Explanation on Osmosis and diffusion:
Osmosis Coursework
Explanation on Osmosis and diffusion:
"Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration, which will then become equal."
Diffusion, although at first may seem insignificant in nature, plays quite a major part. The most obvious example would be in cells, plant and animal alike. They have partially permeable membrane in order to let in things like water, and to prevent unwanted big chemicals such as sucrose from entering. However, diffusion only takes places where there is a diffusion gradient i.e. a high concentration in one place and low in the other, moving down the diffusion gradient. Sometimes there are things which have to work against diffusion e.g. plants who need to absorb mineral ions from the soil, even though the concentration of ions is much higher in the plant that in the soil. The process of absorbing these minerals is called Active Transport.
Hypothesis:
Using all the evidence in the preliminary work, I predict that when distilled water is surrounding the potato after a period of time, the potato will become turgid, and therefore become, stiffened, hard and enlarged, as the water concentration in the beaker would be much higher than the water in the potato.
I also predict that as more molars of sucrose solution after a certain point is used, the potato will become more flaccid, and therefore become more soft, floppy, and smaller, as the water concentration in the potato would be higher than the water concentration in the beaker.
I think this will happen because the lower the concentration of water in the sucrose solution there is, the higher the water concentration in the potato, and the more comparatively higher pressure there is in the potato, the faster osmosis would take place. This would give me a smaller potato after a set period of time, compared to a weaker solution, which would be slower, and make the potato less small over the same period.
As a result, I will be bound to find a point on my conclusive graph whereby the potato's water potential and the sucrose solution's water potential will match, and the potato, in theory, will not change size.
Theory:
In this experiment we will look at the rate of osmosis on pieces of potato. When plant cells are submersed in a solution of sucrose, the process of osmosis occurs. There are three things which could happen to the cell and it all depends on the concentration of the water in the solution.
In this diagram, the plant may become plasmolysed. This is when the concentration of water inside the cell is greater than the concentration of water outside the cell. This diffusion gradient causes water inside the plant to move out and into the sugar solution. It can move out because the plant's cell membrane is permeable to water, and will continue to do so until the concentration of water inside and outside the cells are the same. The plant loses some of the water, The cells lose their turgor, and while the cell wall still remains around the cell, it becomes very weak, or in other words plasmolysed.
In this diagram the plant may not be affected at all. This is when the concentration of water inside and outside of the plant is equal. There is no diffusion gradient and so the plant neither loses nor gains any water.
Finally in this diagram, the plant may become turgid. This is when the concentration of water is greater outside the plant than inside it. The diffusion gradient causes water to enter the plant, causing it to expand, which results in a gain of mass.
All three of these concepts were tested in class by observing a union cell under a microscope. It is the last theory, that I have based the first part of my hypothesis on. If I leave the potato for a long time, there is more time for osmosis to take place, the ...
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Finally in this diagram, the plant may become turgid. This is when the concentration of water is greater outside the plant than inside it. The diffusion gradient causes water to enter the plant, causing it to expand, which results in a gain of mass.
All three of these concepts were tested in class by observing a union cell under a microscope. It is the last theory, that I have based the first part of my hypothesis on. If I leave the potato for a long time, there is more time for osmosis to take place, the potato absorbs water i.e. rate of osmosis increases, and therefore the potato's mass will also increase. So if we double these factors we are doubling the rate of osmosis. They are directionally proportional. Furthermore, in terms of the mass, I do not expect that, when the rate of osmosis doubles, either the mass double as this just would not make sense. However, I do think that the percentage increase for both the mass would be directly proportional, for the same reason that I expect the rate of osmosis.
Visking tube experiment (Preliminary work)
"Visking" tubing is a form of processed cellulose or cellophane, which has pores in it through which water (and other small molecules) can pass, so it can be considered as a partially permeable membrane. If it is sealed at one end, attached to a glass tube, and filled with a liquid such as sugar solution, and immersed in another liquid such as water, then water should pass through the visking tubing and cause the level of liquid to rise inside the glass tube.
After ten minutes, some of the water molecules had left the tubing and moved into the beaker by osmosis. This equalised the water concentrations both inside and outside the visking tubing. The tubing lost weight because water had left it
Onion Cell
Another we experiment we did showed clearly what occurs during osmosis. We put a thin piece of onionskin in a slide. The concentration of water was much higher inside the onionskin than outside it. Therefore water diffused out of the cytoplasm and vacuole through the selectively permeable membrane. The cell shrunk and then became flaccid, then the cell membrane pulled away from the cell wall and the cell became plasmolysed. Under a microscope we could see the space where cytoplasm had been pulled away from the cell wall.
Doing this preliminary work helped me make my hypothesis. It gave me a clear understanding of the process and therefore can give man idea of the rate of osmosis in a certain amount of time and different sucrose concentrations.
Variables
? Mass of potatoes ~ the mass is very important because the bigger and heavier the potato chip, the longer it will take osmosis to occur. If the size of the potato chip varies, so will the amount of mass it loses or gains. The potato chip will be measured before it is put in solution, and after it has been taken out. This will let me see how much osmosis has taken place.
? Concentration of sucrose solution ~ the stronger the solution, the quicker osmosis occurs. More osmosis will occur out of the chip. Because I am investigating different sucrose concentrations, I am going to put the potato chips in different concentrations. These concentrations will be 0m, 0.2m, 0.4m, 0.6m, 0.8m, and 1m.
? Time ~ the longer you have the potato chips in solution, the better, because more osmosis can occur. If you take the chips out after a few minutes, nothing is likely to have happened. This is why the time is important. More or less solution gets absorbed depending on the time.
? Room temperature ~ the colder it is, the longer it will take osmosis to occur. I think that osmosis will occur best at room temperature, because if it is too hot, the semi-permeable membrane will be ruined, so I will do all the experiments at room temperature. The hotter water is, the more energy it would hold and the more movement would take place. This factor was not easy to control, and we were not able to fully control this as we lacked the scientific equipment to do so. However, we did keep each experiment in the same room, which would roughly mean the same room temperature.
? Surface area~ the surface area effects the chips because if there is a large surface area, more surface is exposed so osmosis occurs faster.
? Type of potato~ this effects osmosis because different potatoes have different cell saps which effect the concentration. Different potatoes may absorb at different rates. That is why I will use the same potato.
Apparatus:
I used various apparatus for my experiment. They are listed:
• Tile and scalpel - to cut the chips to size.
• Test-tube rack - so the tubes do not spill solution.
• Test tubes filled with solutions.;
• Stopwatch - to time the experiment.
• Paper towels - to dry the potato chips.
• Ruler - to measure the length of the potato chips.
• Potato borer - to cut potato chips out of the potato (1.4mm).
• Test tubes.
• Weighing scale
Method
• First, I'll get all my equipment together: Sucrose solution, water, potato, potato borer, scales, test tubes, Test-tube rack, ruler, scalpel, tile, stopwatch, paper towels, thermometer, measuring cylinder, and a pipettes
• Next, I will measure the different concentrations of the sucrose and put 0m in the first test tube, 0.2m in the second r tube, 0.4m in the third tube, 0.6m in the fourth boiler tube, 0.8m in the fifth boiler tube, and finally, 1m in the last boiler tube.
This is how the concentrations were mixed:
(In a table)
Volume of water (cm³)
Volume of 1 molar sucrose solution (cm³)
Concentration of sucrose solution (m)
8
.2
0.2
6
4
0.4
4
6
0.6
2
8
0.8
0
0
• I will then cut out chips, using a potato borer. I will use a knife and a ruler to get my measurements exact.
• After doing that, I will rinse all the chips out and dry them with a paper towel.
• Then, I will measured each chip (in mass) and write down the measurements in a table like the one above.
• I will get a stopwatch ready and drop the chips, (one in each tube) and start the timer.
• After the whole 27 hours, I will remove the chips from the boiler tubes, take off the surface solution with a paper towel, and then I'll weigh the chips making sure the scale reads 0.0g before I start - each time.
• I will do the experiment 3 times because I want to get the averages and I want to reduce the effect of anomalous results.
I think that this will be a fair test because I think I have thought of every possible variable and I will everything I can to make it as fair a test as I can.
Fair testing procedures:
* Use the same technique for blotting the potatoes each time
* Use the same species of potato all the time
* Repeat the whole experiment again to get more accurate results if the first set of results are wrong
* Use the same width borer each time
* Keep the temperature of the water consistent
* Same volume of solution
* Same time left for
Table Of Results
.0(sucrose concentration)
Average
% Change
Mass Before
3.01,2.98,3.01
3
-27.7
Mass After
2.21,2.13,2.17
2.17
0.8(sucrose concentration)
Average
% Change
Mass Before
2.99,2.93,2.97
2.96
-19.93
Mass After
2.31,2.39,2.40
2.37
0.6(sucrose concentration)
Average
% Change
Mass Before
3.01, 3.05, 3.04
3.03
-15.73
Mass After
2.52, 2.55, 2.57
2.55
0.4(sucrose concentration)
Average
% Change
Mass Before
3.04, 3.02, 3.10
3.05
-5.57
Mass After
2.86, 2.88, 2.90
2.88
0.2(sucrose concentration)
Average
% Change
Mass Before
3.04, 2.92, 2.97
2.97
3.43
Mass After
3.10, 3.07, 3.04
3.01
The graph shows that as the sucrose molarity increases, the percentage mass change decreases, as water is moving out of the potato. Osmosis has occurred here, and is the process of water molecules moving from a concentrated solution of water to a dilute concentration of water. The sugar molecules move from a dilute concentration of sugar to a more concentrated solution of sugar. This is because the sugar molecules attract water molecules and combine with them, making them no longer free to move, so the free water molecules in the solution are reduced. where the sucrose solution has a lower concentration of water molecules compared to the inside of the potato, so there is a net movement of water molecules out of the potato. The more concentrated the sucrose solution, the more dilute the water concentration is. The more dilute the sucrose solution, the more concentrated the water concentration is.
As the sucrose molarity decreases, the percentage mass increases. This is because water molecules are moving into the potato through osmosis. The water moves across the potato skin, into the potato, as it moves from a high water concentration in the sucrose, to a lower water concentration in the potato. This causes the potato to gain extra mass. As the sucrose has a higher concentration of water molecules compared to the inside of the potato, so there is a net movement of water molecules into the potato. This causes the potato to swell (become turgid), but not burst.
Conclusion
The conclusion that I can draw from the results obtained we can see osmosis at low concentrations causes a gain in mass, at high concentration there is a loss in mass in the potato.
Sucrose Solution lower than 0.6
The water potential in the sucrose solution was larger than the water potential inside the potato. Water went into the potato through osmosis. The potato's cells became turgid. The potato became enlarged. As the solution became stronger, the balance between the two water potentials. became more level. As a result the potato's cells did not have to vary in size as much and so the size of the potato piece did not increase or decrease as much.
Sucrose Solution higher than 0.6
The water potential in the potato was larger than the water potential in the sucrose solution. Water went out of the potato through osmosis. The potato's cells became flaccid forcing it to shrink.
This happened because when more sucrose was added, they were unable to go through the potato's selectively permeable membrane. It also decreased the water molecule pressure pushing onto the potato. At the same time the water potential inside the potato would have been larger with a higher concentration of pure water molecules, and with a higher pressure of water molecules, would have pushed out of the selectively permeable membrane into the solution, making the cells flaccid, and smaller.
Because of osmosis, the potato was more likely to increase in size when the sucrose solution was weaker and decrease in size when the solution was strong.
However, as the solution became more concentrated, the sugar in the beaker per cm eventually became a larger amount than the amount per cm in the potato. Thus, the potato floated because it was less dense.
Factors that were kept the same during this experiment to make the test fair were:
- Strength of solution ff.
- Size and shape of potato
- Volume of solution
- Time in the solution
- Temperature
The factor that we varied was the concentration of sucrose in the solution.
It was not difficult to keep most of these factors the same except for temperature, which we did not measure, as it only required using the same type of materials or making set measurements However, the problems above could have been avoided by:
- Finding a more accurate measuring equipment to measure the potato pieces.
- Control the temperature. Making sure that it doesn't change.
My results are probably not the best example to use to prove osmosis because of the problems that we faced, making them inaccurate and difficult to depict. However, with the problems solved the results may have shown a better idea of osmosis.
- I could experiment with leaving the potatoes in for about 5 hours in order to find a smaller mass difference between each sucrose solution.
- Temperature could also be changed for example the samples could be placed in different water baths and brought up to different temperatures to see if temperature played its part in the osmosis of potatoes. 5 sets of 5 potatoes could be placed in water baths at 10oC, 20oC, 30 oC, 50 oC and 60 oC. Then leave them for 24 hours making sure all the variables in the first experiment still apply however just using one molar solution. Then after 24 hours the samples would be re weighed and results recorded. I would expect that at high temperatures the potato samples would osmose the most. This is because at high temperatures the solutions water molecules would move faster and therefore equal the concentration faster. A preliminary experiment could be set up beforehand to find out how long the experiment should be kept going because if the concentration of the potatoes equalises then the weight of the potatoes will be almost exactly the same.
- Other solutions could be used as well, e.g. glucose solution.
I was not disappointed with my investigation because it achieved accurate results and my hypothesis was correct.
Matthieu Philippe Biology Coursework Mr Robinson
01/05/2007