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I am investigating the effect that different concentrations of salt solution have on the mass of potato tissue due to osmosis.

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Introduction

Plan I am investigating the effect that different concentrations of salt solution have on the mass of potato tissue due to osmosis. Osmosis is the net movement of water molecules across a semi-permeable membrane from an area dilute solution (containing more water molecules) to an area of more concentrated solution (containing less water molecules). Potato tissue is a group of potato cells. I will be weighing each cylinder of potato tissue using a 2 decimal place balance so it will be to 1/100th of a gram before and after I have placed them in the salt solution. I will place them in a range of 5 concentrations of solution starting from distilled water at 0% then 1%, 2%, 3% up to 4% salt solution. I will be placing 3 cylinders in each concentration of solution. I have done some preliminary tests to help me plan this experiment measuring the effect different concentrations of solution had on the length of potato cylinders. In the preliminary test I found that 0% concentration increased the length of the cylinders and 4% decreased the length so 0% had an opposite effect on the length of the potato cylinder than 4% and that 4% is a suitable upper value. I also discovered that a length of 40mm is suitable for each cylinder as at this length I could record the effects of osmosis on the tissue. Also that they should be in the solution for at least 20 minutes because this is how long it took for there to be very clear change in length so that I knew osmosis had taken place. To carry out this experiment I will need: * 5 beakers * 15 potato cylinders * A stop clock * A razor blade for accuracy of cutting the cylinders to 40mm * A millimeter ruler also for accuracy in measuring the cylinders to 40mm * A balance that goes to 2 decimal places for accuracy * 1%, 2%, 3% and 4% salt solutions * Distilled ...read more.

Middle

This is shown as being less than 100% on the first graph and table and being a negative mass change on the second graph and table. The line of best fit that I acquired on my graphs is not a straight line, which shows that the change in mass is not directly proportionate to the change in salt solution concentration. My first graph shows the percentage change of original. My second graph shows the percentage change in mass. They show that when the potato tissue, which is made up of potato cells, was placed in distilled water, of a concentration 0%, the concentration inside the cell vacuole was higher than that of the distilled water. Water molecules moved through the partially permeable cell and vacuole membrane, into the cell vacuole resulting in an increase in mass. This supports my prediction, which was that potato tissue placed in 0% concentration would gain mass. In my experiment, this mass increase was on average 3.38% of the original mass. When the potato tissue was placed in 1% salt solution, the cylinder gained on average 0.47% that means that water molecules have moved into the cell vacuoles. This means that the concentration outside the cells was higher than that inside the cells but only very slightly. The concentration of solutes inside the cell vacuole lies somewhere between 1 and 2% concentration. This supports my prediction that at some point there should be no net movement. It lies much closer to 1% as the cylinders placed in 2% salt solution lost on average 2.36% of their mass which means that water molecules inside the cell vacuole moved out of the vacuole through the partially permeable cell and vacuole membranes. From my second graph, I can see that the concentration inside the cell vacuole is approximately 1.2% because this is the point where there is no mass change. The water molecules are moving in and out across the partially permeable membranes at the same rate, so there is no net movement in one direction. ...read more.

Conclusion

In my experiment the change in mass was smallest at 1% and therefore is roughly the same concentration as the solution in the cell vacuole. For a more accurate result you would need to make improvements to the experiment. The reliability of the results would need to be improved. On the graphs you can see by the error bars that the results for 4% concentration were the most accurate because the bar is smallest. The results obtained at 3% solution however were very inaccurate and I think this definitely effected the reliability and the drawing of the line of best fit. The reason the error bar is so big for 3% concentration is because of the result obtained for the third cylinder. It is an anomalous result, which is a result that is clearly different than the others in the group. The third cylinder in 3% had a percentage mass change of -13.88% whereas the other two had percentage changes of -9.52% and -9.43%. This is a difference of over -5%, which severely effected the average. This result could have been effected by any of the factors mentioned above, for example the cylinder could have been allowed to stay immersed longer than the others allowing more time for water to pass out of the cells by osmosis resulting in a loss of mass greater than the others. To extend this experiment and achieve more accurate and reliable results you could take down a lot more results and repeat the experiment multiple times. You would do this so that anomalous results and extreme values would less effect the average. You could also disregard results that are extreme and anomalous to make it more reliable. Another way to extend it would be to carry out the experiment over a larger range e.g. 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2% etc. This would allow you to more accurately find the concentration of solution at which no net movement of water occurs because the concentration inside and outside the vacuoles is the same. This would mean being able to more accurately know the concentration inside the vacuoles. ...read more.

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