Preliminary Experiment
Method:
The preliminary experiment is to accurately understand and discover the best quantities, time etc. to use to give the best results in the main experiment. In this experiment we will use two cored pieces of potato of the same length, mass and diameter in two different concentrations. One of the pieces will be placed in a beaker of pure, 100% sugar solution and the other piece will be placed in a the same volume of pure distilled water. This complete contrast of solutions is to give the widest range of possible results in the smallest amount of data, so that it is possible to use this for the main experiment. The maximum time that the potato pieces will each be in the solution is for twenty-four hours as this again gives the broadest results that can be gained from this experiment. After this time the mass, length and diameter will be taken again and the results will be compared and explained.
Equipment:
- Same potato used
- Borer of same diameter
- Sucrose solution- same sucrose in solution
- Scalpel to cut cleanly and accurately
- Calliper gauge for accurate measurement for the diameter and length
- Beaker for solutions and potatoes
- Measuring cylinder to keep the amount of solutions the same
- Timer, set for twenty-four hours
- Scales to accurately measure the mass of the pieces of potato
The same borer will be used to cut the potatoes into specific diameters, as they are an accurate piece of equipment compared to cutting the width by hand or even by means of a ruler.
A calliper gauge is a very accurate piece of equipment that can measure to the millimetre the length and diameter of any object. This piece of equipment is to be used as it can hold the object carefully for measuring and so is more accurate than a normal ruler.
Scientific scales will be used for measuring the mass of the potatoes as it can be programmed accurately to measure into decimals the mass of the object. It can be measured to the nearest 0.01g, which is as accurate as possible with the resources available.
The reason for choosing to measure all three is the fact that it is then possible to know what changes the most in different solutions so I can have a wider amount of results to use in my conclusion.
Results:
In general this preliminary shows that in a higher concentration, plant cells use plasmolysis to transport water into the surroundings so decreasing in mass, diameter and length. Whereas if the plant cells are in a lower concentration, they use osmosis to transport water into the cells so increasing in mass, length and should in diameter.
With these results in mind, the main experiment will include a wider range of concentrations and number in each concentration to produce an average. This can then be used for a more accurate result and conclusion about the matter.
Fair Test:
To make sure that the results will be as accurate as possible, the pieces of potato must be the same length, diameter and as close as possible in mass. This can be done by using the same equipment, including the same potato and borer. When diluting the solutions, make sure the pure sugar solution is the same, i.e. sucrose, as the molecules could be larger in other solutions. When diluting measure the amount accurately by looking at the solution at eye level so that the concave of the liquid is what the amount is measured at. This is known as the ‘meniscus’.
As stated in the plan, there are many factors that can affect the rate of osmosis, which must be kept constant so that the experiment is a fair and accurate test. Temperature can be kept constant, even during the twenty-four hours by the solutions being placed in a beaker of water. This is to keep all at the same temperature as the water throughout the time. For this to work the solutions and potato pieces must be put in separate test tubes to then be put in the beakers. The concentration and surface area are remaining constant because of the accurate ways of measuring that stated in the above paragraph. Different plant cells will not be a problem as long as the same potato is used or one of the same types, to make it as accurate as possible with the resources offered. The PH can be kept constant easily by using the same neutral solutions throughout. Finally time of the experiment will for as long as possible, bearing in mind on the schedule of lessons. The time of the experiment needs to be as close to twenty-four hours as possible for an outlook of osmosis in a day. Whatever the time, all potato pieces must be taken in and out of the solutions at exactly the same time to make the experiment fair and precise.
Main Experiment
Prediction:
I predict, with my scientific knowledge, use of the scientific model, and the preliminary experiment as a guide, that as the surrounding solution increases in concentration of sucrose, a decrease will be found in mass, length and diameter. This is known as plasmolysis and so as an opposite effect of if the solution in the surroundings is less concentrated than in the plant cells, then the mass, length and diameter will increase. This is known as osmosis. The change in whether the surrounding solution is less or more is known as the osmotic potential, which will be studied in the main experiment. I also can predict that the shape and correlation of the graph for mass, length and diameter will all increase as the concentration decreases. This will occur, as osmosis will be shown rather than plasmolysis because the point of the experiment is decreasing the concentration not increasing. However if the results were upturned then plasmolysis would be the factor and the graph would decrease in correlation. So the graph can be seen accurately from either angle, it can be inverted.
Method:
In this experiment, five different concentrations will be used:
- 1M (100% sucrose solution)
- 0.75M (three parts sucrose, one part water)
- 0.5M (two parts sucrose, two parts water)
- 0.25M (one part sucrose, three parts water)
- 0M (100% distilled water)
For each concentration there will be four potato pieces, to give an accurate result with mean of the four pieces for each solution. All the potato pieces will begin at the same length, diameter and near to the same mass.
The reason that we will be using five different concentrations and repeating the experiment with four similar chips, is because accuracy and reliability is needed to produce reasonable evidence to back up my knowledge and prediction on osmosis and information that surrounds this topic.
The same volume of liquid will be poured into each test tube, which will have one chip each in there. These test tubes will be separated into groups of their concentrations and put in beakers full of water to keep the temperature constant. This will then be left for a specified time and the difference in mass, length and diameter will be displayed at the end in a table and graphically.
Results: (after twenty hours and two minutes)
In broad terms this table shows what was expected as a whole. In the strongest solution it was much lower in mass, diameter and length than in the weakest solution. The maximum difference was 0.82mm between the average lengths. This shows that the stronger solutions have lost water as their mass etc has decreased. For example in the 1M solution the mass decreased by 0.80g, whereas in the 0M solution it increased by 0.08g, equalling a change of 0.88g.
So, when looking simply at the results table it is possible to view that as the measurements decrease in the stronger solutions, plasmolysis is occurring, whereas when the measurements increase, osmosis is occurring.
However there are anomalous readings within the table that are circled in both the data and in the following graphs. These will be explained fully after the graphical representation of the average results for the different measurements for each concentration.
When observing the data graphically as an overall correlation the graphs successfully show that the lower the concentration of the solute the greater the mass of the potato chips. This conclusion is simply found through the line of best fit on each of the three graphs. As all lines are showing that there is a positive correlation between lower concentration and increase in mass.
Observing the results table more closely and with some calculations it is possible to find the average loss or gain for each concentration to evaluate the overall effect of different concentrations on a plant tissue.
Six factors need to be used at the beginning of the equation for each concentration- original mass, diameter, length and average mass, diameter and length.
As an example the concentration of one molar will be used. Its average mass must be divided into the original mass and then multiplied by one hundred to give a percentage. This is 56% but as the average mass is less than the original it is –56%, as it has lost water so has decreased in mass. This calculation is then taken from one hundred to calculate how much the mass etc has decreased from the original (classed as one hundred in percentage terms). This equation is used for all the other concentrations, with diameter and length calculated also. The following equation shows the overall calculation:
F= Factor that changes (mass, diameter or length)
A= Average
O= Original (A/F) * 100= (?) - 100
? = Answer from first calculation
Each concentration will have three final percentages that will be added together and divided by three. This is so to calculate an overall average percentage for each concentration to observe whether plasmolysis or osmosis occurred. Below is a table that shows the average percentages for each concentration.
This shows that at 1M the chip lost 40% of its structure through water movement, showing plasmolysis occurred. Whereas at 0M the chip gained 5% of structure through water movement, therefore osmosis occurred. The gain or loss of mass, length and diameter determines whether osmosis or plasmolysis occurred as only if the surroundings (concentration) is higher than the chip will water flow out, making the percentage negative. Whereas if the concentration is stronger in the potato chip rather than the surroundings then the water flows inwards, increasing the mass and other structural factors. When osmosis does occur the plant cells should increase in size and become increasingly turgid whereas if plasmolysis occurs the cells will wilt and decrease in structure. This was observed between the chips before and after the experiment, as most did become more flexible and decrease in size. However the pure water experiment showed osmosis as the chips were firmer and slightly larger, but no huge change was observed as the cells only transported 5%.
The average percentage change will be compared graphically with difference in concentration.
The graph gives representation of osmosis and plasmolysis and so backs up what has already been explained. The modal cell showing the flow of water from a low to a high concentration is fully proved by the graphs, as the concentration is the variable of whether osmosis or plasmolysis occurs. The osmotic potential of where neither is increased or decreased can be calculated using the graph. This is a useful piece of information as it calculates exactly when the cell deviates and the concentration is higher or lower enough for either osmosis or plasmolysis to occur. Because of the anomalous readings this osmotic potential is not fully accurate but the basis is helpful for explaining whether osmosis or plasmolysis is occurring. This calculation is found through the use of the line of best fit. The original factor (mass, diameter or length) is read and horizontally joined to the line of best fit, then whatever is vertically read off the concentration line is the osmotic potential. From the graph for mass and the one for diameter, the osmotic potential occurs just before the lowest molar of solute, which could be trailed in a later experiment. However anomalous readings in the length graph made the osmotic potential lower than pure water, which is impossible, so this one is not a reliable source. On the percentage graph the line of best fit crosses the concentration axis and so shows when the osmotic potential occurs, which is at 0.1M.
Through all of these findings of osmotic potential, correlation of graphs and the raw data I can prove that the prediction I made was correct. This is because every point I predicted was proved, as the correlation increased as the concentration decreased. However there were findings I did expect, for instance the calculation of osmotic potential and the fact that most of the data showed plasmolysis occurring rather than osmosis. This may be down to truth on how much concentration is in the plant cell compared to the surroundings or it is probable that many of the readings were inaccurate and so anomalous.
Evaluation:
After observing the results obtained, there are many faults in the data to be accounted for. Reasons for these are the fact that the time for the preparation of the experiment was limited so rushing of the measuring occurred. This meant some chips began smaller or larger than others so the end results would vary, as seen. Different potatoes were used as the amount of chips needed was too many for one potato, but this would not have been a main factor in the anomalous readings. Because the lengths varied this also changed the mass, however the diameter should have started the same. Even though we had proper measuring devices for length and diameter, it was difficult to determine the size when the potato was flaccid and bent inwards when pressure was upon it. The measurement of mass may have been inaccurate because of the fact that the potato chips were wet when taken out of the solute and some may have been surrounded by more solution than others. The chips were not dried as then they may have been dried for a different amount of time, so being inaccurate.
These factors such as of lack of time in preparation are the reason for many of the anomalous readings conveyed in the graphs. With all graphs having two anomalous readings each, with all concentration having at least one it is possible to determine that the results were not very consistent or reliable. On the graph there has been labelled all the primary repeats that were recorded before averages were made. This is to explain why some of the data was anomalous as one or two of the primary results must vary enough to change the position on the graph. On the mass graph the 1M solution had an anomalous reading because of the data that read 1.16g, which heightened the placing of the average on the graph. In the 0.5M solution was greatly decreased in its placing by the reading of 1.26g. In the diameter graph the 0.75M solution was anomalous because of the fact that it was exactly the same as the reading in the stronger solution. This cannot be correct, as the correlation of the graph must change hopefully by increasing to show osmosis. The other anomalous reading is in the 0M solution where two are far away from the average point. Finally the length graph is consistently anomalous, with every reading having one or more repeats distant from the average so reasoning why the osmotic potential states to be past 0M.
When considering the collected data and the method used, improvements could have been made if the preparation and accuracy of the length of the chips had been undergone. This would have lowered the amount of anomalous readings and so a better conclusion could have been drawn on the true correlation and osmotic potential of potato chips in different concentrations. Then the osmotic potential on the graph could have been taken further to find the concentration at which osmosis and plasmolysis occurs. If preparation time had been increased this would have greatened the reliability of results and this could have been increased further if more concentrations were tried with only three chips in each instead of four. This is stated because the amount of chips was too many and so averaged the data in an unreliable way. Finally an extension to this experiment to increase the reliability and accuracy would be to use different plant cells (rhubarb or liver) to have a range of cells to experiment on. This would be useful in discovering how other plants or animals transport water and whether or not they use osmosis in the same consistent way. With this data a conclusion for similarities and differences between different cells could be made and how concentration affects the rate of those cells with osmosis.