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Experiment to find the concentration of sucrose solution where there is no change in mass of potato cylinders.

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Introduction

Experiment to find the concentration of sucrose solution where there is no change in mass of potato cylinders. By Steph Winterbottom 10.D Theory Osmosis is a special kind of diffusion, when water molecules pass through a selectively permeable membrane. This happens when there is a difference in concentration between the two sides of the membrane. The water molecules move along the concentration gradient, from low concentrations of water molecules to high. It does not require energy. Osmosis happens constantly but when there is the same concentration on both sides of the membrane an equal amount of water molecules pass through the membrane on both sides therefore leaving the masses on either side unchanged. Osmosis is used during photosynthesis as the membrane of a root hair cell is selectively permeable, therefore, using osmosis, it can absorb the water it needs to survive from the soil surrounding it. Preliminary Work For my preliminary work I cut 9 cylinders from a potato using a scalpel and a cork borer, size 5, each cylinder was cut precisely 2cm long. I measured up three solutions of sucrose, the first 0.0M the second 0.5M and the third 1.0M. I then weighed the 9 cylinders in groups of three and then put a group in each of the three solutions. After 22 hours I took the cylinders out of the solutions and reweighed them to find the mass lost/gained. Preliminary Results 0.0M Sucrose 0.5M Sucrose 1.0M Sucrose Mass before / g 3.44 3.39 3.53 Mass after/ g 3.96 2.65 2.51 Change in mass/ g +0.52 -0.74 -1.02 % change in mass +15.12 -21.8 -28.91 In the 0.0M sucrose the potato cylinder gained mass therefore the potato must have had a lower concentration of water molecules than the solution (which isn't surprising as the solution was distilled water) therefore the water molecules in the 0.0M Sucrose solution passed through the cell membranes into the potato because of the lower concentration of water in it thus causing the potato to gain in mass as it has acquired more water molecules. ...read more.

Middle

The solutions which caused the potato cylinders to lose mass did so because the concentration of water molecules inside the solution was lower than the concentration of water molecules inside the potato. The potato lost mass in each of the three solutions due to osmosis. The water molecules in the two solutions were of higher concentration than those in the potato cylinders, thus causing the water molecules to pass through the semi-permeable membrane of the potato and diffuse into the solution surrounding the potato. The potato in the 0.5M sucrose solution lost more water molecules (and therefore mass) than the potato in the 0.2M sucrose solution because there was less water molecules in the 0.5M than the 0.2M sucrose therefore causing the 0.5M to have a lower concentration of water molecules, obviously substantially lower than the potato's because of just how much was lost. This is shown by the concentration gradient of the graph. In the 0.2M sucrose solution there is practically no change in mass. The solution which has no change doesn't have any change because the concentration of water molecules is equal on both sides of the potato's semi-permeable membrane (which means that the water molecules in the solution are of equal amount as those in the potato). Therefore, in finding the concentration at which there is no change you have subsequently found the concentration of water molecules in the potato, e.g. if you find that there is no change in mass of a potato cylinder in a 0.2M solution no matter how long you leave it in for, then the concentration in that potato cylinder is 0.2M. However, osmosis still occurs, water molecules still pass through the semi-permeable membrane of the potato cylinder, but they pass through at an equal rate. To find the concentration of the potatoes which I used I would use my graph. At the point where the line crosses the X axis that is the concentration at which osmosis is equal on both sides of the semi-permeable membrane. ...read more.

Conclusion

50%) throughout the whole experiment just in case either of these factors had any effect on the experiments. I could have left the potato cylinders in the solution for longer but I don't think that I needed to as I found quite a substantial change in mass and that was all I needed to plot my results onto a graph and fine the concentration of sucrose in the potato. If I left it for longer it would be a lot longer (e.g. 72 hours) so that I could see if there was any definite difference in leaving the solutions for longer. I could also have expanded my experiments by not only finding the changes in mass but finding out whether the potato cylinders changed in length shape or even colour. Or whether the sucrose solution changed at all, whether that changed in volume and if it did, how much compared to the other experiments? Also I could investigate the cells in the potatoes before and after the experiment. See if any became turgid or flaccid and compare them to the cells in the other experiments. I could even investigate whether the cells become plasmolysed and by how much. For a further understanding of osmosis in potatoes I would like to have seen how temperature effects it. I would have carried out the experiment above in exactly the same way but 5 times, putting one set of the potatoes in a place of 0�C, one in 10�C one at room temperature, as my control, one at 30�C and one at 40�C. I would conduct the weighing and measuring in exactly the same way as the experiment above and this experiment would show me whether or not temperature affects osmosis and whether it is the hotter the more osmosis or the colder the more osmosis. Carrying out any of the extensions of this experiment I have stated above would give me an extremely clear understanding and knowledge of osmosis in potatoes 1 Steph Winterbottom 10.D ...read more.

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