Investigating to find the concentration of potato cells using osmosis
Biology GCSE Coursework
Investigating to find the concentration of potato cells using osmosis
Introduction
Osmosis is the net movement of water molecules from a low concentration (high concentration of water) to a high concentration, (low concentration of water) through a semi permeable membrane. For example2, potato cells in a sugar solution.
Aim
To find out the concentration of potato cells using osmosis, by putting potato cells in different amounts of sugar solution and seeing by a graph when there is no change as the concentration of the solution is the same as the potato.
Factors that effect osmosis
Temperature and light - The molecules will get more kinetic energy and so more osmosis will take place, and also a higher temperature would make the water evaporate, increasing the concentration of the solution
Amount of solution - The larger the surface area, the more osmosis will happen
Size of potato - More solution might also increase the rate of osmosis by increasing the surface area
Type of potato - Different potato cells might change the rate of osmosis
Things that will change with experiment:
Length of potato
Weight of potato
I have decided to look at the difference in mass to find the concentration of potato cells, by seeing when the mass stays the same.
Preliminary Experiment
Apparatus
* 50ml measuring cylinder in 0.5ml steps - to measure accurately
* One large potato - so it is fair and only one potato is used
* Three medium sized test tubes - to put the solution in
* Rack - to hold three test tubes
* Balance that can measure to 0.01 of a gram - to be accurate
* Medium sized corker borer - to cut pieces out of potato which are all the same size
* Scalpel - to cut the pieces to same length
* Ruler in millimetres to measure potato - to be accurate
* Ceramic tile to place potatoes on - to be neat and safe
* Kitchen paper to clot wet potatoes - to be accurate and have each one as dry as each other
* Tap water - to make the solution
* Sugar solution - to make the solution
* Goggles, lab coat - for safety
Method
- Three potato cylinders were cut using a cork borer out of the same potato
- They were measured in mm and cut with the scalpel so they were the same length
- They were each weighed to the nearest 0.01g
- Three test tubes were put in a rack and filled with 1: 10cm3 sugar solution, 2: 10cm3 water and 3: 0.1cm3 sugar solution and 0.9cm3 tap water
- One potato was put in each of the test tubes and a bung put on top to stop evaporation
- The test tubes were left for 30mins
- The potatoes were blotted with kitchen paper and weighed again to the nearest 0.01g
- Results were taken and are in the table below
Results
The sugar concentration % (m)
Mass of potato before (g)
Mass of potato after (g)
Change in mass (g)
0.0
0.1
.0
.43
.45
.44
.46
.43
.24
+ 0.03
- 0.02
- 0.2
Conclusion
The results show that the more sugar solution there is, the mass gets less. There is a dramatic change between 0.1 and 1.0m. For my investigation I will use the range 0.0 to 0.3 going up in 0.05m steps. I have chosen this range because the preliminary results show that somewhere between 0.0 and 0.1 the potato will be at the same concentration as the sugar solution.
Planning
Prediction
I think that the concentration of the potato will be between 0.0 and 0.1m of sugar concentration. My preliminary experiment showed that 0.0m made the change in mass a positive number, while 0.1 made a negative number.
According to the scientific introduction, when water moves out the potato cell it means the potato had a higher concentration of water inside, and when water moves in, the potato has a lower concentration of water.
Therefore, there is a point where the concentration is the same, and water moves in and out from either side, which is called when they are at equilibrium. Half way between +0.03 and -0.02 is +0.05. I think the concentration of the potato will be at that, so in my method I will use the range going up in 0.05m.
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According to the scientific introduction, when water moves out the potato cell it means the potato had a higher concentration of water inside, and when water moves in, the potato has a lower concentration of water.
Therefore, there is a point where the concentration is the same, and water moves in and out from either side, which is called when they are at equilibrium. Half way between +0.03 and -0.02 is +0.05. I think the concentration of the potato will be at that, so in my method I will use the range going up in 0.05m.
I also think that the graph of the results will be a straight line, as the change in mass and sugar concentration are directly proportional. I have shown this roughly in a graph.
Apparatus
My preliminary experiment went well, so I have used the same sort of apparatus:
* 50ml measuring cylinder in 0.5ml steps - so I can measure the solution accurately
* One large potato - so it is fair and only one potato is used
* 7 plastic cups - to put the solutions in
* Balance that can measure to 0.01 of a gram - to be accurate
* Scalpel - to cut the potato squares
* Ruler in millimetres to measure potato - to be accurate and keep the lengths the same
* Ceramic tile to place potatoes on - to be neat and safe
* Kitchen paper to clot wet potatoes -to be accurate and have each one as dry each other
* Tap water - to make solution
* Sugar solution - to make the solution
* Goggles, lab coat - for safety
* Cling film - to cover cups and prevent evaporation of water
* Coloured pins - to tell the difference between potatoes
Planning the method from scientific knowledge and preliminary experiment
I shall use most of the preliminary experiment's method for my method, since it was fair and gave reliable results. From the introduction, I can see that the rate of osmosis changes when different amounts of sugar solution are present. Therefore I will use different amounts of sugar solution.
Making the solutions
Concentration of sucrose (%)
Sucrose 1% (cm3)
Water (cm3)
0.00
0
30
0.05
.5
28.5
0.10
3
27
0.15
4.5
25.5
0.20
6
24
0.25
7.5
22.5
0.30
9
21
I have chosen this range because the preliminary results show that somewhere between 0.0 and 0.1 the potato will be at the same concentration as the sugar solution. This is explained in my prediction.
Fair Test
The things I will keep the same in this experiment to have a fair test:
* Same potato
* Same total amount of solution
* Same balance that weighs accurately
* Same length of potato - measured with a mm ruler
* Same size plastic cups
* Same amount of time for experiment
* Same amount of light and temperature
* Same amount of blotting
Safety
I will wear goggles and a lab coat when I do the experiment. I'll also be careful, and not put things at the edge of tables for people to knock over and I will tie my hair back and also mop up any spillages quickly with a mop before anyone slips.
Method
Set-up diagram
Key
Plastic cup Conc.
0.00
2 0.05
3 0.10
4 0.15
5 0.20
6 0.25
7 0.30
Reliability
There are two potatoes in each plastic cup, because it will be easier to spot anomalous results and will be more reliable. They will be told apart by two different coloured pins stuck in them.
Method
* Fourteen potato squares were cut using a scalpel out of the same potato
* They were measured in mm and made the same length and height
* They were each weighed to the nearest 0.01g using the same balance to be fair
* Seven plastic cups were and filled with 30ml of different amounts of sugar concentration. The amounts used are in the table below:
Plastic Cup
Concentration of Sucrose %
2
3
4
5
6
7
0.00
0.05
0.10
0.15
0.20
0.25
0.30
* Two potato squares were put in each of the plastic cups and a different coloured pin was put on each one to tell them apart and cling film was put on top to stop evaporation.
* The plastic cups were left for four and a half hours.
* The potatoes were blotted with kitchen paper and weighed again, using the same balance, to the nearest 0.01g. Results were taken and the results are in the table below.
Results
Results Tables
A table to show the results of set 1 of potatoes, which were put in different amounts of sugar solution
Sugar concentration %
Mass of potato before (g)
Mass of potato after (g)
Change in mass (g)
Change in mass in percent (%)
0.00
.81
2.34
+0.53
+29.3
0.05
0.82
0.93
+0.11
+13.4
0.10
0.95
.03
+0.08
+8.4
0.15
.34
.43
+0.09
+6.7
0.20
.32
.41
+0.09
+6.8
0.25
.23
.31
+0.16
+6.5
0.30
.04
.06
+0.02
+1.9
A table to show the results of set 2 of potatoes, which were put in the same test tubes as set 1
Sugar concentration %
Mass of potato before (g)
Mass of potato after (g)
Change in mass (g)
Change in mass in percent (%)
0.00
.84
2.22
+0.38
+20.7
0.05
2.13
2.46
+0.33
+15.5
0.10
2.22
2.45
+0.23
+10.4
0.15
.76
.32
-0.44
-23.2
0.20
.63
.78
+0.33
+9.2
0.25
.49
.62
+0.13
+8.7
0.30
.62
.73
+0.11
+6.8
Average of the two sets of results
Change in mass (g)
Change in mass in percent (%)
+0.45
+25.0
+0.22
+14.5
+0.15
+9.4
-0.17
-8.3
+0.21
+8.0
+0.14
+7.6
+0.06
+4.4
Analysis
What was found, trends or patterns
I have found that the more concentration of sucrose, the less the percentage change is. There is a general trend in the lines, and they slope downwards. The slope is a curve, that starts of with a steep gradient, then it gets shallower, and at the end steeper again. The only result that does not follow this pattern is the second result for 1.5 sugar concentration. The purple line is the line of best fit, as it is the average. The dotted line shows where the line of best fit should go.
Prediction
My prediction was correct in the fact that I said the line would slope downwards, but I was not correct where I thought the potato would be at equilibrium - this is because we used different potatoes and so the equilibrium was different. I predicted the results would show a straight line, but it is fact a curve.
Conclusion
I conclude that my results prove that through osmosis, water moves into the potato when there is not a lot of sucrose in the solution, as the potato gains weight. The more sucrose there is, the less the potato gains weight - showing that the more sucrose, the water will eventually move out the potato and it will lose weight. My results haven't gone negative yet, but if I carry on the graph this is what will probably happen:
The predictive graph explained
This graph shows that when the sugar solution is between 3.0 and 4.0 it will go negative. I predict that set one will be at equilibrium at 3.1%, and set 2 at 3.65%. This means that I predict the potato cells concentration, when it is at equilibrium with the sugar solution, will be at an average of 3.3%.
Detailed scientific knowledge
The results go downwards due to osmosis, which I have explained before. Here is a more detailed explanation of what happens to the potato cells and why they expand or shrink.
Osmosis is when water moves through a selectively permeable membrane, the potato cell membrane. It is said to be hypertonic when the sugar concentration in solution is higher and water leaves cell and hypotonic when the sugar concentration in solution is lower and water enters cell. When the solution is at equilibrium, it is called isotonic, when there is the same concentration on either side of membrane.
The potato cells are plant cells, and when in put a hypotonic sugar solution, water moves into the vacuole of the cell. The vacuoles job is to hold water, and so enlarges. The cell doesn't burst, because the cell wall is tough and elastic.
Eventually the cell cannot take any more water in, and it said to be turgid. At this point the cells are at their largest ? the potato expands and the mass increases. When the cells go in a hypertonic sugar solution, water leaves to vacuole, and the cell shrinks -it loses it's turgor and becomes flaccid. If the solution is strong enough, the cytoplasm eventually pulls away from the cellulose wall. This is called plasmolysis ? the potato shrinks and the mass decreases.
When there is an isotonic solution, osmosis occurs both in and out of the cell, and keeps the vacuole at the same size ? when the mass of the potato stays the same, it is not because there is no osmosis, but the rate of osmosis is the same in and out the cell.
Here are pictures of what happens to the cells:
Evaluation
Comment on method and results
I am pleased with how the experiment went, but I was disappointed when the results I got did not go negative.
The method I used was alright, but I had to change quite a lot from the preliminary method. This was because we had different potatoes, and no cork borers. This made it harder to cut the potatoes and make them the same size. Because of this, my results may have been affected in two ways; one being that since all the potatoes had different surface areas, the results could change as a larger surface area increases the rate of osmosis and secondly, because of the difference in potatoes with my preliminary experiment the results could be different as the potato could have started with a different concentration. We also used plastic cups instead of test tubes and this increased surface area, also possibly changing my results.
My results are ok, as there is a clear pattern going downward. Although the pattern is clear, it is not a smooth curve, and some results go down rapidly whilst others more slowly. A few things may have caused this. Firstly, it was a very hot day on the day of the experiment. This meant that the water could have evaporated, and left the solutions a lot more concentrated. The cling film then acted as a mini greenhouse, and kept the heat inside, so the cells may have had more kinetic energy. Another reason was that the potatoes were all different shapes and sizes as it was hard to cut then accurately. This, as explained before, caused the ones with the larger surface area to have an increase in osmosis. This is proven by: one of the largest weight before, 1.81g increased by the largest amount, +0.53g. Other things that may have affected my results where that fact some of the potato squares were on their side, and some were on their front. Also, the position inside the cup might have made a difference, i.e. they have a larger surface area in the middle of the cup than at the edge.
I didn't have a suitable range, but this was because my preliminary test showed the equilibrium would be at roughly 0.6M, and the different potatoes had a different equilibrium. This is also probably due to the fact that they were totally different types of potatoes, as the preliminary ones where very soft and easy to cut, whilst the actual test ones where very hard.
Accuracy + anomalous results
I have one anomalous result. The -0.03 in the second set of results is anomalous. This was probably caused by not reading the balance correctly. My results all follow a pattern that goes downwards in a curve. I think my results are accurate - but temperature, surface area and different tools all were different to the preliminary experiment, which could have caused the anomalous result too. Measuring the correct amount of sugar solution was also quite hard, and I may have made a few errors while making them.
Reliability
I think my results were quite reliable as I did the test twice and both curves were similar, proving they are reliable. The results for my preliminary and actual method -were very different and did not both follow the same pattern. The results are quite strong to support my conclusion, albeit they didn't go negative, as they show a downward curve. They would have gone negative with a few more sugar concentrations.
Changes to improve the reliability
To improve, I would use the same sort of potatoes as used in my preliminary experiment. The potatoes were easier to cut, and a cork borer could be used. This would make all my potatoes the same size. I would also put a thermometer in each plastic cup/test tube to check the temperature stays constant. I would also place each potato in a separate plastic cup/test tube, and not put two together. I would make sure that they were placed roughly in the same place inside the cup to increase fairness. I might even repeat the test again to get three sets of results. I would also increase the range of concentrations by two or three concentrations e.g. 3.5, 4.0 and 4.5 so the results would go negative.
Further work
For further work I could investigate what would happen if I carried out my experiment further, and do a few more concentrations, e.g. 0.35, 0.40 and 0.45. I could see if my prediction on where the potato is at equilibrium was correct. I could also investigate whether surface area does affect osmosis, and use different sizes of potato at the same concentration. I could also see if temperature increases the rate of osmosis.
Sarah Lowman 10W