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Osmosis is the passage of water molecules from a weaker solution into a stronger solution through a partially permeable membrane.

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Introduction

Introduction Osmosis is the passage of water molecules from a weaker solution into a stronger solution through a partially permeable membrane. In this case, Osmosis applies to our practical because we are trying to see if a sugared solution (sucrose) has an effect on potatoes. I aim to investigate the effect of varying concentration of a certain sugar solution on the amount of osmotic activity between the solution and a potato chip of a given size. I will also endeavour to find the exact (or as near as possible) concentration of sucrose solution when no osmosis occurs and the concentrations are equal. Preliminary Work In a sense, preliminary work is as important as the actual experiment. It allows you to make hypotheses before performing actual tests. For my preliminary work, I used a 50 mm piece of potato. It was easy to cut 50 mm (using a Vernier calliper) and to weigh them (using electronic scales to two decimal places). We used 10 mls of sugar solution/distilled water- this was the amount needed to cover one piece of potato. This was put in a test tube along with a piece of the potato and these were then put in a test tube rack. The first thing we needed to observe was how different concentrations affected the weight and length of the potato. We therefore needed to observe each end of the spectrum. We tried 0.0 molar concentration (the lowest) and 0.8 molar (the highest.) We then left each test until the next lesson, and then took the measurement of the potato chips once more. This is my results table: Concentration Initial length Final Length Percentage difference Initial mass Final mass Percentage difference 0 molar (water) 5 cm 5.15 cm 3 % 1.69 g 1.76 g 4.14 % 0 molar (water) 5 cm 5.2 cm 4 % 1.63 g 1.70 g 4.29 % 0 molar (water) ...read more.

Middle

If done with potato cells the cells would increase in length volume and mass because of the extra water. If these potato cells were placed in a solution with a low water concentration, then the opposite would happen. Water would move out of the cell into the solution. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as plasmolysed. The potato cells will have decreased in length, volume and mass. The greater the concentration of water in the external solution the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution the greater the amount of water that leaves the cell. However, there will be a point where the concentrations of water inside and outside the potato cells are equal (isotonic). At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred. I predict that the higher the sugar concentration, the more water will transfer from the potato into the sucrose solution making the potato decrease in mass and length, and vice versa for a lower sugar concentrations. I believe this will happen because the solution is diluted with water; the water molecules will transfer over to the stronger solution: whether this is the solution or the potato. Thus occurring osmosis. I also predict that if a sample floats in the solution then it will gain weight and osmosise and if it sinks then the potato will ex-osmosise. Results Table Concentration Initial mass/ g Final mass/ g Percentage change in mass/ % Average percentage change in mass/ % 0.0 1.04 1.20 +15.38 0.0 1.03 1.17 +13.60 0.0 1.01 1.18 +16.83 +15.27 0.2 1.03 1.08 +4.85 0.2 1.03 1.04 +0.97 0.2 1.01 1.03 +1.98 +2.6 0.3 1.00 0.98 -2.00 0.3 1.01 0.98 -2.97 0.3 1.04 1.00 -3.85 -2.94 0.4 1.03 0.92 -10.68 0.4 1.06 1.01 -4.75 ...read more.

Conclusion

There are no averages that are too far out indicating that the results are fairly accurate: there is not too much variability. The results in my results table are fairly similar within the concentrations with the maximum difference only being 5 percent between runs in each concentration. I would have preferred if it was less but there was not time to repeat the experiment over any more times. I am confident about my values, trend and therefore my conclusions to the extent that I believe the negative correlation trend to be accurate. I am not completely positive that a straight line is truly the line of best fit and I am also not sure as to whether my line is accurate as it is difficult to plot a straight line of best fit. If my results were slightly different I may have decided to draw a curve but then this would not fit in with the logic that mass percentage loss and gain are directly proportional to the concentration of sugar solution. I also had to round off my average percentage change in mass to make it fit onto graph paper and also be easier to draw: two decimal places were rounded off to one which has made my graph and therefore possibly my conclusions less accurate. To continue the experiment further I would like to have been able to do more concentrations and to actually test out the conclusion that there would be 0 percentage change with a solution of 0.24 moles. It would also improve accuracy to have repeated the experiment with perhaps different lengths and diameters of potato chips: as we are measuring percentage change then the size would not matter if the whole experiment was re-run with the same size and then the results compared to the experiment which has been done. Secondary sources include: Biology lessons Internet (various biology and coursework sites) Encyclopaedias Textbooks (various but mainly The Living World by Michael Roberts) ...read more.

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