How Does The Concentration of a Sucrose Solution Affect The Mass of a Potato Chip?
Biology Investigation Into Osmosis: How Does The Concentration of a Sucrose Solution Affect The Mass of a Potato Chip?
Introduction
I am going to investigate how changing the concentration of sucrose in a solution will alter the mass of a potato chip placed in it for a certain time. To do this I need to set up a set of experiments where potato chips of known masses are placed in sucrose solutions of different concentrations and left for a time, after which their mass is recorded again. By carrying about the method below I will be able to observe and record the changes in a potato chip's weight that occur with different concentrations of sucrose solution then calculate the average percentage change in mass of each set of chips and draw a line graph.
Method (For Graphical Illustration, see next page)
) I will cut out twelve potato chips, with a scalpel, all with the identical dimensions of 8x7x20mm.
2) I will weigh each of the potato chips and record their masses in a table, labelling half of them A and the other half B, a pink streak of felt tip pen will denote B.
3) I will attach all the A chips to their corresponding B chips with a pin.
4) At one minute intervals I will place each of the pairs into six different boiling tubes containing sucrose solutions, varying from a concentration of 0.1M to 0.6M and increasing in 0.1M intervals.
5) After twenty minutes is up for the first pair I will carefully remove them with a mounted needle and dry each chip with three strokes of filter paper to remove excess solution.
6) I will separate the A chip the B chip, weigh them and record their new masses in a table.
7) I will reattach A to B and replace it in the sucrose solution.
8) I will repeat numbers 5-7 for each of the six chip pairs.
9) I will leave the solutions for approximately two days then return and repeat steps 5-6 then dispose of everything.
0) I will work out the changes in mass for each result, percentage change in mass and average percentage change in mass and record the results in my table.
1) I will plot a graph of concentration against average percentage mass change after two days and look for patterns in it. I will also plot the same graph but for the results after twenty minutes and compare the two sets of data,
To make this a fair test the input variable will be the concentration of the sucrose solution and the output variable will be the change in the potato chips' mass, the only measurements I will take will be the initial masses of all twelve potato chips, their masses after twenty minutes and finally their masses after two days. I will always use potato chips of exactly the same dimensions, to make this possible I used a hand lens to help me get all the measurements exact to the nearest millimetre. In this way all the chips will have the same surface area exposed to the sucrose solution. Unfortunately the chips' initial mass will vary slightly between them but I will take this into account by calculating the percentage mass change using each chip's end mass and individual initial mass. The volume of the solution will always be 25ml to ensure that each pair of chips is fully submerged in it. The temperature during all the experiments will remain constant, so that liquid is not evaporated or frozen, and I will avoid agitating the solutions in any way in case any liquid is spilled or the arrangement of the chips changes. All the potato chips will be left in the solutions for exactly twenty minutes before being weighed. After this they will all be left for two days or approximately fifty hours, however I do not believe that time will make too much of a difference after this long period because all the chips will have had plenty of time to gain or lose as much mass as they can, (see prediction). When drying my potato chips each one will always receive three strokes, lengthways, widthways and height ways, with a piece of filter paper to remove excess liquid that is still attached to the chip. As soon as I have recorded each of the potato chip' masses after twenty minute in the solution I will immediately replace them in their respective boiling tubes and leave them again.
To increase the accuracy of the experiments each solution will contain two identical potato chips, I will obtain measurements for both of them then calculate an average that I will plot on my two graphs. I believe that this experiment will give results that are quite accurate as I will use a balance that calculates mass in grams to two decimal places. I will also use a hand lens to ensure that all chips are cut with the same dimensions and I will measure out the solutions using a measuring cylinder so that I always get the same volume of liquid. I will make sure I read the scale at the meniscus. I think that the six results I will obtain are enough to draw a good graph as most maths text books say that you need a minimum of three to get an accurate curve or line that will answer the question. Although the solutions have been made up for me I believe the ones chosen are the most logical as they move up in uniform steps from 0.1M to 0.6M and would be fairly easy to prepare. I have already done an experiment similar to this that has helped me prepare for this investigation. By doing the previous experiment I was able to learn how to dry and cut the potato chips effectively. I also learnt how to make different concentrations of sucrose solution, I do not have to do this for this investigation but I can now understand the theory behind it which will allow me to interpret and explain by results more successfully. I have also read about several similar experiments in Biology text books that have enabled me to get a clearer picture of what I am going to do and given be a greater understanding of why I am doing this investigation. I believe that the above method is a good way to do my investigation because it will be simple and produce a good spread of results that will be quite accurate and reliable.
Safety
To be safe I will hold the scalpel properly when cutting the potato chips out to avoid cutting myself. I will also place all the boiling tubes in a rack during the experiment to prevent spillage. I will be careful not spill any of the sucrose solution when measuring it out or accidentally ingest it and I will wash my hands after the handling the chips.
Prediction
I believe that the concentration of a sucrose solution will affect the mass of a potato chip. My hypothesis is, "If the solution of sucrose is weaker ...
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Safety
To be safe I will hold the scalpel properly when cutting the potato chips out to avoid cutting myself. I will also place all the boiling tubes in a rack during the experiment to prevent spillage. I will be careful not spill any of the sucrose solution when measuring it out or accidentally ingest it and I will wash my hands after the handling the chips.
Prediction
I believe that the concentration of a sucrose solution will affect the mass of a potato chip. My hypothesis is, "If the solution of sucrose is weaker then more mass will be gained by the potato chip."
I can explain this prediction by applying the theory of osmosis to the changes that will occur to the potato chip during the experiments. GCSE Biology by D.G. Mackean states that, " If a dilute solution is separated from a concentrated solution by a partial permeable membrane, then water diffuses across the membrane from the dilute to concentrated solution. This is known as osmosis."
This occurs because when a substance like sucrose dissolves in water its molecules attract some of the water molecules and prevent them from moving freely which effectively reduces the concentration of them in the solution. This means that if a weaker solution is placed next to it and separated by a semi-permeable membrane then the concentration of free water molecules in it will be higher so they will diffuse from it to the other solution that has a higher concentration of sucrose, (diffusion is the movement of a substance from an area of high concentration to low concentration until an equilibrium is established). The direction of movement of free water molecules is called the diffusion gradient, if the more concentrated solution is on the right and the weaker one on the left then the gradient will be left to right. Some water molecules however will pass from the more concentrated sucrose solution to the weaker one but there will be fewer of them as less of them are free. Sucrose molecules will also diffuse through the membrane but as they are bigger the process will be much slower.
Plant cells contain vacuoles that hold solutions of sugars salts and proteins thereby reducing the concentration of free water molecules within them. The cell wall acts as a partially-permeable membrane.
If I placed my potato chip, made up of plant cells, in a container holding pure water I would expect the potato to gain mass. This would occur because pure water has the highest possible concentration of free water molecules therefore some of them would diffuse into the potato cells giving it its extra mass. Referring to GCSE Biology again it states that, "If a plant cell is surrounded by water or a solution more dilute than its contents, water will pass into the vacuole by osmosis." Therefore when doing my investigation I can expect to see a graph looking like this for the average percentage change in mass after two days in relation to the concentration of the sucrose solution:
When the solution of sucrose is weak the chip will gain mass as there are more free water molecules flowing in than out. As the solution gets stronger the amount of mass gained will decrease until mass starts to be lost, when there is a higher concentration of free water molecules on the inside of the cell. When the sucrose solution is at its strongest the most mass will be lost from the potato chip and the diffusion gradient will be in to out. At the point where the line crosses the x-axis this will be where the two solutions, inside and outside the cell, have an equal concentration, as there is little or no change in mass at this point. Calculating the value of this point will tell me what the water potential of the solution inside the potato's cells is. The water potential is a measure of whether a solution is likely to gain or lose water molecules from another solution. If I continued the experiment with even higher concentrations of sucrose solution I would expect the expect the curve to level out as the cell can only hold so much excess liquid. With regards to the results I obtain after twenty minutes I do not believe that these will show as clear a pattern as those for two days, as there may not be enough time for the chips to gain as much extra liquid as they are able to. However I will plot a graph of these results but I don't expect to see much of a pattern. As I mentioned above I have already obtained results from a previous experiment similar to this one that support my prediction. These results were taken after leaving the potato chips in the solutions for only twenty minutes however so they may not explain and show the changes fully.
Results
Potato Chips After Twenty Minutes
Concentration of sucrose solution (molar)
Initial mass (g)
Mass after twenty minutes (g)
Change in mass (g)
Percentage change in mass
Average percentage change in mass
A B
A B
A B
A B
0.1
.40 1.42
.65 1.45
0.25 0.03
8.1 2
0.1
0.2
.41 1.40
.51 1.56
0.10 0.16
7.1 11.4
9.3
0.3
.50 1.44
.56 1.63
0.06 0.19
4.1 12.9
8.5
0.4
.40 1.40
.42 1.58
0.02 0.18
.4 12.9
7.2
0.5
.33 1.41
.48 1.54
0.15 0.13
1.3 9.2
6.1
0.6
.39 1.40
.37 1.31
-0.02 -0.09
-1.4 -6.6
-4.0
Potato Chips After Two Days (About Fifty Hours)
Concentration of sucrose solution (molar)
Initial potato chip mass (g)
Mass of potato chip after two days (g)
Change in mass (g)
Percentage change in mass
Average percentage change in mass
A B
A B
A B
A B
0.1
.40 1.42
.74 1.68
0.34 0.26
24.3 18.3
21.3
0.2
.41 1.40
.52 1.64
0.11 0.24
7.8 17.1
2.5
0.3
.50 1.44
.58 1.72
0.08 0.28
5.3 19.4
2.4
0.4
.40 1.40
.33 1.49
-0.07 0.09
-5.0 6.4
0.7
0.5
.33 1.41
.41 1.34
0.08 -0.07
6.0 -5.0
0.5
0.6
.39 1.40
.30 1.22
-0.09 -0.18
-6.5 -12.9
-9.7
NB. To work out the percentage change in mass, I took the change in mass, divided it by the initial mass and multiplied the answer by one hundred.
Graph Notes
I have decided to put all the data on one graph so that I can compare the two sets more effectively. Line One is the line of best fit for the results for the chips' percentage mass change after twenty minutes. Line Two is the line of best fit the results for the chips' percentage mass change after two days, a time span of approximately 52 hours. Point P denotes the water potential of the cells while results that are circled are ones that I believe do not entirely correspond with the overall pattern. Red crosses indicate change in mass after twenty minutes, blue points show change in mass after two days.
Conclusions
Part One-Potato Chips After Twenty Minutes:
The graph of average percentage change in mass against sucrose solution strength shows a rough pattern of negative correlation, the change in mass is high and positive when the solution is weak, the highest value being 10.1%, and gradually decreases in value until the last result where the change in mass, measured as -4%, is negative. I was able to draw an approximate line of best fit onto the graph and it crosses the x-axis when the sucrose solutions concentration is around 0.5 Molar. Therefore, I have found out that before this point, as the solution gets stronger the chip gains progressively less mass and after this the point the chip starts to lose mass. However the results for 0.4 and 0.5M do not entirely agree with this, for an explanation see Data Comparison.
Part Two-Potato Chips After Two Days
The graph for this shows quite a clear pattern. When the sucrose solutions' strength is relatively low, the percentage change in mass is high and positive, the highest value being 21.3%. However when the solution is becomes stronger the percentage change in mass gets lower. Only the very last result, when the concentration was 0.6 Molar featured a negative mass change, it was -9.7%. Therefore the change in mass of a potato chip is inversely proportional to the strength of the sucrose solution.
Referring back to my plan I predicted the changes that would happen correctly. I also stated that by looking at my graph's gradient I would be able to get a rough idea of the water potential inside the potato cells. It appears to occur when the solution's strength is just under 0.5 Molar at around 0.48, I.E this is the point where the line of best fit cuts the x-axis. Therefore around this point the solutions inside and outside the cells have the same concentration. I believe this is why the result for 0.5M is positive because as the concentrations are very similar water is moving in and out. I have therefore found out the same about the effect of changing the solution as in the first part, only after the two days the chips have had all the time they need to achieve a balance in concentration so the pattern on the graph is much more clear. In my prediction I stated that the graph would curve before levelling out however it actually appeared to stay as a straight line in actuality. I think this happened because the concentration of the solution can not get any bigger than 100% so this limits the amount of mass the cell can lose to its total water content. If the solution's concentration is zero then the cell simply gains as much water until it becomes turgid. If the looked at after a time the mass of the chip is inversely proportional to the solution concentration. To obtain a curved graph I believe you would have to take one strength solution and plot a graph of change in mass against time. I would expect then expect the graph to begin quite steeply, possibly fluctuating a bit if the strength is comparable to the cell's water potential, before getting flatter until dm/dt is zero and the cell is turgid.
Data Comparison
Both sets of data show that there is an inversely proportional relationship between the percentage mass change of a potato chip and the concentration of the sucrose solution. However in the first set the correlation is much weaker. It is interesting to see that my line of best fit for the first set cuts the x-axis near the point where it does on the second one. This suggests that the two sets of results show the same, correct pattern, even though the gradient of the line of best fit is a bit different on the first. I expected that the positive results for the first set of measurements would be lower than on the two day graph and I believe that this is occurred because the solutions did not have enough time to balance fully in only twenty minutes. The same also goes for the one negative result except that I expected it to be higher than the second value, which it was; -4% to -9.7%. With regards to the results at 0.4 and 0.5 M strengths, I believe they differed because of the similarity in concentrations between the solutions inside and outside of the cells which caused water to flow both ways, hence in some cases one chip lost mass while the other gained it. In my prediction I said that I expected the results after two days to show a more clear pattern and this proved to be correct.
Explanation
I believe that the results showed the above pattern because of the osmosis taking place inside the potato cells, (refer back to Introduction for full explanation of osmosis). As the concentration of sucrose molecules inside the solution increased they attracted water molecules and so they moved further away from each other, creating more gaps and an overall lower concentration of free water molecules. This meant that if, in the solutions in the potato cells, there were a higher proportion of free water molecules then they were able to diffuse out through the semi-permeable cell membrane and into the solution, making the chip's mass decrease. This was what should have occurred on strengths 0.5 and 0.6M sucrose solution. On the other hand, if the solutions of sugars and salts in the potato cells were more concentrated than the sucrose solution, as they were in most cases, then there would be a higher concentration of free water molecules on the outside of the cells. They would then be able to diffuse into the cells to fill in the gaps and the potato chip would gain mass as a consequence. This happened in solutions of strengths 0.1-0.3M, before the water potential mark I think there was a certain amount of ambiguity about whether the chips were gaining or losing mass around the 0.4-0.5 Molar mark because the solutions were quite similar in concentration here so water would be moving in and out all the time trying to reach an equilibrium. Therefore most of my original prediction was correct. Although the results did not entirely conform to the pattern I feel confident that if I conducted the investigation again in a more meticulous way I would get better, more accurate data that would support my prediction completely, this is discussed further in the Evaluation.
Evaluation
I believe that my investigation was quite accurate and allowed be to gather enough data to write a reasonable conclusion. I think that two sets of six results is an adequate number to make a graph out of and draw a reasonable conclusion from. By doing the experiments, using my chosen method, I was able to answer my original question and find out further about the nature of the potato cells. The method enabled me to determine the chips masses, before and after the experiment, and make the test a fair one by keeping everything, bar the sucrose concentration, constant. I think the experiments were preformed accurately because we measured all the masses to two decimal places and made sure they all had exactly the same surface area. Another reason why I believe my results to be quite accurate is because most of my colleagues in my class obtained data that showed a similar pattern. However I did not manage to dry all the chips as accurately as I'd hoped so some excess water may have accidentally measured along with the chips' masses. It was also hard in a few cases to distinguish between chip A and chip B, as the marker had been partially removed, so I was forced to make a guess as to which was which. The results that do not seem to fit the pattern are highlighted with a circle. I believe that these results may not fit in because I might not have dried the chips in the same way as the others and therefore might have accidentally measured excess solution . As I have mentioned above, in the conclusion, the results close to the water potential mark appear to be less accurate. I believe that is because there would be water movement both ways in the cells around that point so I may have taken my measurements at a time when the general flow was out rather than in or vice-versa. If I repeated the experiments again with many more repeats I could probably overcome this problem and get a true average. The diffusion of sucrose molecules also has to be taken into account, although this process is quite slow it may have affected the final mass results. It is also true that the solutions in the cells are much more complicated then the external ones I was using and I think it would be interesting to find out what the constituents of a potato cell's vacuole are exactly. Having said this, the concentration of the solutions inside the potato chip may vary slightly from cell to cell, this could be another reason why my results did not come out perfectly. Another reason for any inaccuracy that came up is the measurement after twenty minutes. This might have disturbed the osmotic process and could have introduced foreign traces into the solutions as well as agitating them. At twenty minutes the results may have seemed to have a weaker correlation because the cells had had less time to establish a balance between the concentrations inside and out.
To get more accurate results I could firstly use a greater number of different concentrations of sucrose solution, perhaps going up in fives over a wider range from 0-100%, this might give a better idea of the cells' water potential and give me more evidence to support my conclusion. I could increase the number of repeats I did to get a better average and bring in some statistical analysis. I think I should also make sure that all the potatoes way the same at the start as this would make all the calculations easier. I could use a balance that measured mass to more decimal places than two and also be more rigorous in drying all the potato chips uniformly. One thing I would definitely change, if I did the experiment again and kept everything else the same, would be the observation after twenty minutes. I believe it would be good to see what the results after two days looked like if the chips were completely left alone for that amount of time, rather than being taken out, tampered with and replaced after twenty minutes. I could then compare the two graphs and find out doing measurements at twenty minutes altered the final results. Following on from this point, I think I should time the period I left the chips for exactly although I am not sure that the length of time makes any difference after a certain point, (something else I could investigate).
To extend my investigation further I could try repeating the experiments using a different solution such as glucose or protein or I could use a different source of plant cells and see if their water potential differs from the potato. It might be interesting to investigate the relationship between the mass change and the surface area of a potato chip while keeping the solution's concentration constant. I might also be able to investigate the effects of changing the temperature of the solution. If I wanted to find out how the change in mass changed over time I think I would make up many identical solutions and cut out chips that were identical in every way. I could then measure one batch after a certain length of time then dispose them while still leaving many other unopened sets that I would measure later. The ideal investigation would be to do the experiments on a single plant cell, however I no not believe this is possible in practice unless you have some very hi-tech and expensive equipment.
I do not think that my conclusion could always be counted on, with regards to the numerical data values, because although I thought my results were quite accurate and they agreed with those collected by others, I would need to collect more evidence and do more experiment and repeats to verify it. There is also the possibility that the measuring balance malfunctioned and gave incorrect results, either at the start or the end of the experiments, and I did not make up the solutions myself so some of them could have been prepared incorrectly and so given false results. Finally, in the period when I left my experiment at school it could have been tampered with my someone. However I believe my overall conclusion and explanation of the pattern is the right one as Biology For Life describes a similar, more basic experiment. It relates it to plant cells and gives the same reasons for the occurrences as I have said in my prediction and conclusion.
