Taylor wrote, in 1972, that osmotic potential is defined as, “the portion of the water potential that results from the combined effect of all solute species present in the soil or plant system.” A solution has a high osmotic potential if it has a high proportion of water molecules. Conversely, a solution has a low osmotic potential if it has a relatively low amount of water molecules and is therefore a concentrated solution. The solution with the highest osmotic potential is pure water. In an isotonic solution, the concentration of the solutions is the same inside the cell and outside the membrane. A cell placed in an isotonic solution neither gains or loses water.
Prediction-
I predict that, if the potato is put in a solution and they are both isotonic, no change occurs in mass.
If the potato is put in a dilute solution that has a lower concentration than the cell sap, the mass of the potato would increase. The lower the concentration of the surrounding solution, the greater the increase in the mass of the potato.
Conversely, the greater the concentration of the surrounding solution, then the greater the decrease in the mass of the potato. This would happen if the potato is put in a solution which is more concentrated than the cell sap.
Linking prediction with theory-
As the concentration of the solution decreases, more water molecules diffuse in the potato chip therefore they gain in mass.
If the cell sap is less concentrated , there is a loss of mass as water will diffuse out of the cell sap to the surrounding, more concentrated solution. This occurs because, osmosis is the diffusion of water molecules from a less concentrated solution to a more concentrated solution or a solution that has a low osmotic potential. The greater the concentration of the surrounding solution, the greater the loss of water molecules from the less concentrated cell sap so the greater the decrease in mass meaning that the potato chip will become lighter. The lower the concentration of the surrounding solution, the lower the loss of water molecules from the more concentrated cell sap so the greater the increase in mass meaning that the potato chip will become heavier.
Basic Method-
Apparatus and materials needed-
Detailed method-
In my investigation, there are variable factors that are likely to effect my experiment in some way. The variable factors are; the surface area of the potato chip, the mass of the chip, the temperature of the solution, the concentration of the solution, volume of the solution and the length of time that the potato chip is immersed in the solution. Some variables will be kept the same, for example, the temperature of the solution and the volume of the solution, however, only the concentration of the solution will change. In this investigation, our aim is to find out what the concentration of the cell sap is and to measure the mass of the potato chip before and after it is immersed in different glucose solutions.
It is important to keep the variables I have decide to keep the same, the same to make this a fair test. The surface area of the potato chip should remain the same throughout the experiment. We can calculate the surface area through the use of this formula;
2Πr (r + h)
If the surface area is larger then osmosis will take place quicker meaning more water diffuses in or out so the mass is considered unreliable and inaccurate. Similarly, the shape of the potato chip could cause problems. If it is shorter, longer, wider or narrower then this also makes the results unreliable.
The mass of the chip should be the same for each chip as we used the borer with the 8 mm diameter, if problems have occurred then the size of the potato chip could be different. If the potato chip is heavier then there are more vacuoles so more water could diffuse in or out.
Similarly, if the chip is left in the solution for less than the allocated time then this means that less water has entered so there is only a small increase in mass. Conversely, if it is left in the solution for a longer time then more water molecules have diffused in the vacuole and as a result, the experiment is inaccurate and unreliable.
If the solution is warmer then osmosis will occur faster as the particles will have more energy so there will be a greater increase in mass, however if the solution is colder, the particles will have less energy and as a result, the potato chip will have a lower change in mass.
The volume of the solution is to be kept the same throughout the experiment. Although it is not clear why this would effect the experiment, to make it a fair test, the volume is to be kept 50 cm³ regardless of the concentration of the solution.
The only variable we will be changing throughout the experiment is the concentration of the solution. The aim of this investigation is to find out what the concentration of the cell sap is, by changing the concentration of the solution, recording the results, plotting a graph and a line of best fit, we can find out the concentration of the cell sap.
Anomalous results-
Errors are likely to occur throughout the experiment that will effect the results we obtain. As a result, anomalous results will be generated and to rectify this I had to repeat the experiment for 0.5M until I got a result that fitted in with the general trend. This could have been the result of not fully controlling all the variables that were meant to be kept the same. When I obtained my final result I had to generate an average for each percentage change in mass and plot a line graph connecting as many points as possible with the Concentration of Glucose (M) on the x-axis against the Average percentage change in mass (%). This way I was able to pin-point any graphical anomalous results. A graphical anomalous result is an unreliable result that does not fit in with the general trend of the line. As a result, I had to repeat the experiment for 0.5 M. I had to repeat the experiment for 0.5M as I had generated an unwanted anomalous result. When I repeated it, I generated new results and a new average, which when plotted, fitted in perfectly with the line.
A concentration of surrounding solution gave no percentage change in mass. This was the concentration of the cell sap, being 0.3 M.
Conclusion-
Through analysing my results, I can say that my prediction was precise; if the potato is put in a solution and they are both isotonic, no change occurs in mass. The concentration of the cell sap was 0.3 M. In surrounding solutions that were more concentrated than the cell sap, the potato chip decreased in mass. From the graph, we can say that, the greater the concentration of the solution, the greater the decrease in mass. On the contrary, in surrounding solutions that are more dilute than the cell sap, there is an increase in mass on the potato chip. The lower the concentration of the surrounding, the greater the increase in mass by the potato chip.
Osmosis is the diffusion of water molecules from a dilute solution to a more concentrated solution. Osmosis stops only when the solutions are isotonic. That means that the concentrations of the solutions are equal.
In certain concentrated solutions, the solutions are nearly isotonic so there are only small changes in mass. We get a large change in mass when the solutions are at extreme differences so a lot of water is lost by the cell sap or a lot of water is diffused in the cell sap. However, a small change in mass would occur if the solutions are nearly isotonic. Osmosis is still happening in that solution but not to a great extent that is very noticeable.
The graph that I drew with the results I generated showed a negative correlation.
Evaluation-
Overall, the experiment I conducted was successful and I found out that the concentration of the cell sap was 0.32 M. However, throughout my experiment, I expected errors to occur which would effect my results. The anomalous result I generated was at 0.5 M which meant that I had to repeat the experiment until I generated a result that fitted in with the general trend of the graph. If all the variables were controlled to the fullest, I would have expected the percentage change in mass for the two potato chips to have been the same for a surrounding solution of a particular concentration. However, I was unable to control all of the variables completely.
When we removed the potato chip from the solution we had to blot the excess water however not all the potato chips were blotted to the same degree therefore there is a possibility that too much water may have been blotted off one of the potato chips meaning that it will be lighter and not enough water may have been blotted off another potato chip so that it was heavier than it should have been. Therefore producing unreliable mass results. To avoid this effecting the results, the potato chips should have been rolled across the tissue paper a fixed amount of times e.g.only 5 times.
After blotting the potato chips and after cutting them out of the potato we had to weigh them using the measuring scales correctly to 2 decimal places. However, some measuring scales are more accurate than others to 2 decimal places therefore the weights generated would have been unreliable. The same measuring scales should have been used when measuring all the potato chips.
Even if the potato chips were correctly blotted, there was often a queue of people waiting to weigh their potato chips on the measuring scales. As a result, some of the water would have dried up therefore the potato chip would have become lighter so the weight generated would be unreliable. To overcome this problem, the potato chip should be weighed at a fixed length of time after in is blotted, e.g. 30 seconds.
We recorded the temperature of the solution and we assumed that it would be the same each day. However we didn’t take into account that fluctuations of temperature could occur. If the solution is warmer, osmosis would occur faster meaning that the potato chip would be heavier than it should be. If the solution is colder, osmosis will be carried out at a lower rate therefore that potato chip would be lighter. Similarly, this would generate unreliable weights. We should have carried out the experiment in a water bath so that the solution would be at a constant temperature.
To cut out the potato chips we used borers which had a certain diameter, however due to the constant use of the borers, most of them were worn or miss shaped. As a result, when the potato chips were cut with different borers, the potato chips would not be exactly the same as each other. Meaning that the surface area would have changed and osmosis would have occurred at different rates. If the potato chip was smaller, osmosis would have been carried out slower, conversely, if the chip was larger, the increased surface area would have resulted in osmosis occurring faster. The same borer should have been used to cut out each potato chip to avoid this having an effect on the results.
We used different potatoes as there was not a big enough potato to supply all the potato chips. However, different potatoes have different cell sap concentrations therefore anomalous results would be generated if different potatoes were used. We should have found a big enough potato to be used in the experiment and to cut all the pieces from.
Once having cut the potato pieces, we measured them with a ruler to check the length. The ruler we used was an ordinary ruler which is not very accurate. A vernire should have been used to measure the potato chip more accurately.
Similarly, we cut the edge of the potato chip to try and make it at a 90° angle however this was not always accurate. A set square should have been used which would have give the potato chip an accurate 90° angle.
