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Weighing method of finding water potential

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Karl Byrne 137 Mrs. Knight Prediction I predict that the potato and carrot samples will lose mass proportionately to the concentration of the sucrose solution. This is because of water potential, and the fact that the higher the solute potential of the solution the lower its overall water potential. Results Change in Mass of Potato Tissue Placed in Varying Concentrations of Sucrose Solution Molarity/mol dm-3 of sucrose solution Weight of potatoes Before (In Grams) Weight of Potatoes After (In Grams) Difference Between the two masses Percentage Change in Mass 0.0 0.52 0.80 +0.28 +53.84 0.1 0.68 0.67 -0.01 -1.47 0.2 0.70 0.64 -0.06 -8.57 0.3 0.61 0.46 -0.15 -24.59 0.4 0.61 0.47 -0.14 -22.95 0.5 0.52 0.42 -0.10 -19.23 Change in Mass of Potato Tissue Placed in Varying Concentrations of Sucrose Solution(Class Averages) Molarity Of Solution Percentage change in Mass 0.0 +18.47 0.1 +3.33 0.2 +0.08 0.3 -11.79 0.4 -21.28 0.5 -22.45 Change in Mass of Carrot Tissue Placed in Varying Concentrations of Sucrose Solution Molarity Of Solution Percentage change in Mass 0.0 +15.40 0.2 +12.68 0.4 +5.43 0.6 -4.07 0.8 -4.85 Interpretation Water can move very easily into and out of a cell by a process called osmosis. This process involves the movement of water from a high concentration to a low concentration across a semi-permeable membrane. In the above example water can be seen moving from the right hand side where there are more water molecules to the left hand side which has fewer water molecules. ...read more.


This is because the carrot contains more soluble sugar, such as monosaccharides or disaccharides, which are reducing sugars. We know that it is a reducing sugar, due to the fact when Benedicts is added to it and brought to the boil; a yellow/brown precipitate is form, which shows presence of reducing sugar (e.g. monosaccharides). This means that the carrot has a relatively lower water concentration than the potato. This is because carrots naturally have more soluble sugars and this affects the water potential. Because water molecules have kinetic energy, they are constantly moving around in gaseous or liquid form, moving randomly from one place or another. The sucrose in the solution lowers the total kinetic energy of the free diffusing water molecules will decrease. This means that the water molecules are not able to move around freely and their tendency to diffuse out of a solution will decrease. However potatoes do not contain as much soluble sugar as the carrot does, it mainly has insoluble macromolecules. It contains starch molecules. Starch is stored very compactly as coils and is very easily stored. It is a very effective store of energy as it can be easily hydrolyzed to give glucose and is made up of thousands of glucose molecules linked up together by hydrogen bonds in a compact spiral or coil shape. It is used as a storage polymer of glucose in plants. It is a non- reducing sugar we know this because when it is boiled with HCl and Benedicts, it again has a Yellow / Brown precipitate. ...read more.


To keep the test fair, we have to cut all the potato discs the same size, surface area etc. But putting this theory into action wasn't easy and mistakes were made. But this was down to Human Error. When it came to weighing the potato tissue after the experiment, we dabbed the discs with a paper towel. However, in doing this we could have removed water that was not excess, and altered our results. Another way of improving the results would have been to leave the experiment running longer, this would have enabled me to find the saturation point (when the potato can no longer take in any more water) and dehydration point (when the potato cannot lose any more water) and therefore get a more accurate result. Finally, I could extend the experiment to a more exact level by looking at the potato cylinders under a microscope e.g. incipient plasmolysis. An example of this is immersing onion cells in a range of sucrose solutions of different molarities and watch them till at least 50% of the cells have been plasolytised. Incipient plasmolysis is the point when plasmolysis is about to happen and at this point the pressure potential is equal to zero so the water potential and the solute potential are equal and the water potential of both the cell and the surrounding solution are the same.. At incipient plasmolysis the protoplast has just ceased to exert any pressure against the cell wall, so becomes flaccid. With incipient plasmolysis I would be able to see the cells in greater detail and draw some more observational results. ...read more.

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