- N.B: When selecting suitable potato chips, choose those that are longer than 7cm so that they can be cut down to the exact length.
- Using a ruler, measure 5cm across from the end highlighted by the red arrow above.
- Using a pen, mark off 5cm on each potato chip.
- Place ruler horizontally so that it joins the pen marks each showing 5cm, together.
- Run the scalpel firmly along the ruler so that excess potato is severed from the chips that are now 5cm and will be used for the experiments.
- Dispose of the surplus potato which has just cut and unneeded in the remainder of the experiments.
- Rub each potato chip with a tissue to wipe away moisture from the vegetable which might affect the results.
- When dry, remove the tissue and weigh each potato chip separately upon a balance. Record the weight in grams (g) to two decimal places i.e. 3.57g.
- N.B. Record the weight for each potato chip as well as recording the cup number that it will be placed into so as not to confuse the wrong chip weights with the wrong concentrations after the tests.
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Ensure that the stop – clock is set at 0:00 seconds. Place one potato chip in each cup of fluid correspondingly and begin the time on the stop – clock at the same time. Set the experiment out as shown below:
- After 40 minutes, pause the stop – clock whilst removing each potato chip from each cup.
- Dry excess moisture off each potato chip using tissues.
- Once again, weigh each potato chip separately upon the balance recording the weight to two decimal places. Notify the cup number as well as the weight so as not to create confusion as mentioned above.
- Put the potato chips back into the cups (ensure that the potato chip goes back into the cup that it cam from beforehand) at the same time and restart the stop – clock.
- After a further 20 minutes, once the potato chips have been equally immersed for a total of 60 minutes, remove them from each cup and repeat the instructions that are highlighted in blue.
- Repeat these instructions every 20minutes until the results recorded show that the weight of each potato chip has neither increased nor decreased.
- Once the experiment is complete, dispose of the polystyrene cups containing either distilled water or varied solutions of sucrose concentration, the potato chips featured in the experiment as well as wet tissues and repeat the whole test once more. The experiment is repeated in order to find an average and improve the accuracy of the results found. It is possible that results can be anomalous yet these cannot be detected if the experiment is not repeated as there are no other results to compare them to.
- Once the second experiments has been completed record the first and second results obtained in a table as shown below:
- The percentage change in mass is obtained by using the following sum:
Original Mass (g) x 100
- Once both series of experiments are complete, the average change in mass must be solved. This is worked out by adding the two initial masses before the experiment was completed, for the same concentration. This figure is then divided by 2.
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The mass after(g) in 1st series of experiment is added to the mass after (g) in 2nd series of experiments for the same concentration. This figure should be divided by 2 also.
- The two answers obtained from finding the average mass (g) before and the average mass (g) after, are then added and divided by two which gives the average percentage change in mass (%).
- Record the averages in a table like so:
- Dispose of all solutions and potato chips and clear away work surface when finished.
- Plot results on a line graph and draw a curve of best fit.
Fair Test:
In order to obtain accurate results, it is essential that a fair test is sustained throughout the procedure.
There are numerous factors that affect the fairness of an experiment.
In this osmosis investigation, I will try to keep constant the factors that affect the rate of osmosis apart from the concentration of sucrose solution (and water) for this is what I am investigating through this report.
I shall use potato chips from the same potato because it is presumed that they will share fairly symmetrical genetic make – up as opposed to selecting potato chips from various potatoes all with different characteristics dissimilar to one another. Although living matter shows variation, the variation is greater between two potato chips each from different types of potatoes than the variation between two potato chips from the same potato. I presume that because the genetic make – up of the chips from the same type of potato will be similar, the partially permeable membranes should be alike also. Hopefully the results will show a relationship between the concentration of sucrose concentration and the rate of osmosis in potato chips.
I shall use the same volume of solution for the experiment involving distilled water and each concentration of sucrose solution (100ml). I will leave the potato chips immersed within the cups of fluids, for the same lengths of time at each concentration of solution as well as the cup containing only distilled water.
I will dry every potato chip off individually, using tissues, before they are weighed so as to reduce the amount of surplus liquid that could affect the results. I will use the same scales for every weighing of a potato chip.
I will perform all of the experiments at room temperature (approximately 37oC) for I am not investigating how temperature affects the rate of osmosis in potato chips but the effect of sucrose concentration.
I will keep the surface area for each potato chip constant for I am not investigating this factor either during the investigation. Variation between surface area and temperature would have affected the rate of osmosis and the focus that was intended to be upon the effect of sucrose concentration would no longer exist.
I shall immerse each potato chip within each cup of either distilled water or sucrose concentration for the same amount of time so that osmosis cannot occur for longer in one cup than the other. Yet again the factors of time, temperature and surface area are irrelevant to the core of my investigation so I won’t vary them but keep them balanced in order to concentrate solely on the effect of sucrose concentration.
When measuring either distilled water or sucrose concentration with the measuring cylinders, ensure that the meniscus is on the exact line that verifies the amount required. If it is below, it means that there is not enough of a substance and if it is above the wanted quantity, there is too much.
As there is only one measuring cylinder, I shall rinse it out thoroughly between each experiment so that droplets of fluid that were left behind from the previous measurements do not affect the concentration of the solution that shall be measured next.
When using the balance, no weight apart from that of the potato chip must be forced onto the balance or the surrounding surface for it is highly sensitive and can display a total weight other than that of the potato chip.
Safety Procedures:
- Be aware of the blades on the chipper when cutting the potato into chips.
- When cutting the potato chips to precise lengths, take care using the scalpel and be aware of its blades trying not to cut you or someone else.
- Hair must be tied back during experiments so that there is no chance of it falling into any of the solutions or being cut by sharp blades on the chipper or the scalpel.
- Place all apparatus, especially measuring cylinders and beakers, onto the centre of the table so there is little risk of people walking past briskly and knocking over and smashing equipment.
Preliminary Work:
Before the main investigation, I carried out preliminary experiments. I analysed the rate of osmosis in a beaker of pure water and the rate in another beaker of saturated sucrose concentration.
I obtained the following results, the length of the potato chip that I used at the beginning of the experiment was 70mm.
The preliminary results show that between 20 minutes and 40 minutes, there is not a change in length. Therefore I have decided that I shall remove each potato chip from the solution after 40minutes because it appears that osmosis is quite a slow process in relation to the time I have to complete the investigation. There is no need to disturb the process after 20 minutes because there is no change in mass yet. The results show that osmosis usually happens between 20 and 40 minutes. Therefore I shall remove each potato chip from each solution after 40minutes and then I shall test it again after a further 20 minutes so that each potato chip has been submerged within a solution for an hour.
Before doing the preliminary work, I was prepared to measure the lengths of each potato chip after 20 minutes. Nevertheless the preliminary results show that after an hour, the lengths of potato chips irrespective of what strength of concentration they are in, do not change very much – only 0.1mm in the distilled water and no change in the saturated sucrose concentration.
Therefore I have decided that I shall weigh each potato chip instead because the balance will detect any change in mass (g) because it displays to a greater degree of accuracy (three decimal places). Therefore I will be able to prove that there is a change in mass over an hour because the balance shall show significant changes. The ruler, only measuring length, will not take into account that the width of each potato chip might change due to the effect of sucrose concentration. The balance will show a change in mass because the area of the potato chip is noted for when it exhibits the mass, whereas a ruler shall only show a change in how long the potato chip is.
I shall record each weight to two decimal places as this is accurate. There is no need to show the results to three decimal places. This would also reduce the accuracy of the points plotted on my graph for there is not enough space on an A4 sheet of paper to draw axis that are precise to three decimal places.
Previously, I intended to use potato chips of 7cm in length. I have decided against this because the preliminary results showed that the potato chips tended to float in solutions of strong sucrose concentration because the potato was less dense in the sucrose concentration. This did not happen in the water because the potato chip was denser than the pure distilled water. Therefore I have reduced the size of the chips so that they are the same lengths as the cups’ diameters (5cm) halfway down the cup. This means that the each chip in each liquid will fit precisely and wedge between the cup’s sides, completely submerged. It is essential that each surface of each chip be exposed to either the sucrose concentration or the distilled water so that osmosis can occur in these areas and the test remains fair throughout. I would obtain unrealistic averages if I were to use two figures from experiments where one chip had had five of its surfaces exposed to whatever solution and the other was completely submerged.
I have applied the changes decided from the preliminary work to the procedure above.
Background Knowledge and Prediction:
The potato chips used in this investigation are made up of cells like all other organisms. Each cell in each potato is a plant cell and is surrounded by a cell membrane - a thin skin separating the contents of the cell from the outside and controls what goes in and out.
The cell membrane has tiny holes in it allowing small molecules to pass through but not the larger molecules. The cell membrane in each potato cell is said to be partially permeable.
Osmosis is a special type of diffusion involving water molecules. It happens when a partially permeable membrane separates two solutions. In this experiment, the partially permeable cell membrane of the potato chip cells will separate the water in the potato chip from the solution it is immersed in, either pure, distilled water or a solution of sucrose concentration.
Osmosis is the crossing of water molecules from a weaker solution into a stronger solution through a partially permeable membrane.
A solution is a solute dissolved in a solvent. The solution in this investigation is sucrose concentration. It is a solution because the sugar, which is a solute, is dissolved in water, which is a solvent.
The diagram shows that the weaker solution contains fewer sugar (solute) particles than the stronger solution which has a higher sucrose concentration. If the sugar and water particles were added up together, those in the weaker solution would make up a larger fraction or proportion of the total, than in the stronger solution. This means that the weaker solution will have a higher concentration of water molecules than the stronger solution.
If the selectively permeable membrane was removed, the water and sucrose particles would move from one side to the other. This is called diffusion. As there is a greater concentration of water in the weaker solution, the water molecules usually diffuse into the stronger solution and the sucrose particles would move from the stronger solution to the weaker.
If there is a barrier that is a selectively permeable membrane which only allows water molecules pass through it, the result is that only water moves – from the weaker solution to the stronger solution. This process is called osmosis, the solute particles are unable to diffuse from the stronger solution to the weaker one because the particles are too large to pass through the membrane.
If the water in the experiment was replaced with a dilute sugar solution, there would still be a concentration gradient and water would diffuse in the same direction into the sucrose solution of high concentration because the concentration of water is lower there.
Water will move into plant cells like those in the potato chips because:
- The cell membrane of the plant cell acts as a partially permeable membrane.
- The cell sap inside the vacuole is a strong solution.
- Water advances into the plant cell by osmosis.
- The concentration of the sap in the vacuole is now weaker.
- Water passes from the weak solution into the strong solution in the next cell by osmosis.
When plant cells are submerged into water, the water enters the cells. Osmosis causes this. As water goes into the cell, it causes the cell to swell up. As there is more water on the outside of the cell than on the inside, it creates a high pressure upon the outside that forces the water into the cell where the pressure is lower and there is enough room for the water to be stored.
Once the cell has taken in as much water as possible, it is said to be turgid – swelled up although the strong cell wall prevents the cell from bursting. The cell is said to be turgid.
Nevertheless when plant cells are submerged in a solution of high sucrose concentration, the cells become flaccid. As there is a lower water concentration on the outside of the cell, the water molecules want to move from the area of high concentration to the area to this area where there are fewer water molecules. The insides of the cells exert a high pressure upon the outside of the cell where the pressure is lower forcing the water from the cells into the sucrose solution. As water passes out, the sap vacuole begins to shrink. The cells are now limp and it is said that they are flaccid. As more water leaves the cells the cytoplasm shreds away from the cell wall causing the cell to become plasmolysed.
Using the above information, I can make predictions about the outcome of the experiments.
I predict that when a potato chip is submerged in either a sucrose solution with a concentration of 0.0mol/litre (i.e. pure distilled water) or a sucrose solution with a concentration of 0.2mol/litre, the cells in the potato chips will be turgid (moreso in cup 1where the sucrose concentration is 0.0mol/litre) and increase in mass (g).
Osmosis is the diffusion of water from a solution of high concentration to a solution of lower concentration. When the concentration of sucrose is 0.0 mol/litre, there is a higher concentration of water in the solution than there is inside the potato chip. Therefore the water from the outside of the potato cells exerts a high pressure onto the cell where the pressure is lower inside it. This pressure forces water through the partially permeable membrane and into the potato cells where the water concentration is lower and there is room for the water to be stored. I expect that an increase of water retention within the potato chip cells mean that the mass of the potato chips in solutions where there is more water than sucrose in the solution (i.e there is a low concentration of sucrose), will increase once osmosis is complete.
I believe that the change in mass will be greater when the concentration of sucrose is 0.0mol/litre in cup 1rather than when it is 0.2mol.litre in cup 2. This is because the solution will be weaker in cup 1 and so the water concentration will be higher on the outside of the potato chip cells than the concentration of sucrose. Therefore the water particles will move into the potato chip cells where the water concentration is lower.
In the cup where the sucrose solution has a concentration of 0.2mol/litre, the concentration of water in the solution will not be as high as the water concentration in the solution of cup 1. Therefore less water will move into the potato cell because the water concentration is not as high as that of cup 1 hence there will still be an increase in mass in potato chip 2 but not as great as that of cup 1.
Absorption of water by osmosis makes the cells in the potato chips firm and turgid when the concentration of sucrose solution is low. I expect that the potato chips will increase in mass (g).
When the concentration of sucrose solution is, 0.6mol/litre or 0.8mol/litre I expect that the mass of the potato chip submerged into this solution will decrease in mass once osmosis is complete. This is because the concentration of sucrose in the solution is higher than the concentration of water which is only 0.4mol/litre or 0.2mol/litre. The concentration of water inside of the cell will be greater than the concentration on the outside of the cell. The water shall exert a high pressure from the inside of the cell onto the outside of the cell (where the pressure is low). The water will osmose out of the potato chips’ cells and into the sucrose concentration and the mass of the potato chip, in grams, will decrease. It is probable that the potato chip cells will become plasmolysed due to the inundation of water as it diffuses from the area of low concentration to high concentration.
When the concentration of sucrose concentration is 1.0mol/litre, I expect that the mass of the potato chip within this solution will decrease and become plasmolysed. This is because the concentration of the sucrose solution is very high, therefore the water within the potato chip cells will exert a high pressure from the inside upon the outside of the cell where there is not a lot of water. The water molecules will diffuse through the partially permeable membrane and into the area of high concentration, i.e. the strong sucrose solution.
The cells lose water and become flaccid (flabby). Water has diffused out by osmosis and therefore the potato chips where the sucrose concentration has been high, have decreased in mass.
Results:
These are the results that I obtained during this investigation:
The following table shows the average masses before and after the investigation using varied concentrations of sucrose solution, as well as the average percentage mass change:
- The percentage change in mass is obtained by using the following sum:
Original Mass (g) x 100
In order to find out the average percentage change for a potato chip immersed in a solution with a concentration of, for example, 0.4mol/litre, I calculated:
(Change in mass)
(Average mass before experiment) x 100 =
-0.2
4.05 x 100 = - 4.94%.
Analysis:
I have devised a graph showing the results which I obtained during this osmosis investigation. It shows the average percentage change as well as the percentage changes for the 1st and 2nd series of experiments.
Analysis:
The results obtained show that when the concentration of sucrose in a solution is 0.0mol/litre, the average percentage change in mass largely increases by 5.12% and the potato chip becomes 4.25g. When the concentration of sucrose solution is 0.2mol/litre, the average percentage mass falls to an increase of only 1.26% so that the potato chip submerged in this solution is 4.02g.
The increase in mass when the sucrose concentration is 0.0mol/litre (i.e. distilled water) is caused because the concentration of water is higher on the outside of the potato chip cells than within the cells. Therefore, the water on the outside exerts a high pressure onto the inside of the cell where there is some room for the water to be stored. The pressure forces water from the outside of the cells where the water concentration is high, to inside the cells where the water concentration is lower. This causes the potato to increase in mass and become turgid.
This concentration gradient therefore causes water molecules to osmose down the concentration gradient, across the partially permeable membrane and into the plant cells (those of the potato chips). This was the case for when the solution concentrations were 0.0mol/litre and 0.2mol/litre because the potato chips gained mass causing the cells to become stiff due to their filling of water.
The water molecules in the solution where the sucrose concentration was 0.2mol/litre diffused through the partially permeable membrane from the outside of the potato chip cells where the water concentration was higher, into the inside of the cells were the water concentration was lower. The reasons mentioned in the paragraph above apply in this case also, to why the mass of the potato chip increased.
Looking at my results, I can see that the isotonic point is 0.26mol/litre. This is the concentration of sucrose solution when osmosis is not occurring because the concentrations of water are the same both inside and outside of the cell. Therefore water molecules do not move from inside or outside of the cell because the condition are exactly the same, this means that the potato chip could be left in this concentration for sucrose for a long time but its mass would not change at all. At this point the net movement of water is 0.0.
The results show that when the concentration of the sucrose solution is 0.4mol/litre, the average percentage change in mass is –4.94 %. This means that the initial mass of the potato chip soaked in this solution was 4.05g but this reduced to 3.85g.
This decrease was caused because in the solution the concentration of water was higher in the inside of the potato cells than on the outside. This meant that the water in the inside of the cells exerts a high pressure upon the outside of the cells and so the water is forced out from the potato chip causing a decrease in its mass.
For the same reasons, when the sucrose solution was 0.6mol/litre, the potato chip immersed in this solution decreased in mass. I have calculated an average from the two experiments, the average mass was 3.95 but lost 8.10%of its original weight to become 3.63g.
Also for the same reasons, when the sucrose solution was 0.8mol/litre, the average mass beforehand was 3.86g. On average, this reduces by 15.80% and the final mass is 3.25. This is because the water from inside the cell moves from the area of low sucrose concentration to the area of high concentration which causes a decrease the potato chip’s mass.
When the sucrose concentration was 1.0mol/litre, the potato chip’s mass decreased on average by approximately 18.04% from 3.88g to 3.18g. This was due to the water from inside the potato cells exerting a pressure upon the outside of the cells forcing water through from a higher water concentration to that of a lower concentration.
The loss of mass in when the sucrose concentrations are either 0.4, 0.6, 0.8, or 1.0 (all in mol.litre) causes the cells in each potato chip featured in each of these to become plasmolysed (and the chips quite soft) due to the loss of water in the cells.
Evaluation:
The results obtained from the investigation show that as the concentration of sucrose solution increases, the average percentage change in mass (g) decreases.
The results obtained showing the average percentage change in mass are accurate to an extent. They are accurate because along with the results of the percentage change in mass for varied concentrations of sucrose concentration in the 1st and 2nd sets of experiments, they show a trend.
The results show that when the sucrose concentration is low and the water concentration is higher outside the potato cells, the mass of the potato chip, after an hour increases. As I said before, this is due to the water from the outside of the cells exerting a high pressure onto the inside of the cells forcing water into the potato chip. This happens as there is room inside the cell and the potato chip increases in mass (g). The cells become turgid as they take in more water.
The affect that this has on the shape of the graph is that at the beginning, the line is relatively straight because osmosis has only just begun.
However when the concentration of sucrose increases, the potato chips lose mass because there is a higher water concentration on the inside of the cell than on the outside. Therefore the water from the inside of the cell exerts a pressure upon the outside and water is forced outside of the potato chip to the area where the concentration of sugar is higher than within the potato.
The decrease in average percentage change in mass (g) causes the graph to adopt a good negative correlation. The line begins to stoop downwards until it cuts the x – axis at approximately 0.26mol/litre – the isotonic point. This is where the net movement of water is 0 because the concentrations of water are the same on either side of the partially permeable membrane.
From this point, the graph line continues to move downwards, and according to the average percentage change in mass (g), at approximately – 15.80% (the figure shown on the y – axis), the graph line once more begins to straighten out proving that osmosis is nearly complete.
The results show that osmosis takes longer when the concentration of sucrose solution is lower. This is evident because the average percentage changes in mass for each concentration value of sucrose, are greater when the solution has a higher concentration. If I were able to continue with experiments, the line on the graph would have been straight because osmosis would have been complete.
For example when the average weight of a 50mm potato chip, weighing 4.04g, is immersed into a sucrose solution with a concentration of 0.0 for an hour, the average percentage change in mass is only a 5.12% increase. Nevertheless when the concentration of sucrose is 1.0mol/litre, a 50mm potato chip weighing 3.88g, is soaked into this for one hour and the average percentage change in mass is an 18.04% decrease. This shows that the mass of a potato has a greater change in the same period of time when the concentration of sucrose solution is higher.
Although the weights in these two potatoes were different at the beginning of the experiment, the difference was only slight and would not drastically affect the changes in mass that have been caused by the variation in sucrose concentration in each solution.
The results were also inaccurate because the results showing percentage changes in mass were different for the 1st and 2nd series of experiments. These differences might have been caused because each potato had a different weight before the experiment.
It is unlikely that one could repeat this experiment using potatoes of exactly the same weight in order to retrieve exact results. This is because we assume that potatoes own unique genetic make – up dissimilar to other potatoes. Therefore a person could certainly cut chips from a number of potatoes and use those with the same weight. However as all organisms show variation, the sugar concentration in one potato might be different to that of another irrespective of them sharing the same weight. This would possibly distort the results and it would be useless to do the experiment.
The experiment proves that there is variation even within the same potato because I used chips from the same vegetable with the same length but their masses were still different to one another. It is likely that there are different sugar concentrations in different parts of a potato.
The graph produced was not a linear graph but had a curve of best fit. In the 2nd experiment, 5 out of 6 of the results showing percentage change in mass were higher than the results of percentage change in mass in the 1st experiment. I obtained one anomalous result that didn’t conform to the curve of best fit which I drew. It claimed that when the concentration of sucrose solution was 0.6mol/litre, the average percentage change in mass was –8.10%.
This result might have been caused due to the oxidation of some potato chips in the air. I am not exactly sure how the oxidation of chips affected the experiment yet it offered visual evidence that proved that a chemical reaction was occurring, for example, the potato was turning brown because of its exposure to oxygen.
Due to time restraints, I was unable to complete the osmosis investigation to a stage where the potato chip was weighed at twenty minute intervals after the hour, and the change in mass had not varied between this time. No variation in mass would have proved that osmosis was complete and I could terminate that experiment because the potato cells were either completely plasmolysed or turgid. Ideally, a whole day would have been best to finish the experiment.
There was only one chipper and two balances between a class of thirty people. This meant that the whole potato had to be chipped and all of the chips used in the 1st series of experiments had to be weighed as the same time so as not to slow down the pace of work amongst other people in the class. The problems this creates was that the chips were exposed to the air for a longer duration of time whilst each one was weighed. Time restraints, once again did not permit us to store them in a plastic bag and remove them when needed. Therefore, the chips had excessive exposure to oxygen which possibly caused the anomaly in my results.
I could have prevented the problem of the oxidation of potatoes by chipping only half the potato for the chips for the 1st series of experiments and then chip the other half of the potato (after slicing off the area that was exposed to air throughout that time) after the first set of tests were complete and I was ready to start the next.
If I were to repeat the experiment, I would like to use a chipper and a balance per group. As a lot of people were queuing to use the balance at the same time, it meant that inevitably they were leaning upon the work surfaces where the balances were based on. As they are extremely sensitive, it is possible that they picked up and displayed the wrong masses which might have distorted the calculations when working out the percentage mass changes.
It was extremely difficult, as there were only two people per group, to drop 6 chips into 6 separate cups as well as start the stop – clock all at the same time. Theoretically, the potatoes were not immersed into the solutions for exactly the same times which means that osmosis might have continued for longer in some cups and shorter in others.
Therefore, if I was to do the experiment again and had more time, I would like to use 6 separate stop – clocks and (for each experiment) begin one experiment first and then begin the next with another stop – clock ten minutes later and so on. This idea would have assured that the experiment remained fair throughout and osmosis occurred for exactly the same duration of time for each experiment.
The results show that a larger concentration gradient means that there is a greater difference between the water concentrations inside and outside of the potato cells. Likewise a smaller concentration gradient means that the difference between the water concentrations inside and outside of the potato cells is small also.
I did not monitor the temperature during the entirety of the investigation but instead assumed that the temperature was approximately room temperature (37o C). I doubt there would have been a dramatic difference nevertheless. Another group measure the temperature and their results were ever so slight therefore I don’t think this is essential because the change in air temperature would have been greater than the temperature of liquid. As the air temperature was so small, the liquid temperature would have been lower and therefore this didn’t affect the results.
In order to extend my knowledge of osmosis, I could investigate the concentration of sugar in other vegetables such as carrots, parsnips, turnips etc. I would repeat the same procedure but I would take into account the problems I cam across and use the solutions offered in the evaluation. This would be an interesting investigation because it could then lead to being able to compare the rates of osmosis in different vegetables when affected by the concentration of sucrose solution.
