The Consequences of Osmosis

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The Consequences of Osmosis

Firstly what happens to plant cells:

Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what make the green parts of the plant "stand up" into the sunlight.

When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed.

When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. "Incipient" means "about to be". When I forget to water the potted plants in my study you will see their leaves droop. Although their cells are not plasmolsysed, they are not turgid and so they do not hold the leaves up into the sunlight.

Now to explain osmosis:

When you put an animal or plant cell into a liquid containing water one of three things will happen.

If the medium surrounding the cell has a higher water concentration than the cell (a very dilute solution) the cell will gain water by osmosis.

Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up.

If the medium is exactly the same water concentration as the cell there will be no net movement of water across the cell membrane.

Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.

If the medium has a lower concentration of water than the cell (a very concentrated solution) the cell will lose water by osmosis.

Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink.

Skill Area E: Evaluation

The experiment was very successful in my opinion. I obtained a large quantity of very accurate results from which I was able to create informative graphs. I think I took easily enough results for the amount of concentrations that I was using, and the time that I used for the experiment to last was enough to allow sufficient osmosis to occur. However if I was to repeat the experiment I might well increase the time of the result to allow more osmosis to happen and possibly find out the saturation point of the chips. The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have more varied results, i.e. 0.10m, 1.15m, 1.20m, and so on. This way would have allowed me to also find out the isotonic point far more accurately as the one that I estimated is very approximate.

The cutting of the potatoes was the most difficult part of the experiment as although I was recording my results by mass, it could well have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment I would have possibly found a machine to cut the potato as it would ensure that all potatoes would be the same weight and dimensions. As well as the potato I could have found a more accurate way to measure out the solutions and to determine the molar concentrations. Perhaps I could have used a burette. This would ensure that I have an accurate amount of fluid in each test tube. I could also weigh each chip on a more accurate scale, e.g. not to 0.00g but to 0.0000g.

There were not any out of the ordinary results, but some were not as close to the line as others. This may have been caused by human. When the potato chips were removed from the test tubes and dried I may well have dried some potatoes more thoroughly than others and so some would have more excess water, which would add to the mass. If the experiment was repeated I could find another way to dry the potatoes that would ensure that all were dried in the same way for the same time. However with all this said I think that the experiment was truly successful and I was very pleased with the complete comparison of my results with my initial prediction.

 

Skill Area E: Evaluation


This experiment was very successful in my opinion. I obtained accurate results from which I was able to create informative graphs. I think I took enough results for the concentrations that I was using, and the time that I used for the experiment to last was enough to allow sufficient osmosis to occur.
However if I was to repeat the experiment I could possibly try to find out the saturation point of the potato. The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have a larger amount of results to test this idea further.
The cutting of the potatoes was the most difficult part of the experiment as although I was recording my results by mass, it could well have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment I would take more care when cutting the potato to ensure that all potatoes would be the same weight and dimensions. I will also use the same balance to weigh my potato chips. This is because the measurements may slightly vary between the top pan balances. I would have liked to repeat this experiment again to obtain a second set of results. This will hopefully produce more accurate results from which I will be able to draw a more accurate conclusion.

There were not any out of the ordinary results, but some were not as close to the line as others. This may have been caused when the potato chips were removed from the test tubes and dried I may well have dried some potatoes more thoroughly than others and so some would have more excess water, which would add to the mass. If the experiment was repeated I could find another way to dry the potatoes that would ensure that all were dried in the same way for the same time. However with all this said I think that the experiment was truly successful and I was very pleased with the complete comparison of my results with my initial prediction.

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Evaluation

The experiment was easy to do , but all the results I had to take had to be accurate had to change my plan several times. Things like having the potato chip 5cm long changed to 3cm because it was it was hard to have all the chips 5cm long, and 3cm was sufficient. Also, I changed the amount of solution I would keep the potato chips in, because I only needed enough to cover the potato chip. These changes were easy to make, because I did a preliminary experiment, so I could iron out any errors that ...

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