Undertake an experiment to understand the effect of varying sucrose concentration on osmosis in potato chips, and to try and find where incipient plasmolysis lies between these experiments.

Biology Coursework

Aim

Background Knowledge

Osmosis is the movement of water molecules from regions of high water potential to regions of lower water potential through a semi-permeable membrane until equilibrium is reached.

Semi-permeable membranes hold back larger molecules that water, not allowing them through, and this is particularly important in plant cells for maintaining turgor.

Plant cells are surrounded by a cell wall, and cell walls are fully permeable which means that it will let any molecules through it; therefore osmosis will not occur across it. A plant cell also has a cell membrane, and just like an animal cell it is selectively permeable. A plant cell, in the same way as an animal cell, will take in the water by the process of osmosis through its selectively permeable cell membrane if placed in pure water. As the water enters the cell, the cytoplasm and vacuole will expand and swell.

However plant cells have a very strong cell wall. Cell walls are a lot stronger than cell membranes, and they stop the cell from bursting. When there is a higher water potential outside the cell and a lower water potential inside, and water has entered the plant cell through the semi-permeable membrane by osmosis, the cytoplasm presses out against the cell wall, but the cell wall resists this pressure and pushes back onto the contents. At this stage the cell is said to be turgid.

Hence, when there is higher water potential inside the cell as opposed to outside, water will exit the cell through the semi-permeable membrane by osmosis and the cell will shrink. At this point the cell is said to be flaccid.

Incipient Plasmolysis is when the concentration of water on both sides of the semi-permeable membrane is equal, in equilibrium; therefore there will be no gain or loss in weight.

An experiment to show what happens to plant cells in pure water and strong sugar solution.

Prediction

I predict that in the experiment with the strongest concentrated sucrose solution, the potato chip within the solution will shrink due to water moving from the potato cells through the semi-permeable membrane and into the sucrose solution causing the cells to become flaccid, hence the potato shrinking. This is because there is a higher water potential inside the potato cells than in the sucrose solution, and osmosis tells me that water moves from a high water potential to a lower, therefore the potato will lose water and shrink.

I predict that in the experiment with the weakest concentrated sucrose solution, the potato chip will expand due to water entering the cells of the potato from the sucrose solution through the semi-permeable membrane of the cells, causing the potato cells to become turgid. This is because there is higher water potential outside the chip in the sucrose solution than in the chips, and in osmosis water moves from areas of higher water potentials to areas of lower water potential.

Therefore, I also predict that the experiment where there will be incipient plasmolysis, where the potato chip will ...

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Therefore, I also predict that the experiment where there will be incipient plasmolysis, where the potato chip will remain the same shape, and weight will not be gained nor lost, will lie between the experiment with 0m(molars) of sucrose solution and the experiment with 10m of sucrose solution, and this may lie somewhere in the middle.

Loss or Gain

In weight (g)

Concentration of sucrose

Solution (m)

My prediction Where

Incipient plasmolysis

Will lie

Equipment Used Reasons

1 molar solution of sucrose - To use as the variable in the experiment
Beakers/Test tubes - What the experiment will take place in
Measuring Cylinders (ml) - To measure the liquids, sucrose and water
Rulers/ Balance 0.01g - To measure and weigh the potato chips
Scalpels/ Chipper - To cut the potato chips to right shape
Distilled Water - To use as another variable in the experiment
Bungs/ Test tube rack - To hold and protect the test tube and

Contents of experiment

Potato - To use in the experiment

Method

Cut and shape with a scalpel 12 potato chips, ensuring each chip is 3cm cubed, and dry the chips with paper towels, and then weigh the chips recording their weights.
Measure out 6 different mixtures with different concentrations and amounts of sucrose solution and water…

(The brackets at the end of the measurements tell you the molar of the sucrose solution when mixed with the water)

- Test tube one with 30ml sucrose solution (1m)

- Test tube two with 24ml sucrose solution and 6ml distilled water (0.8m)

- Test tube three with 18ml sucrose solution and 12ml distilled water (0.6m)

- Test tube four with 12ml sucrose solution and 18ml distilled water (0.4m)

- Test tube five with 6ml sucrose solution and 24ml distilled water (0.2m)

- Test tube six with 30ml distilled water (0m)

Place the test tubes with the solutions in, in a test tube rack and put two potato chips in each solution. Whilst doing this record which weights of potato chips went in which test tube.
Leave the experiment to undertake for 24hrs
After 24hrs dry the chips and evaluate them by weighing them, here we can work out the change in weight by taking away the weight of the chips after the experiment from the weight of the chips before the experiment.

Resources

Internet (Search Engines)
Encarta Encyclopaedia

Variables – Fair Test

The main variable in this experiment is the concentration of sucrose solution that we put in the test tubes. This will be measured very accurately to ensure it is fair.

Another variable will be the weight of the potato chips, but this is not a problem because we can work out a percentage change in weight.

The constants are the size and shape of the potato chips, they will be cut very carefully to the right shape, because if they were different sizes more osmosis would take place in the bigger chips due to surface area which will make the experiment unfair.

Another constant is the volume of liquid in the test tubes, they will always be the same and will cover the chips, if they didn’t cover the potato chips then osmosis couldn’t take place to its full potential and this will also make the test unfair.

Another constant will be the temperature because if the temperature is hotter, the heat will make the molecules move faster and more osmosis will take place, the temperature will act as a catalyst, and this also makes it an unfair test.

The size of the test tubes will also stay the same, again emphasising the chips being covered in the solution, and different size test tubes may alter this, and the chips may not be covered.

I am also placing two chips in each test tube and this will make it a more accurate test because we can take an average mass of the chips, and if one of the chips go drastically wrong we can rely on the other chip. This will give me a more precise and accurate result and conclusion.

To ensure that this experiment is safe we will take precautions to prevent any injuries or accidents from happening, such as wearing protection goggles to protect the eyes, cutting the chips on a cutting board in case we cut our fingers, washing our hands after using the solutions, and make sure we do not drink or put any solutions near our mouth.

Results Table

There were two chips in each test tube I have simply taken an average change in mass which is done by adding the two masses of the chip which went in one of the sucrose solutions and dividing that total by two.

We find the percentage change in mass by using the following formulae:

Change in Mass

Initial Mass x 100 = % Change

Conclusion

I have found out that the point where incipient plasmolysis starts in potato chips according to my results is when there is 0.15m of sucrose solution. This is also the isotonic point, where on both sides of the cell membrane the water potentials are equal, in equilibrium.

My graph tells me that as the concentration of sucrose solution increases the percentage change in mass decreases, and as the concentration of sucrose solution decreases the percentage change in mass increases.

It also tells me that when there is a lower concentration of sucrose solution that 0.15m, the cells in the potato chips become turgid because there is higher water outside the chips, therefore the process osmosis allows water into the cells.

When there is a higher concentration of sucrose solution above 0.15m, the cells in the potato chips become flaccid, the shrink because there is higher water potential inside the cells than outside, so osmosis allows the water out of the cells until equilibrium is reached, hence the cells shrinking.

On my graph of the results, I have inserted a line of best fit, and this allowed me to identify an anomaly, which was a result that was unexpected and did not follow the pattern, was quite a bit outside the line of best fit. This could have happened for many reasons: I may not have dried the chip enough before placing it into the test tube before the experiment, I may not have dried it well enough after the experiment, the chip may have been a bad chip to use in the experiment, and there may have been a fault when weighing the chip that I made.

Before I undertook this experiment I predicted that the point where incipient plasmolysis would take place in the potato chips would be somewhere between 0m and 1m, perhaps in the middle. I was right about it being between 0m and1m, but it was not in the middle. The point where incipient plasmolysis took place was at 0.15m of sucrose solution. This is because this was the point where the water potentials were equal on both sides of the cell membranes of the potato chips.

Evaluation

In the experiment I did not make any modifications, I followed my method through every step of the way. I thought my method through properly and felt there was no need to modify my experiment.

In my results there was one anomaly, and this may have been for the many reasons I gave above in the conclusion. I don’t feel my results were of that good a quality, and this could have been because I didn’t take enough care or time doing the experiment, e.g. taking time to dry the chips in the same way before and after the experiment.

If the experiment was undertaken more carefully I feel my results could have been better. I compared my results with the rest of my group, and found out that we all had roughly the same results, so they were quite reliable results.

I think the experiment was suitable for our coursework, but if we were to look at osmosis in potato chips in greater depth then we would have undertaken a more accurate and detailed experiment, to obtain more accurate results. I also know that potatoes are not the best and most reliable of vegetables to use, so a different vegetable would have been more appropriate.

I also do not know where the chips came from, they may well have came from different places, with different levels of water potential, so if I were to take this experiment again this would be one of the things I would change: make sure the potatoes come from the same place.

I feel that the results were reliable enough to justify a conclusion, but I would have liked to repeat the experiment a couple more times to compare the results before making an overall conclusion. If I were to do the experiment again I would ensure that the potatoes came from the same place, they all had the same weight and size, I dried them all thoroughly and all the same before and after the experiment, and I would repeat the experiment a few times to obtain a more precise conclusion.