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:

I think that, as the concentration increases so will the mass of the potato. This is due to osmosis. I believe that because there will be a higher water concentration in the solution than in the potato then water will enter the potato to make the concentrations both inside and outside the potato equal.

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 equal. The plant loses some of the water, which was in it, and the cell contents shrink away from the cell wall. The cells lose their turgor, and while the cell wall still remains around the cell, it becomes very weak or in other words, plasmolysed.

The plant might 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 doesn’t lose nor or gain any water.

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, which will result in a mass increase.

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These diagrams helped me make my prediction. For example, if I leave the potato for a long time, there is more time for osmosis to occur. The potato absorbs water thus the rate of osmosis increases, and that is why the potato’s mass will also increase. They are directionally proportional; I think that the percentage increase for the mass would be directly proportional, for the same reason that I expect for 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. This would be the same for the potato. (Semi permeable membrane.)

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: 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.

◊ 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)

• 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 reliable results if the first set of results are wrong
Use the same width cork borer each time
Keep the temperature of the water consistent
Same volume of solution
Same time left for

As long as I follow these points mentioned above, I should get accurate results.

As I expected. There was a greater percentage mass change as the concentration increased. This shows my prediction was correct.

Conclusion

I have found from my repeated results, that my prediction was correct. As the sucrose concentration rose the potato lost weight. Alternatively, as the water concentration increased the potato gained weight. This shows that osmosis happened.

For example in the case of 100% sucrose solution:

1. There was more sucrose (low water concentration) outside the potato than in it. There was more water in the potato (high water concentration).

2. Water left the potato and passed through the semi-permeable potato cell membranes.

3. The levels of water concentration were then equal both inside and outside the potato

4. The weight of the potato decreased when water left it. For example, when we had an increasing concentration of water, the low water concentration solution diffused into the potato, this balanced the water concentrations inside and outside the potato.

Teacher Reviews

Here's what a teacher thought of this essay

Luke Smithen

29th of May 2013

This is a good report that has a clear structure and uses research to guide the process. 1. The introduction and background knowledge sections are well researched but this information needs to be referenced and rewritten. 2. The variables section contains a good level of detail. 3. The preliminary work is a good lead in to the investigation. 4. The results table needs to be readdressed. 5. The conclusion shows a good understanding but lacks structure and evidence. 6. There is no evaluation. *** (3 stars)

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