In the case of the potatoes that grew, the higher the concentration of water there was, the longer the potatoes grew. This is because the greater the concentration of water in the external solution the greater the amount of water that enters the cell through osmosis.
The same happened with the potatoes that shrunk, the lower the concentration of water there was in the solution, the more the potatoes shrunk. This is because the smaller the concentration of water in the external solution the greater the amount of water that leaves the cell through osmosis.
Aim [Hypothesis]
We aim to investigate the effects of osmosis on cylinders of potato to prove that if the concentration of a solution into which a cylinder of potato is placed is greater than a certain level the cylinder will contract, and if the concentration is less than that level it will expand.
Plan
I will carry out this experiment based upon the method used for my preliminary experiment (Appendix A). I have, however, noticed there are a number of things I can change about the way the experiment should be carried out to achieve more accurate results. They are as follows –
- As the results for mass were slightly inaccurate I have decided to repeat the experiment 3 times for each concentration and then take an average.
-
The concentrations used were not evenly distributed. I shall therefore use concentrations that increase by 0.2 mol dm-3 in this experiment.
-
The potato cylinder in distilled water grew in length and mass while the potato cylinder in the solution 0.5 mol dm-3 remained the same length and changed very little in mass. My control shall therefore be 0.5 mol dm-3.
By carrying out three identical experiments and calculating the average change in length and mass, I hope to be able to reduce the effect of anomalies and therefore provide more accurate data.
Preliminary experiments showed that the cylinders do increase in length as the salt concentration decreases and reduce in length after a certain point, thus backing up the hypothesis I outlined earlier. The mass, however, did not. The mass of each cylinder did decrease as the concentration of salt in the solution increased, but not in the same pattern as the lengths.
I do believe that this was due to slightly inaccurate measurements of mass in the preliminary experiments. I also believe that by carrying out three experiments and taking an average will overcome this problem.
Apparatus
I will use the following equipment in my investigation –
- 21 x Potato cylinders
- 21 x Boiling tubes
- 6 x Salt solutions of varying concentrations
- 4 x Boiling tube rack
- Electronic balance
- Calliper
- Measuring cylinder
- Distilled water
- Cutting tool
- White tile
Safety
There are many potential hazards in this investigation that must be taken into account. They are outlined in the table below –
Method
With the changes from my preliminary experiment outlined in my plan, I shall use the following method in my final investigation –
Set Up
- Set up six boiling tubes in a boiling tube rack
- Label them with the concentration of solution they will contain
- In five of the tubes place 35ml of each of the chosen solutions
- In the remaining tube place 35ml of distilled water
- Select six approximately equal length cylinders of potato
- Allocate each cylinder to a concentration of solution
- Measure each cylinder individually (noting the length)
- Then weigh each of the cylinders (noting the mass)
- Place all six cylinders in their respective solutions (note the time)
- Leave for 2 hours
This must be repeated three times, all three experiments can run at the same time so long as each rack is numbered 1, 2 or 3 (respective of their experiment number)
Obtaining results
- Remove the cylinders from their solutions, removing any excess solution on the outside of the cylinder
- Immediately weigh the cylinders (noting the mass)
- Then measure each of the cylinders (noting their length)
- Calculate the change in mass and length (noting these)
- Repeat for all three experiments
- Calculate the average change in weight and mass
Results
Graphs
Conclusion
Evaluation
Appendix A – Preliminary Experiment
Method
- We selected five cylinders of potato and trimmed them so they were approximately the same length
- Then we weighed them and measured them
- We then placed each of them into a boiling tube
- We then poured 40ml of a different concentration salt solution into four boiling tubes and 40ml of distilled water into the remaining.
- We then left these for an hour
- After an hour we removed the potatoes from the solutions, reweighed and re-measured them
- We then calculated the change in length and mass
Results
Evaluation
The above results show that cylinders placed in lower concentrations of salt gained length while cylinders placed in higher concentrations lost length. Once cylinder neither gained nor lost length, this could be because the water was in equilibrium. The changes in weight also show a pattern of decreasing as the concentration of salt increased. There is, however, no obvious relationship between change in length and mass.
Appendix B – Raw Data
Experiment 1
Experiment 2
Experiment 3
Appendix C - Glossary
Equilibrium
Isotonic is when a cell has the osmotic pressure to that of the surrounding medium.
Osmosis is the passage of water from a region of high water concentration to a region of low water concentration through a semi-permeable membrane.
Plasmolysed is when a cell has suffered from plasmolysis (see below).
Plasmolysis is the loss of water by from a plant cell to the extent that the cytoplasm shrinks away from the cell wall. This happens when the cell is placed in a solution that has a higher solute concentration than that of the cell sap.
Turgid is when a cell is swollen and inflated
Bibliography
Oxford Reference Online
