What effect does the concentration of salt solution have on the amount of osmotic activity in a potato chip?
What effect does the concentration of salt solution have on the amount of osmotic activity in a potato chip?
Skill Area P - Planning Experimental Procedures
The aim of this experiment is to investigate the movement of water into and out of potato cells by osmosis, and what effect the concentration of salt in that water has on the rate of osmosis.
Variables: To ensure that the experiment is fair, certain aspects of it will have to be kept constant, whilst one key variable is changed. I have chosen to vary the concentration of the salt solution because this will give me a varied set of results from which I hope to make a decent conclusion. This means that in order to get accurate results I must keep all of the other non-variables the same. If any of the non-variables below were not kept constant, it would mean it would not be a fair test.
* I shall ensure that all the potato chips are the same length and that all the ends are cut to 90 degrees. If one of the potato chips was longer than the rest, the surface area of the chip would be larger and there would therefore be more space for osmosis to occur.
* I shall carry out all of my experiments at room temperature, because different temperatures would cause osmosis to be carried out at different rates.
* I shall keep the water potential of the potato initially the same by using the same variety of potato, which have been treated in the same way, e.g. have all been cut without being washed. This is because washing them would be subjecting them to water, which could affect the initial water potential.
* The mass of the potato is a dependent variable, which means that it will be measured throughout the experiment. I will measure the mass in grams, and shall weigh the potato chip before it is put in the solution, and after. This will allow me to see whether osmosis has taken place, and to what extent.
* The volume of the solution that the potato cylinders are placed in must be constant. The potatoes must be totally immersed in the solution to ensure that the whole of the surface area is being used for osmosis.
* I am going to use the same set of scales to weigh my potato chips. This is because the readings given may vary slightly between different sets of scales.
* I am going to leave all the potato cylinders in the solutions for the same length of time, to allow them to carry out osmosis for the same length of time. If one was left for longer that the others then it would carry out more osmosis and therefore give an anomalous result.
* I shall keep the light intensity the same by doing all of the experiments in the same room. No extra light will be shone on the experiments, and none of them will be kept in the dark.
List of Equipment: Scalpel
White Tile
Cork Borer
6 Test Tubes
Ruler
Test Tube Rack
Method: I shall select 3 or 4 large ground potatoes and check that they are firm and healthy. I do not want soft or old potatoes because these could give inaccurate results. I shall then use a clean cork borer to cut 12 cylinders of potato out of them. These cylinders will be of varying lengths so I shall use a sharp scalpel and a ruler to cut them all to the same length (45mm), ensuring that the ends are all cut to 90 degrees. I shall cut the potato chips on a white tile with a scalpel so that no one will be cut. Also while using scalpels everyone will have to wear safety goggles as a standard safety precaution.
I shall fill the 6 test tubes labelled A-F with varying amounts of distilled water and salt solution. The quantities shall be: -
Distilled Water
Salt Solution
A -
20 ml
0 ml
B -
6 ml
4 ml
C -
2 ml
8 ml
D -
8 ml
2 ml
E -
4 ml
6 ml
F -
0 ml
20 ml
I shall then weigh each individual potato chip on a set of electronic scales ...
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I shall fill the 6 test tubes labelled A-F with varying amounts of distilled water and salt solution. The quantities shall be: -
Distilled Water
Salt Solution
A -
20 ml
0 ml
B -
6 ml
4 ml
C -
2 ml
8 ml
D -
8 ml
2 ml
E -
4 ml
6 ml
F -
0 ml
20 ml
I shall then weigh each individual potato chip on a set of electronic scales and then put them into the test tubes, ensuring that they are completely covered in the solution. To ensure safety, the test tubes will be kept in test tube racks to reduce the risk of them being knocked over. I shall put two chips in each test tube because this will be a way of repeating the experiment to increase the accuracy of my results by averaging the 2 sets of results at the end. I shall then label the test tube rack so that I know that it is mine and leave it for 24 hours to let osmosis take place.
When the 24 hours is up, I shall remove the potato chips from the test tubes one at a time so I don't get confused as to which chip came from which solution, place them on the paper towel, dry them off and weigh them on the same set of electronic scales. I shall make sure that when I handle the potatoes my hands are clean and dry to prevent any contamination and to make sure I do not pass any extra water onto the potato as this could cause inaccuracy.
Hypothesis: Osmosis is defined as "the flow of the solvent of a solution, e.g. water, across a selectively permeable membrane, e.g. a cell wall, down a concentration gradient". A selectively permeable membrane is one which lets smaller molecules, such as water, through, but does not let larger ones, such as solute molecules to pass through. Only some membranes permit selective flow, or osmosis. Many membranes allow all or none of the constituents of a solution to pass through. As the potato is a plant cell, it contains a vacuole and a cytoplasm. The cell membrane is partially permeable. This means that water particles can diffuse into and out of the cells by osmosis.
The water potential of a system is the tendency for water to leave the system. The higher the concentration of water, the higher the water potential, and therefore the more likely it is that water will leave the system if it has the chance. As distilled water has a water potential of zero, adding salt to it will reduce the water potential by reducing the concentration of water in it. If it is lowered below the water potential of the cytoplasm in the potato cells (hypertonic solution), then water from the potato cells will diffuse through the cell wall into the solution until they both have equal concentrations of water molecules. This loss of water will cause the potato cells to shrink and become flaccid. This means that the potato cylinders will shrink and become floppy because the hydrostatic pressure inside the cells has become so small that the cell loses its shape. When a cell is like this it is said to be plasmolysed. If the water potential of the potato cells' cytoplasm is less than that of the solution it is in (hypotonic solution), then water molecules will diffuse from the solution into the potato cells. This increase in the volume of the cells' cytoplasm causes the cells to swell and become turgid. This means that the cell has become swollen and hard and the potato cylinder will increase in size and become stiff and rigid. This is because the hydrostatic pressure inside the potato cells rises, the cells swell, but they have a strong cell wall around them, which prevents them from bursting. This means that there is a limit to how much water can be absorbed, and it will not just absorb water until the water potentials are equal. If the concentration of water molecules is exactly the same in both the potato cells and the solution, then no water will diffuse in either direction because there will be no concentration gradient. In this case the solution is known as isotonic.
Prediction: I predict that in the distilled water the potato will gain water and become turgid because the concentration of water in the potato must be lower than distilled water. As the concentration of salt in the solution increases, I predict that the potato cylinder will become smaller because the cells will become more and more plasmolysed due to more water leaving the cells. I predict that by doubling the concentration of salt in the solution, you will also double the percentage decrease in the mass of the potato cylinder, because doubling the concentration of salt would halve the water potential, and therefore double the difference in the water potential between the potato cylinder and the solution that it is in. I predict therefore that gain/loss in mass and strength of salt solution are directly proportional.
Skill Area O: Obtaining Evidence
Volume of salt solution - mass before (g), mass after (g), %age change in mass
0ml - 1st attempt - 2.95, 3.39, +14.9
2nd attempt - 3.03, 3.50, +15.5
4ml - 1st attempt - 3.05, 2.93, -3.9
2nd attempt - 3.02, 2.91, -3.6
8ml - 1st attempt - 3.09, 2.78, -10.0
2nd attempt - 3.11, 2.81, -9.6
2ml - 1st attempt - 3.01, 2.53, -15.9
2nd attempt - 2.95, 2.49, -15.6
6ml - 1st attempt - 3.03, 2.39, -21.1
2nd attempt - 3.05, 2.43, -20.3
20ml - 1st attempt - 3.09, 2.40, -22.3
2nd attempt - 3.00, 2.32, -22.7
Average %age change in mass
0ml - +15.2
4ml - -3.75
8ml - -9.8
2ml - -15.75
6ml - -20.7
20ml - -22.5
Skill Area A: Analysing Evidence and Drawing Conclusions
My graph gives the line of best fit for the percentage change in mass of the potato chips over the course of the experiment. The graph is a curve that slopes downwards which means that as the concentration of the solution increases the percentage change in mass decreases. The graph shows that the percentage gain and loss is not directly proportional to the concentration of the solution, because if it were then the graph would be a straight line. My graph gets less steep as X axis gets bigger because the potato chip is becoming closer to as plasmolysed as it possibly can be, and so the change in mass with each increase of salt concentration is becoming smaller and smaller. From the line of best fit, you can see that all of my points were fairly close to creating a perfectly smooth curve. This shows that my results are fairly reliable. However my graph does not fit in perfectly with my prediction of the experiment graph. I predicted that it would be a straight-line graph, but in fact it is a curve. The reason my prediction was incorrect was because I did not take into account the fact the change in mass would decrease as you got closer to the stage of complete plasmolysis.
My graph shows that the potato cells increase in mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration. From the graph an estimate to the concentration of the potato cell can be made as 2.8ml. This is the point where the potato is not increasing or decreasing in mass, and is known as the isotonic point. This is where no osmosis is taking place because both the potato and the solution have identical water potentials. The next point, 4ml, loses approximately 4.0 %. This shows that the water potential of the salt solution in the beaker is lower than that of the potato chip. The next, 8ml, looses approximately 10.0 % in mass. This shows that this salt solution has an even lower water potential than 4ml and that therefore more osmosis took place. This is why the potato lost even more mass. This pattern carries on through the graph, and even more mass is lost, as more water moves out of the potato into the solution. My results match with my initial predictions.
This graph of the change in mass helps to prove the point of complete plasmolysis, whereby the potato cannot lose any more water. As you can see as the molar concentration increases, the change in mass decreases. From left to right the first two points on the graph are very spread out indicating that there was a large change in the mass (18.75%). This decreases throughout the increasing molar concentration until the change is very small (1.8%)
The graph above gives a clear indication that there was an overall decrease in mass during the experiment. At the point of distilled water, the line for after the experiment is above the line for before the experiment unlike any of the others. This is because the water potential of the salt solution is lower than that of the potato chip, whereas the water potential of distilled water is higher.
Skill Area E: Evaluating Evidence
The experiment was very successful in my opinion. I obtained a large quantity of very accurate results from which I was able to create informative graphs. I think, however, that I could have taken more results, e.g. 4chips at each concentration rather that 2, as this would have greatly increased the accuracy of the experiment. But I think that the time that I left the experiment for was enough to allow sufficient osmosis to occur. However if I was to repeat the experiment I might well increase the number of the results because I do not think that the range of concentrations was quite adequate. I would take readings at more concentrations and at stronger salt solutions to increase the accuracy and to try and find the point of complete plasmolysis. If I repeated the experiment I would probably take extra readings at 1ml, 2ml, 3ml, 6ml, 10ml, 14ml 22ml and 24ml. This would also have allowed me to also find out the isotonic point far more accurately as the one that I estimated is very approximate.
The cutting of the potatoes was one of the most difficult parts of the experiment as although I was recording my results by mass, it could well have affected the surface area and so therefore the overall rate of osmosis. If I were to repeat the experiment I would have possibly found a machine like a guillotine to cut the potato as it would ensure that all potatoes would be the same weight and dimensions. As well as the potato I could have found a more accurate way to measure out the solutions and to determine the molar concentrations. Perhaps I could have used a pipette or burette. This would ensure that I have an accurate amount of fluid in each test tube. I could also weigh each chip on a more accurate scale, e.g. not to 0.00g but to 0.0000g.
There were 2 anomalous results on my graph at 8 and 12 ml. This may have been caused by human error, such as adding too much water or too little salt to the solution, or cutting them differently from the others. When the potato chips were removed from the test tubes and dried, I might have dried some of the chips more thoroughly than others, and so some would have excess water on them, 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 the experiment was truly successful and I was very pleased with the outcome.
