3. ANALYSIS
The graph clearly shows a negative correlation between the concentration of the solute solution and the change in mass of the potato chips over one hour, demonstrating that as the concentration of the solute is increased, the mass of the potato chips decreases. However, this trend does not follow a linear path due to the process of osmosis, which takes place within the potato cells.
Osmosis is the overall movement of solvent molecules through a semi-permeable membrane (permeable to the solvent but not to the solute) into a region of higher solute concentration (see Figure 1) in an attempt to balance the solute concentrations on either side and achieve equilibrium, a steady state. It can be seen as the opposite of active transport. The reason why osmosis is considered to involve an overall movement of solvent molecules in one direction is because the particles are actually moving both ways, but as there are a lot more solute particles in the more concentrated area, there is a net (overall) movement in solvent particles to that side. In our case, the solvent was water and the solute was salt.
Figure 1:
Using the graph and my understanding of osmosis I can infer that after one hour, the cells of the potato chips which were placed in the 0.0M and 0.5M salt concentrations had become turgid while the cells of the potato chips which were placed in the 1.5M and 2.0M salt concentrations became flaccid. I can also use the line of best fit on the graph to estimate the isotonic point with reasonable accuracy, which should be at a 0.98M salt concentration. Below this point, the solutions were hypertonic as the concentration of salt was greater on the inside of the potato cells than on the outside, meaning that there was an overall movement of water into the potato cells from the surrounding environment, which explains why they gained mass and became turgid. Alternatively, above the isotonic point, the solutions were hypotonic as the concentration was of salt was greater on the outside of the potato cells than on the inside, meaning that there was an overall movement of water out of the potato cells into the surrounding environment, which explains why they lost mass and became flaccid. The water in a plant cell is stored in its vacuole (see Figure 2).
Figure 2:
The term turgid refers to the swelling up of a cell because it has gained too much water as a result of osmosis while the term flaccid refers to the shrivelling up of a cell because it has lost too much water as a result of osmosis. In distilled water (0.0M), potato cells can become lysed, meaning that they are so turgid that they burst due to the overwhelming water pressure on the cell membrane. In concentrated solutions such as 2.0M, potato cells can become plasmolysed, meaning that they are so flaccid that their cell membranes completely leave the cell wall due to lack of water pressure on it (see Figures 3 & 4). I believe both occurred in my experiment, which would explain the curve in the line of best fit.
Figure 3: Figure 4:
Osmosis is of prime importance in humans and all other living organisms because it is necessary for homeostasis, the ability of the body to regulate its internal environment and maintain equilibrium. Living cells of both plants and animals are enclosed by a cell membrane which regulates the flow of liquids and of dissolved solids and gases into and out of the cell through the process of osmosis. The membrane forms a selective barrier between the cell and its environment as not all substances can pass through the membrane with equal facility. Without this selectivity provided by osmosis, homeostasis would collapse and the substances vital to the life of the cell would diffuse uniformly into the cell’s surroundings while toxic materials would enter the cell from its surroundings. Although osmosis is a natural tendency and does not require an input of energy to start it, it does release kinetic energy and can be made to do work. Nevertheless, it is a passive process, like diffusion.
4. EVALUATION
I am generally confident with the results because they match my prediction and clearly show a trend which is representative of osmosis. My results are consistent and have small range bars, suggesting that I achieved a relatively high level of accuracy and reliability.
Before I began my investigation into osmosis, I did some preliminary experiments. The purpose of these was not to record results but to optimise the method and establish a range. I was working with concentrations from 0.0M to 2.0M, so I decided to take five measurements at equal intervals across this range. During my preliminaries I also realised that I would achieve a greater level of accuracy and reliability if I took repeat measurements, so for my actual experiment I repeated every measurement three times. I then calculated the average of the three readings and converted this into a percentage change in mass using the formula:
Percentage change from a to b =
However, there are also signs that the experiment did not go as well as I had hoped. For example, I believe that potato chip D1 (see results table) was an outlier because it does not follow the line of best fit and has corrupted the result for the average percentage change in the mass of the potato chip at a 1.5M salt concentration, as indicated by the large range bars for this reading. It is probably due to human error or environmental changes. I can prove that D1 was the outlier rather than D2 or D3 by removing it from the data set and recalculating the average for the remaining measurements, a method known as a best estimate. The best estimate percentage increase in mass for chips D2 and D3 equals -7.82%, which is much closer to the trend line.
If I was to do this experiment again, I would make a number of changes to my method to improve my results. Firstly, I would take more repeat measurements because this would give me a more accurate representation of the average. I would also take greater care when handling the potato chips so that they did not lose water as they were taken to be weighed. Finally, I would want to work in a more controlled environment than a classroom, as the temperature of the room can have an effect on the speed of osmosis.
5. CONCLUSION
In conclusion, this experiment has shown that increasing the concentration of the solute (in this case salt) decreases the mass of the potato chips. I am generally confident with the results, but I would recommend that further work to investigate how long it would take before the solutions reached equilibrium (see Figure 5) at different concentrations should be taken in order for me to build a comprehensive understanding of osmosis.
Figure 5:
6. BIBLIOGRAPHY
Below is a list of sources that I have referred to throughout this investigation. I have detailed the full URLs of these websites and the full titles of any books used in my research.
[1]
[2]
[3]
[4] Oxford Dictionary
