11. 01 Osmosis Investigation
To investigate the effects of changing the sucrose concentration on osmosis in plant cells.
Water passes into cells through a special type of diffusion called osmosis. Water molecules diffuse through the membrane from a weak solution into a strong solution until the concentration is the same on both sides. A membrane that allows only certain molecules to pass through is called a semi-permeable membrane. In a plant, water passes from a weak cell sap solution to an adjoining cell with a stronger solution, as water passes in, the volume of the sap vacuole increases. When a full sap vacuole presses against the cell wall, it is said to be turgid. If water that is lost is not replaced the sap vacuole shrinks and pulls on the cell wall, the cell becomes flaccid; this is known as plasmolysis.
In the cells shown below, water molecules will diffuse from the turgid cell into the flaccid cell, until the cells contain equal concentrations of cell sap.
I intend to use potatoes for my investigation because these are sufficiently large, to enable all cores to be taken from the same potato, which will assist in ensuring a fair test.
The concentration of sap in the sap vacuole of a potato cell is approximately
10% - 15%. I intend to place a predefined weight of potato cells (0.15g) in varying concentrations of sucrose solution (0%, 20%, 40%, 60%, 80%), to see the effects of osmosis in the cells of a potato in varying levels of sucrose solution. The potato cores will be prepared, weighed and then placed in the solution and left for a certain amount of time, they will then be removed, re-weighed and the difference in weights calculated and plotted, and a conclusion reached.
I have done some preliminary work into osmosis in potato cells. I weighed six potato cores, and then put three into sucrose solution and three into distilled water. After 25 minutes, I removed the potato cores from the solutions and re-weighed them. I discovered that the three in water had increased in mass and the three in sucrose had decreased in mass. This decrease was due to osmosis.
I therefore predict that the potato cores in distilled water will increase in mass because the water molecules will diffuse from the solution into the sap vacuoles of the cells in the core. The water molecules will diffuse across the semi-permeable membrane, because the sucrose concentration is higher in the sap vacuole than in the distilled water. This is shown by the diagram (below left), water molecules enter the sap vacuole of the cell due to osmosis, and make the cell turgid.
However, the potato cores in sucrose solution will lose water molecules to the sucrose solution, causing the sap vacuole to shrink and the cell to become flaccid. The water molecules will diffuse because the sucrose solution has a higher concentration than the cell sap. As there are varying concentrations of sucrose solution, I think that the solution with the least concentration will have the cores which lose the least mass; and the solution with the strongest concentration will have the cells which lose the most mass. I think this will occur because the larger the difference in the osmotic pressure, the faster the osmotic diffusion will proceed.
A measuring cylinder was taken, and filled with 20ml of distilled water, distilled water was used, as it contained no impurities which could have caused anomalous readings. This water was transferred into a boiling tube, which was labelled and then placed in a boiling tube rack. The above steps were repeated twice, so there were three boiling tubes, each with 0% concentration of sucrose. The measuring cylinder was dried and 4ml of saturated sucrose solution was added, this was emptied into a boiling tube and then 16ml of distilled water was measured into the measuring cylinder and then added to the boiling tube to give a 20% concentration of sucrose. (It was decided that saturated sucrose solution could be taken as 100%, for the purposes of this experiment.) This boiling tube was then labelled, and placed in the rack, and the above steps repeated twice. The above steps were repeated until there were 15 boiling tubes; 3 with 0% sucrose concentration, 3 with 20%, 3 with 40%, 3 with 60% and 3 with 80%. There were three of each solution to enable three readings from each level of sucrose concentrationto be made, and therefore an average reading could be calculated.
A potato from the Desiree family was taken, a tile was put on the bench and a core remover was used to take fifteen cores from the potato. All cores were taken from the same potato to help ensure a fair test.
Safety precautions were put into practise. We used a tile on the bench so that the bench would not be damaged, and were careful not to push the corer into our hands. Boiling tubes were held safely in racks.
The cores were then sliced up on a new tile using a scalpel, until they were all approximately the same length and weighed using a top-pan balance to check that they all weighed in the region of 0.65g. The cores were then put into the solutions in the boiling tubes, and a stopwatch was started. The cores were started with two minute intervals between each start time to allow suffiicient time when they were finishing to be weighed and disposed of. If this time had not been allowed, some cores would have had more time in the solutions than others causing an unfair test and a false conclusion.
After one hour, the potato cores were removed, dried on a paper towel to remove any excess surface water and re-weighed on the top-pan balance. The results were noted in a table and the difference in mass was calculated by dividing the final weight by the original weight, multiplying this by one hundred, and then calculating the difference between the number and one hundred.
first weight = 0.65g
second weight = 0.50g
divided = 0.50/0.65 = 0.77
multiplied x 100 = 77
difference = -23%
When all the differences in mass had been calculated, they were plotted on a graph.
The apparatus was set out as shown below;
A total of fifteen measurements were made during the investigation, three for each of the sucrose concentrations. This was undertaken to help improve the reliability of the results and prevent any anomalous results giving a false conclusion. On the table below, the average mass lost/gained for each sucrose concentration has been calculated.
Sucrose Concentration Before (g) After
Difference % Before (g) After
Difference % Before (g) After
Difference % Average
0.63 0.74 + 17 0.64 0.77 + 20 0.69 0.81 + 17 +18
0.69 0.66 - 4 0.67 0.63 - 6 0.69 0.67 - 3 -4
0.68 0.55 - 19 0.64 0.52 - 19 0.69 0.55 - 20 -19
0.64 0.48 - 25 0.65 0.50 - 23 0.64 0.49 - 23 -24
0.66 0.47 - 29 0.67 0.49 - 27 0.66 0.48 -28 -28
The potato cores left in distilled water have increased in size; they were very firm or turgid, water had diffused into the potato cells by osmosis.
The potato cores left in concentrations of sugar solution have decreased in size, they were very soft or flaccid, and water had diffused out of the potato cells by osmosis.
From the results of this investigation, the conclusion may be drawn that the higher the level of sucrose concentration, the more rapidly osmosis takes place.
The bar graph above compares the mass difference at the different levels of sucrose concentration. From this graph, we can see that as the sucrose concentration increases, the mass percentage decreases.
When this is shown on a line graph, we find that there is a correlation between the results, supporting the conclusion that the stronger the concentration of the solution outside the cell, the faster plasmolysis occurs.
The conclusion of this investigation supports the original hypothesis that the potato cores in distilled water will increase in mass and that the potato cores in sucrose solution will lose mass. Also, the solution with the lowest sucrose concentration will have the cores which lose the least mass, and the solution with the strongest sucrose concentration will have the cells which lose the most mass.
I think that this investigation went well and that the results were fairly accurate.
There were a few anomalous results, but because several readings at each concentration were done, these anomalous results did not disrupt the overall graph or the conclusions. The anomalous results were probably caused by a difference in the surface area of the cores which was due to imprecision during the cutting procedure. If this experiment was repeated, or a similar one undertaken, greater accuracy during cutting could probably prevent these anomalous readings occurring.
I have several proposals for further experiments to provide additional evidence for my conclusion. I would like to undertake another experiment with sucrose concentrations at 5% intervals from 0% to 100%. I would like to do this as I feel it would provide a better and more reliable set of results and a more justified conclusion. When there are more concentrations the shape of the curve shape can be seen more clearly, I suspect that at the lower concentrations of sucrose the graph falls more steeply, flattening off at higher concentrations. I would also like to conduct an investigation, in conjunction with this experiment, to calculate the concentration of sap within the vacuole.
To do this, I would find between which parameters the curve crosses the x-axis of sucrose concentration and retest at every 1% between these two parameters until I have found the concentration which contains a core that neither gains nor loses mass. This osmotic concentration would be the equivalent of the sap in the vacuole.
As an extension to this investigation, I would run two experiments in parallel. All cores would be from the same potato, but one would run for an hour, as this one did and for the other the cores would be left in the solutions for longer, perhaps 24 hours, to establish if one hour is the end point of the osmotic diffusion.