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The Effect of Solute Concentration on the Rate of Osmosis.

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Biology coursework - The Effect of Solute Concentration on the Rate of Osmosis. Aim: To test and observe how the concentration gradient between a potato and water & sugar solution will affect the rate of osmosis. Introduction: Osmosis is defined as, diffusion, or net movement, of free water molecules from high to low concentration through a semi-permeable membrane. When a substance, such as sugar (which we will be using in the experiment we are about to analyse), dissolves in water, it attracts free water molecules to itself, and in doing so, stops them from moving freely. The effect of this, is that the concentration of (free) water molecules in that environment goes down. There are less free water molecules, and therefore less water molecules to pass across a semi-permeable membrane, through which sugar molecules and other molecules attached to them are too big to diffuse across with ease. In the diagram below on the right, we see two solutions divided by a partially/selectively permeable membrane (i.e. one that is porous, but allows water molecules through faster than dissolved substances). Originally, the two solutions were; pure water, on the left, and sugar solution with a high sugar concentration, on the right. The pure water solution is said to have higher water potential than the concentrated sugar solution, because the water will flow from the area of high concentration of free water molecules (the dilute solution), to the area of low concentration of free water molecules (the concentrated solution). In other words, to the sugar solution. However, in this diagram we see that osmosis has been taking place for a short while, because water molecules have started to diffuse to the right, across the membrane, so that there are now many present on the right side of the membrane, and a few sugar molecules are starting to diffuse across the membrane in the opposite direction, to the left side of the membrane as we see it. ...read more.


To tell the pieces of potato apart, we made a different mark, in pen, on each of the three chunks. The effect of the pen, on the permeability of the potato block, was minimal. * Once all the results had been written down in the table (see Results), we got rid of the actual values, and changed them to '% Mass Change', to eliminate any bias caused by the sizes and masses of the blocks being slightly different to start off with. Diagram of the Apparatus: Results: Molarity of solution: 0.0M Time Mass (1) % Mass Change (1) Mass (2) %Mass Change (2) Mass (3) % Mass Change (3) Average % Mass Change 0 1.23 0.00 1.20 0.00 1.22 0.00 0.00 15 1.26 2.44 1.27 5.83 1.30 6.56 4.94 30 1.35 9.76 1.35 12.50 1.35 10.66 10.97 45 1.37 11.38 1.37 14.17 1.39 13.93 13.16 60 1.38 12.20 1.39 15.83 1.42 16.39 14.81 Molarity of solution: 0.2M Time Mass (1) % Mass Change (1) Mass (2) %Mass Change (2) Mass (3) % Mass Change (3) Average % Mass Change 0 1.27 0.00 1.24 0.00 1.11 0.00 0.00 15 1.28 0.79 1.26 1.61 1.20 8.11 3.50 30 1.33 4.72 1.32 6.45 1.30 17.12 9.43 45 1.35 6.30 1.33 7.26 1.33 19.82 11.13 60 1.37 7.87 1.33 7.26 1.35 21.62 12.25 Molarity of solution: 0.4M Time Mass (1) % Mass Change (1) Mass (2) %Mass Change (2) Mass (3) % Mass Change (3) Average % Mass Change 0 1.07 0.00 1.21 0.00 1.31 0.00 0.00 15 1.05 -1.87 1.19 -1.65 1.27 -3.05 -2.19 30 1.03 -3.74 1.18 -2.48 1.25 -4.58 -3.60 45 0.98 -8.41 1.14 -5.79 1.25 -4.58 -6.26 60 0.96 -10.28 1.09 -9.92 1.21 -7.63 -9.28 Molarity of solution: 0.6M Time Mass (1) % Mass Change (1) Mass (2) %Mass Change (2) Mass (3) % Mass Change (3) Average % Mass Change 0 1.00 0.00 1.15 0.00 1.22 0.00 0.00 15 0.95 -5.00 1.10 -4.35 1.19 -2.46 -3.94 30 0.94 -6.00 1.04 -9.57 1.15 -5.74 ...read more.


2. Type of cell - all of the chunks came from just two potatoes, both of which had come from the same field, and were in the same packaging. 3. Pressure - again, because we did all aspects of the experiment in the same room, there wouldn't have been any differences in air pressure. As well as inaccuracies, as I was doing the experiment, I saw clearly where improvements could be made to make it more successful and accurate: * We could have further mitigated the effects of anomalies (such as the anomaly at 1.0M) by using even more potato chunks in each solution, and taking an average of them. * The inaccuracies to do with evaporation, drying potatoes and water on the scales were all to do with our being rushed, careless and suffering, because of what other people had done. If we had had more scales, we could have taken our time over things and perhaps erased these inaccuracies. * Instead of having to rely on a rough line to estimate the molarity of the potato, we could have used more concentrations, all around the 0.25M, 0.3M, 0.35M mark, and then have a more concrete foundation from which to analyse the results. * On the topic of concentrations, we could have maybe gone above 1.0M, and then the level-off point at which the rate of osmosis can be no faster would have been more clearly illustrated, and anomalies would also be easier to spot. There are two ways I think we could extend this coursework so that there is more to talk about, and we can learn more about the conditions osmosis thrives in: * We could investigate the other variables and see if their effects on the rate of osmosis are lesser or greater than concentration gradient factors, and if so, why? * The other option would be to investigate rate of osmosis in other plants, and see maybe if there are specific cell structures etc. under which the rate of osmosis changes. ...read more.

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