Investigate the effect of increasing the concentration of sucrose solution on the rate of osmosis in potato cells, and to investigate the internal concentration of potato cells in terms of sucrose solution.

Plan On Osmosis Experiment

Jonathan Anderson

Introduction

Knowing that osmosis (a diffusion of water) will occur across a semi-permeable membrane whenever there is a difference between the water concentration on the two sides of the membrane. And knowing that when this happens to all they will either become turgid if water flows into them, or plasmolysed if the water flows out of them, and therefore change their volume, we are going to test the hypothesis that:

“If the concentration of a solution into which a cylinder of potato cylinders are placed is greater than a certain level the cylinder will contract, and if the concentration is less than that it will expand.”

Aim

To investigate the effect of increasing the concentration of sucrose solution on the rate of osmosis in potato cells.
To investigate the internal concentration of potato cells in terms of sucrose solution.

Hypothesis

For this investigation I think that the lower the concentration of the sugar solution in the test tube the larger the mass of the potato will be. This is because the water molecules pass from a high concentration, i.e. in the water itself, to allow concentration, i.e. in the potato cylinder.

Therefore the cylinders in higher water concentration will have a larger mass than in the higher sugar concentration.

Variables

To create a fair test certain aspects of the experiment will have to be kept the same whilst one key variable is changed. I have chosen to vary the concentration of the sugar solution. This will give me a varied set of results from which I hope to make my conclusion. If any of the non-variables are not kept constant however it would mean that it would not be a fair test. For instance if one of the potato cylinders was 1cm longer than the rest the surface area of the cylinder would be greater and therefore would have more “space” for the osmosis to occur. Doing the test at the same temperature will control the temperature as well, and for the same reason of keeping it a fair test.

The factors involved that would affect the experiment are:

Time

The time span in which the potatoes are exposed to the different solutions will have a large impact on the experiment. For instance if we put the potatoes in the range of concentrations for an hour, more osmosis would take place than if we put them in the same concentrations for half an hour.

Test samples

The test samples (potatoes) will need to be the same size (surface area) for all of the tests that I choose to do, and they must be from the same potato. This is because when there is a larger surface area, more osmosis will take place, because there are more cells for the water to diffuse through across the selectively permeable membrane (Surface area is proportional to osmosis). The test samples need to be from the same potato because different potatoes may have cells with different water potentials and therefore giving us a false result

Temperature

For the duration of this experiment, the temperature will have to be kept constant because osmosis may occur at a higher or lower rate at different temperatures. This is because heated water molecules have more energy than non-heated water molecules. We will therefore need to make sure that all the samples are kept at the same temperature when they are being tested.

Concentrations

Concentrations of sucrose will have a big affect on the outcome of the experiment. The higher the concentration of the solution, the less heavy the potato becomes. This is the crucial variable that is going to be tested.

Volumes

The volume of the solution that the potato chips are kept in must be the same. The chip must be totally covered in the solution, and the amount of solution will be kept the same because all the chips are the same size.

Other factors involved which might affect my results are:

Temperature of water
Size of chip
Light
Mass of potato
Volume of solution potato is in
Type of potato
Time left in solution
Surface area of potato chip
Using the same balance to measure chip

The light moderation can’t actually be measured or kept the same, as the pieces will be moved from one room to another for storage. Yet, I will try to keep the pieces in the same position whilst in storage.

For the purpose of my experiment I will use the same ...

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Other factors involved which might affect my results are:

Temperature of water
Size of chip
Light
Mass of potato
Volume of solution potato is in
Type of potato
Time left in solution
Surface area of potato chip
Using the same balance to measure chip

For the purpose of my experiment I will use the same balance to weigh my potato cylinders, this is because the measurements may slightly vary between scales. While using the balance, I will make sure that it is reading zero before I weigh each potato so that I don’t get any false readings. To make the mass readings a lot fairer, I will take each piece; roll it gently on a paper towel, to remove all extra solution whilst not squeezing the potato piece. I’m also going to use the same balance to weigh my chips. This is because the measurements may vary slightly between scales.

The volume of the solution that the potato cylinders are kept in must be fair, they must be totally covered in the solution, and measured as accurately as possible, either using syringes or a burette, because all the potato cylinders are the same size. The mass of the potato is a dependant variable, and this means that it will be measured throughout the experiment. I will measure the mass in grams the potato cylinder will be measured before it is put in the solution, and after. This will allow us to see whether osmosis has taken place and to what extent. I will also make sure that the temperature is constant throughout the experiment.

To increase the chances of my experiment being a fair test I will also use a wide range of sucrose sugar solution 0 molar, 0.25 molar, 0.50 molar, 0.75 molar and 1 molar and repeat the experiment at least twice, preferable three time so I can take an average result.

Preliminary work

Before carrying out the final experiment, I carried out a preliminary test to give me an overall impression on the change in mass gain or loss when placed in varying concentrations of sucrose solutions, my results are show in the previous results table. To increase the chances of the experiment being a fair test I kept the potato cylinders submerged in water for five minutes as they reached equilibrium and this meant that they will all absorb the same amount of water. I also repeated the experiment to give me a chance of gaining affair test and these results can be seen on the next page.

When looking at the above results you can see that the percentage change in accordance to the verifying solutions, the concentration average change is:

0.00M 4.94

0.25M -2.11

0.50M -5.16

0.75M -8.93

1.00M -9.67

Equipment

10cm cubed measuring cylinder
Balance
Burette
Cork Borer to gain potato cylinders
Distilled water in a beaker
Glass rod
Potato
Safety goggles
Science Overall
Stop clock to make sure the potatoes are submerged for the same time
Sucrose solution.

Safety Factors

Before the experiment commences I must make sure that I am wearing safety goggles and my science overall.

Whilst cutting the potato, extreme care and precisions must be taken and if any s dropped on the floor it must be tidied up immediately otherwise someone may slip on it.

I must also make sure that nay spillages of water or sucrose solution are mopped up and any glass breakages are reported to a teacher. When I finish the experiment I must make sure that that I tidy away my apparatus used and clean my work area.

Method

I will take1 average sized ground potato and I will check it is both healthy and hard.

Using a cork borer I will then bore holes into this potato, after cutting it in half. I will use the glass rod to push the potato out of the borer if necessary.

Using a scalpel and ruler I will cut the potato “cylinders”, which are 5cm long, t care must be taken as the scalpel is exceptionally sharp. I will then have 15 cylinders.

I will then place the cylinders in distilled water until they are ready to be used.

Taking a test tube rack I will pace 5 test tubes and then label them 0 molar, 0.25 molar, 0.50 molar, 0.75 molar and 1 molar.

Using a measuring cylinder I will measure out different amounts of sucrose solution and distilled water which I will then pour into the test tubes in a percentage ratio giving me various molar concentrations. The measuring will be done using a burette.

I will then weigh every potato cylinder on a electronic balance and record the weights, but before doing so,” blott” them on a piece of paper towel, and make sure blotting process is exactly the same for each one.

I will then put 2 potato chips into each beaker and start the stop clock, 2 cylinders were used to create an average giving me a better set of results and more accurate graphs.

Whilst waiting I will set new paper towels with which I am going to dry the potato.

After 1 day (approx) I will drain out the solutions in the sink and place all the cylinders on the paper towel in the order that I put them in the test tubes as not to confuse myself as to which chip came from which solution.

After I have dried each chip I will place them on the balance so I can record their mass.

Each potato will be accurately measured using electronic scales.

If there is time after carrying out the first experiment I will redo the experiment under exactly the same conditions and this will give me a secondary set of results and this will enable me to have a more accurate view on the changes.

Prediction

When basing my prediction on preliminary experiments and my knowledge of osmosis, I think that the concentration of the solution increasing will give a decrease in the percentage change in mass. I also think that the potato cylinders will become as flaccid as possible, and so the change in mass of each molar concentration is becoming closer and closer together.

Each cell of the potato is surrounded by a semi permeable cell membrane. Semi permeable means that water molecules can travel through the membrane while sugar molecules cannot because they are too large. This will mean that over a period of time water will diffuse through the membrane from a weaker solution to stronger solution. Where the cells are in a weaker solution than that of the cells’ cytoplasm I would expect the cells to loose water by osmosis and the cytoplasm to shrink. Where the concentrations are the other way round I would expect the cells to take on water through osmosis and the cytoplasm to expand.

I feel that the potato cells increase in mass in solution with a high water concentration and decrease in mass in solutions with a low water concentration.

At point A the graph suggests that no osmosis has occurred, suggesting that the concentration of water inside the cell is equal to the solution outside.

At point B (high water concentration), there is no indication that the cell is increasing further in size. This is because the cell is fully turgid and no more water can enter.

At point C (low water concentrations), there is no indication that the cell is decreasing further in size. This is because the cell is fully plasmolysed and no more water can leave the cell.

Background Knowledge

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.

When plant cells take up water by osmosis they start to swell, but the strong 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 and eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid works against osmosis. Turgidity is very important to plants because this is what makes the green parts of the plant “stand up” into the sunlight helping photosynthesis occur.

When plant cells are places in concentrated sugar solutions they lose water by osmosis and they become “flaccid”. This is the exact opposite of “turgid”. The content of the potato cells shrinks and pulls away from the cell wall. These cells are said to be plasmolysed.

Obtaining Evidence for Osmosis Experiment

Jonathan Anderson

Obtaining Evidence

When carrying out the experiment I made sure that I kept it a fair test and also designed a table in which I could record my results clearly and accurately.

See figure 0.1 for my results table and when looking at t you will be able to see that although I carried out the experiment only once, I took three readings of each different concentration and this enabled me to find a average result and have more accurate results.

Analysis of Osmosis Experiment

Jonathan Anderson

Analysis

I feel that my results give me enough information to support my prediction.

You can see by looking at my results table in figure 0.1 that there is an overall decrease in mass during the experiment.

The results show that in strong sugar solutions the chips shrink while in distilled water they actually increase in length.

The graphs I have drawn show that for the potato cylinders placed in the higher concentration of molar, there is a lower % change in length and in mass. This shows that the percentage gain and loss are inversely proportional to the concentration.

This is because when the potato cylinders are in the lower concentration of molar, are more flaccid, as they cannot absorb as many sugar molecules into their membrane, therefore, making them “less hard” than those in the higher concentration of molar. The cylinders in the higher concentration of molar, can absorb more sugar molecules, and therefore attract more water molecules.

The stronger the sugar solution used, the more the potato chips will shrink in length. In distilled water the potato chips actually increase in length. This means that in a graph there would be a negative correlation between the size of the chips after the experiment and the strength of the sugar solution used.

The graphs show the change in mass, and this helps prove the prediction of complete plasmolysis, which is when the potato cannot absorb anymore water.

So, the cylinders in the higher concentration of molar have a smaller change in mass and length, because they have reached plasmolysis, and therefore cannot absorb anymore water to make them larger in length or mass. However, the cylinders in the lower concentration, reach plasmolysis more slowly, and therefore have time to absorb more water, increasing there mass and length.

Conclusion

The results show clearly that the length of potato chips is affected by the strength of the sugar solution they are put in. The way the chips shrink in strong solutions is consistent with the cells of the potato loosing water through osmosis and shrinking so that the chip becomes smaller. Similarly the way chips in distilled water, increase in length is consistent with the cells of the potato gaining water through osmosis and expanding to make the chip longer.

As I predicted, if the concentration of sugar in the liquid surrounding the potato chip has a higher sugar content than inside the potato then the chip will shrink. This prediction appears to have proved correct.

On the graph, I have circled certain anomalous results, which occur away from the general trend of results. These could of occurred due to inaccuracies and differences in the carrying out of the experiment. For example, the sucrose solution could have been measured differently in the different molar solutions. Also the original mass of all the cylinders could have been “miss-weighed”, and the blotting techniques for each cylinder could have differed enough to give these anomalous results. This is because in most experiments, despite best efforts to make them fair tests, errors are made.

Evalutaion of Osmosis Experiment

Jonathan Anderson

Evaluation

I feel that the experiment was a success; I obtained a large quantity of accurate results from which I was able to create informative graphs. I think that I gained enough results for the amount of concentrations that I was using. I also feel that the time I used for the experiments to take place was enough to allow sufficient osmosis to occur. However, if I was to repeat the experiment I may think about increasing 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 may also consider increasing the amount of concentrations as I would get more varied results, i.e. 0.10m, 1.15m, 1.20m, and so on. This way would have allowed me to find out more information.

I did not find any part of the experiment particularly difficult but perhaps I would have weighed each chip on a more accurate scale, for example 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 due to a number of things. 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 overall the experiment was a success and I was pleased with the complete comparison of my results with my initial prediction.

I think the results my group and I have gathered show a reasonable level of accuracy. This is because they tie in with what I predicted from my scientific knowledge.

To do further research into this topic which is relevant to the investigation, we could:

Measure different potato’s sugar contents to see how it compared with the solution strength where no change in length was measured
See how different breeds of potatoes sugar contents differ.
Try different solutes to sugar to see which give the best results.
Look at cells under a microscope in different sugar concentrations, to see their visible differences (see if the cells were plasmolysed).
Try and find a way of measuring turgidity of the cells, perhaps by seeing how ‘bendy’ the chips are.