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Biology Coursework: Osmosis

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Introduction

Osmosis What is osmosis? Osmosis is a type of diffusion, or where the passage of a solvent (water for e.g.) diffuses from a place where they are in a higher water concentration, (more water molecules) to a place where they are in lower water concentration (less water molecules) through a partially permeable membrane. Partially permeable membranes mean that they will let some substances pass through them, but not all. High water concentration can be also called as having a high water potential, as both mean there are more water molecules, like a dilute solution. The same, low water potential is another term meaning the same as low water concentration, like when in a concentrated solution. Osmosis is the process responsible for the turgidity of plant cells as it causes osmotic pressure. For example, plants may wilt because of osmosis. In concentrated solutions, the water will escape from (diffuses out) the large vacuole in a plant cell through the cell membrane (partially permeable membrane) with osmosis. The cytoplasm and vacuole will therefore shrink and stop its pushing outwards on the cell wall. This resembles a tyre when the air leaks out of it. It becomes floppy, which in scientific terms, 'flaccid'. When the cells in a plant become flaccid, the plant is not strong enough and will start to wilt. When the solution outside the plant cell is very concentrated, then more water will diffuse out of the large vacuole through the cell membrane. The cytoplasm and vacuole will shrink though the cell wall is too firm to shrink that much. As the cytoplasm and vacuole shrink in the inside of the cell wall, the cell wall will be what's left in the end. The cell membrane, which surrounds the cytoplasm, will then split up with the cell wall. When this occurs, it is said to be plasmolysed. However, when a plant cell is in a dilute solution, like pure water for example, the plant cell will take in water by osmosis through its cell membrane, which is partially permeable. ...read more.

Middle

Method: -Make potatoes same diameter with a borer -Cut 21 pieces of potato chips to all the same length in millimetres (20mm) -Weigh all the potato chips on a scale using grams (g) to 2 decimal places -Record the masses -Pour the same amount of 7 different solutions (0.2M, 0.4M, 0.6M, 0.8M, 1.0M, 1.5M sucrose solution & 1 water) into 7 test tubes using a measuring cylinder (8 millilitres). All the solution's temperature are kept the same (room temperature, 23�C) to keep fair test -Put 3 chips in one group, each marked with an individual mark in the group to help later identify which potato is which (There will be 7 groups of 3) -Put a group of 3 chips into each test tube with a different solution -Leave for a few hours, all the same time to keep it a fair test -Weigh on scale and record the final mass using grams (g) to 2 decimal places Concentration of sucrose (M) Initial mass of chip 1 (g) Initial Mass of chip 2 (g) Initial Mass of chip 3 (g) Initial Average Mass of chip (g) Final mass of chip 1 (g) Final Mass of chip 2 (g) Final Mass of chip 3 (g) Final Average Mass of chip (g) Change in initial to final Mass of chip (g) % change in Mass of chip Water (0) 1.23 1.23 1.12 1.19 1.33 1.36 1.23 1.31 +0.12 +10.1% 0.2 1.20 1.14 1.13 1.16 1.22 1.14 1.10 1.15 -0.01 -0.86% 0.4 1.19 1.16 1.20 1.18 1.13 1.06 1.08 1.09 -0.09 -7.6% 0.6 1.18 1.20 1.22 1.20 1.03 1.05 1.04 1.04 -0.16 -13.3% 0.8 1.17 1.19 1.20 1.19 0.89 0.88 0.97 0.91 -0.28 -23.5% 1.0 1.22 1.23 1.23 1.23 0.87 0.90 0.87 0.88 -0.35 -28.5% 1.5 1.20 1.20 1.15 1.18 0.81 0.83 0.81 0.82 -0.36 -30.5% Analysis According to my results, osmosis did occur. Evidence is shown on my results and seen clearly with my graph, as there is a trend. ...read more.

Conclusion

Between 0.2M-1.5M, all the points are very close to line except for 0.6M. It should have lost more mass, but it was probably because I did not dab off enough liquid when I weighed it after putting it in the solution. Having three potatoes for each solution also makes this experiment more reliable because I can find an average from it. Improvements: The potatoes were not all the same mass due to limit of time. I could improve this issue next time if I were to have more time. I also did not dab off enough water, so next time I should carefully dab each to make sure all liquid are off. Timing was not exact as I put all the chips in different times. I don't feel that I shall need to improve this next time as the duration of the experiments is hours long, so having a few seconds in difference will not affect my results very much at all. If I were to take this experiment again next time, I could have more concentrations to improve my work, specifically in the area where the potato chip would be isotonic. To do this, I would put more sucrose concentrations in between 0.15M to 0.25M, as the line on my graph seems to cross the x-axis somewhere near 0.2 M sucrose solution. (The line through the x-axis means it would be isotonic, no net gain or loss of mass) Using 0.15M, 0.175M, 0.25M sucrose concentrations would help me find almost precisely where the potato would have no net gain or loss of mass. When the potato chip is isotonic, I could therefore find out its concentration too, hence developing my experiment into a higher level. Furthermore, I could also carry the experiment on other fruits and vegetables such as a pear or an apple. An apple or banana for example, would have a much different result as it has more sugar in it than a potato does. Monica Liaw 10JG ...read more.

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Here's what a teacher thought of this essay

4 star(s)

****

A well structured essay with some good scientific theory.

The main area to improve on would be in the analysis of the results. It is clear that the results show water moves in to and out of the cell and different concentrations but you repeat this point many times.

The aim of the experiment is fairly ambiguous also, investigating the effect of osmosis does not really give you a purpose or issue to solve; investigating the effect of osmosis to determine the water potential of the potato cells would be better.

Marked by teacher Jon Borrell 08/01/2013

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