- Skin- If the potato has a skin it may cause more of barrier for the water molecules to go through thus decreasing the rate of osmosis.
- Age- if the potato is old some of the cells may be damaged or be starting to decay.
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Time Left- If a piece of potato is left in the solution for a longer period of time more water molecules may have the chance to move across the membrane.
- Cover- if there is a cover over the pot containing the potato and solution it could stop evaporation of water form the solution which could change the concentration of the solution which would affect the rate of osmosis.
- S.A- If the cell has a larger surface area more cells will be exposed to the solution.
Preliminary Experiment:
Aim: I will use my preliminary experiment to test any flaws in my method and to ensure I have an appropriate range of sugar concentrations.
Method:
- I will have three galli pots.
- I will cut three pieces of potato using the number 5 potato corer.
- I will trim these potato pieces until they each have a mass of 2g, weighing them using a balance.
- I will place each potato piece in a galli pot. Each galli pot will have 30ml of solution in it of varying sugar concentrations. The sugar solution I will be using is 340g/l sugar solution, which is equal to 1 molar, which I will be mixing with distilled water to create different concentrations.
- I will leave the solutions for 24 hours
- I will then weigh the potato pieces using the balance.
- I will calculate the difference in potato mass and will record my results.
Diagram
Results:
What I have learnt from my preliminary:
- In order to fit three pieces of potato in each pot I will have to use smaller pieces of potato. I will use 1gram pieces instead of 2gram pieces.
- I will cover my pots with Clingfilm to stop any evaporation
- 24 hours is a suitable time to allow for recordable changes in potato mass.
- 30ml of solution is a good amount, it covers the potato yet fits in the galli pot.
- Size 5 potato cutter produces a piece of potato with a good surface area.
Full Investigation:
Method:
- I will have five galli pots.
- I will cut fifteen pieces of potato using the number 5 potato corer.
- I will trim these potato pieces until they each have a mass of 1g, weighing them using a balance.
- I will place three pieces of potato in each galli pot. Each galli pot will have 30ml of solution in it of varying sugar concentrations. The sugar solution I will be using is 340g/l sugar solution, which is equal to 1 molar, which I will be mixing with distilled water to create different concentrations.
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I will cover each pot with cling film.
- I Will leave the solutions for 24 hours
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I will weigh the potato pieces using the balance.
- I will calculate the difference in potato mass, recording my results.
Diagram
Prediction: I predict that pot one, with the highest concentration of sugar solution, will have the lowest mass after 24 hours. I think that pot five, with the lowest sugar concentration, will have the greatest mass after 24 hours.
I think this is because in pot five there is no sugar outside of the potato cells. Inside the cells there is a weak sugar solution inside the central vacuole know as sugary sap, therefore the sugar concentration inside the cell is greater than outside the cell. This means that outside the cell there is a higher water potential and therefore the majority of the water will move into the cell causing the potatoes mass to increase. The water inside the cell is slightly attracted to the sugar molecules and has a lower water potential so the majority of molecules stay inside the cell.
Pot one has a higher concentration of sugar solution outside the cell and so the net movement of water molecules will go from inside the cell to outside the cell because they are ‘free’ to move around. The water outside the cell is slightly attracted to the sugar molecules and have less water potential therefore they do not move into the cell.
Predictive Graph:
Fair Test:
- I will put three pieces of potato in each galli pot and take an average of my results to ensure each result is accurate.
- I will keep the same conditions, i.e. light intensity, temperature.
- I will use the same balance to weigh my potato before and after osmosis because there maybe a variation in the accuracy of the balance.
- I will use distilled water to ensure no other chemicals can effect the rate of osmosis.
- I will measure the mass of the potato and the concentrations of the solutions accurately
- I will leave each pot for 24 hours.
- I will take my potato pieces from the same potato
- I will not have any skin on my potato pieces.
- I will ensure the surface area of each piece is the same by using the same diameter potato corer.
Results Table.
Conclusion:
My graph to show results was very similar to my predictive graph suggesting my prediction was correct.
The pot with the highest concentration of sugar solution did have the lowest mass after 24 hours and the pot with the lowest sugar concentration did have the greatest mass after 24 hours.
My table of results showed that the potato in pots 4 and 5 on average increased in mass. This is because there was little or no sugar outside of the potato cells therefore the weak concentration of sugar solution inside the central vacuole was greater than the concentration outside the cell. This means that outside the cell there was a higher water potential and therefore the majority of the water moved into the cell causing the potatoes mass to increase and the potato cells to become turgid. The water inside the cell was also slightly attracted to the sugar molecules and had a lower water potential so the majority of molecules stayed inside the cell. However when I put a line of best fit on my graph, which didn’t touch on my suggested outlier (the average difference in mass for 0.25M solution), it suggested that any pot with a concentration of less than 0.15 molar would increase in mass which suggests that the potato in the 0.25M solution should have really decreased in mass as apposed to slightly increasing suggesting my result for this molarity might have been slightly inaccurate, possible due to contamination of the solution.
My table of results and my line of best fit both show that pots one, two and three decreased in weight. This is because they had a higher concentration of sugar solution outside the cell than inside the cell and so the net movement of water molecules went from inside the cell to outside the cell because they were ‘free’ to move around. The water outside the cell was slightly attracted to the sugar molecules and had less water potential therefore they did not move into the cell.
So taking into account my suggested outlier after looking at my line of best fit; this particular variety of potato if left in a sugar solution for 24 hours with a cling film cover will increase in mass in the molarity of the solution is less than approximately 0.15 and will decrease in mass if the molarity is greater than approximately 0.15. There will be no change in mass if the solution is approximately 0.15 Molar, because the concentration of sugar solution will be equal inside and outside of the potato cells i.e. an isotonic state.
Evaluation:
I feel my experiment was quite successful. I obtained a good range of results which enabled me to come to a reasoned conclusion. However I had one outlier, as indicated on the graph. I think that this could be eradicated by using three different pots for each molarity of solution, each containing one piece of potato, rather than one pot per molarity of solution each containing three pieces of potato. This would mean that if one of the pots some how became contaminated and the concentration of the solution was changed we would still be able to use the other two pots to find an accurate average of that molarity.
I tried hard to ensure that I kept the experiment a fair test and although I had one outlier I think I kept the conditions constant.
If I were to continue this experiment I could look at different factors that affect the rate of osmosis in this variety of potato. Alternatively I could look at different varieties of potatoes and compare my results. I could also look at the rate of osmosis in other root vegetables such as carrots and compare these results.
