To investigate how the concentration of water in a sucrose solution affects a potato using different concentrations of sucrose solution and measuring the effects.
Aim: To investigate how the concentration of water in a sucrose solution affects a potato using different concentrations of sucrose solution and measuring the effects.
Hypothesis:
Osmosis is the net movement of water molecules across a selectively permeable membrane from an area of high concentration to low concentration.
The selectively permeable membrane.
A selectively permeable membrane is a type of barrier that will only allow certain molecules through it. It acts like a sieve in which microscopic holes only let small molecules through it and keep out larger molecules, therefore 'selecting' smaller molecules.
An example of the usage of a selectively permeable membrane is in a dialysis machine. Used as an artificial kidney, a dialysis machine allows urea through its membrane but does not allow protein to escape. This is because protein molecules are larger than urea molecules. Urea is mainly made up of water molecules, H2O, which are possibly the smallest compound atoms as hydrogen consists of only one electron, proton and neutron.
In the same way, water would have smaller molecules than sugar, which means that out of the two main ingredients for a sucrose solution, (water and sugar), water molecules would be more able to pass through
a selectively permeable membrane than sugar molecules.
Potato cells are surrounded by their own type of selectively permeable membrane. Inside the potato cells they contain a concentrated solution of sugars, proteins and other substances in water. As protein, sugars and other substances other than water have large molecules, they are unlikely to be able to pass through the selectively permeable membrane. However, water could.
The water concentration
Pure water has a maximum water potential of 100%. The more water molecules there are in one area the more pressure there is and energy to move the water particles around, called water potential. This means they collide and tumble around each other quite freely.
When water and sugar molecules are dissolved together (sucrose solution), this creates a smaller concentration of water molecules as the sugar molecules combine with some of the water molecules which makes them too big to penetrate the membrane. This decreases its water potential and applies less pressure for movement.
The same applies to the potato, which contains the other various substances. When these substances are mixed with the water inside the potato, its water potential also decreases as the protein and substances combine with some of the water molecules to make them too big to move through the selectively permeable membrane. Therefore the pressure of the water, its water potential, decreases, and less pressure is applied for movement.
Because of this imbalance of water molecules and according to other forms of molecular movement (where a high pressure of molecules will move to a lower pressure of molecules), water will therefore have a greater tendency to move from pure water (high pressure) to a more concentrated solution of water (low pressure). This is defined through osmosis whereby the higher concentration of water will move to a lower concentration of water.
The other molecules
The sugar molecules in the first region (the sucrose solution) will not move because they cannot penetrate the selectively permeable membrane that surrounds the potato cells. Nor will the substances within the potato, as they are also too large (the second region). However, they both play an active part in osmosis as they affect the water potential of both regions to lower and raise the water potential of each region.
As potato cells contain a concentrated solution of substances other than water, this will affect which direction osmosis occurs. As there is water in both substances, osmosis could occur two ways: through the potato's membrane into the cell when the higher pressure of water (water potential) is in the sucrose solution , or out of the potato's membrane into the sucrose solution when the higher pressure of water (water potential) is in the potato.
Therefore, when the higher water potential is in the sucrose solution compared to the potato, water will leave the region of the ...
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As potato cells contain a concentrated solution of substances other than water, this will affect which direction osmosis occurs. As there is water in both substances, osmosis could occur two ways: through the potato's membrane into the cell when the higher pressure of water (water potential) is in the sucrose solution , or out of the potato's membrane into the sucrose solution when the higher pressure of water (water potential) is in the potato.
Therefore, when the higher water potential is in the sucrose solution compared to the potato, water will leave the region of the sucrose solution and go into the potato. When the water potential is lower in the sucrose solution than in the potato, water will leave the potato and go into the sucrose solution.
This creates the effect of turgid and flaccid potato cells.
When potato cells gain water due to osmosis, the cytoplasm inside the cell swells up to take in the extra water, (as the water potential outside the cell is bigger than inside the cell). This makes the cell turgid, as the extra water stretches the tough cell wall that surrounds the cell:
When potato cells lose water due to osmosis, the cytoplasm inside the cell is drawn out (as the water potential is higher inside the cell than outside). The cell loses its turgor as the cell wall is no longer stretched by water and the cell becomes flaccid, the process of plasmolysis:
Prediction
Using all the evidence above, I predict that when distilled water is surrounding the potato after a period of time, the potato will become turgid, and therefore become, stiffened, hard and enlarged, as the water concentration in the beaker would be much higher than the water in the potato.
I also predict that as more molars of sucrose solution after a certain point is used, the potato will become more flaccid, and therefore become more soft, floppy, and smaller, as the water concentration in the potato would be higher than the water concentration in the beaker.
I think this will happen because the lower the concentration of water in the sucrose solution there is, the higher the water concentration in the potato, and the more comparatively higher pressure there is in the potato, the faster osmosis would take place. This would give me a smaller potato after a set period of time, compared to a weaker solution, which would be slower, and make the potato less small over the same period.
As a result, I will be bound to find a point on my conclusive graph whereby the potato's water potential and the sucrose solution's water potential will match, and the potato, in theory, will not change size.
Key factors
All the key factors that could affect this experiment are:
Strength of solution
The strength of the solution would inevitably affect which direction osmosis takes place, as discussed in the hypothesis.
Size and shape of potato.
The size of the potato pieces would affect how much water each piece could take in, and therefore affect my results as the size I measured afterwards would change. The same potato size was used each time using a potato borer in order to give each sucrose solution I used the same amount of water that could possibly be taken in (or out) of the potato, as well as give me results that I could accurately compare.
Volume of solution
The amount of solution used would affect osmosis. The amount used would affect how much water potential the solution had compared to the potato, and therefore affect how osmosis takes place. The same amount of solution was used each time in order to avoid this problem.
Time in the solution
As osmosis is a type of diffusion, it would no doubt take time to take effect. Also, because it takes time for osmosis to take place, it is possible that the potato's size would be affected by how long it stays in the solution, as the more time it stays in there, the longer osmosis can take place and the more net movement takes place. This would affect the size of the potato and the end result. So all the potatoes were left in the solution for the same amount of time to avoid osmosis being allowed to take longer on some potato pieces than others.
Temperature
The hotter water is, the more energy it would hold and the more movement would take place. This factor was not easy to control, and we were not able to fully control this as we lacked the scientific equipment to do so. However, we did keep each experiment in the same room, which would roughly mean the same room temperature.
The factor I will investigate is the size of the potato before and after osmosis with 5 different types of sucrose solution.
To make this a fair investigation, I tried to keep each other factor the same.
I used the same amount of sucrose (100ml) for each set of potatoes.
I used the same potato borer to get the same size of potato, and measured each piece to be 3cm long.
I left the pieces in the solution for 45 minutes and stopped them all at the same time.
I did the experiment in the same room the whole time.
Apparatus
Potato borer with an 8mm diameter - to remove the potato pieces
Two potatoes
Knife - to cut the pieces into 3cm length
White tile
Tweezers
5 beakers (one for each solution)
Stopwatch - to make sure they are all left in at the same time
Ruler - to measure the results
5 different strengths of sucrose solution - to compare the affects of osmosis on the potato:
0.0M - distilled water
0.5M
.0M
.5M
2.0M
Method:
* Use the potato core to take 15 pieces out of the potatoes
* Peel the skin off the edges of the potato pieces using the knife (as this barrier is not selectively permeable) and cut the pieces so that they are all 3cm in length. Thanks to the potato borer, each piece should have the same diameter of approximately 8mm.
* Pour the 5 different strengths of sucrose solution into the 5 beakers.
* Put three pieces of potato in each beaker.
Due to lack of equipment, we were only able to use 5 beakers for each solution. However, after some discussion, we decided that this would still be a fair test if each beaker had the same amount of potatoes. This is because each solution would still have the same amount of comparative water potential, as the same amount of potato is added to each one.
* Leave for 45 minutes and then take out.
* Measure each potato piece and record the results.
Due to the amount of time we had, we were not able to repeat the experiment 3 times. However, we decided instead to use 3 pieces of potato each time instead to get the average normally expected. So in this way we still got 3 results for each solution.
Safety first
To make this a safe experiment, we used a white tile to cut the potato pieces on.
We also used tweezers to remove the potato from the solution to avoid corrosion.
All potato leftovers were put in the bin.
Results
Molar of sucrose solution (M)
Length of potato to start (mm)
Experiment 1 (mm)
Experiment 2 (mm)
Experiment 3 (mm)
Average
(mm)
Difference
(mm)
0.0
30
35
32
30
32
+2
0.5
30
25
28
28
27
-3
.0
30
29
29
29
29
-1
.5
30
27
29
28
28
-2
2.0
30
30
28
26
28
-2
To work out the average change in size, I added the three experiments for each solution and divided the answer by three. I deducted this result from the original size (30mm) to find my difference.
Graph
To draw this graph, I took the original measurement of 3cm as the horizontal axis (0) and drew according to this in order to show how the change in molars had affected the original size of the potato. I have also split the graph into sections. This would help prove (or not prove) the prediction in an easier way as the line of best fit could be easily related to the original size.
My line of best fit shows me that as the solution gets more concentrated, the potato shrank in size.
At 0.0M (distilled water) the graph generally showed an increase in length, the average being 2mm increase.
The graph slopes down at section A.
At 0.5M my line of best-fit shows there is still an increase in length of approx. 0.4 mm. However, my results show me that it decreased in length by an average of 3mm (an anomalous result).
The graph slopes downward at section B.
At 1.0M the graph generally shows a decrease in length, the average being a 1mm decrease.
At section C the line slopes downwards.
At 1.5M the graph showed a decrease in length, the average being a 2mm decrease.
At section D the line slopes downwards.
At 2.0M my line of best fit shows there is a decrease in length of approx. 2.8mm. However, my results show me that it decreased in length by an average of 2mm (an anomalous result).
In theory, at 0.625M the potato would not have changed size.
On my graph I have found two anomalous results at 0.5M and 2.0M. I think this may have happened because of the way the measurements were taken and the fact that each beaker had shared 3 potato pieces instead of one. I shall explain this in my evaluation.
Conclusion
My results and analysis shows me that the more molars of sucrose solution used after 0.625M the smaller the potato pieces because. Any solution that was weaker made the potato increase in length. This proves my prediction, as I had stated this would happen
Lower than 0.625M
The water potential in the sucrose solution was larger
than the water potential inside the potato.
Water went into the potato through osmosis
The potato's cells became turgid.
The potato became enlarged.
As the solution became more strong, the
balance between the two water potentials
became more level. As a result the potato's
cells did not have to vary in size as much
and so the size of the potato piece did not
increase or decrease as much.
Higher than 0.625M
The water potential in the potato was larger than the
water potential in the sucrose solution.
Water went out of the potato through osmosis.
The potato's cells became flaccid.
The potato shrank.
This happened because when more sugar molecules were added, they attached themselves to the water molecules and these water molecules were unable to go through the potato's selectively permeable membrane. It also decreased the water molecule pressure pushing onto the potato. At the same time the water potential inside the potato would have been larger with a higher concentration of pure water molecules, and with a higher pressure of water molecules, would have pushed out of the selectively permeable membrane into the solution, making the cells flaccid, and smaller.
Because of osmosis, the potato was more likely to increase in size when the sucrose solution was weaker and decrease in size when the solution was strong.
Also, one result I found was that at 2.0M the potato pieces initially floated instead of sinking like the other sucrose solutions. Although I thought this was an error, it appeared in other experiments round the class. I think this happened because of the density of the sugar in the solution and the density of the potato.
When there was less sugar in the beaker per cm
than substances in the potato per cm, the potato
was more dense than the solution. This made it sink.
However, as the solution became more concentrated, the sugar in the beaker per cm eventually became a larger amount than the amount per cm in the potato. Thus, the potato floated because it was less dense.
Evaluation
The investigation did not go as planned, but I managed to show some sort of pattern using a line of best fit.
We came across a few difficulties when setting out this experiment.
* Although it is possible to say that using 3 pieces in each jar of sucrose solution would be a fair test, using 3 pieces gave the sucrose solution more surface area to work on. Because there was more surface area less of the sucrose solution would have entered the potato (or less come out of the potato into the solution), as the concentrations would alter.
Therefore, it is less likely that the size of the potato would change a great deal, making it more difficult for us to compare
* Our measuring equipment was not as accurate as we'd hope. We used a millimetre ruler to measure the potato pieces. Although usually this would be reasonably sufficient for measurements, dues to the fact that there were only slight changes in size, a potato piece that looked the same size as another potato piece may have been slightly bigger or smaller, affecting my results.
Factors that were kept the same during this experiment to make the test fair were:
* Strength of solution
* Size and shape of potato
* Volume of solution
* Time in the solution
* Temperature
The factor that we varied was the concentration of sucrose in the solution.
It was not difficult to keep most of these factors the same except for temperature, which we did not measure, as it only required using the same type of materials or making set measurements However, the problems above could have been avoided by:
* Using one jar of solution per piece of potato
* Finding a more accurate measuring equipment to measure the potato pieces.
My results are probably not the best example to use to prove osmosis because of the problems that we faced making them inaccurate and difficult to depict. However, with the problems solved the results may have shown a better idea of osmosis.
I do not completely trust my results. They do not show the pattern that I had predicted, or any scientifically proven pattern associated with osmosis. Repeating the experiment would probably show a different set of results each and every time, which is why I used an average, as I knew it would change. However even this didn't fit the pattern I had expected, and it was only through making a line of best fit that I was able to come to my conclusions as I did.
To further this enquiry I would solve the problems I had above and repeat the experiment to see if the pattern actually occurs (although I strongly predict it will).
I would also experiment with leaving the potatoes in for about a 5 hours in order to find a larger size difference between each sucrose solution.
Another idea that might be worth investigating is to try out 0.625M on a potato piece to see if the potato changes size or not, to coincide with my result from my graph that showed 0.625M to be the amount of molars which wouldn't alter the potato's size. If this doesn't work, perhaps finding out what the actual strength is would be useful.
Jing Ting Lau Osmosis Investigation 10MP
