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Explanation of Osmosis.

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Theory Osmosis If a dilute solution is separated from a concentrated solution by a partially permeable membrane water diffuses across the membrane from the dilute solution to the concentrated solution. This process is known as osmosis. A partially permeable membrane is porous but allows water to pass through it more rapidly than dissolved substances. Because a dilute solution contains more water molecules than a concentrated solution there is a diffusion gradient. This means that the water moves from the dilute to the concentrated solution. In living cells the cell membrane is partially permeable and the cytoplasm and vacuole (in plant cells) contain certain dissolved substances. Because of this water tends to diffuse into cells by osmosis if they are surrounded by a weak solution, like fresh water, if the cells are surrounded by a stronger solution, like sea water, then the cells will lose water by osmosis. Explanation of Osmosis When a substance such as sugar dissolves in water it attracts some of the water molecules and stops them moving. This, in effect, increases the concentration of water molecules. The sugar molecules on the right have captured some of the water molecules. There are more free water molecules on the left so diffusion will go more rapidly from left to right. The partially permeable membrane does not act like a sieve in this case. The sugar molecules can diffuse from right to left, it just does this a lot slower because the molecules are larger and surrounded by a cloud of water molecules. ...read more.


In some cases the cell membrane breaks away from the cell wall and then the cell is called plasmolysed. These pieces of potato will decrease in length, volume and mass because of the loss of water. The greater the concentration of water in the external solution, the greater the amount of water that enters the potato cells by osmosis. The smaller the concentration of water in the external solution, the smaller the amount of water that enters the cells by osmosis. When the water concentration is equal outside and inside the potato cells there will be no change in the length, volume or mass. This is known as isotonic. Method Equipment * Beakers * Scales * Core Borer * Knife * Labels Method First of all I will measure 200ml of water into each of 5 beakers. The first beaker will be left without sugar, the second will have 0.5oz of sugar in it, the third will have 1oz of sugar in it, the forth will have 1.5oz of sugar in it and the fifth will have 2oz of sugar in it. I will then label the beaker so I know which one has which sugar solution in it. I will then use the cork borer to cut 5 cylinders of potato which I will then weigh to ensure they are all equal and record their weights in a table. ...read more.


This is because the cell is fully plasmolysed and no water can leave the cell. Evaluation The results obtained bear a strong resemblance to the predications. This suggests the experiment was fairly accurate. The accuracy of the experiment was fairly adequate; however, there was one anomaly in second set of results. The result for the solution containing 1.5Oz of sugar differed from the pattern of the other results. This anomaly could have been due to a number of reasons such as; when this beaker was left for 24 hours its location could have had a slightly different temperature to the other beakers. To get rid of this anomaly you could repeat the experiment again, keeping all the beakers in the same temperature controlled room. Another way to improve the accuracy of the experiment would be to have measured the length of the potato pieces. Although they were all of equal weight they could have been of different lengths which means that some of the pieces would have had a larger surface area than others, therefore increasing the speed of osmosis. To make the results and graph even more accurate you could decrease the difference in water concentration of the solutions therefore getting more results. Although in the experiment no alterations were made to the original method, we did record the results in ounces, not gram as had been originally planned, this was because the scales we used were different to the ones we normally used and we were unfamiliar with how to change them to display in grams. This did not affect the accuracy however. ...read more.

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Here's what a teacher thought of this essay

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In this report of an experiment to investigate the effect of changing sugar concentration on the process of osmosis, the writer gives us a good account of the theory of osmosis but does not make it clear as to the aims of the investigation. Unlike most investigations involving a range of sucrose solutions and potato tissue, this one does not go on to find the water potential of the potato tissue. Instead, it restricts itself to demonstrating the process of osmosis and showing that water leaves living tissue into solutions with lower water potentials.

The report (and indeed the whole investigation) would have benefited from clearer aims, together with a discussion of previous work carried out by other students (accessible online). The data collected here is unusual and deviates from most of the data collected in similar experiments on potato tissue. However, a direct comparison is difficult owing to the use of ounces rather than grams in the method.

3 stars

Marked by teacher Ross Robertson 10/05/2013

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