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Osmosis in Potato Tubers Andrew Dickson Background When a plant cell is bathed in a solution of the same concentration (isotonic) as its intracellular environment, its mass and volume remain the same. This is because water enters and leaves the cells at the same rate. There is no net loss or gain of water by osmosis. Samples of cells can be placed in a range of solutions of different concentration. The cells will gain water by osmosis when placed in solutions which are more dilute (hypotonic) than the intracellular environment. They will therefore gain mass.
The cells will lose water in those solutions that are more concentrated (hypertonic) than the intracellular environment and so lose mass. The concentration of the intracellular environment of the cell can be determined by placing a sample of cells in solutions of different concentrations and determining at which concentration the cells neither gain nor lose mass. This external solution will have the same concentration (be isotonic with) as the intracellular environment of the cells. Aim The aim of this practical is to determine the isotonic concentration of sucrose in potato cells. Hypothesis
If the concentration of the sucrose solution in which the potato cylinder is bathed in changes, then there will also be a change in the mass of the potato cylinder. Variables Independent: Sucrose concentration Dependant: Percentage change in mass Other variables in the experiment are the potato which is to be controlled by having identical shaped cylinders of each. If the potato cylinders in different test tubes change in mass corresponding to the sucrose solution they are bathed in then the hypothesis will be supported. Materials ???Sucrose solution ???Water ???Potato ???Beaker ???5 boiling tubes ???Stoppers ???Razor blade ???Apple corer ???Paper towel Petri dish ???Scales ???Forceps Method 1. Label 5 boiling tubes with the concentration of sucrose solutions provided 2. Place 15cm3 of distilled water or sucrose solution in each of the appropriate tubes 3. Stopper the tubes 4. Using a razor blade, prepare 5 potato cylinders, each about 4cm in length. Cut off the ends if there is potato skin present 5. Place the cylinders on pieces of filter paper labeled in pencil with the concentrations of sucrose corresponding to the solution the cylinder will be placed in 6. Dry each cylinder on the filter paper dabbing (no squeezing/pressing allowed) 7. Record the mass of each cylinder on its own.
A pre-weighed Petri dish will be present on the balance to keep it clean. Make sure the balance is zeroed before you weigh 8. Transfer the pairs of cylinders to the appropriate boiling tube with forceps 9. Stopper the tubes. Note the time 10. After 30 minutes remove the cylinders from the tubes in turn, using the same order that you inserted them. Note the time 11. Remove any surplus fluid quickly and gently dab with filter paper. Do not squeeze the cylinders or they will loose all water. Reweigh the cylinders. Record their mass. Record any observations you make about changes in the nature of the cylinders in your table 12.
Work out % change in mass of each pair of cylinders. Record this in your table. 13. % change in mass = change in mass/original mass x 100% Results Test TubeSucrose Concentration (%)Mass of potato cylinder (g)Mass of potato cylinder after (g)Changes in mass (g)Percentage change in mass (%) A0%3. 663. 740. 082. 10% B5%3. 473. 530. 061. 73% C10%3. 793. 80. 010. 27% D15%3. 663. 45-0. 21-5. 73% E20%3. 993. 75-0. 24-6. 02% Discussion The results show that the different concentrations of sucrose solution in each test tube affected the resulting mass of each of the potato cylinders.
Some of the potatoes expanded and increased in mass whilst others shrunk and decreased in mass. This was caused by the diffusion of water from the cells to the sucrose solution or vice-versa through the process of osmosis. During osmosis water will diffuse across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration. The potato cylinders in both test tubes A and B with sucrose concentrations of 0% and 5% respectively, had a noticeable increase in mass, whereas the potato in test tube C even though having an increase in mass, was only marginal.
This suggests that the sucrose concentration in test tube C, which was 10%, is almost the same concentration (isotonic) as the potato’s cells intracellular environment because the mass remains relatively the same. This is because the movement of water into and out of the cells occurs at the same rate and results in no net loss or gain of water by osmosis. This also implies that the potato cylinders in test tubes A and B where bathed in a sucrose solution that was more dilute (hypotonic) than the intracellular environment of the potato’s cells.
The cells of the potato in test tubes A and B gained water by osmosis and therefore gained mass. However, the results show that in test tubes D and E with sucrose concentrations of 15% and 20% respectively, that there was a decrease in mass of the potato cylinders. This was because the sucrose solution was more concentrated (hypertonic) than the intracellular environment of the cells causing the cells to lose water therefore lose mass due to osmosis. Evaluation Some of the weaknesses of this experiment were that the process involved in removing the surplus fluid from the potato cylinders using paper towels was not very consistent.
Depending on how firm the paper towel was applied to the potato cylinders each time determined how much of the surplus fluid was removed which would have altered the results slightly. Also, the paper towel could have absorbed some of the water from the potato changing the mass of it giving an inaccurate measurement. Another weakness was that the amount of time each potato cylinder was bathed in the sucrose solution for was slightly different and was not timed using an accurate timing measure.
Next time each potato cylinder should be placed in the test tube of sucrose solution at the same time and a stop watch used to time the correct duration that they should be bathed for. Another weakness of the experiment was that the use of different potatoes could have altered the results as they may very from each other. A way to improve the experiment and make it more consistent would have been to use the same potato. Some of the strengths of the experiment were that it produced results that were expected based on research and gave in insight to the process of osmosis and the effect it has on cells.
Conclusion The concentration of sucrose in the intracellular environment of potato cells was tested by placing a sample of potato in different solutions of different concentrations of sucrose and then weighed to determine at which concentration the cells neither gained nor lost mass. This would determine the isotonic concentration of sucrose in potato cells which came out to be around 10% from the results. This supports the hypothesis of if the concentration of the sucrose solution in which the potato cylinder is bathed in changes, then there will also be a change in the mass of the potato cylinder.