Investigating Osmosis In A Potato

Investigating Osmosis In A Potato
Introduction: "Osmosis is typically defines as the flow of one constituent of a solution through a membrane while the other constituents are blocked and unable to pass through the membrane. Experimentation is necessary to determine which membranes permit selective flow, or osmosis, because not all membranes act in this way. Many membranes allow all or none of the constituents of a solution to pass through; only a few allow a selective flow. In a classic demonstration of osmosis, a vertical tube containing a solution of sugar, with its lower end closed off by a semi-permeable membrane, is placed in a container of water. As the water passes through the membrane into the tube, the level of sugar solution in the tube visibly rises. A semi-permeable membrane that may be used for such a demonstration is the membrane found just inside the shell of an egg, that is, the film that keeps the white of the egg from direct contact with the shell. In this demonstration, the water moves in both directions through the membrane; the flow is greater from the vessel of pure water, however, because the concentration of water is greater there, that is, fewer dissolved substances exist in this solution than in the sugar solution.
The level of liquid in the tube of sugar solution will eventually rise until the flow of water from the tube of sugar solution, under the influence of hydrostatic pressure, equals the flow of water into the tube." -From Encarta 2001 In osmosis, a solvent (often water) moves from a region of low concentration to a region of high concentration through a semi-permeable membrane. Here, water molecules diffuse into the concentrated sugar solution because the water molecules are small enough to move through the membrane. The larger sugar molecules are unable to move through the membrane into the water solution. Osmosis will stop when the two solutions reach equal concentrations on either side of the membrane[image] . In the investigation I will be doing, it is osmosis in plant cells that I will be examining. I will be investigating how the osmotic rate changes and how the cells of a potato are affected by changing concentrations of sucrose solutions. That will be the basis for my main investigation. Investigating the effect of various concentrations of sucrose solution, on the osmotic rate and point of potato samples Aim: To investigate the effect of various concentrations of sucrose solution, on the osmotic rate and point of potato samples. Diagram: Apparatus: In this investigation, I used the following apparatus in order to carry out the tests fairly and effectively: ? Potato Samples- I took all of the samples from the same potato to ensure a fair test. ? Potato Borer- This seemed the most appropriate way to obtain equally sized samples for all of the tests. ? Test Tubes- I needed 15 test tubes to contain all the tests. That was three repeats for each of five different sucrose concentrations. All the test tubes were the identical to ensure a fair test. ? Sucrose Solution- I decided to used 8ml of each concentration for each test. So I had to collect in the test tubes, 8ml of each of the concentrations, three times (for the repeats). ? White tile- To cut the potato samples on. ? Syringe- To accurately measure the 8ml of each solution I needed. I decided that a syringe was the best method of accurately obtaining the correct volume of solution. ? Test tube Rack- To hold all of the 15 tests in order, so that the results could be collected easily and efficiently. ? Knife- To cut the skin off the end of the samples and to make sure that all the samples were the same length, thus ensuring a fair test. ? Paper towel- To remove the excess moisture from the potato samples surfaces. ? Electronic scales- This is the most accurate way to measure the mass before and after the soaking process. We will be able to use these masses to calculate the percentage mass change of each sample. ? Rubber Bung- To stop the solution evaporating out of the test tubes, and to seal the tests. Prediction: I believe that as the concentration of the sucrose solutions increases, the average percentage mass change of each sample will decrease. Thereby suggesting that in higher sucrose solutions, the potato cells will take in less, and in some cases, even lose water, thus decreasing the mass of the whole sample. Hypothesis: This prediction is based on a number of ideas. Firstly, when the concentration of sucrose outside the potato is low, a diluted sucrose solution, there will be a more concentrated sugar solution inside the potato than out. This is why the water will enter the potato cells via osmosis, against the concentration gradient, from a dilute solution to a concentrated solution, to dilute the sucrose in the potato. Water will diffuse into the cell by osmosis and water molecules will diffuse into the cytoplasm and vacuole through the partially permeable cell membrane. By taking in water, each cell will be swelling up and consequently the potato will increase in mass. Similarly, when the potato samples are in a higher sucrose solution, there will be a concentrated sucrose solution outside the potato and therefore less water. The solution inside the potato will be much lower as it is more diluted. Therefore the water will exit the potato cells via osmosis, against the concentration gradient, from a dilute solution to a concentrated solution, to dilute the sucrose outside the potato. Water will diffuse out of the cytoplasm and vacuole through the partially permeable membrane. By losing in water, each cell will be become flaccid and consequently the potato will decrease in mass. [image]Preliminary Investigation: I will need to carry out a preliminary investigation in order to have a general idea about the main investigation and plan the parameters of the main investigation. Therefore, my aim for this preliminary experiment will be to investigate the effect of various concentrations of sucrose on the mass of six potato samples. I will use concentrations between 0.0 and 1.0 moles/dm³ of sucrose solution to test this change in mass. I will need to stick to the following conditions in all of my six tests in order to make the results reliable and the tests fair: Volume or Sucrose solution used: 10 ml Number of pats on each potato sample after ?extraction? (to remove surface moisture): 4 Mass of potato samples used: 1.00 (+/- 0.05g) Obviously, I will take all of the samples from the same potato and use to same potato borer to obtain my samples. This will ensure similar samples for all the tests, with little variation in water or sugar concentrations in the samples. Preliminary Investigation Results: Molar Concentration of Sucrose (moles/dm³) Mass after 0 hours (g) Mass after 24 hours (g) Total Mass Change (g) Percentage mass change (%) 0 0.2 0.4 0.6 0.8 1.0 1.05 0.98 1.04 1.04 1.01 1.05 1.26 1.07 1.00 0.88 0.77 0.70 0.21 0.09 -0.04 -0.16 -0.24 -0.35 20.0% 9.18 % -3.85 % -15.8 % -23.8 % -33.3 % These results clearly show that the osmotic point (point where the percentage mass change is 0.0) is between 0.2 and 0.4 molar concentrations of sucrose solutions and so in my main investigation, I will only test concentrations of sucrose between 0.2 molar and 0.4 molar to create a more specific and accurate test environment. Quantitative Prediction: From my preliminary investigation, I have concurred that the rough osmotic point of the potato samples I took was between 0.2 moles/dm³ and 0.4 moles/dm³. This means that the rate at which the osmosis simultaneously transferring water into the potato, and also out of it, are equal, when the solution is somewhere between 0.2 and 0.4 molar solutions of sucrose. For this to happen, the solution of sucrose inside the potato must be around this point, so that the mass change is zero and the osmosis in and out balances equally. Therefore from these findings, I can predict that as the concentration of the sucrose solutions outside the potato increases, the potato samples will lose mass and have a lower (then going into negative) percentage mass change. I can also predict, that the osmotic point of my potato samples will be somewhere between 0.2 moles/dm³ and 0.4 moles/dm³, due to the fact that between these values, the 0 % mass change point will occur, thus showing the osmotic point, where the osmosis in and out of the potato is equal. Fair test Control: In order to obtain a fair test and thereby make my results reliable, I will need to keep the following controls on the tests: ? The potato samples I will use will all 1. Be from the same potato, to ensure that the sucrose concentration inside the potato is the same throughout all the samples. 2. Have the same masses: 0.93g +/- 0.05g. 3. Be the same length, to ensure an equal surface area: 3.50cm. ? I will use the same potato borer, to make sure that all of the samples have the same diameter and therefore should be the same size. ? The test tubes are the same size to ensure that all the samples are subject to the same amount of sucrose solution. ? I will use the same volume of each solution in each test: That is 8ml of- 0.20 M, 0.25 M, 0.30 M, 0.35 M and 0.40 M. ? The temperature of the tests will be kept as constant as possible, at room temperature, to ensure a consistent environment for the tests. ? The time that each test is left in their solutions is equal at 24 hours. ? After being cut out of the potato and after the 24 hour soaking period, each potato will be patted 5 times to remove excess moisture of the surface. This will make the tests fairer as the mass of the samples will become more uniform after the surface moisture has been removed. Variables: The following factors, I will need to change accordingly for each test: ? The concentration of sucrose solution used in each test. Constants: I will need to keep the following factors in the investigation the same throughout all the tests: ? The mass of the potato samples. ? The length, volume and surface area of the potato samples. ? The potato from which the samples are taken. ? The size of the potato borer I use to cut the samples out. ? The size of the test tubes used to contain the samples and sucrose solution. ? The volume of the solutions used. ? The concentration of the solution used within each three repeated tests. ? The syringe used to measure the solution. (This will have to be rinsed out after each test so that the solutions are not contaminated and the concentrations are what I have said they are). ? The total time I leave the samples in the solutions. ? The temperature that the samples are in for the duration of the test time. ? The number of pats on each sample after cutting and the soaking period, aswell as the type of towel I use to remove the excess water from the samples. ? The scales we use to weigh each sample, to ensure consistency. Range and Scientific Measure: In this investigation we will be using the following ranges for each of the factors: ? Concentrations of sucrose solutions- 0.20 M à 0.40 M (at 0.05 M intervals) ? Mass of each potato sample- 0.93g +/- 0.05g. ? All of our results and following calculations will be taken to an accuracy of 2 decimal places to ensure a consistent level of accuracy throughout all the results and calculations. By doing this, I will be able to have a set of consistent, accurate and well arranged results. This will enable me to make strong conclusions about my investigation and the suitability of the results as I have put strict parameters on the range and level of accuracy I will be examining my results at. Method: In order to complete my investigation successfully, I had to go about obtaining the evidence in the following way. 1. Draw up a results table containing the headings: Molar Concentration of Sucrose (moles/dm³), Repeat No., Mass after 0 hours (g), Mass after 24 hours (g), Total Mass Change (g), Percentage mass change (%) and Average % Mass Change. 2. Take the chosen potato and using a potato borer, cut 15 roughly equally sized tubes of potato from it. Try to make sure that all of the tubes are of a similar size and condition. Any that are ?broken? or otherwise different to the rest, replace with a new one. 3. Once 15 samples have been taken, cut the skin off the end off any samples with skin remaining on the ends. 4. Using a paper towel, pat each sample five times to remove the excess moisture. The precise pressure used to pat the samples does not matter, but just make sure to pat each sample equally. 5. Next, line them all up against the edge of a ruler. Then cut all the samples to the same length. Now, there should be 15 uniform potato samples of the same length and size. 6. Get 15 test tubes and put them all into a test tube rack. Fill the first three tubes with 8ml of 0.20 M sucrose solution, the next three with 0.25 M solution and so on, until the last three contain 8ml of 0.40 M sucrose solution. Now all the test tubes should have 8ml of sucrose solution in them. At this point it may be wise to label each tube with the solution number (e.g. 0.25 M etc.) and the repeat number (e.g. 1, 2, 3). This will make collating the results easier and more efficient later. 7. Now, take your 15 equal potato samples and measure one of them with a ruler. Write down the length near the results table. 8. Using the electronic scales, measure the mass of each sample, and write it down in the Mass after 0 hours column in the results table. All of the samples should be of a similar mass. 9. After each sample has been weighed. Put the corresponding samples into the test tubes with the solutions and make sure that each one is fully submerged by the sucrose solution. Then put the rubber bung on the top of each test tube and return each tube to the test tube rack. 10. Leave the samples in the test tubes for 24 hours. 11. After the 24 hours has passed, remove the bung from each test tube and drain off the solution inside the test tube. Remove the samples from each tube. 12. Carefully pat the first sample five times as before, to remove the excess moisture and again place on the electronic scales. Write down the mass in the Mass after 24 hours column in the results table. Repeat this for each sample and dispose of each one after its mass has been recorded. 13. Once all the masses have been collected, the total mass change, percentage mass change and average percentage mass change (an average of the three repeats) can be calculated and put into the table. Results Table: Molar Concentration of Sucrose (moles/dm³) Repeat No. Mass after 0 hours (g) Mass after 24 hours (g) Total Mass Change (g) Percentage mass change (%) Average % Mass Change 0.20M 0.25M 0.30M 0.35M 0.40M 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 0.97 0.89 0.96 0.89 0.98 0.90 0.94 0.89 0.93 0.91 0.97 0.93 0.92 0.94 0.89 1.02 1.02 1.03 0.95 0.96 1.02 0.92 0.90 0.93 0.85 0.90 0.87 0.81 0.80 0.77 +0.03 +0.13 +0.07 +0.06 -0.02 +0.12 -0.02 -0.01 0.00 -0.06 -0.07 -0.06 -0.11 -0.14 -0.12 +0.03% +0.15% +0.07% +0.07% -0.02% +0.13% -0.02% -0.01% 0.00% -0.07% -0.07% -0.06% -0.12% -0.15% -0.13% +0.08% +0.10% / (+0.06%) -0.01% -0.07 -0.13 Analysis of Results and Graph: The graph clearly shows that as the concentration of the solution outside the samples increases, the percentage mass change decreases and goes into negatives. The percentage mass change gradually decreases and passes the 0.00 % point (i.e. the osmotic point) between 0.25 and o.3 molar sucrose solution. From our graph we can see a strong correlation in this sense, showing very solidly that as the sucrose solution is increased in concentration, the percentage mass change decreases. The samples put into the low concentration solution began by gaining in mass, as shown in the high values in the graph. Then as the solution concentration is increased, the samples begin to lose mass in the form of water. This is because of many things. This prediction is based on a number of ideas. Firstly, when the concentration of sucrose outside the potato is low, a diluted sucrose solution, there will be a more concentrated sugar solution inside the potato than out. This is why the water will enter the potato cells via osmosis, against the concentration gradient, from a dilute solution to a concentrated solution, to dilute the sucrose in the potato. Water will diffuse into the cell by osmosis and water molecules will diffuse into the cytoplasm and vacuole through the partially permeable cell membrane. By taking in water, each cell will be swelling up and consequently the potato will increase in mass. Similarly, when the potato samples are in a higher sucrose solution, there will be a concentrated sucrose solution outside the potato and therefore less water. The solution inside the potato will be much lower as it is more diluted. Therefore the water will exit the potato cells via osmosis, against the concentration gradient, from a dilute solution to a concentrated solution, to dilute the sucrose outside the potato. Water will diffuse out of the cytoplasm and vacuole through the partially permeable membrane. By losing in water, each cell will be become flaccid and consequently the potato will decrease in mass. From our graph, after using a line of best fit, we can see that the osmotic point has come out as being roughly 0.275 moles/dm³. This means that in this concentration, the rate of osmosis in and out of the potato cells is equal. This does not mean that there is no osmosis, because some sort of water transfer is always taking place, it just means that the water in and out of the cells are being transferred at the same rate. Therefore, if the water going in and out of the potato is equal, it must mean that the concentration of sucrose inside and outside the potato cells is equal. Hence, the point where the percentage mass change is 0.00%, the concentration of the sucrose solution outside the potato will tell us the concentration or sucrose inside the potato cells. From the data collected from the investigation, and shown in the results table and graph, I can draw the following conclusions. Firstly, I can say that as the sucrose solution outside the potato increases in concentration, the percentage mass change of the samples will decrease, and at around 2.75 M sucrose solution, the samples will begin to lose mass. Secondly, the point at which the line of best fit on the graph, crosses the x-axis, the osmotic point, will show the sucrose concentration inside the potato cells. These points do support my prediction strongly and also show that the osmotic point did occur between 0.20 M and 0.40 M. All the other points also support my prediction strongly. Evaluation: The reliability and consistency of my results prove that the procedure used and the way that I undertook the investigation was efficient and successful. The evidence I have collected is of a high quality and show clear patterns as I predicted them before the investigation. My results to follow a clear data set and this clearly shows a strong link between the two factors being investigated. (That is the concentration of the sucrose solution outside the potato and the percentage change in mass of the potato sample.) By looking at my results table, the data appears to descend in value as the concentration of the sucrose solution increases. It would appear to do so at a steady rate, with the values at either end of the scale as we would expect them, considering the osmotic point. (Which did not directly in the middle of the concentrations). However, the set of results we collected for the 0.25 Molar sucrose solution did appear to have some flaws. Firstly, the second repeat of this concentration did come out as a negative percentage change, which clearly does not fit with the rest of the sequence. So, primarily I decided to leave this value out of the average, as it was obviously an anomalous result. However, when an average was then taken with the other two values only, it came out as a very high value, so high infact that it exceeded that of the previous set of results at 0.20 M. This made little sense, so I decided again, to change my mind and include the 2nd repeat result. The value the came out as +0.06%, which seemed to fit the data sequence more suitably. After considering these three values points in relation to the other two sets of data around it (i.e. the 0.20 M and 0.30 M) I decided that all three results were probably unreliable. From this I can conclude that all the tests we took from the 0.25 M set were infact anomalous. The 1st and third were oddly high and the 2nd was very low, but when averaged out, the data does fit with the rest of the sequence. This is also reflected on the graph, were I have shown the two-value average. The reasons for this huge difference in results for this concentration are unknown, as the rest of the results seem to be reliable enough. However, I do have some ideas as to reasons for these odd results, and how I could improve the reliability of my results. Firstly, whenever using solutions, it is always hard to get the exact concentrations that we have planned to use. If the solutions I used had not been mixed, the concentration may have been inconsistent across the whole solution, thus affecting our results, and making them vastly different for the supposed ?one concentration? There may have been some water in the test tubes when we put the solutions in, which would have diluted the solution and, again, affected the results. If we had not secured the bung on the top of the test tube properly, some of the water from the solution could have evaporated, leaving the solution more concentrated and therefore altering the reliability of the results. Another thing that I noticed that was specific to this investigation, was the following point. In a diluted sucrose solution, water is being taken into the potato cells by osmosis, to make the solution inside the potato more dilute, and the outside more concentrated. Surely this is then making the solution outside the potato more concentrated as water has been taken into the potato. This also works the other way, and would therefore suggest that the investigation is infact very unreliable as it is investigating constantly changing solutions. In each test, the solution will be changing concentrations at different rates and so the investigation is unfair. All of these things could have lead to the strange results we collected. There are endless human- related errors and other errors which could be blamed for bad results in an investigation and a combination of these is probably the reason for the anomalies that occurred in my investigation. There are also areas that we could target to improve the investigation. Things like just checking the consistency of the solutions we are using, and the temperature around the test tubes to limit evaporation. Obviously, the last point I mentioned cannot be dealt with as it is a matter of science and not anything that the investigator can change. I definitely think that my results were strong enough to support the conclusions and the prediction that I made at the beginning. I think that in any investigation, some anomalies are imminent and so I am not particularly concerned about the ones that occurred in my investigation. They were In the end the three values averaged out to fit the sequence anyway, and the rest of the results were good enough to be able to strongly support the prediction and the conclusions that I made from them. I could also extend this investigation into further work if I wanted. I could use an even more specific range of concentrations. For example, only using concentrations between 0.25 M and 0.30 M solutions. This could help us to pinpoint the osmotic point of the potato samples even more specifically. We could also change the investigated factor of the investigation. For example, keep the concentration of the solutions constant at 0.40 M for example, and investigating how the surface area of the potato samples affected the percentage change in mass. By using different sized potato borers and using simple maths, we could work out how the surface area of the potato samples (i.e. the number of cells exposed to the solution), made a difference to the osmotic point and osmotic rate etc. of the samples. Any of these extensions could be investigated to provide additional relevant evidence.

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