It may be expected that factors such as osmosis (the diffusion of water molecules) and diffusion (the random movement of particles from areas of high concentration to low concentration) may affect the rate in which the particles move. They may speed up or slow down this rate and in turn, affect the rate of osmosis. I also predict that the molarity of the isotonic solution should be the same.
Variables-
For this experiment to be a fair test, certain aspects of the experiment will have to be kept constant whilst one key variable is changed. The independent variable for his experiment is the molarity of the sucrose solution. This will be changed and should cause changes in the dependant variables. I chose to vary the concentration as it is relatively easy and I will also be able to obtain varied results from which I can make a decent conclusion. The concentrations that I shall be using in the experiment are shown in the table below.
In this experiment the dependant variables are the changes in the mass of the potato cores, will result after they have been put into the sucrose solution (refer to method). I will be able to measure the change in mass of the potato cores using laboratory equipment such as a balance.
If any of the non-variables (shown below) were not kept constant it would mean that the experiment was not a fair test. For example one potato chip having a slightly larger mass would not be fair as there would be more space for osmosis to occur.
To ensure that the experiment is a fair test I will:
- Carry out the experiment at room temperature. This is because by increasing temperatures, the kinetic energy of the molecules is increasing and as a result the diffusion rate will also increase.
- Keep the water potential of the potato initially the same by using the same type of potato throughout the experiment.
- Measure the dependent variable (the mass of the potato chip) throughout the experiment. I will measure this in grams and the mass of the potato chips will be measured before and after the experiment. This will allow me to see to what extent osmosis has taken place.
- Use the same mass for each potato chip so that the experiment is fair.
- Use the same volume of sucrose solution for each concentration. The potato chips must be covered in the solution so that osmosis can take place. This will allow for consistency.
- Use the same balance to weight the potato chips this is because the accuracy of the scales may vary between scales.
- Time the experiment accurately as different times of exposure to the sucrose solution will lead to different amounts of osmosis taking place.
- Only handle the potato cores will tweezers in order to minimise contact with the cell surface membranes.
Apparatus –
Below is a list of the apparatus that I shall be using throughout the experiment.
Table to show reasons for choice of apparatus –
Diagram –
Method –
- Prepare the 6 different concentration of the sucrose solution; refer to the tale showing the concentrations. This will be done by adding varying amounts of distilled water to varying amounts of sucrose solution.
- Place the test tubes in the test tube rack.
- Cut sections of the potato using a scalpel and measure using a ruler. This will have to be done very accurately as the change in surface area may allow more or less osmosis to occur.
- The cores will then be weighed to three significant figures and the mass will be noted down so that more accurate results can be obtained.
- The chips will then be placed into the six different concentrations and left overnight for about 18 hours.
- I will use 10ml of each concentration of sugar solution and the test tubes will be labelled.
- The timing of the experiment will be done using a stopwatch
- After the 18 hours is u I will remove the cores and remove the surface solution using paper towels.
- I will weigh them again and note any changes in mass.
- If I have time I will then repeat the experiment to obtain a second set of results so I can draw a more accurate conclusion.
- After the experiment is complete I will put the equipment away and all of the potato cores will be disposed of.
- I will then record the results obtained in a results table as follows:
- Concentration of sucrose solution/mol dm³ - Initial mass of potato core (grams) final mass of potato core (grams) – change in mass of potato core (grams) – Percentage change in mass of potato core (%).
- I will then plot the percentage change in weight against the molarity of the sucrose solution and then I will estimate the water potential of the cells of the potato cores.
Reliability –
In order to carry out the experiment and obtain the most reliable results as possible, reducing the chance of any anomalous results occurring I will make sure that the factors making the test a fair test are followed. I will also repeat the experiment if there is time available.
Risk Assessment –
- Whilst cutting the potato take extreme care. Precision will need to be taken with the scalpel as it is very sharp and could easy cause a serious wound.
- After handling the potatoes wash hands to ensure that there is no risk of any contamination occurring. Also this will stop any extra water absorbing into the potato.
- There is no risk of the sucrose solution as long as it is handled with car.
- When using the core borer use it facing away form the eyes.
- Be cautious of any glassware around the laboratory.
- Where a lab coat and protective eye goggles when cutting the potato.
Preliminary results –
I carried out the experiment as shown above and the results were as follows:
The results show that as the amount of sucrose in the solution increases the cell has less mass. This is because water always moves to the cell with lower water potential. I will see if in my actual experiment this is the case again.
Analysis -
Results -
The results that I obtained from my experiment are as follows:
- Each time I weighed the potato cores I deducted the mass of the filter paper, as this would have affected the end results.
- The equation I used to find the percentage mass was –
Mass after/mass before * 100% – 100 = %change.
Analysis -
The graph shown gives the line of best fit for the percentage change in mass of the potato chips over the course of the experiment. The graph is a curve that slopes downwards and does not go through the origin. This means that the percentage gain and loss in mass and concentration are not directly proportional. However there is a trend on my graph – as the concentration of the solution increases the percentage change in mass decreases. The graph shows that the percentage loss and gain is inversely proportional to the concentration. There is a change in gradient on my graph; it becomes steeper between the concentrations of 0.2M and 0.4M. Thereafter the graph becomes less steep. This is because the potato chip is becoming as flaccid as possible and therefore the change in mass of each molar concentration becomes closer together. The line of best fit has also been added, we can see that some of the points were slightly out but the general trend can be seen. My graph fits in with the prediction that I made at the beginning of the experiment.
The water potential of pure distilled water is zero, as there are no solutes present. The graph shows that the potato cells increase in mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration due to the water potential inside the potato cells being far less than that of the water. The increases in mass of the potato cores meant that the cells were in various levels of turgidity in the different molar concentrations. From the graph an estimate to the concentration of the potato cell can b made as 0.24M, as this is the point where the potato cell is not increasing or decreasing in mass. This is known as the isotonic point. This is when no osmosis is taken place; both the potato and the solution have an identical molar concentration. The next point, 0.30M looses approximately 12.5%. This shows that the water potential of the sucrose solution in the test tube is weaker then that of the potato chip. At 0.40M there is a further percentage loss of 13.75% totally a total loss of 26.25%. This shows that the sucrose solution has even weaker water potential than 0.30M and that osmosis took place. This is why the potato chip lost even more mass, and it shows that the water potential in the beaker is less than that of the potato chip. This pattern continues and more mass is lost as more water moves out of the potato into the solution. My results confirm my hypothesis.
The graph of the change in mass shows that plasmolysis occurs in the potato cores, whereby the potato cannot expand and take in any more water. From the graph you can see that as the molar concentration increases the change in mass decreases.
The graph clearly shows that there was an overall decrease in mass during the experiment. The graph does not go through the point (0,0) because the water potential of the sugar solution is higher then that of the potato chip.
Evaluation –
In my opinion the experiment that I carried out was successful. I obtained accurate results from which I was able to produce informative graphs and turn make a good analysis. I think the right amount of results to obtain an accurate percentage change for the different molar concentrations I used. I think that the amount of time (overnight) I used for the experiment to last was enough to allow sufficient osmosis to occur. However, if I were to repeat the experiment I would carry out the experiment for a longer period of time to allow more osmosis to happen. I could then possibly find out the saturation point for each of the potato cores. The range of concentrations that I used was adequate for the experiment but if I repeated the experiment I would use more concentrations and a bigger range. Therefore I would obtain more varied results and the results would give me a more in depth study. The isotonic point that would be obtained in doing so would be far more accurate than the one that my current results have concluded.
The main sources of error in order of importance are:
- Cutting the potatoes
- Drying the potato cores
- The concentrations of the solutions
- The accuracy of the balance
- The room temperature and pressure
The limitation in the experimental procedure when cutting the potatoes was that the cores were not cut accurately. Ideally the samples should have come from the same part of the potato and this would have decreased the chances of variation in the texture. Although I was recording my results by mass, it could have affected the surface area and therefore the overall rate of osmosis may have been affected. If I were to repeat this experiment I would find a more accurate way of cutting the potato cores as the cored were cut by hand using a ruler for measurement. I may possibly have found a machine to cut the potato, as this would ensure that all the potato cores would be the same weight and dimensions and this would make the experiment a fairer test. Even a template of some description may have been ideal.
When the potato cores were removed from the test tubes and dried I may have dried some of the potato cores more thoroughly than others and so some of them may have had more excess water then others, this may halved added to the mass. If the experiment were repeated I would find another way to dry the potato cores that would ensure that they were all dried in the same way for the same amount of time. For example using a hand drier.
I measured out the solutions and determined the molar solutions accurately using a burette. There may have been a slight degree of error but this would have been an insignificant factor to affect the results. Improvements to increase the accuracy of the concentrations are to be more accurate when measuring out the solutions.
The experiment was also limited by the accuracy of the balance used, although I used the sane balance each time I was recording a mass and therefore there was the same amount of error each time. I could have weighed the potato cores more accurately on a more accurate scale e.g. to 0.0000g.
It was unlikely that the room temperature and pressure remained consistent during the experiment and changes may have slightly altered the rate of diffusion. The room temperature could have been monitored using a thermometer throughout the experiment. Using a heating system or by open windows this may have been kept more constant.
The results and evidence I have produced were reliable to complete my investigation. I didn’t have any anomalous results, which were out of the ordinary, but some of the results were not close to the line of best fit as others.
It may have been more beneficial to repeat the experiment to prove that the results were not obtained through chance or by an external factor. However my results confirmed my initial hypothesis and I am pleased that I have produced accurate results from which I have analysed and drawn conclusion from.