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Osmosis: Isotonic Point of Potato

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Aim The aim of this investigation is to find the isotonic point of cell sap in a potato. Introduction Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration. A partially permeable membrane is one were only certain particles can pas through. An example of this is visking tubing. Water molecules are really the only substance with small enough particles to pass through the membrane, and they do it in a two-way traffic. However, because there are more on one side than the other, there is a steady net flow into the side with fewer molecules. The glucose rich side fills up with water, to even up both sides. The process of osmosis makes plant cells swell up, and if they are surrounded by weak solution they become turgid, which is important for supporting the green plant tissue and for opening guard cells. Animal cells do not have cell walls, so therefore they could easily burst. The following diagram shows water molecules moving by the concentration gradient. This experiment involves potato cylinders, which will be used to find the isotonic point of potato cell sap. Potatoes are plants with large vacuoles inside them - which contain a lot of cell sap. The potato has a semi-permeable membrane around the vacuole and around the cytoplasm. ...read more.


The potato was left in the solution for a total period of 60 minutes, and it was decided that this was a sufficient enough time to get the good results. The aim of the experiment is to find the point where the potatoes neither gain nor lose weight. For the concentration of sugar solution I have decided to use a range of different molar solutions - Dilute Water, 0.2M, 0.25M, 0.3M, 0.35M and 0.4M. I think that from this experiment a potato piece that is left in dilute water will gain mass because the water molecules enter the cells by osmosis. Then as the concentration of solution increases there are less water molecules in the solution so the potato cylinders will shrink. This is because the net movement of water molecules in the potato will decrease and water molecules will leave the potato. This will therefore cause a mass decrease. This mass loss will be proportional to the increase in concentration of the solution. At the same time, there will only be a gain in mass when the sugar solution is 0M, i.e. dilute water. Results of the Investigation The following table shows results of the main experiment. Molar concentration Mass before (g) Average Mass after for 3 Chips (g) Change (%) 0.0 2.5 3.6 44.0% 0.2 2.5 2.89 15.6% 0.4 2.5 2.41 -3.6% 0.6 2.5 2.02 -19.2% 0.8 2.5 1.89 -24.4% 1.0 2.5 1.67 -33.2% The graph above shows a general trend with relation to the concentration and percentage change in mass of the potato chip. ...read more.


The preliminary experiment contained one where the percentage change in mass increased even though the concentration increased. One problem with this experiment was that not enough results were taken to the stage where the graph was drawn, so the isotonic point was not as reliable as it could have been. However, the experiment was done to the fairest possible. Every precaution was taken to make sure that all the control factors were kept the same. The results are very reliable because there were 3 potato chips for each concentration so that the average could have been taken. This allowed the risk of anomalous results to be reduced. So the evidence in the form of the results and graph can support a very firm conclusion. To improve the investigation, I would have taken the isotonic point that I concluded to be, and experimented with this concentration. So I would have taken the 4.6 mole concentration and seen how accurate this experiment was in finding this as the isotonic point. If I were to do this project again I would change one thing to extend the experiment. This would be to use another plant to compare the results with. For example, we could have used an onion or carrot as they are both under-growing vegetables, so they have a lot in common with each other. By doing the same experiment again with a similar vegetable we could have seen whether it is a similar case for other similar plants. ?? ?? ?? ?? Investigation into finding the Isotonic Point of Cell Sap in a Potato ...read more.

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