RESULTS FROM PILOT
ANALYSIS OF PILOT
I obtained 2 anomalous results from my pilot experiment; the two anomalous results were for the 1.0M and 0.5M sucrose solutions. Even thought the potato cylinders in these two sucrose solutions were left in the sucrose solutions for the same amount of time as the other 3 potato cylinders, they did not change in length like the other 3 potato cylinders by remained the same length. Theoretically speaking the two potato cylinders in 0.5M and 1M sucrose solution should have decreased in length which is why I decided they were anomalous results. There are two possible reasons to why these anomalous results were gained, the first reason could be that the potato cylinders weren’t left in the sucrose solution for long enough for osmosis to take place, or that if some osmosis did occur it was too little to show a actual change in length. The second reason could be that these potato cylinders did not increase in length but in width, but I was not measuring width.
By looking at the 3 remaining results I can still see that my prediction was correct, because even from the results I have it is visible that as concentration increases the length of the potatoes decreases. This is because for 1.5M sucrose solution the potato cylinder decreased in length by 1mm. Then when the concentration increased by 0.5M to 2.0M the potato decreased by 2mm which is 1mm more than the potato cylinder in the previous concentration. My prediction is further proven when I look at the result for the potato cylinder in 0.0M sucrose solution (distilled water). This is because I predicted that the potato cylinder in 0.0M sucrose solution (distilled water) would increase in length and it did by 1mm and may have increased by more if the potato cylinder had been left in the sucrose solution longer. However in my prediction I also predicted that the potato cylinder in 0.0M sucrose solution (distilled water) would be the only potato cylinder that increased in mass. Unfortunately my pilot is unable to prove or deny this because I obtained no results for 0.5M and 1.0M, thus I don’t know if the potato cylinders in these concentrations would have increased or decreased in length. My qauntative prediction in which I stated ‘as concentration doubles the decrease in length will also double’ also cannot be proven due to obtaining no results for 0.5M and 1.0M sucrose solution.
MODIFICATIONS
After carrying out my pilot experiment I can see by the results and they way my experiment was carried out that there are certain modifications I would like do to improve my experiment further. Firstly I would like to change the time the potato cylinders are left in the solution for from 30 minutes to 50 minutes. This is because my results show very little change in length either decreasing or increasing and two results for 0.5 and 1.0 molar sucrose solutions show no change in length which is surprising as theoretically speaking there should be a decreased in length. Therefore I have drawn the conclusion that the potato cylinders were not left in the sucrose solutions for long enough for enough osmosis to take place. The second modification I think that I must carry out is to instead of measuring the length of the potato in mm I will measure the mass of the potato in grams by weighting it on a top pan balance. This is because measuring length is not good form of measuring for this experiment because it does not include width of the potato cylinder which is vital too because during osmosis the potato chip can not only get longer lengthways but also wider. This could also have been the reason why the two potato cylinders in my pilot showed no change in length and were categorised as anomalous, because maybe they didn’t increase in length like the remaining potato cylinder but increased in width, which I didn’t measure. Therefore measuring length is inaccurate so to increase the accuracy as I said earlier it would me more sensible to weigh the potato chips using a top pan balance because weight is a uniform measurement and a top pan balance is accurate to 2dp.
PREDICTION FOR MAIN INVESTIGATION (very similar to prediction for pilot experiment)
For this investigation I predict that as the concentration of the sucrose solution increases the mass of the potatoes cylinders will decrease. I can predict this because during osmosis water moves down a diffusion gradient from high water potential through a partially permeable membrane to a low water potential. Therefore the more concentrated the sucrose solution the lower the water potential hence the water will move from high water potential in the potato cylinder through a partially permeable membrane to low water potential in the sucrose solution. The potato cylinder therefore decreases in mass because it has lost water by osmosis
I can predict that the largest decrease in mass will be in the potato cylinder in the most concentration sucrose solution (2M) because the potato will have to loose the largest volume of water so that both sides can reached a state of equilibrium, which is when the water potential is the same in both the potato cylinder and in the sucrose solution.
I can further predict that the potato chip in 0.0M sucrose solution which is distilled water will not decrease in mass but increase in mass because the distilled water has a higher water potential than the potato. Hence the water will be moving from a high water potential in the 0.0M sucrose solution (distilled water) through a partially permeable membrane to low water potential in the potato cylinder. Therefore the potato cylinder will increase in mass because it is gained water by osmosis.
I can quantify my prediction by saying that, as concentration doubles the decrease in mass will also double until a limiting factor operates. The limiting factor being that each potato chip has only a certain volume of water to loose, however I am not sure if the limiting factor will operate during my experiment because I may not be using a large enough range of sucrose solution concentrations.
GRAPH TO SUPPORT MY PREDICTION
Above is a graph to support my prediction it is also the graph I expect my results to yield. The graph shows that the potato cylinder in distilled water (0.0m sucrose solution) increases in mass while all the remaining potato cylinders steadily decrease in mass as the concentration steadily increases. However if the limiting factor ‘each potato chip has only a certain volume of water to loose’ does begin to operate in my experiment the graph will be that same as the one above but will curve at the end. The dotted line drawn on my prediction graph represents this curve which may or may not be obtained in the graph my results will yield.
VARIABLES
For a fair test I must only vary 1 variable and that is the thing I am investigating (independent variable), all the remaining variables must be kept the same. I have to make sure this experiment is a fair test because if not it will affect my results causing them to be inaccurate meaning I will not be able to compare my results fairly to draw and accurate conclusion that will support my prediction.
The independent variable
The independent variable is the variable I change and in this investigation it is the 5 different concentrations of sucrose solution. The concentrations of sucrose solution are at intervals of 0.5Mand the concentrations I am using are: 0.0M (distilled water); 0.5M; 1.0M; 1.5M and 2.0M.
The dependant variable
The dependant variable is the variable I measure and in this investigation it is the change in mass of the potato chips after osmosis has occurred, allowing me to see whether osmosis has taken place and to what extent. I will measure the change in mass in grams by weighing the potato chips on a top pan balance (accurate to 2dp) before the experiment and after the experiment.
CONTROLLED VARIABLES: - are variables I must under all circumstances keep the same to ensure I am conducting a fair test.
- The size (length and width) and therefore also the surface area of the potato cylinders must be kept the same for all the potato cylinders. This is because if the size of any of the potato cylinders will vary so will the surface area over which osmosis would occur which will affect the volume of water the potato cylinders loose or gain during osmosis. For example if one of the potato cylinders was 1cm longer the surface area of the chip would be larger hence there would be more area over which osmosis would occur so the potato cylinder would loose or gain a larger volume of water than the remaining potato cylinders. I will control this variable by always using the same size borer (size 5) and always keeping the length of the potato cylinders constant at 2cm/20mm.
- The temperature at which the experiment takes place must be kept constant because the temperature will affect the rate at which osmosis will occur. For example if the temperature increases at all during the experiment the molecules will have more kinetic energy meaning there will be more collisions and the rate of osmosis will therefore be faster. I will control this variable by carrying out the whole experiment in the same lab which should be at room temperature.
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The volume of sucrose solution the potato chips are in must be kept the same because the volume of sucrose solution will affect the volume of water the potato cylinders loose or gain during osmosis. I will control this variable by always using 10cm3 of sucrose solution measured using a syringe. The sucrose solution must always cover the potato cylinder because if any part of the potato cylinder is uncovered it decrease the surface area over which osmosis will occur therefore decreasing the volume of water the potato gains or looses in relation to the other potato cylinders.
- The type of the potato must be kept the same because different potatoes may absorb at different rates and will contain different volumes of water (water potentials) that will affect if the potato looses or gains water and the volume of water the potato looses or gains. I will control this variable by using the same brand of potato for all the experiments and treating all the potatoes in the same way i.e. all have been cut without being washed and peeled.
- The time the potato cylinders are left in the solution must be the same for all the chips. This is because the time the potatoes are left in the solution will affect the volume of water the potato looses or gains. For example if any of the potato cylinders is left in the solution longer it will gain or loose more water because it will have longer to do so than the reaming potato cylinders. Hence I will control this variable by leaving all the potato cylinders for exactly 50 minutes in the sucrose solutions and I will measure the time the potato chip is left in the solution with a stopwatch. I will start the stopwatch as soon as the potato cylinders are put into the solutions and I will stop the clock as soon as 50 minutes has passed and immediately removing all the potato cylinders so that no further osmosis can take place.
- The light intensity during the experiment must be kept the same, e.g. no extra light must be shone on the experiment. This is because light is heat which would increase the temperature around the test tubes, causing the temperature to increase, which in turn would cause the molecules to gain more kinetic energy, meaning that they would move faster and there would be more collisions so the rate of osmosis will be faster.
- The same top pan balance (accurate to 2 decimal places) must be used to measure all the potato chips because the measurements may slightly vary between scales. I will control this variable by always using the same top pan balance to measure the mass of my potato chips before and after the experiment. When using the top pan balance I must make sure that the balance is reading zero before I begin to weigh each potato cylinder so that I do not get an inaccurate reading.
MAIN INVESTIGATION
For my main investigation I will use the same concentrations of sucrose solution as for my pilot. There will be a range of 5 concentrations at intervals of 0.5M. The concentrations of sucrose solutions will be 0.0M (which is distilled water); 0.5 M; 1.0M; 1.5M; and 2.0M.
I will repeat the experiment three times so that I obtain 3 sets of precise and reliable results for each sucrose solution, which I will then average. In doing this I hope that any anomalous results will have a chance to show. It is important to repeat any experiments more than once because if they were conducted only once, then an anomalous result might be gained, and I wouldn’t even know because I wouldn’t have any data to compare it with. Anomalous result need to be repeated because they are inaccurate results and therefore cause any graphs and further calculations also to be inaccurate which in turn cause that a precise conclusion cannot be made. If I get any anomalous results, which are inaccurate results that don’t fit in with the pattern of the rest of the results I will repeat the procedure again until I get an accurate reliable set of results. I will include the anomalous results in my results table, but ring them and not include them in the average. After I have obtained all my results I will calculate the % change in mass of each set of results and then further calculate the average % change in mass which I will use to plot a graph of concentration of sucrose solution against average % change in mass.
SAFETY
Safety is an important aspect in every experiment, therefore all solutions and equipment should be used carefully and the whole experiment should be conducted with care. The points that must be followed when carrying out this experiment are:
- Handle all glassware (e.g. test tubes) with care because glass is easily breakable. If breakages to occur, notify the teacher and once and make sure all the glass is cleared away before continuing with the experiment. When clearing away avoid handling broken glass.
- Handle the scalpel with care because it is sharp and could easily cause a wound.
- Act sensibly and do not run in the lab.
- Test tube holders should be used to hold test tubes.
CHEMICALS AND APPARATUS
- Sucrose solutions of these concentrations: 0 molar (distilled water), 0.5 molar, 1 molar. 1.5 molar and 2 molar.
- 2 large hard and healthy potatoes – to act as the plant cell in osmosis.
- 5 large beakers – to hold the five different concentrations of sucrose solution.
- 15 test tubes – to hold the potato chips and sucrose solutions and in which osmosis will occur.
- 5 test tube racks (3 test tubes to a rack) – to hold the test tubes.
- Cling film – to cover the openings of the test tubes.
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10cm3 syringe (accurate to ) – to measure the volume of sucrose solution required.
- Scalpel – to cut the potato.
- Cutting board – to protect the work surface when cutting the potato.
- 5 paper towels (1 towel to 3 test tubes/3 potato chips) – to remove excess water from the potato.
- Cork borer (size 5) – to bore out potato cylinder from the potato.
- Glass rod – to push the potato cylinders out after using the cork borer.
- Plastic covered graph paper – to measure the length of the potato cylinders.
- Stop watch (accurate to 1/100 of a second) - to time the durancy of time the potato cylinders are left in the sucrose solution for.
- Permanent marker – to mark the concentrations of sucrose solutions on the appropriate test tubes containing these concentrations so as not to mix the different test tube up.
- Top pan balance (accurate to 2dp) – to weigh the potato cylinders before the experiment and after the experiment.
DIAGRAM
METHOD
- Take an averaged sized potato which is hard and healthy, and on a cutting board so as to protect the work surface cut one end of the potato off using a scalpel.
- With care push the borer (size 5) down vertically through the potato, using a glass rod to push the potato cylinder out. Repeat this until you have bored three cylinders out of the potato.
- Cut all the ends of the potato cylinders square and using plastic covered graph paper measure the potato cylinders to the nearest mm cutting off any excess potato on a cutting board using a scalpel so that all three potato cylinders are exactly 2cm/20mm in length.
- Weigh each potato cylinder on a top pan balance recording the weight in grams.
- Place each potato cylinder in a separate test tube.
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Using a 10cm3 syringe cover each chip with 0.0M sucrose solution (distilled water).
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Cover all three test tubes with cling film. The test tubes are covered with cling film so that none of the water in either the potato or the sucrose solution will evaporate.
- Leave like this for 50 minutes, timing the time using a stopwatch.
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After 50 minutes remove all three potato cylinders from their test tubes wiping off any excess water using a paper towel. The excess water on the surface of the potato cylinders needs to be wiped away so that it wont be included in the final mass reading of the potato.
- Reweigh each potato cylinder recording the weight in grams in a table.
- Now repeat point 1 to 10 using 0.5molar, 1 molar, 1.5 molar and 2 molar sucrose solutions recording all weights.
RESULTS
ANALYSIS
Using the results I obtained from my main investigation I calculated the % change in mass of each potato cylinder by dividing the change in mass of the potato cylinders by the initial mass of the potato cylinders and multiplying the result by a 100. When I had calculated the % change in mass for each potato cylinder, I then calculated the average % change in mass for each set of results for each concentration and plotted a graph with concentration (the independent variable) on the x-axis and average % change in mass of the potato cylinders (the dependant variable) on the y-axis.
The line of best fit on my graph just like I predicted in my prediction graph is a curve that slopes downwards, it also doesn’t not go through the origin, which means that the % change in mass and the concentration of sucrose solution are not directly proportional.
From my graph I can see that as the x-axis increases the y-axis decreases in other words that as the concentration of sucrose solution increases the % change in mass decreases. I can see this because for 0.5M the change in mass is -7%, then as the concentration increases to 1M the change in mass decrease to 13%, when the concentration again increases to 1.5M the change in mass further decreases to 18% and lastly when the concentration increases to 2M, the change in mass decreases yet further to 22%. This complies with my prediction and like I stated in my prediction occurs because during osmosis water moves down a diffusion gradient from high water potential through a partially permeable membrane to a low water potential. Therefore the more concentrated the sucrose solution the lower the water potential. Thus the water will move from the high water potential in the potato cylinder through a partially permeable membrane to low water potential in the sucrose solution. The potato cylinder therefore decreases in mass because it has lost water.
My graph does not support my quantitative prediction because as the concentration of the sucrose solution doubles from 1M to 2M the decrease in mass does not double. I think my results didn’t support my quantitative prediction because they weren’t entirely accurate, due to the many errors that occurred during the experiment and also due to variables not being controlled. However I will discuss this later in my evaluation.
In my prediction I mentioned a limiting factor saying that it may or may not operate in my experiment because I wasn’t sure if I had a large enough range of concentrations of sucrose solutions. However my graph did begin to curve slighting at the end, meaning that the limiting factor was in fact beginning to operate. Therefore if I had used a wider range of concentrations i.e. 2.5M and 3.0M the graph would have curved further until it finished in a horizontal line. This is because the mass of the potatoes cannot just keep decreasing as concentration increases, because every potato cylinder has only a certain volume of water to loose. This meaning that eventually there will be a point at which the potatoes will stop decreasing in mass even thought concentration is increasing because they will have no more water to loose.
From the graph I can approximately estimate the concentration of the potato cylinder as 0.1M, this is because at this point on the graph the potato is neither increasing nor decreasing in mass. This is known as the isotonic point and it is when no osmosis is taking place only random diffusion of particles because both the potato cylinder and the sucrose solution have the same water potential.
EVALUATION
Overall I can confidently say that my investigation was successful because the experiment was carried out well and produced a good set of accurate and reliable results which yielded a informative graph from which I could draw an accurate conclusion.
I believe I did enough repeats (three) for each concentration of sucrose solution I was using. I left all the potato cylinders in the sucrose solutions for 50 minutes and this was enough to allow sufficient osmosis to occur. The range and interval of concentrations of sucrose solution I used were adequate, but I were to repeat the experiment I would use a larger range of concentrations and more concentrations at smaller intervals, so that my results would be more gradual and show a clearer pattern, but also so I can see clearly when the limiting factor begins to operate.
Apart from one anomalous result (circled in my results table) all the remaining results that I obtained are consistent and quite accurate. I believe they are quite accurate for a three reasons, the first reason is that I did three repeats for each concentration of sucrose solution and all the repeats were fairly similar. The second reason is that the line of best fit in my graph runs through all the plotted points and the third reason is that both my results and the graph plotted from them are theoretically supportable therefore meaning they are fairly correct. The reason why I said the result were ‘quite accurate’ or ‘fairly correct’ is because there is a moderately big difference in values between repeated results, which means the size of error between these repeated results is bigger than it should be. I believe there are two things to blame for this; the first thing is errors which occurred in the experiment and the second thing is variables which weren’t controlled. Both these things affected the accuracy of the results and the difference in values between repeated results. However errors and uncontrolled variables will be discussed further on in my evaluation, but all I can say is that if these errors were reduced and variables were controlled then there would be only a very small difference in values between repeated results. Repeated results cannot be exactly the same as this is a biology experiment involving biological variation.
I can say that my conclusion is fairly reliable but not as reliable as it should have been firstly because what I have stated in the previous paragraph and secondly because the graph I plotted from my results was not as accurate as it could have been. This is due to the fact that I plotted my points to the nearest whole numbers and not to 2 decimal places as in the table. I couldn’t include the 2 decimal places because I was unable to fit in a scale on my graph paper which would allow me to plot to two decimal places. As I already mentioned earlier my best-fit line runs through all my plotted points hence I have no need to consider different best fit curves for points on my graph.
The anomalous result I obtained was for 0.5M sucrose solution and it was –13.38%. I decided that this particular result was anomalous because the difference between it and the other results for 0.5M sucrose solution was much a too large, the other results being-7.46, -6.91 and -7.27. The reason why this anomalous result occurred is very simple to see because it is very similar to the results obtained for 1.0M sucrose solution and could easily have been one of the results obtained for 1.0M sucrose solution. Hence the anomalous result was due to human error because instead of covering the chip with 0.5M sucrose solution I must of by mistake covered it with 1.0M sucrose solution. Which is why the result is much too large for its set and is similar to the results obtained for 1.0M sucrose solution. After I had repeated this result using the correct concentration of sucrose solution I obtained a more suitable looking result. When I plotted my graph I looked for further anomalous results but found none.
Looking back at the experiment I can see that many errors occurred. The first error was caused when the potato cylinders were removed from their test tubes and dried using a paper towel to remove excess water. I may well have dried some potatoes more thoroughly than others, meaning that I did not just remove the excess water from the outside of the potato but also from the inside, which is the water it had gained during osmosis. Which of course meant that when the potato cylinders were weighed they weighed less, therefore due to this my results were inaccurate because they lead me to conclude that the potato cylinders had lost more than they actually had. Vice versa I may well have not removed all the excess water from the outside of some of the potato cylinders, which would have added to the mass of the potato. Meaning that when the potato chips were weighed they would weigh more due to the excess water, this again causing my results to be inaccurate because they lead me to conclude that the potato cylinders had gained more or lost less than they actually had done. For this error it is difficult to come up with an improvement, however one way to reduce the size of error would be to ensure that all the potatoes are rolled the same amount of times on the paper towels and as far as possible with the same pressure. If this is done it would not eliminate the error but reduce it to a minimum
The second error which occurred was due to the fact that I was doing many sets of potatoes at the same time. Which meant that after the set time the potatoes were supposed to be in the sucrose solution for was up ideally I had to remove all the potatoes from their test tubes at the same times and wipe away any excess water on the outside immediately so that no further osmosis took place in any of the potato cylinders. However this was not the case as it was impossible to remove all the potato cylinders from their test tubes and wipe any excess water off at the same time because there were just too many potato cylinders. This means that while I was removing the potatoes and wiping them the potatoes still left in the test tube waiting their turn had more time to gain or loose water. Meaning that they had a smaller or larger mass than they should have had if all the potatoes had been removed at the same time. This of course made my results in accurate but only slightly because it was only a matter of 1 minute.
The third error is bound to occur in very experiment where measuring is concerned and in this experiment small measuring errors when measuring the sucrose solution using the syringe could have easily occurred, as I am only human. For example an air bubble could have become trapped in the syringe and this would affect the measurement. If any measuring errors occurred they were however only very tiny and could have only had a tiny or no affect at all on my results. However I could reduce this error by using a burette (accurate to 0.05cm3) to measure the volume of sucrose solution required instead of a syringe because a burette is far more accurate than a syringe.
The forth and last error but also what I think was the largest source of error in this experiment is to do with the biological variation of the potatoes. The initial water potential of the potato cylinders should have been the same for all the potato cylinders. However this was not the case because the potato cylinders all of the same length and dimensions were not the same weight, or even close in many cases. Therefore because the potato cylinders were bored from the outside and inside of the potato I can conclude that the cells in the center of the potato have a larger capacity of water than the cells in the outside of the potato. Meaning that the potato cylinders bored from the outside had lower potential than the potato cylinders bored from the inside of the potato. If I were to repeat this experiment to reduce the error discussed above I would not cut potato cylinders instead I would cut the potato into thin doughnut shaped discs, so that I would only be using the out side of the potato, leaving out the middle where the water potential is higher. Also because the potato discs would be thin but wide I would be able to cut all the required potato discs from one potato. Meaning that the water potential was the same in all the discs, and not slightly varied if I had used a different potato.
I managed to control all my variables (but one) because they were all easily controllable. I left all the potato cylinders in the sucrose solution for exactly the same amount of time 50 minutes, and to makes sure this time was timed accurately I used a stopwatch which is accurate to 1/100 of a second. All the potato cylinders I used were the same size and had the same surface area. This is because firstly I used the same size borer (size 5) to bore all the cylinders out which ensured they all had the same circumference/width and secondly I measured the potato cylinders using plastic covered graph paper and to nearest mm so that they were all 2cm/20mm in length. The volume of sucrose solution I used was kept constant at 10cm3 for all the potato cylinders and covered all the potato cylinders eliminating and error in that respect. The light intensity was kept the same because no classroom or extra lights were shone during or at the experiment. All the potatoes I used were the same brand and I weighed the potato cylinders always on the same top pan balance. However if I were to repeat the experiment I could increase accuracy by weighing each chip using a more accurate scale, e.g. not to 0.00g but to 0.0000g.
The one variable which was not controlled successfully was temperature. Temperature was not kept constant for all the experiments because the experiment was carried out during the summer term over two lessons all of which were on different days and at different times of the day. Thus the temperature in the room varied from each lessons because one day could have been warmer or colder than the other and mornings were colder than afternoons, which in summer were hot. Therefore I can conclude that in some sets of experiments the rate of osmosis either decreased or increased depending whether it was colder or warmer. This would have an effect on the accuracy of my results.
This investigation can be extended further in the following ways:
- Using different plant cells i.e. different vegetable, perhaps celery or cucumber and investigating whether osmosis occurs at the same speed and in the same way in all plant cells.
- The experiment could be carried out using an animal cell, so that osmosis in plant and animal can be compared and differences observed.
- The experiment could be carried out not only using sucrose solution but also using other solutions for example salt solution to see if the solution affects the rate of osmosis and how much osmosis takes place.
- I could extend the range of the independent variable to see at what concentration the limiting factor would apply.
- The potatoes could be left in the sucrose solution longer, enabling me to find the saturation point and dehydration point. The saturation point is when the potato can no longer take in any more water and the dehydration point is when the potato cannot lose any more water.
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I could extend the experiment to a more exact level by looking at the potato cylinders under a microscope, then I would be able to see the cells in greater detail and draw some observational results.
Resources used:
- Encarta DVD
- Britannica
- Biology – Mary Jones and Geoff Jones
- Biology (Key Science) – David Applin