Knowing that osmosis (a diffusion of water) will occur across a semi-permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane, and knowing that when this happens to cells they will either become turgid if water flows into them, or plasmolysed if water flows out of them, and thus change their volume, we want to test the hypothesis 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.
We have studied turgidity and plasmolysis in a textbook (Key Science-Biology, pages 143-144) and in a preliminary experiment, where we first added 2% sucrose solution to rhubarb epidermal cells, and saw them become plasmolysed, and then added water, and saw them become turgid. However, we did not use different solution concentrations, and did not measure the amount of contraction or expansion that took place. From our results in the main experiment, we should be able to work out not only the amount of contraction or expansion caused by each strength of solution, but also the concentration of the sap inside the cells.
For the experiment we will require:
Either cylinders of potato with a diameter of 6.5mm and a height of 5mm, or a potato, a borer with a diameter of 6.5mm and a scalpel. (To allow us to make our own).
Solutions of varying strengths (of sucrose and NaCl), or a solution of a known strength and distilled water. (To allow us to make our own).
Pins (To ensure that cylinders remain separate while in the solutions.)
Callipers (To measure cylinder height and diameter.)
Diagram One of the test-tubes during the experiment. Three potato discs on a pin, not touching.
We take a cylinder of potato, with a diameter of 6.5mm, from the potato, and cut it into separate cylinders each with a height of 5mm. We then thread at least three of the cylinders, to make the experiment fair (in case one of the cylinders is abnormal or damaged), on to a pin, keeping them apart from each other. We then make up solutions of either sucrose or sodium chloride, either by % strength or by molarity, and place 4 millilitres of each strength into a separate test-tube. We used a range of % sucrose solutions, going from distilled water (0%) to 2% (which we knew from earlier experiments would plasmolyse the cells), and a range of sodium chloride solutions from distilled water (0) to 0.4 molar (which would again be enough to plasmolyse the cells). We then place each of the sets of three cylinders on a pin into each of the different solutions, making sure that the cylinders are covered by the solution, and leave all of the test-tubes close to each other for 24 hours.
We assume that this means that the pressure and temperature in each case is the same, as these are factors which could affect osmosis, and we know that the volume, size and surface area of each cylinder is the same, and as they are all from the same potato, the only variable that we are altering is the concentration of the solution. Although ideally the experiment would be repeated several times, we were not able to do this as we did not have sufficient time.
After 24 hours we remove the cylinders from solution and, with callipers, which are more accurate than a ruler and would cover the likely range of sizes (from 4mm to 7mm), measure the new diameter and height of the cylinders. The results, in table and graph form are recorded below in the Results section.
ConcentrationCylinder Diameter/mmCylinder Height/mmVolume/mm3 (2dp)Ave. Cylinder Volume/mm3
Sodium Chloride solution
0.4 Molar-18.84 Concentration of SolutionAverage % Change in Volume from Original
The results show that, in accordance with our hypothesis, the cylinders will expand when external solute concentration is low (high water concentration), and contract in strong solutions (low water concentration). This is due to osmosis, where water passes from weak solutions to strong solutions across a semi-permeable membrane, such as a cell membrane. The graphs of % change against solution strength show that the results tend to form a curve, crossing the x axis (where there is no change in volume), at approximately 0.07 molar concentration for the sodium chloride solution, and at approximately 0.2 % for the sucrose solution. This concentration is the osmolar concentration (the total solute concentration) of the sap inside the cell. The volume change forms a curve when plotted against solute concentration because the cells, which have cellulose cell walls in addition to a cell membrane, will not expand or contract indefinitely, and will be held in shape within certain limits. However, the relatively low number of solutions tested (5) means that there is a range of possible values for the osmolar concentration of sap in the cell, and means that we cannot accurately predict values for volume change at different concentrations. To conclude, therefore, the results support our hypothesis, and we were also able to discover the approximate concentration of the sap in the cell.
Although the results of the sodium chloride and sucrose experiments support the hypothesis, there are several anomalous results and a large deviation for each result. These could be improved by altering the experiment, for example by keeping the test-tubes in a water bath at a set temperature, by keeping them at a constant pressure, and by measuring the sizes of potato cylinders before and after with a more accurate method, e.g. accurate weight measurement or volumetric displacement. The test might also be more accurate if the potato cylinders were left in the solutions for a longer period of time to allow the solution to penetrate fully to the core of the sample. The test could also be repeated more times for each concentration of solution, and with a greater number of concentrations, as this would decrease the error – a disadvantage of our experiment was that one anomalous result affected the others significantly (e.g. NaCl 0.2 molar concentration). Another factor is that the potato from which the cylinders are taken could be abnormal – this could be prevented by amalgamating sets of results, for example of a whole class, where each experimenter used a different potato.
Results that were not as I would have expected occurred with NaCl solution at 0.2 molar concentration (see above), where the range of results appeared too low. However, although this is apparently an anomalous result, it could have been caused by either experimental error – more significant when a small number of results are used, or a difference in the potato for those cylinders. Either of these would easily be recognised if a larger number of results were collected and used. Another result that appeared unusual was the ¡¥step¡¦ in the graph for the sucrose solution between 0.25% and 1% solutions – here different results for each cylinder pulled the average upwards by a noticeable amount, a problem that possibly would not occur if more measurements were taken.
For future experimentation we could repeat this experiment using a range of solution strengths very close to the value discovered here of sap osmolarity, to define more exactly its true value. We could also extend the experiment to use tissue samples from other plants, to discover whether the hypothesis is also correct for other tubers, and even for other plant tissues. We would then also be able to compare osmotic pressures inside different plants.