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Osmosis GCSE Biology

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The effects of osmosis on potatoes placed in different salt concentration Introduction: Osmosis is a movement of water across a semi-permeable membrane. This may only happen when there is a difference in concentration between the two sides of the membrane. The water moves from low concentrations to high concentrations. That is because the low concentrated area tries to dilute the opposite via osmosis. The effects osmosis has on a cell are either that it becomes turgid and hard as water enters the cell or it becomes dehydrated (this is called plasmolysing in plant cells) as water leaves the cell and the solution enters. There is a change of mass, volume, texture and length. Aim: The aim is to investigate the effects of varying concentrations of a salt solution on the amount of osmotic activity between the solution and a potato of a given size. Osmosis: Process of Osmosis Osmosis is the process of the flow of one constituent of a solution (in this case salt) through a membrane while the other constituents are blocked and unable to pass through the membrane. Experimentation is necessary to determine which membranes permit selective flow, or osmosis, because not all membranes act in this way. Many membranes allow all or none of the constituents of a solution to pass through; only a few allow a selective flow. That is why I have chosen potatoes as my semi-permeable membrane. In osmosis, a solvent (often water) moves from a region of low concentration to a region of high concentration through a semi-permeable membrane. Here, water molecules diffuse into the concentrated salt solution because the water molecules are small enough to move through the semi permeable membrane. The larger salt molecules are unable to move through the membrane into the water solution because the membrane acts as a net. Osmosis will stop when the two solutions reach equal concentrations on either side of the membrane. ...read more.


When the experiment was first attempted time ran out because of lesson clashes. The following lesson a new bag of potatoes was opened meaning that the potatoes in the second bag might have been different. The results were different from the hypothesis because the potatoes in the new bag were very new and fresh. This was shown by the fact that the potato cracked when the core borer was put into it. It was also very hydrated or turgid, the water on the cylinders that came out of the potato helped in the answer to this it showed that the cells were very turgid. This is one explanation is that the potatoes were so turgid due to being watered a lot during growth. This effected a major change in the results. The isotonic solution was found to be roughly 0.15 when it was predicted to be 4.0. The reason that it affected our results so much was that the potato could hardly absorb any more water because the cell wall was stopping it. In an animal cell the cell would probably have burst because animal cells do not have a rigid cell wall like the plant cells. The reason that the potatoes were losing water at 0.2 molars was that it had a stronger concentration of water in its cells and therefore a weaker solution of sucrose in it. Therefore water left the potato cells, to go down the concentration gradient and dilute the stronger sucrose solution outside the cells. The reason that the potato gained water at 0.0 molar is that even though the potato was fully turgid it would have to have other chemicals in it that make up the potatoes. So it is impossible for a potato or any other organism to have an isotonic solution of 0.0 molar. This is why even though the potato was nearly fully turgid, water entered it when it was put in this solution. ...read more.


In this demonstration, the water moves in both directions through the membrane; the flow is greater from the vessel of pure water, however, because the concentration of water is greater there, that is, fewer dissolved substances exist in this solution than in the sugar solution. The level of liquid in the tube of sugar solution will eventually rise until the flow of water from the tube of sugar solution, under the influence of hydrostatic pressure, equals the flow of water into the tube. The hydrostatic pressure establishing this equality of flow is called osmotic pressure. A variety of physical and chemical principles are involved in the phenomenon of osmosis in animals and plants. Conclusion: From all these facts and figures we can ascertain that osmosis has taken place because it has a semi-permeable membrane. As quoted in the hypothesis, the solution is stronger than the isotonic solution then the cell will lose water. The amount of water in the cells of a potato has an effect on its isotonic solution. The isotonic solution of the potato in this experiment was 0.08. The results showed that as the strength of the solution increases the % mass difference falls. Water has left the cell to go and try to dilute the stronger solution that is outside the cell. It has tried to make the concentration gradient the same. As the strength of the solution increases, the more water leaves the potato. The potato cell has a cell wall, however, so the cell has only become dehydrated or plasmolysed to a certain extent because of this unlike animal cells. The prediction of a change, for higher and for lower, at about 0.4 on the concentration solution was not close, but, as the results of the experiment show, that it was inaccurate by 3.8 molar. Perhaps the water potential of the particular potatoes was slightly higher than average. Even though the prediction was inaccurate i can deduct that it was a highly successful experiment. ...read more.

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