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By Kimberley Snaith
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Contains results table but got scrambled, may need redoing or decoding!
Osmosis is the movement of water molecules from a high water concentration to a low water concentration, passing through a partially-permeable membrane. A partially-permeable membrane (or semi-permeable) has very tiny holes which allows only certain substances to pass through. This means that water molecules can pass through at a steady flow. The glucose-rich region will collect a high amount of water, acting as if to even up the concentration. Osmosis makes plant cells swell up and become turgid.
I am going to investigate the rate of expansion of a piece of potato using different concentrations of sugar solution. I am going to use six different solutions - thus being my input variable. Each solution will have three pieces of potato soaking in it, so I will have a more accurate result at the end. The six concentrations I will be using are
Purely water
0.2 molar of sugar solution
0.4 molar of sugar solution
0.6 molar of sugar solution
0.8 molar of sugar solution
1 molar of sugar solution
To ensure that my investigation will be a fair test, there are some aspects I need to take into consideration. These are called ‘variables’ and are factors that are within my control, which could affect the results of my experiment. I need to make sure all the beakers are the same size, and that equal amounts of solution are in each beaker. If there is more in one beaker than the other, that could mean more water molecules will pass into (or out of) the potato, therefore the results would not be accurate. The potato pieces should all start off the same size. If one piece was 1cm and another was about 5cm, that would mean that there would be room for more water/sugar molecules, and could affect the rate that it would grow or shrink. The potato pieces also need to be soaking in the solution for the same amount of time. If one was soaking longer than the others, it would mean that there was more time for osmosis to happen; therefore the size may have changed more than it would have in the same timescale as the other pieces of potato.
I also need to keep in mind that there are elements which may affect my experiment that are out of my control. As I am leaving the investigation overnight, it may get touched by other people, and possibly become disorganised. To minimise the chances of this happening, the beakers will be placed in a hard-to-reach place.
To conduct my investigation successfully, I will need the use of certain apparatus throughout. The items I will need are as follows
Six beakers, all of the same size
Potato
Scales
The solutions, (water, 0.2 molar of sugar solution, 0.4, 0.6, 0.8 and 1)
Core-borer
I need to construct a method to follow when undertaking my investigation. This will make sure I act in the same way for each solution, and will make it easier to know what I am doing. Below is an overview of the steps I will be taking
1) Cut the required number of potatoes using a core-borer. I need eighteen (three for each solution � this will give me a more accurate final result.) Each piece needs to be cut as close as possible to the same size.
2) Pour the solutions into beakers. Make sure they all have the same amount in them - 100ml.
3) Weigh each piece of potato and record the mass.
4) Label each potato piece in a way so each is distinguishable from the others. I am going to use pins; for each piece of potato in the same solution, I am going to put pins in at different angles. Make sure that the position of the pins has been recorded with the corresponding weight of the potato, so when I weigh them again, I know which one is which.
5) Take each piece of potato and put it into the solution. Make sure all go in at the same time, and leave them overnight. Put in a hard to reach place, to minimise meddling.
6) After twenty four hours, take the potato pieces out of the solution. Dry them off and remove the pins before weighing them. Record the weight next to the prior data entry.
I predict that my results of my investigation will show that in the higher solution, the potato will shrink, and in the water and lower solutions the potato will grow. The scientific definition for osmosis is the flow of water from a high water concentration to a low water concentration. In the pure water solution and low-sugar solutions, the potato will have a lower water concentration. This means that water molecules from the solution will pass through the partially permeable membrane into the potato, expanding it and making it turgid. In the high-sugar solutions, the potato will have a higher water concentration; therefore water will leave the potato into the solution, this time making it shrink.
I have carried out my investigation according to my method. I have recorded my results in the table below.
Start Weight (g) Starting Weight Average End weight (g) Ending Weight Average Weight Diff. (g) Average Weight Diff. Percentage Weight Diff.
Conc. 1 1 1
HO 1. 0. 0.7 0. 1.4 1.1 1 1.17 +0. +0. +0. +0. 5%
0.2 m 0.8 0. 1 0. 0.8 1.1 1. 1.07 0 +0.2 +0.2 +0.17 18.8%
0.4 m 0.7 0.8 1.1 0.87 0.8 0.6 1 0.8 +0.1 -0.2 -0.1 -0.067 -7.7%
0.6 m 0.8 1.1 1 0.7 0.8 0.8 0.8 0.8 0 -0.2 -0.2 -0.167 -17.16%
0.8 m 1 0. 0.8 0. 0.7 0.5 0.5 0.57 -0. -0.4 -0.2 -0.2 -6.67%
1.0 m 1.1 1. 1. 1. 0.7 0.7 0.6 0.67 -0.4 -0.5 -0.7 -0.5 -44.167%
I have checked through my results to see if I have any anomalous results. An anomalous result is one that appears to be out of place. For example, in the end weights, if weight 1 was 1.4, weight was 1. and weight was .6, weight would certainly be an anomalous result as it is extremely higher than the others. In my results I do not appear to have any that are too far out of place. This means that I will not have to retake any part of the investigation.
According to my prediction, my results seem correct. They match to what I thought they would be potato pieces in the low sugar solutions grew after twenty-four hours, and potato pieces in the high sugar solutions shrank.
I have plotted a graph to show which will show the difference between the average starting weight and the average end weight of the potato pieces in each of the solutions.
You can clearly see that the potato grew over the 4 hour period in the water solution, and the 0.2 molar solution. After the two lines cross (labelled point A) is where the weight of the potato shrunk in the 4 hour period. You can see how the difference in size increases as the lines on the graph get further apart.
The second graph I plotted shows the percentage difference between the starting mass and the end mass.
The line which goes through the centre of the graph is the line of best fit. This shows the trend of which most of the results follow. Most of the results follow the line of best fit closely, but the result circled is an anomalous result. It is further away from the line than the others, therefore does not follow the trend.
At point A in the graph, there is a high percentage increase from the starting weight of the potato and the end weight of the potato. That means that a high amount of water was passed from the solution into the potato.
At point B in the graph, there is a very high percentage decrease from the starting weight and the end weight. That means that a high amount of water was passed out of the potato into the solution.
This matches up with my prediction. I predicted that there would be a larger increase in the size of the potato the lower the concentration would be and the graph clearly shows this. The line of best fit on the graph decreases at a steep, constant rate which proves that as the sugar solution becomes a higher concentration, the flow of water molecules changes direction from entering the potato to leaving the potato, thus meaning that the lower the concentration, the larger the potato piece will be.
The procedure of my investigation was successful and accurate. The data I obtained answered the task fully, providing me with the evidence I required. Because I measured the growth of three difference pieces of potato in each solution, this has provided me with a very precise result, making my graphs and conclusions more accurate.
Because I took note of variables, my investigation was fair and there was nothing that I did different for one beaker from another. This has also contributed to getting accurate results.
If I was to improve my investigation, I would use more solutions. If I did this, when it came to plotting a graph, I would have a more accurate line of best fit, therefore making it easier to see if any results were out of place, or anomalous. I would also measure the growth at different intervals, thus giving me extra data of the speed of the growth. For example, I could have left the potato pieces soaking in the solution for 48 hours, and record their weights after 4 hours as well as 48 hours. Or if I was still to do it in twenty-hour hours, I could measure them after 1 hours. This would give evidence on when the majority of osmosis happened.
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