Osmosis in Potato Chips

Year 11 GCSE Biology Coursework – Osmosis in Potato Chips

Aim

This experiment is to find out how osmosis in potato chips is affected by the concentration of sucrose solution present in the solution around it and also the ways in which osmosis occurs i.e. where the water molecules involved are travelling.

Prediction

I think that the higher the concentration of sucrose in the solution around the potato chip, the more weight the chip will lose as the experiment is run. I also think that in a concentration of sucrose where there is a lot more water than there is sucrose that the chip will gain weight as the experiment goes on. I think this because in a solution where there is a low water concentration, where a potato chip with a higher water concentration is placed, osmosis dictates that water will pass out of the potato chip (down the concentration gradient), out through the cell’s semi-permeable membrane and into the solution.

This is how osmosis works. Osmosis is:

The movement of water molecules from a high concentration of water, through a semi-permeable membrane into a low concentration of water.

In this case, there would have to be a higher concentration of sucrose in the solution to give the solution a low water concentration. This would mean that the potato chip would have a higher concentration of water. The movement of water molecules from the chip into the solution would culminate in the mass of the potato chip decreasing as the test goes on.

In the other case, where a low concentration of sucrose is added to the solution, the water concentration would be much greater in the solution than in the potato chip, so the effects would be reversed and water molecules from the solution would start moving in through the potato chip cells semi-permeable membranes. This movement of the water molecules from the solution and into the potato chip would result in an increase in the chips weight as the test runs on.

However, there must be a stage at which osmosis cannot occur because the water concentration in both the potato chip and the solution around it is balanced. I aim to investigate this.

Diagram 1 – Osmosis occurring when a potato chip is placed in a solution containing 100% sucrose.

Diagram 2 – Osmosis occurring when a potato chip is placed in a solution containing 0% sucrose.

Plan

I will set out by setting up 21 test tubes. I am going to have 3 test tubes with each different solution in them to make this a fair test. I will have 7 different solutions in all because I am changing the concentration of sucrose present in each of them. These concentrations will be 0%, 20%, 40%, 50%, 60%, 80% and 100%. I am putting 20cm(3) of solution into each test tube so I will be putting these amounts in:

I aim to leave the potato chips inside the test tubes for 24 hours, checking them every 2 hours (in the time I’m at school). Each potato chip will be cut to 5cm long. This is so that I can make sure that each potato chip has more or less the same surface area (the chipper cuts potatoes to 1 cm wide cuboids) as the surface area will play a major role in how osmosis occurs during the experiment. If the chip has a large surface area, it will mean that more water molecules can enter/leave through the chips semi-permeable membrane at the same time, meaning that the surface area affects the speed at which osmosis occurs. It would not, therefore, be a fair test if I started chucking random bits of potato into my test tubes and hoping they would all turn out similar results.

My experiment will look something like this:

Equipment Needed

21 stoppered test tubes
Syringe
Mounted needle
Beaker
21 potato tubes (cut with chipper)
70cm(3) sucrose solution
70cm(3) distilled water
Tile
Razor blade
Ruler
Marker pen

Preliminary experiment

The preliminary experiment is necessary in helping me to decide what the best way to approach my main experiment will be. I will be able to asses how careful I will need to be with certain aspects such as weighing and drying and I will be able to see how well my method works.

Preliminary method

Set up three test tubes. One will contain 20cm(3) of distilled water, one will contain 10cm(3) of distilled water and 10cm(3) of sucrose solution and the third will contain 20cm(3) of sucrose solution.
Cut three potato chips each one must be 5 cm long and 1 cm thick.
Weigh each potato chip and record the weight accurately.
Place each potato chip into a test tube and label the test tube so that you know which chip is in which tube.
Stopper the test tubes and start a stopwatch.
Leave the chips for 6 hours, checking back to weigh them every 2 hours. When weighing, dry the water from the outside of the chips with blotting paper or a paper towel and then place them on the scales to weigh the,.
Record all results accurately.
Dispose of all equipment used safely and appropriately.

Preliminary test results

These results show that the higher the concentration of sucrose, the less the potato chip weighed afterwards. This supports my prediction so far. However, these results were only a preliminary test and may not reflect fairly the actual experiment.

How did the preliminary test help me?

The prelim helped because it gave me a chance to test out the method I had decided to use for my final experiment and it helped me to select my controlled variables and how they will affect the overall result.

I set up three test ...

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Preliminary test results

How did the preliminary test help me?

I set up three test tubes. One had a 0% sucrose concentration, one had 50% sucrose concentration and one had 100% sucrose concentration. I left them for six hours in total, checking up on the weights of the potato chips every two hours. My results show that my prediction was justified, but I cannot be sure that they are 100% accurate because this was only a preliminary test. I decided that there was quite a difference in the weights of the potato chip in the 0% concentration and the potato chip in the 50% concentration so I chose to use concentrations of 0% (0 mol), 20% (0.2 mol), 40% (0.4 mol), 50% (0.5 mol), 60% (0.6 mol), 80% (0.8 mol) and 100% (1 mol) in my final experiment. Also I learnt that I would have to use potato chips that are the same size so they have the same sized surface area so that there is an equal area for water molecules to travel in or out through the semi-permeable membrane of the potato chip.

Plan Part II

I will be looking to see whether different concentrations of sucrose solution affect the way in which osmosis works. I will do this by weighing the potato chips at intervals of 2 hours until 24 hours is up (I can only measure them in school time but I should be able to construct a suitable trend line on a line graph when I have the results to show an approximation of what the chips would have measured at any given time.) I would expect that the chips in the sucrose solution would weigh less at each weigh-in because water molecules will be moving out of the potato cells and into the sucrose solution because there is a higher water concentration in the potato than there is in the solution. The water molecules will be travelling down the concentration gradient, like this:

I will be able to tell if this has happened by the weight of the potato chip. If it weighs less than before it must have lost water. One the other hand if it weighs more than it did when it was put in the test tube it means that it has gained water through osmosis. I would expect to see this from a potato chip that is in a 0% sucrose concentration solution, where the water potential is higher in the potato chip. In osmosis, water molecules only ever move down the concentration gradient but in experiments we are likely to get a few strange results. To combat this and to help make it a fair test I am running 3 test tubes with the same concentration of sucrose for each concentration level. This means I will be able to take an average of three and end up with a more reliable value.

The controlled variables in this experiment are:

Amount of overall solution in each test tube – This makes sure that there is no more water or sucrose than there should be in each test tube, meaning that the opportunity for osmosis to occur and in which way it occurs remains as it should be, depending on the concentration of sucrose present.
Size of potato chip – This is a big contributing factor because it means that all the potato chips should have about the same amount of water inside them and should all have about the same sized surface area. Surface area is a big factor of osmosis as it means there is more of an area to take in or release water molecules, meaning that osmosis should happen quicker than it would with a potato chip with a smaller surface area.
Amount of time potato chips are left in solution for – This will mean that they all have the same amount of time for osmosis to occur in. Hopefully it means that the results for each group of three concentrations should be similar.

Hopefully this will be enough to make this as fair a test as possible.

My independent variable will be:

Concentration of sucrose solution in each test tube – This means that there will be different things happening in each concentration. For instance in a 0% concentration, the potato chip should gain weight and in a 100% concentration it should lose weight.

My dependent variable (or output) is going to be the weight of the potato chip after each weigh-in.

Method

Set up twenty one test tubes; 3 with 20cm(3) of distilled water, 3 with 16cm(3) of distilled water and 4cm(3) of sucrose solution, 3 with 12cm(3) of distilled water and 8cm(3) of sucrose solution, 3 with 10cm(3) of each, 3 with 8cm(3) of distilled water and 12cm(3) of sucrose solution, 3 with 4cm(3) of distilled water and 16cm(3) of sucrose solution and 3 with 20cm(3) of sucrose solution only.
Cut twenty-one potato chips with the cutter and use a razor blade to cut each one to 5cm in length. Use a tile so as not to damage the bench. It is best to wear gloves when using a razor blade but if you cannot just make sure that you are using it properly. Always make sure that you hold it with the blade facing downward and your forefinger and thumb resting on either side of the blunt edge at the top.
Place each potato chip on the scales and measure it before putting each one into a different test tube. Record all weights and label each tube so that you know which chip is which. It is very wise to start a timer when you have put all your chips into their solutions so that you know when to come back and check up on them. Make sure to stopper all the tubes. Again there are safety issues here. Make sure you do not force the stopper into the test tube as you may break it. Try and find a stopper that fits easily into the test tube.
After every two hours, take the chips out and measure their weights. Make sure to dry them with blotting paper before weighing as this could affect the results, making the potato chips appear to be heavier than they are. The water on the outside of the chip doesn’t count because the chip has not absorbed it.
After 24 hours the experiment will be finished. Take the final measurements and then dispose of all equipment safely and correctly. All test tubes, syringes, razor blades, tiles, mounted needles and boiling tubes must be put into a box for cleaning.
Record all results accurately and work out an average for each concentration (average weight of chip after testing).

I will need to make sure that my results are precise so I end up with a correct and fair experiment. Extra care must be taken when weighing the chips so that mistakes are not made in the recording.

I will then be able to see if my prediction is justified by the final results. The preliminary results suggest that my prediction was right because they show that when there is more sucrose present in the solution, the chip will weigh less because it has a lower water potential than the solution.

My preliminary test wasn’t as accurate as it could have been as it was not very thorough and I only tested one test tube of each solution. This means that I will have to wait for my main results to be able to see if it is correct or not.

Results

This shows the results of my main experiment. I will now show the change in mass and the percentage change in mass to help give me an idea of how much water the potato chip is gaining or losing through osmosis.

This shows me how much the potato chip has lost or gained in weight. The minus numbers tell me that in these particular cases, the potato chip has lost weight since its last weigh in. All the positive numbers suggest that it has increased in weight. I will now attempt to work out the percentage change in mass. I can get this by dividing the change in mass by the starting mass and then multiplying by 100. Here is the percentage change in mass.

Analysis

We can see from this that in the third test tube I ran that had 60% concentration of sucrose in there is a strange result in which between 4 and 6 hours the potato chip gained weight instead of losing it like it had before. This affected the percentage change in mass and meant that instead of the percentage change being minus like the rest of that group, it was positive. This meant that I couldn’t include this in my average because it would have made the average into a strange result, so instead I took the average of the three other normal results and used that as the value for the change in mass in that particular test tube. This then changed the value of my percentage change in mass and therefore affected the average of percentage change in mass.

This means that I will be able to draw an accurate graph to show fair results that are a true representation of the experiment I have done.

My results definitely support my prediction because I stated that because osmosis is the movement of water molecules from a high water concentration to a low water concentration through a semi-permeable membrane, the more sucrose present in the solution, the higher the water potential there will be and the more water will move out of the chip, meaning that the chip will weigh less. Likewise I predicted that when there is a low concentration of sucrose present in the solution, the water potential will be higher in the potato chip and thus the chip will gain weight. My results show this to be true. The trend line on my graph shows that all concentrations of sucrose above 47% (or 0.47 mol) resulted in the chip losing weight and that all concentrations above 4.7 mol resulted in it gaining weight.

My graph shows a trend line that curves quite rapidly at the top and then straightens out as it goes down. This suggests that the results were less varied for concentrations of over 40% sucrose solution. The trend line crosses the x axis at 47, suggesting that a 47% concentration of sucrose or 0.47 mol of sucrose in the solution would have no effect on the weight of the potato. This means that according to my graph, osmosis would not occur in a solution containing 0.47 mol of sucrose and 0.53 mol of distilled water when a potato chip with a surface area of 20cm(2) is left in for 24 hours. This means that the water potential in the solution and the water potential in the potato chip must be the same at that time so osmosis would not happen. I propose that because osmosis is water molecules moving from a high concentration of water to a low concentration of water through a semi-permeable membrane that when both concentrations are the same, osmosis does not occur because it would be needless.

One thing I noticed in my experiment was that when I took the potato chips out of the concentrations below 0.5 mol and they weighed more than they had, the chips were quite hard and rigid. I think that this is because when water molecules flow through the semi-permeable membrane and into the potato cells, it causes a build up of turgor pressure and the cells become turgid. This means that there is a higher pressure inside the cells than there is outside the cells and so the cells expand like balloons and become harder. I think that eventually there comes a point where no more water can enter the potato cells because turgor pressure is too high and this means that the water potential has been equalled out. In contrast the potato chips in the higher sucrose concentrations that had lost weight became floppy and quite squashy. I think this is because the pressure inside the potato cells had decreased when the water had flowed out of them, meaning that the pressure was less inside than it was outside. This would cause the cell walls of the cells to be squashed inwards, thus making the potato chip floppy and soft.

Can I be certain that my results justify my prediction?

I am confident that my results justify my prediction because they have a very wide range and it would be difficult to mess up all of these results to the point where they were displaying the opposite trend to what they should be showing. Although I am confident of the science behind my prediction and the results I have gained, I cannot be absolutely certain that these results justify my prediction because I have not been able to physically observe osmosis occurring through a microscope during the experiment. I am relying on basic scientific principles on osmosis to form a scientifically justified prediction that I am almost certain my results display.

Evaluation

Strange results

The only strange result I observed during this experiment was in the third test tube I tested with a sucrose concentration of 0.6 mol (60%). My results suggested that between 4 and 6 hours the potato chip had gained weight through osmosis and had increased in mass by 0.33 grams. I decided to leave this out of the average change in mass for that test, instead taking the average of the other three results for change in mass and substituting that value for the strange value so that I got a good average of the 4 change in mass results for that particular test tube. I think this result may have been down to carelessness when I weighed the potato chip because I don’t think it would have been possible for the potato chip to have taken up water through osmosis because it would go against the principle foundations of osmosis in that water molecules must always travel from a high concentration of water to a low concentration of water during osmosis. I think I could have avoided this result by changing the method slightly so that when I dried the potato chips I made sure that I was drying them properly until the outside of the potato was not dripping. This would then ensure that the potato chip would only weigh what was inside the cells and not have the additional weight of water on the outside of it to affect the results.

Gaining further results

I think that drawing a graph trend line helps to display a good approximation of how the results are supposed to appear but sometimes trend lines can be misleading. If I were looking for additional evidence to support my prediction I would want to take concentrations of 0.1 mol, 0.3 mol, 0.7 mol and 0.9 mol as well as those I have already done to make sure that my trend line was right and still supported my prediction. I think I would have also liked to continue the experiment by weighing each potato chip every two hours instead of just during school hours and I would have liked to leave the experiment for 48 hours to see when osmosis reaches a point where it has to stop. I believe that osmosis is limited because it works in trying to balance the concentrations of water present in both the solution and the potato chip and so I think that there must be a point perhaps during 48 hours that it has to stop and cannot go any further. I have already explained my theory on turgid cells and that there must be a point where the cells have too high a pressure that any more water can enter them and that this is when water potential becomes obsolete because the water concentrations have become the same. I also think that when a cell loses water there must be a point where the water concentrations are balanced between the potato chip and the solution and so it does not have to give up any more water molecules. However I would like to have found out at what point osmosis stops and how long it takes to balance out the water concentrations in solutions with different concentrations of sucrose. Also I would have liked to find out if any other factors affect osmosis and how they affect them. By changing my controlled variables to something like heat of water or surface area of potato chip I would be able to find out more about the factors that limit osmosis and in which way they limit it.