could lose. I did not originally plan to make a bar chart of the temperature in the science laboratory, but I feel that if I had done the experiments at different times, the rate of osmosis may have differed. The bar chart would have been useful in explaining any anomalous results, but was not really relevant to the way in which I carried out my experiment.
My final average graph did not completely match the one which I sketched in my prediction, but I feel that the overall trend shown has proved the theory of osmosis.
In experiment 3 there was one anomalous result, which was the mass reading for the potato that had been in the 15% sucrose solution. I recorded that it had decreased in mass by 12.7%, and the point on the graph was a long way from the line of best fit. Although all of the other results did not fit exactly on the lines of best fit, they were no all far off.
I did not expect every point to be on the line, as I know from other biology I have studied that unlike in chemistry, there will always be an amount of natural variation. The anomalous reading that I obtained could be due to a number of things, such as maybe not measuring the cylinder accurately, because I know that surface area affects osmosis. Something that could have altered the reliability of that particular reading could be that when I was putting the solutions into the boiling tube, I topped them up with syringes so that there looked to be the same amount. I could have easily accidentally put some of the wrong solution into that boiling tube, although I doubt that would have had a very large impact on the reading.
Apart from that one anomalous result, the rest of my results all fit with my prediction, which indicates that my results are valid. I also took careful precautions to make sure that the only factor that I altered was the concentrations of the solutions. I tried hard to keep the following variables the same throughout the investigation:
- Length of potato cylinders
- Surface Area of potato cylinders
- Size of boiling tubes
- Volume of Solution
- Although I was not able to control the temperature in the lab, we opened the windows when an increase in temperature was measured, therefore trying to keep it as constant as possible.
I think that I collected sufficient evidence to support my conclusion, but if I had had more time I would have maybe carried out more experiments. I would have used a wider range of concentrations, and taken more readings, just so that I had more results to draw conclusions from. This may have made my results more reliable and scientifically accurate.
I did not have many difficulties when carrying out the investigating, as not much specialist equipment was used. However, it was quite hard to try and keep all variables other than concentration the same, for example cutting the cylinders exactly the same size was rather difficult.
It was hard to try and judge how much I should roll the cylinders on the paper towels before I weighed them, as I knew that they were so small that I bit of water could change the mass dramatically, and therefore compromise the reliability of my results.
If I were to repeat the investigation I would not significantly change my method because I felt it was a good way of measuring osmosis. I would probably change the readings that I took, so that I had a lot more results to analyse and draw conclusions from. I would obviously need more time to take more readings, and the concentrations I would use are:
- 2.5% sucrose
- 5.0% sucrose
- 7.5% sucrose
- 10.0% sucrose
- 12.5% sucrose
- 15.0% sucrose
- 17.5% sucrose
- 20.0% sucrose
- 22.5% sucrose
- 25.0% sucrose
I am unsure whether it is possible to have more than 20% sucrose,
but I would like to try, as it would be interesting to see if the line produced on the graph would level out and become horizontal if the cells could not lose any more water.
To support my conclusion by obtaining more evidence I could still investigate change in concentration of the solutions, but as well as measuring the percentage change in mass, I could measure the percentage change in length of the potato cylinders. I would use callipers to measure the change, and according to the theory behind osmosis, the cells would also expand and become turgid, causing an increase in size and length. I would plot a graph of my results, and like this investigation I could plot change in concentration against percentage change in length.
This additional work would help make me more certain of my conclusion as it would show me that the cells expand in size as they take in water by osmosis.
