Students in Biology find it difficult to understand the concept of tonicity and osmolarity in a real time situation. In this investigation, several concentrations of sucrose were used to determine the osmolarity of a potato. It was found that the concentration of sucrose was close to 3.6 in the potato used showing that it was isotonic. Anything above caused the potato to gain weight showing that it was a hypotonic solution and anything below caused the potato to lose weight showing that it is a hypertonic solution. Teachers can use this experiment to help teach students the concept of osmolarity and tonicity in a real time situation.
Sucrose is a carbohydrate that is used for energy. Potatoes contain 1% fructose, glucose, and sucrose by weight . That is a very small amount to how much weight it contains. Molecules are constantly moving and tend to move from a region of higher concentration to a region of lower concentration. Diffusion is the net movement of molecules down their concentration gradient. Diffusion can occur in gases, in liquids, or through solids. Osmosis is the process of water moving across the membrane from an area of high concentration to low concentration. Osmosis is a special case of diffusion where now water is moving across the membrane. T
his process continues until both sides of the membrane are at a dynamic equilibrium state. A hypertonic solution is a solution that contains more solutes on the outside than the inside of the cell, which causes the water move from a high concentration to a low concentration area. If this happens in the potato then the potato will lose weight because the water will have left its cells. If there was a potato cell in a hypotonic solution it will have gained weight because it would be gaining water.
The water is moving from outside the cell to inside the cell because of the low concentration of water in the cell. It has a low concentration of water inside the potato cell because of the many solutes it contains. If an isotonic solution is interacting with a cell then that means that the concentration of water outside and inside of the cell is equal. If the potato cell is in this solution then it will not lose weight or gain weight because its cells are not expanding or shrinking from the concentration of water because it is equal on both sides of the cell membrane  Shown below is graph of the results of another experiment similar to this one (Fig 1).
Figure 1. Effect of different molarities on potato mass by other students.
Materials and Methods
The independent variable for this experiment was the concentration of sucrose measured by its molarity. The dependent variable was the mass of the potato cuboids, measured in grams. The conditions that held constant were air pressure and temperature. The same type of solution which was sucrose was used and the surface area to volume ratio was the same in all potato cuboids. We extracted the cuboids from a Yukon gold potato and these same cuboids were used in the treatments.
The process included cutting up a potato into twenty five 2 cm cuboids and grouping those cuboids into 5. A french-fry cutter was used to cut up these potatoes into strips and placed into water. In the meantime each beaker was filled with 40ml of sucrose solutions. Each beaker was labeled with the different molarities that included 0.2, 0.4, 0.6, 0.8, and 1.0. Once the beakers were labeled and filled with solution, the potatoes were taken out of the water and placed on the lab bench. Using a ruler the length was measured to 2cm and cut the same length using a scalpel. Those potato cuboids mass were measured in groups of 5 then placed those same 5 grouped cuboids in 5 different beakers filled with the different solutions of sucrose. Each different molarity represented the different treatments. Final measurement of weight was recorded after 48 hours. There were 12 repetitions of each treatment.
The potato cuboids placed into the sucrose solution of 0.2 and 0.4 shows an average positive percent change in mass indicating that they gained weight. This shows that the solution was hypotonic. On the other hand, the potato cuboids placed into the sucrose solutions of 0.6, 0.8 and 1.0 show an average negative change in percent mass indicating that they lost weight. This shows that they were in a hypertonic solution. Between the molarities of 0.2 and 0.4, the percent mass change was zero indicating the solution at that point was isotonic. The x intercept of the graph is 0.36 which is the point where the solution was isotonic (Fig.2).
Figure 2.Effect of different molarities of sucrose solutions on mass of a potato.
The potato cuboids placed into the sucrose solution with the molarities of 0.2 and 0.4 gained weight showing that this is a hypotonic solution because a hypotonic solution is a solution that contains more solutes on the outside than the inside which causes the water to move from outside the cell to inside the cell due to the low concentration of water in the cell. Therefore the potato gained weight because it was gaining water. The potato cuboids placed into the sucrose solution with the molarities of 0.6, 0.8 and 1.0 lost weight showing that this is a hypertonic solution because it has a high concentration of water inside the potato cell due to the many solutes it contains.
This causes the water move from a high concentration to a low concentration area. This happened in the potato because the water left its cells causing it to lose weight. When the average percent mass is 0, the concentration is 0.36 showing that this is isotonic because an isotonic solution is when the cell is interacting which means that the concentration of water outside and inside of the cell is equal. This shows that the potato in this solution then will not lose weight or gain weight because its cells are not expanding or shrinking from the concentration of water because it is equal on both sides of the cell membrane. Therefore when the percent mass change is at the value of 0 then the cell hasn’t gained or lost weight.
The results of the investigation are consistent with the results shown in the background overall. For instance both data gains weight from the molarity of 0.2 to 0.3 and both lose weight between 0.4 and 0.6. The concentration when both potatoes are isotonic are different but at close range. The investigation shows the potato being isotonic at 0.36 while the other investigation shows the potato being isotonic at 0.29. The concentration at which potatoes seems to be isotonic lies between 0.3 and 0.4 looking at the data in both findings. The concentration at which potatoes are hypotonic lies between 0.2 and 0.3 and the concentration at which potatoes are hypertonic lies between 0.4 and 0.6 looking at the data in both findings.
A weakness in design was in the manner data was collected. The weakness was that multiple persons collected the data therefore only the trial that person was working on did the person know about what happened and if any issues came up which affected the data. When the data was combined between group 1 and group 2 there was a drastic change in the values which is unexplainable because group 1 and group 2 collected data separately. Even in the groups the data was collected separately.
Further research might include researching other plants or even other types of eukaryotes regarding their osmolarity. Different plants or eukaryote can have different osmolarity but it will be interesting to show the pattern in other real time situations.
The hypothesis was supported by the results. When the potato was in a hypotonic solution it gained weight while when it was in a hypertonic solution it lost weight to balance out the solute concentration. As for when it was isotonic, it gained no weight. Further research regarding other plants and other Eukaryotes can be used to the pattern continue in real time situation. This is helpful towards understanding the relationship between tonicity and osmolarity.
1-Bruso, Jessica. “The Natural Sugar Concentration in Potatoes.” SFGate. Hearst Communications, n.d. Web. 23 Oct. 2014.
. 2-Kosinski, Robert. “Osmosis and the Water Potential of Potato Tissue.” Oct. 2014. MS. 3-Holtzclaw, Theresa Knapp. “Diffusion and Osmosis.” The Biology Place. Pearson, n.d. Web. 19 Oct. 2014. .
4-Education, Clayton. “The Effect of Osmosis on Potatoes.” 19 Sept. 2011. Raw data.
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