- Water Particles can get through these holes, so that if a membrane separates a strong solution from a weak solution, the water may pass through the membrane to dilute the strong solution until both solutions are the same concentration. Diffusion of water molecules across a partially permeable membrane like this is called OSMOSIS.
- The diagram below shows Osmosis at work. The water molecules move through the partially permeable membrane from a high water concentration to a lower water concentration until, both is equal. As the water moves from a high water concentration solution to a low water concentration, the process of Osmosis is occurring:
Below is how plant cells use osmosis:
- In plant cells osmosis is used to take up water, as water moves into the low concentration of water in the plant cells.
- The cell membrane of a plant cell acts as a partially permeable membrane.
- The cell sap inside the vacuole is a strong solution, so water passes from the ground or water source into the plant cell by osmosis until the solution concentrations are equal.
- By this time the concentration of the sap in the vacuole is now weaker.
- The cells now divide the water, so water passes from the weak solution of one cell into the strong solution in the next cell by osmosis:
After this the cell becomes turgid:
- As water enters the cell it makes the cell swell up. The water pushes against the cell wall. Eventually the cell contains as much water as it can hold. The strong cell wall stops the cell from bursting. The cell is now Turgid, it has gained mass:
Whereas when the plant cell looses water, it decreases in mass:
- When plant cells are placed into strong sugar solutions water passes out the cells by osmosis.
- After water passes out, the sap vacuole starts to shrink.
- The cells are no longer firm, they are limp. We say that they are flaccid.
- As more water leaves the cells the cytoplasm starts to peel away from the cell walls, causing the cell to decrease in mass.
- The cells are now plasmolysed:
# All information is from exercise book and Biology for you text book
Apparatus:
- 6 pieces of potato (used getting cork borer)
- Scalpel
- Cork borer
- Tweezers
- Small tile
- Sucrose solution
- Distilled Water
- 5 boiling tubes
- Test-tube holder
- Stop clock
- Electric scale which measure to 2 decimal places
- 30cm ruler
- Measuring cylinder X2
Experiment
- Start by getting your potato and using the cork borer to cut through the potato to get 6 equal pieces of potato.
- Place your potato slices on the tile and cut of any remaining potato skin.
- Measure each potato piece with a ruler (they do not have to all the same length) and cut them at their half lengths, to give you 2 equal pieces for each of the 6 original pieces you had.
- Place these potato pieces on a paper towel- remember do not muddle them up.
- Now you can make the six concentration of the solution.
- Firstly get you test tubes and place them in the rack.
- Label each test tube the following: 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0.
- According to the table below fill each test tube with their designated quantities- do not use the same measuring cylinder to measure water and sucrose as this may influence you results.
- After you have filled the test tubes you should find that all the test tubes are all at the same level in volume.
- Now choose which pair of potato pieces will go in each test tube.
- For each pair of potato pieces, dry the potato gently against a paper towel to remove excess water
- Then weigh them all and record on a piece of paper there masses (grams)
- Now place all the pairs of pieces of potatoes in the tubes that you decided to put them into.
- Start timing for 15 minutes using the stop clock.
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- After 15 minutes use the tweezers to gently take out each pair of potato.
- Repeat step 11
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Now weigh them with the electric scale and record their new weights under a new heading of mass after (grams)
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Now work out the change in mass and the percentage change in mass Use the equation %change= original length X 100
Change in length
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Now you have retrieved your 1st set of results.
- If you would like to get an average then repeat steps 1 to17 but remember not to mix the content.
Sample of a table:
Next page preliminary work>
The work on the next page allowed me to conclude that osmosis was occurring at different concentrations as the length changed. Additionally I used the preliminary work to help me know when osmosis would stop, which is after 15 minutes making the main experiment easier as there was no need to repeat the measurements every 5 minutes.
Safety:
- To make sure that the no accidents occur several measures have been taken
- I will be standing during the experiment to prevent me from being harmed if any spillages occur.
- The scalpel will be placed in a piece of cork to prevent it from harming anyone who walks by and myself.
- Overall these safety measures will allow the main experiment of the investigation to run smoothly.
ANALYSIS OF RESULTS
The General Pattern:
- When looking at my results a general pattern can be seen.
- As the molarity increases the percentage change in mass decreases.
- For example, when the molar is 0.2 the percentage change in mass is 2.02% but when the molar increases to a higher value of 0.8 molar the percentage change in mass drastically decreases to -6.25%.
- Therefore this fully agrees with my predictions.
- The reason for this is because by the process of osmosis the water is moving from a high water concentration to a low water concentration.
- The amount of change depends on the difference between the two water concentrations, the bigger the difference, the more the change and the opposite for small difference.
The diagram below shows how when the concentrations are equal no movement takes place, how when the concentration of the solution is higher causes the movement of water into the potato and finally how when the concentration in the potato is higher causes the movement of water out of the potato. Remember the water concentration in the potato remains at the same level until and that by looking at the graph we can see that the concentration are equal at 0.35 molar, which is the estimated concentration of the potato:
- To conclude with we can see by looking at the graph that the concentration of the solution and that of the potato are the same at 0.35 molar, which indicates to us that the concentration of water inside each slice of the potato was 0.35 molar.
EVALUATION OF EVIDENCE AND RESULTS
Success of Experiment and Problems encountered:
- I think that by looking at my results we can see that my experiment went very well. Evidence of this is that my results, nearly all, followed along the line of best fit of the graph, considering that I was dealing with quite small and precise quantities which were difficult to control and maintain. Additionally, the general predicted change and outcome can be seen with the results being precisely as I expected.
- During the experiment I found that I did encounter one problem. That problem being the drying of the potato slices with the paper towel, as it was very hard to precisely dry each piece of potato to the same degree. I might have accidentally squeezed out some water by applying too much pressure to the potato, which could have easily influenced the mass and how much it changed. Overall it was very hard to make all the potato pieces equally dry, it was a variable which I could not control.
Accuracy of Results:
- The results from the experiment were quite accurate. I made these results accurate by doing one particular thing. I repeated the experiment, this therefore allowed me to get accurate results and obtain an even more accurate average in the change in percentage mass. To show that these results are accurate we can look at the percentage changes (two results as it was repeated) in mass for 0.2 molar. By looking here I can see that the two results for the same molar are very similar, as the first is -0.01 grams and the second is 0.05 grams, which are very close considering that they are only 0.06 grams away from each other. Similarly, at 0.6 molar the first change is -0.03 grams and the second change is -0.11 grams, which are also very close like above, but this time there is only a difference of -0.08 grams.
Reliability of Results for evidence:
The results retrieved have given very good evidence for what I predicted in my prediction. As they follow the prediction word for word, ‘the increase in molarity will cause a decrease in the average percentage change in masses’.
There are many reasons why I think I have good results to be used in evidence:
- Firstly I took enough measurements, with a wide range from 0 molar to 1 molar, with six different values used in total (0.0, 0.2, 0.4, 0.6, 0.8, and 1.0).
- Secondly, I checked my measurements by repeating them twice, which enabled me to gain an average change in mass and an average percentage change in mass.
- Although I did not get the same results when I repeated the measurements for each concentration, as the mass of each of the potato did vary even if their lengths were equal. So the overall change in masses could not be the same, but could only be similar, as said in the accuracy of results.
- Also the range of measurements which I took were not too wide or too narrow as they went up in equal intervals of 0.2, ranging from the lowest possible molar of 0.0 to the highest possible molar of 1.0. Overall they were evenly spaced out and equally diverse.
- To sum up I think that I can safely use my results for evidence, as they are accurate, precise, evenly spaced out and clearly in agreement with my conclusion and prediction. As they show a general pattern that as the molarity increases the percentage change in mass decreases. For example, when the molar is 0.2 the percentage change in mass is 2.02% but when the molar increases to a higher value of 0.8 molar the percentage change in mass drastically decreases to -6.25%. Therefore as said above this fully agrees with my predictions.
Improvements:
- Many improvements can be made to the investigation as a whole and the main experiment as the experiment did not go perfectly, because I did get a light anomaly at 0.8 molar, although as the point still lay at least 0.5% within the line of best fit, I did not count it as a major anomaly.
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I could have repeated the experiment for a 3rd or even 4th time to define weather or not the anomaly was an anomaly. This repetition would have also allowed me to get an even more accurate percentage change in mass, as there would have been more values to observe and use.
Further evidence from other experiments:
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I could have investigated the change in mass again, using the same method but using the values of the concentrations which I did not use e.g. 0.1, 0.3, 0.5, 0.7 and 0.9 molar.
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By looking at the graph we can estimate where no change in mass occurs and when dynamic equilibrium osmosis is taken place. But a further experiment can be carried out to find the exact point/molar at which both the concentration in the potato and the solution are the same. As the estimated value of molar is 0.35 molar, I would investigate the exact molarities of 0.26 to 0.36, going up in intervals of 0.01 molar. However it would be difficult to precisely calculate the molars as they are all to two decimal places e.g. 0.26, 0.27, 0.28…….0.35, and 0.36 molar.
- We could investigate the exact same area of how molarity affects change in mass however we could use a different material to test with except from potato, and further see how different materials differ in their change in masses, after about 5 minutes. The new materials will have to be hard when raw just like potato, for example Swede, carrots, onion, garlic etc.
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Lastly we could further investigate the effect of temperature on osmosis, by placing different pieces of a suitable material in different tubes filled with the constant molar of 0.0 (e.g. pure distilled water) at different temperatures. The temperatures would range from 5◦C to 30◦C in intervals of 5◦C (e.g. 5◦C, 10◦C, 15◦C,….25◦C and 30◦C)