Experiment Essay Sample on Osmosis
Abstract
Osmosis, the diffusion of water through a cell membrane, is a method of cellular transport that can be affected by different solutions. Depending on the concentration of a solution outside of the cell, a solution can either be classified as hypotonic, hypertonic, or isotonic. Red onion skin cells were submerged in 5 different solutions and studied under a microscope to observe the levels of shrinkage caused by different solutions. Depending on the solution, various levels of shrinkage were observed; in some samples, there was swelling or no change. When compared to each other, the NaCl solutions showed more shrinkage than the glycerol and sucrose solution samples.
Introduction
Cell transport is the movement of a solute or solution across a cell membrane. Osmosis is a subsection of cell transport, and it is specifically the movement of water across a membrane. This membrane is semipermeable, meaning that water is allowed through but other substances, like salt, aren’t allowed through (Ndiaye et al. 2021). The tonicity of a cell refers to the effect that the movement of water in osmosis has on a cell; the three classifications of tonicity are hypertonic, hypotonic, and isotonic. If a solution is hypertonic, that means that the concentration of the solution outside the cell is greater than the solution inside the cell. Due to the pressure difference, the movement of water is out of the cell and results in a shriveled appearance. In a hypotonic solution, the concentration of the solution outside the cell is lower than the solution in the cell. This causes water to move inside of the cell and creates a swelled appearance. If a solution is isotonic, that means that the concentrations both inside and outside the cell membrane are equal. Because of this, there is no noticeable physical change to the cell. Overall, the movement of water is dictated by the concentration inside and outside of the cell; it will move towards the solution with greater concentration (Freeman et al. 2020). In this experiment, the cell transport of water, glycerol, sucrose, 5% NaCl, and 10% NaCl will be observed in red onion skin cells. I specifically studied the effect of glycerol on red onion skin cell transport. I hypothesized that all of the solutions, except water, would be hypertonic. I assumed that water would be hypotonic and cause the cells to swell. Since the 5% and 10% NaCl solutions contained salt, they would be more hypertonic than the other solutions.
Methods and Materials
First, a scalpel was obtained to cut off a small slice of red onion; it was no larger than 1cm x 1cm. Tweezers were then used to peel off a very thin layer of the outer red onion skin, which was promptly placed on a slide; if the sample taken was too thick, the coverslip wouldn’t be able to secure properly and the experiment would not work. Now that the slide was prepared, one drop of glycerol was added to the slide and secured with a coverslip. Other members tested different solutions in the same manner; water, sucrose, 5% NaCl, and 10% NaCl were tested by others. Immediately after it was prepared, the slide was observed under a microscope in order to observe shrinkage at 0 minutes. Further signs of shrinkage were observed at 5, 20, 40, and 60 minute intervals. Data was recorded based on qualitative observations in intervals of slight shrinkage, medium shrinkage, and large shrinkage. Not only was shrinkage observed, but cell volume recovery had to be kept in mind as well. Once 60 minutes had passed, the experiment was concluded and the slide was cleaned.
Results
The “+” symbol was used to record shrinkage, as seen in the table below. For no shrinkage, “0” was used. The “-” symbol represents growth since it is the opposite of the symbol denoting shrinkage. The more symbols consecutively used, the more shrinkage that had been observed. The results were based on qualitative observations and the number of symbols used was meant to make the results quantitative for future analysis.
Solution
Shrinkage at 0 Minutes
Shrinkage at 5 Minutes
Shrinkage at 20 Minutes
Shrinkage at 40 minutes
Shrinkage at 60 minutes
Water
0
0
0
0
0
Sucrose
0
0
+
0
0
Glycerol
0
0
-
0
0
5% NaCl
0
+
+
+
+
510% NaCl
0
+
+++
+
+
The red onion skin cells showed no signs of shrinkage when in water; water was the only sample that showed no changes. When it was exposed to sucrose, slight shrinkage was seen at the 20 minute mark, but it remained relatively unchanged after. Rather than show signs of shrinkage, the cells submerged in glycerol showed signs of expanding rather than shrinking. The sample exposed to glycerol was the only solution that displayed swelling rather than shrinkage. Shrinkage was observed in both NaCl solutions, but it was more prominent in the 10% NaCl solution. The 5% NaCl solution showed a steady increase in shrinkage. At 20 minutes, 10% NaCl showed large amounts of shrinkage; out of all of the solutions, it displayed the most shrinkage. Compared to the rest of the solutions, the 5% NaCl solution was the only sample to show shrinkage at a constant rate over time.
In the graph below, shrinkages among all solutions are compared together to better illustrate the results stated previously. The “+” was converted to 1, and “-” was converted to -1. Graphically, it is clearer to see the drastic difference in shrinkages of the NaCl solutions compared to the other samples.
Discussion
In theory, glycerol is a hypertonic solution and shrinkage should have been observed. While it had been initially assumed that glycerol was hypotonic based on the results, that could have been a display of cell recovery instead. It is also hydrophobic, so there may have been water on the red onion I took a slice from that altered my results. The NaCl solutions were hypothesized to be hypertonic, and they were. Since the 10% NaCl solution showed more shrinkage than the 5% NaCl solution, the former is more hypertonic than the latter. The results of this experiment indicate the higher the concentration of NaCl, the more hypertonic the solution will be. In my hypothesis, I originally assumed that water would be hypertonic, but it was actually isotonic according to my results. According to a replication of the experiment done by Lankford and Friedrichsen, water moved inside of the cell and caused the vacuoles to swell, indicating that it is hypotonic. They also used a 20% sucrose solution, which was a different concentration from the sucrose solution used in the lab, and shrinkage was observed. The usage of different concentrations of solutions could potentially be explored in a different experiment. Since it was already explored in the 5% and 10% NaCl solutions, different concentrations of sucrose or glycerol could be used to test the effects of different solution concentrations on the rate of osmosis in red onion cell skin.
One limitation I came across when conducting this experiment was obtaining the slice of red onion skin cells. It was difficult to get a piece thin enough to fit under the coverslip; I had to redo my slide twice because I kept getting samples that were too big. The tweezers provided were thick, which caused the red onion skin to tear easier. I believe that a thinner pair of tweezers would more effectively peel off the layer of red onion skin. Regarding the red onion skin sample, a limitation of this study is that multiple red onions had been used; different lab members potentially took samples from different onions. Since different samples were taken, it could have potentially altered the results of other findings. The onions could have come from different sources, and it is possible that the age of the onion could have affected the quality of the onion skin sample.
In a future study, this experiment could be replicated utilizing different parts of the onion. Potentially, the inner layers of the red onion could be used as samples to see how different layers would affect cell transport. This experiment could also be replicated using different samples or solutions. In an article by Lankford and Friedrichen, they explored the pros and cons of using different types of samples to observe osmosis. Not only are red onion skin cells tested, but potato skin cells and decalcified chicken eggs were used as well. However, the differences in samples could potentially lead to different representations. In the experiment I performed, osmosis was observed on a microscopic level. Lankford and Friedrichen introduce different levels of viewing osmosis: Macroscopic, Microscopic, and Virtual. The level of viewing would depend entirely on the accessibility of materials required for the experiment (Lankford, Friedrichsen, 2012).
