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Osmosis Practical This practical is designed to investigate how osmotic activity is related to the surface area to volume ratio of a potato. The surface area to volume ratio is very important in living organisms. For example, Amoebas are uni-cellular organisms with a very large surface area to volume ratio. This is crucial for them, as they absorb all of their oxygen through their membrane. If they were any bigger the cells in the middle would die because the diffusion distance would be too big. They would then need lungs, like mammals have. These have a large surface area to volume ratio so that the oxygen can diffuse into their blood stream and be transported around the organism. Apart from the diffusion of substances, surface area to volume ratio is also important in heat loss. A small mammal like a mouse has a large surface area to volume ratio, so it will loose a lot of heat through its skin. This is why they have fur coats, to keep the heat from escaping. But a large mammal like an elephant needs a larger surface area to volume ratio to disperse of its body heat, so it has large ears with a huge surface area to volume ratio. Research: Osmosis is the movement of water across a semi-permiable membrane from a high concentration of water to a low concentration of water; in cells water would diffuse from a dilute cell solution to a higher concentrated medium. This is particularly important in animal cells, because if an animal cell is placed in a high concentration of water, it will absorb the water until it bursts.
Then weighed them again after. Because I'm using the same size cubes for each experiment. Their surface areas and volumes won't change. So I will write them all at the beginning: 1cm3: SA = 6cm2 Vol = 1cm3 SA: Vol = 6cm2 2cm3: SA = 24cm2 Vol = 8cm3 SA: Vol = 3cm2 3cm3: SA = 54cm2 Vol = 27cm3 SA: Vol = 2cm2 Results: Experiment 1 Before: Cube 3cm3 2cm3 1cm3 Beaker 1 31.35g 9.56g 1.47g Beaker 2 24.15g 7.34g 1.64g After: Cube 3cm3 2cm3 1cm3 Beaker 1 31.00g 9.47g 1.34g Beaker 2 23.70g 7.07g 1.54g Percentage Change: Cube 3cm3 2cm3 1cm3 Beaker 1 1.1% 1% 8.8% Beaker 2 1.9% 3.7% 6.1% Average 1.5% 2.4% 7.45% I then repeated this experiment two more times but with an extra beaker, and 3 more potato samples. I have kept the results separate because I performed the experiments on different days, so there may be slight variations to the results. Here are those results: Experiment 2 Before: Cube 3cm3 2cm3 1cm3 Beaker 1 19.96g 9.02g 1.00g Beaker 2 29.62g 5.66g 1.06g Beaker 3 32.94g 8.39g 2.09g After: Cube 3cm3 2cm3 1cm3 Beaker 1 19.80g 8.91g 0.97g Beaker 2 29.33g 5.55g 1.00g Beaker 3 32.54g 8.29g 2.07g Percentage Change: Cube 3cm3 2cm3 1cm3 Beaker 1 0.8% 1.2% 3% Beaker 2 0.98% 1.94% 5.7% Beaker 3 1.2% 1.2% 0.95% Average 0.99% 1.5% 4.35% Experiment 3 Before: Cube 3cm3 2cm3 1cm3 Beaker 1 35.23g 11.09g 1.28g Beaker 2 36.4g 9.79g 2.13g Beaker 3 22.81g 9.79g 1.38g After: Cube 3cm3 2cm3 1cm3 Beaker 1 34.83g 10.80g 1.22g Beaker 2 35.54g 9.4g 2.03g Beaker 3 22.44g 9.59g 1.31g Percentage Change: Cube 3cm3 2cm3 1cm3 Beaker 1 1.13% 2.5% 1.6% Beaker 2 2.6% 3.9% 2% Beaker 3 4.7% 4.7% 5% Average 1.87% 2.8% 3.5% Analysis: As you can see from the results.
If I had longer I could pre-soak the potatoes to ensure that they were all fully turgid. Because I didn't have time to do that, the different potatoes would have different dilutions in their cells, effecting the rate of osmotic activity. By taking into account some of these circumstances I could have minimised the amount of anomalous results. If I were to do this experiment again I would use a different shaped piece of potato. With the cubes there was not a very large range of sizes that I could use. Because I only used three different sizes, it is very difficult to come to a firm conclusion, whilst if I had more results I could draw much more accurate conclusions. Therefore I would use a different shape, a cuboid for instance, this would give me a greater range of sizes without having to use a very large potato. An experiment I could perform to extend the enquiry would be to keep the size of the cube constant and to change the concentration of the sucrose. By doing this I could discover the concentration of the sucrose inside the potato cell. I would do this by varying the concentration of the sucrose solution. I would weigh the potatoes before and after the experiment and see how their masses changed. When I use a concentrated sucrose solution I would expect the potato to loose water, and when I use a dilute solution I would expect them to gain mass. Somewhere in the middle the mass of the potato would not change. This would be the same concentration as the sucrose inside the potato cells. This way you can investigate whether the size of the potato or the difference in concentration has a larger effect on the rate of osmotic activity.
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