Variables
- Concentration of the sucrose solution - This will need to be changed in order to find out how the plant cells are affected by different concentrations of sucrose.
- Light intensity – All the potato chips will be in the same room and the same place so although we will leave them in the room for 24 hours, they will all get the same amount of light intensity.
- Same amount of sucrose solution placed in each test tube – This measure will be taken because even if the sucrose solution is dilute, if there is a lot of it then there could still be more sucrose outside the cells than inside and therefore would distort results even if the solution was dilute.
- Using the same potato throughout the experiment – this measure will be taken because one potato might have more water inside the cells than another and this could distort results.
- Temperature – Similar to light intensity, the temperature during the 24 hours varied but they were all subjected to the same amounts therefore this is still fair.
- Amount of time the potato chips are left in solution
- Using the same balance to measure chip
Safety
To ensure safety, the 6 test tubes will be kept in beakers so that they can not be knocked over. The potato chips will be cut on a white tile with a scalpel so that no one will be cut.
Preliminary Experiment
I conducted a preliminary test which was a smaller version of what the experiment I am hoping to conduct. I cut potato of equal size and then I placed them into molar sucrose solution of 0M, 0.2M, 0.4M, 0.6M, 0.8M and 1M. The test was to see what solutions I should use in the experiment to get a fair spread of results. When I plotted the results in a table, which is also shown below, I found that the only increase in mass was at 0M and the rest of the results were decreases in mass. This means that on plotting a graph I will only have one point which reflects an increase in mass and the other will all be decrease and so I would not get a good spread of results. From this preliminary test, I therefore decided that I would take the results of the change in mass of potato chips in sucrose solutions of 0, 0.1, 0.2, 0.3, 0.4 and 0.5. This way I will get a more even spread on my graph and if the potato in 0.1M solution causes an increase in mass, I will have the graph that I want.
Method
Apparatus Needed
Distilled Water
Forceps
Knife
Test Tubes
Tile (To cut potatoes on)
Weighing Machine
Tissue
Molar Sucrose
Potatoes
Measuring Cylinder
I will first prepare potato chip ready. In order to do this I will use a knife to cut the potato chips to the same size. I will have 2 sets of 11 different molar sucrose solutions ranging from 0M up to 1M going up in intervals of 0.1M. Then I will place the already prepared potatoes into the different solutions after first weighing and recording their mass and their flexibility. The potatoes will be left in the same corner of a room overnight to ensure that osmosis has taken place fully. Then I will take the potatoes out of the test tubes and dry them. I will re-weigh them recording any changes in mass and in flexibility. I will have to measure the change in mass as well as the percentage change in mass. Then I will have to work out the average percentage change in mass for each result as there will be two potatoes going into each solution. This is to ensure that any errors are kept to a minimum and reduces the likelihood of having anomalous results.
Results
The table below is the set of results obtained from the experiment that I conducted. I took two sets of results for potato chips in each sucrose solution.
Observations
The potatoes which were placed in distilled water were absolutely straight and hard. They were longer and I also found that they were lighter in colour. As the sugar solution became more concentrated the “bendability” of the potato chips increased and the potatoes were floppier, shorter, softer and darker in colour.
The potato cells placed in pure water were harder because their cells took in water by osmosis and were turgid. This led them to be longer (as they had expanded) and lighter in colour. To explain why the potatoes were lighter in colour we can use the example of a balloon. As the balloon is blown, the rubber expands and gets lighter. This is what has happened to the potato cells as their cell walls were stretched to their full capacity. The potatoes which were placed into sugar solutions got gradually softer as the concentration of sugar increased. This is because water has osmosed out of the potato cells causing them to shrink and therefore to be shorter, darker and floppier.
Analysis
The evidence that I have obtained from this experiment fully supports the prediction I made. It shows that potato cells increase in mass when placed in solution with a higher water concentration and they decrease in mass when placed in solutions with a lower water concentration. The graph, which plots percentage change in mass against molarity, shows that as sucrose solution increases, the percentage increase in mass is an increase of 16.53%. This drops to a 4.11% increase at 0.1M and a 2.29% decrease at a concentration of 0.2M. At 0.17M, which is between 0.1M and 0.2M, there was no change in the mass of the potato and we can therefore say that this was also the concentration of sucrose inside the potato. There is a continuous fall in the change of mass, and by 1M the potato chips had lost 43.78% (almost 50%) of their mass.
The reason for the changes in mass of the potato is osmosis. The direction of osmosis is also different at varying concentrations of sucrose. At 0M sucrose, the concentration of water was higher outside the potato than inside the potato and so water evidently moved into the potato cells. However, moving on to 0.2M sucrose, we can see from the graph that the concentration of water inside the potato cells was greater then the concentration of water outside the potato cells and therefore water osmosed out of the potato and into the sucrose solution.
Why was the graph a curve?
In the high sucrose solutions the decrease in mass inside the potato chips was less than expected. Because water came out of the potato in the sucrose solutions, it diluted the sucrose. The ‘dilution’ effect could explain why the potato chips did not lose as much as I expected.
Evaluation
Although I tried to make the test as fair as possible, there were several possible sources of error.
- We did not have a cork borer to cut the potato chips to the same size. This could have meant that osmosis could have taken place quicker in potatoes which were smaller in size or potatoes which had a larger surface area than others. This source of error was minimized by calculating the percentage change in mass.
- We left the test tubes open for over 24 hours, so this could have led to the evaporation of the water in the solution and therefore resulting in an increase in concentration of the sucrose solutions, although in the case of the 0M solution, where no sucrose was present, osmosis may have not taken place fully because maybe not all of the potato would be surrounded by water. Although, the solutions may have evaporated a little, as the test tubes were kept in the same room and therefore at the same temperature, the “evaporation effect” would have happened to all the test tubes even though the overall results of all the test tubes may not have been what was expected.
- We only put 10cm of solution in the test tubes and so this could mean that the solution could easily get weaker due to the “dilution effect”. The dilution effect occurs when osmosis takes place. In stronger sucrose solutions, water leaves the potato cells, causing the solution to become weaker and more dilute, meaning that not as water would come out as expected. In weaker solutions, this is not really a problem. To overcome this, one could increase the volume of the solution used so that the dilution effect would be less, or the solution could be refreshed every so often to prevent the solution from getting weaker during the experiment.
- We did not cut the potato chips from the same part of the potato so the concentrations of sucrose in the potato chips could have been different depending on which part of the potato we cut the chips from.
If I conducted the experiment again I would change the following things to improve the experiment:
- The volume of sucrose solution – I would put more sucrose solution in the test tubes or refresh the solution every ten minutes or so to ensure that the solution hadn’t been diluted by the water coming out of the potato cells. A larger volume would mean that the solution is diluted less.
