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Osmosis
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Biology Coursework: Investigating osmosis in potato cells AIM: To investigate the affect of changing the concentration of a solution on a potato. BACKGROUND INFORMATION: Osmosis is the diffusion of water from a dilute to a concentrated solution through a selectively permeable membrane, which allows water through but not the solute. Diagram to show Osmosis Information on potato plant cells: Turgidity: when plant cells are turgid, the cell looks like this: When plant cells are placed in water the water enters the cells. This is because their cell sap contains a strong solution. Therefore water passes into the cells by osmosis. Plasmolysis: When plant cells are placed into a stronger sugar or salt solution, water passes out of the cells by osmosis. As water passes out the sap vacuole starts to shrink. These cells are no longer firm, they are limp and we say they are flaccid. As more water leaves the cells the cytoplasm starts to peel away from the cell wall. These cells are now plasmolysed. The diagram below shows this. I have chosen to use sugar as my independent variable. I have chosen this substance because it worked well in a class experiment. Here are the results of that experiment. Solution Length before (cm) Length after (cm) Mass before (g) Mass after (g) Change in mass (%) Distilled water 40 41 1.40 1.41 0.71 Weak sugar solution 0.5m 40 39 1.39 1.37 -1.44 Strong sugar solution 1.0m 40 39 1.41 1.34 -4.96 I have chosen to use a range of sucrose solution from 0.0molar to 1.0 molar. I will increase the concentration of the sucrose solution by 0.1 each time. Therefore, I will take 11 different measurements in total. To get more accurate results, I will repeat each measurement three times and them take the average percentage change. I have chosen to measure the mass of the potato chip. I have chosen this because previous in experiments, found on www.freestudentstuff.co.uk, show that measuring the change in mass is more reliable than measuring the change in length of the potato. From my own knowledge, I know that the mass is more reliable than the length because of the apparatus you would use, i.e. when measuring the length you can only go up to the nearest millimetre, but electronic scales can weigh to a much more accurate degree. Also, the measuring the change in length is not an accurate measurement of the overall change in size of the potato, because the width of the potato is also likely to change. Therefore, I will calculate the change in mass of the potato to get accurate results. PREDICTION: I predict that as the sucrose solution becomes more concentrated the mass of the potato will decrease. I predict this because osmosis is the diffusion of water from a dilute to a concentrated solution. Therefore, the more concentrated the solution is the more water will diffuse by osmosis from the potato to the sucrose solution. The more the water diffuses, the more the mass will decrease. RISK ASSESMENT: I must be very careful when cutting the potato, as the scalpel is very sharp. I will use a tile to cut on and be aware of people around me. Apart from this risk, normal laboratory rules will be followed. I have already decided that my independent variable will be the sucrose solution. Here is a table showing the variables I will control. Control Variables Why control it? How I will control it Temperature More kinetic energy in the water molecules. Incubate the potatoes to keep the temperature constant. Surface area It will affect the rate of osmosis. Use the same cork borer. Time I will incubate the potatoes for It will affect the overall change in mass of the potato. Keep all the potatoes in the incubated for the same amount of time.
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