Biology Investigation
Osmosis in Potatoes
OBSERVATION-
I have observed that when a potato cylinder is left in water for a period of time the cylinder becomes larger and firmer. When an identical cylinder is left in sugar solution the potato becomes softer and smaller.
HYPOTHESIS-
I think that the potato cylinder is becoming turgid because it is absorbing water by osmosis; through it's semi-permeable cell membrane. I think that the second cylinder is losing water by the same method; the water inside the cylinder is being absorbed through the semi-permeable cell membrane by the sugar solution, which results in the potato being left soft and slightly withered.
KEY FACTORS-
Temperature-
If the potato cylinder becomes too cold then it may freeze and then water would not be able to enter through the membranes. However if it gets too warm then the potato may shrivel in the solutions, if the temperature gets very hot then the potato may actually cook, this would kill the cells inside the potato and prevent osmosis occurring through the damaged cell membranes
Different Potato-
The same type of potato must be used, one type may contain more water to begin with than another and so would weigh more in the beginning, mixing the results of two potato's may distort my results. Some types of potato may have less permeable membranes; this may mean that less water would diffuse through the membrane. Also if two ages of potato are used then an older potato would probably be sprouting shoots, these shoots require much sugar to grow, the potato would be transferring a large amount of the starch it contains into sugar in order that these shoots may grow properly, these means that the potato will contain less starch, this lack of starch may affect the membranes which would distort my results.
INDEPENDENT VARIABLE-
The independent variable is the strength of the sugar solution I will be placing each cylinder of potato into, the strength of the sugar solution will affect how soft or turgid the potato goes.
DEPENDANT VARIABLE-
The length, width and weight of the potato cylinders all depends upon the strength of the solution, the more sugar there is in the solution then the more water will probably be absorbed out of the potato, this will leave the cylinder smaller and possibly lighter than the potato that has been placed in 0% sugar solution.
CONTROLLED VARIABLES-
Time-
The time between measuring the weight and length must be the same otherwise the potato cylinder may have absorbed more or less solution or lost more or less of the water already inside the potato. If one cylinder is left longer than the others then the results will not be correct and the osmosis that has occurred may be more extreme than the others, the same would be reversed if the cylinder were left for less time than the others.
Temperature-
The temperature is essential, as I have explained in the Key Factors section, if the temperatures are too extreme then the potato may either cook or freeze.
Type of Potato-
The type of potato must be the same. (See key factors section)
Amount of solution-
This is also important, as if one cylinder is left in very little solution then the solution will only be able to diffuse through the base of the cylinder, also if one is left in more solution than another then it will have more chance to ...
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Temperature-
The temperature is essential, as I have explained in the Key Factors section, if the temperatures are too extreme then the potato may either cook or freeze.
Type of Potato-
The type of potato must be the same. (See key factors section)
Amount of solution-
This is also important, as if one cylinder is left in very little solution then the solution will only be able to diffuse through the base of the cylinder, also if one is left in more solution than another then it will have more chance to absorb a larger amount of the solution.
Size of potato-
The size of the potato cylinder is also important; if the cylinders are all different sizes then they may be able to absorb more or less solution than a larger or smaller cylinder would be able to.
PREDICTION-
I predict that the stronger the sugar solution is then the softer and smaller the potato will be. The weaker the solution is then the larger and more turgid the cylinder will be.
SCIENTIFIC KNOWLEDGE-
The more sugar there is in the solution then the more water will be absorbed out of the potato cells by osmosis- the water will diffuse out of the cell, through the semi-permeable cell membrane and out into the solution. The cells inside the potato will shrink, causing the cylinder to become smaller and more flaccid as water is lost from the contents of the cell, if the cells were inside a plant then the plant would wilt, because the cells are no longer turgid enough to hold the plant upright.
The less sugar there is in a solution then the larger more turgid the cylinder will be, the water from the weaker solution will diffuse into the lower concentration area, inside the cell, this will cause the cells to expand and in turn the potato cylinder will become more turgid and grow in
size and weight due to the excess water inside it. Unlike animal cells however, plant cells will not burst under the pressure, the cell wall holds the contents firmly in place and prevents explosion of cells.
Preliminary Work-
Prepare potatoes- we cut them into equally sized cylinders and measured them (length and width) we then weighed them so that we would have a result before we put them into any solution. Then the solution was measured out using a syringe as we found that this gave the most accurate measurement. To make the 0.75, 0.50 and 0.25 Mole solutions we added (for the 0.50M solution) 5cm of water and 5cm of 1M-sugar solution. The same process was repeated with different measures of each solution to make up the other solutions. We also decided on the time that we would leave the potato cylinders in the different solutions before measuring, 10 minutes was decided on as there would have been time for a noticeable change in the size and weight of the potatoes.
The first time we did the experiment we decided that it would be easier to use the syringe as during our tests on measuring equipment it proved the most accurate. In order to cut the potatoes to the same size we used a cork borer which ensured that we got the same size cylinder each time; the borer was a size 6.
Diagram showing prepared cylinders. 0M solution=Pure water.
Measuring the Independent Variable-
To measure the increase or decrease in the size of the potato cylinders I am going to measure the length, width and weight of the cylinder at 10 minute intervals (this time slot gives the potato cylinders enough time to change in size, any less would not allow them enough time to change noticeably) I will measure the cylinders in the following ways in order to get the most accurate results possible.
Number and Range of instruments-
To measure the size of the potato cylinder we are using a cork borer, this measures exactly the size of the potato cylinder and is much more accurate than attempting it free hand with a scalpel.
To measure out the amount of solution (10cm) we will use a 10cm syringe, this is more accurate than using a larger measure as it is harder to over measure. A syringe was proved to be more accurate than a measuring cylinder in the preliminary work that we did.
To measure the width and length of the potato cylinders we are using a 15cm ruler.
To measure the weight of the cylinders we are using an electronic top-pan balance, this is the most accurate balance that we have access to.
Keeping the Controlled Variables constant-
The controlled variables are- time the potato is left in solution, temperature of the test, type of potato, amount of solution and size of potato.
To control the time the potato is left in the solution we are simply starting a stop clock as soon as all the potatoes are in the solutions, we then time 10 minutes and measure the potatoes, then the process is repeated again as soon as we put the cylinders back into the solution.
It is very difficult to accurately control the temperature in lab. Conditions but we will try to keep the potatoes at room temperature (approximately 20c) We will not heat or cool the tubes deliberately.
The type of potato will always be the same, so it should contain the same amount of water to start with.
The size of the potato is controlled by the borer, we will always use the same type (size 6) this should accurately control the size of the cylinder.
Safety-
Care should be taken when working with flat-bottomed tubes, as they may break easily.
Results-
In order to get a clearer representation of the end results the table below shows the original weights, lengths and widths of the potato cylinders and then shows the measurements at ten 10-minute intervals.
Sugar Solution
Weight-start
0mins
20mins
30mins
5 days
M
4.27
4.14
3.91
3.72
2.1
0.75M
4.3
4.29
4.24
3.79
2.6
0.50M
4.46
4.48
4.27
4.16
3.24
0.25M
4.31
4
3.96
4.24
4.26
0M
4.32
4.45
4.43
4.49
4.83
Sugar Solution
Length-start
0mins
20mins
30mins
5 days
M
4
4.4
4.4
4.5
3.7
0.75M
4
4.5
4.5
4.4
3.8
0.50M
4
4.5
4.6
4.7
4.5
0.25M
4
4.5
4.5
4.4
4.5
0M
4
4.5
4.6
4.5
4.6
Sugar Solution
Width-start
0mins
20mins
30mins
5 days
M
0.9
0.7
0.75M
0.9
0.50M
.1
0.25M
.1
0M
.1
.1
0.8
There appear to be no anomalous results amongst those that I have collected and so I will not have to remove any data.
The results show that in the stronger sugar solutions (1M and 0.75M) the potato cylinders had a tendency to shrink in weight, length and width. In the weaker solutions however the potato cylinders expanded and became denser, this is shown by an increase in weight, length and width.
The first graph shows a trend towards negative correlation in the weight of the potatoes in the 1M, and 0.75M solutions, this means that as the time goes on the cylinders became lighter. The potatoes in the 0M and 0.25M solutions, the graph goes up, this means that as the time goes on then the weight increases. The weight of the 0.50M solution is very uneven, but seems mainly to fall; this may be because I have some anomalous results.
The second graph shows the length of the individual cylinders after different lengths of time in the solutions, again the potatoes in the 1M and the 0.75M solutions seem to fall in length whereas the length of the potatoes in the 0M and 0.25M solutions seem to be rising, the potato in the 0.5M solution stays roughly the same length.
The graph of the widths show that the widths of all the potato cylinders changed very little during the 30-minute period that they were left. After 20 minutes in their sugar solutions, the widths began to change very gradually. The potatoes in the 0M and 1M solutions began to increase in width and the potatoes in the 0.25M and 0.50M began to increase, the 0.75M potato stayed virtually the same throughout. This shows that the changing width of the potatoes is not very reliable, as the potato in the 1M solution has become thinner which is not what I predicted or expected. The potato in the 0M sugar solution has fallen in weight which is not what I predicted either.
SCIENTIFIC KNOWLEDGE SUPPORTING CONCLUSION:
The results that I have gathered show that my earlier predictions were correct, the potatoes seem to be gaining and losing weight by osmosis. The potatoes in the strong sugar solutions (1M and 0.75M) have lost weight because the sugar solution surrounding them is absorbing liquid from inside the potato cylinder by way of the semi-permeable membranes surrounding each cell. The potatoes in the weak sugar solutions have gained weight because they are absorbing the solution into themselves, again through their semi-permeable membranes. (For additional information see earlier section on scientific knowledge)
EVALUATION:
I think that the experiments went quite well although the results seem to be a little odd. There were two of us working on the experiments, which made it difficult for us to tell what the other was doing. Also the potatoes were not put back into their solutions exactly on time as we had to measure each one and make sure that it went back into it's respective tube. This made it a bit hard to get the timing exactly right and also meant that every so often the potatoes were dried out so that they could be weighed and measured.
If did the experiment again I would try and increase the amount of time that the potatoes were in the solutions for and also try to speed up the weighing and measuring process as this was very slow and awkward which made it hard to tell whether we were getting true results or not.
Also at the beginning of the experiment I could have measured the potato cylinders more accurately so that they began at roughly or even exactly the same weight, width and length, as the difference between them made it harder to compare the graphs of the different molar solutions.
In order to gather more information to get better averages and lines of best fit on my graphs I could have spent more time gathering larger amounts of data by repeating my experiment more times which would give me more results in order that I could draw a more accurate line of best fit and also by trying the experiment with different strength sugar solutions in addition to the ones I have already used. This would mean that my results would be more accurate as I would have more results to compare and this may bring to light more anomalous results that I have not noticed this time.
On my graphs and through my results collection I have not noticed any anomalous results so I have not had to deal with any exclusion of results. Any anomalous results I might have had could have been caused by a mix-up between the different solutions or by inaccurate weighing or measuring.
Sarah Webb Year 10
