Biology Coursework: Osmosis

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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. As the water enters the cell membrane, the cytoplasm will boost, though not burst because plant cells do not burst in pure water because its cell wall is very strong. Though the cytoplasm will push outwards against the cell wall, the cell wall is firm enough to resist the pressure. When this occurs, a plant cell is said to be turgid, like a well-blown-up tyre.

Preliminary Experiment

Aim

I will be investigating the effect of osmosis using different concentrations of glucose solution on plant cells. (A potato for example in this case)

Method:

-Cut 3 pieces of chips about the same size using a knife

-Measure their lengths and mass with a scale and record it down on a table

-fill three test tubes of the same volume, one with distilled water, one with weak sugar solution, and one in strong sugar solution.

-Put one potato chip into each test-tube

-leave it for 30 minutes

-after 30 minutes retrieve the chips from the test tubes

-wipe them carefully with dry tissue and then re-measure the length and mass

-record onto table

Results:

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What unit I am choosing to do for the next experiment?

I chose mass, because on the results table it shows that the % change in mass is far more obvious than the % change in length. In the % change in length table, we can see that in two out of three test tubes, the % change in length is zero. The unit mm is still far too big for this experiment. As for the % change in mass table, we can see that all three test tubes have a more accurate and clear number to show ...

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**** 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.