Diffusion June 6th, 2017
Movement of materials from regions of high concentration to region of low concentration
Powered by differences in potential energy
Osmosis is a kind of passive diffusion; diffusion of water
Osmosis
Osmoregulation is essential for survival: cell can lyse or shrivel
Demonstration Prediction: The green dye will diffuse evenly through the beaker.
Factors that affect rate of diffusion:
- Temperature
- Concentration Gradient
Experiment 1: The Effect of a Solute Concentration Gradient on Rate of Osmosis
- Sample A: 0% inside cell, 20% outside cell
- Sample B: 0% inside cell, 60% outside cell
- Sample C: 20% inside cell, 0% outside cell
Experiment 2: The Effect of Temperature on Rate of Osmosis
- All samples have 60% inside cell, 0% outside cell.
- Sample A: 0 degrees Celsius
- Sample B: 21 degrees Celsius
- Sample C: 37 degrees Celsius
- Sample D: 60 degrees Celsius
General Biology I Lab
Osmosis Lab Report
July 12th, 2017
The Effect of Solute Concentration on Rate of Osmosis
Introduction
The purpose of this experiment was to investigate the effect of a solute concentration gradient on the rate of osmosis across a semipermeable membrane. It was hypothesized that an increased difference in solute concentration across the concentration gradient would result in an increased rate of osmosis. Furthermore, it was predicted that if the hypothesis was correct, then Sample B would decrease more in mass than Sample A, while Sample D would increase more in mass than Sample C. Sample A and Sample B both contained cells with lower solute concentrations inside the cell than outside the cell, while Sample C and Sample D contain cells with higher solute concentrations inside the cells than outside the cell. This prediction was made using information on osmoregulation, osmosis, and diffusion behavior in cells.
Osmosis, a variety of passive diffusion, is the movement of water in and out of a cell (Laboratory Manual for General Biology I BSC 2010L., page 27). Osmosis can occur, while active diffusion can not, because the cell membrane is semipermeable. This means the membrane will allow water through naturally, but not charged ions- without the input of energy (Urry et al, page 132). A cell uses osmotic regulation, or osmoregulation, to direct free water in an environment to the area with the highest concentration to decrease the concentration gradient (Urry et al, page 132). A concentration gradient in a cell is the difference in potential energy created by a difference in concentrations on either side of a semi-permeable membrane (Urry et al, page 132). When free water moves out of the cell to a more concentrated environment, it is hypertonic (Laboratory Manual for General Biology I BSC 2010L., page 27). When free water moves into a cell because it is the more concentrated than the environment, it is called hypotonic (Laboratory Manual for General Biology I BSC 2010L., page 28). If the solute concentrations are the same both inside and outside the cell, then it is called isotonic.
In this experiment, Sample A and Sample B both contained cells with lower solute concentrations inside the cell than outside the cell, making them hypertonic. However, Sample B was placed in a more concentrated environment of 60 percent solute, which means osmosis should occur at a quicker rate. Sample C and Sample D both contained cells with higher solute concentrations inside the cell than outside the cell, making them hypotonic. However, Sample D contained a more concentrated cell of 60 percent solute, which means osmosis should occur at a quicker rate. Sample E, the isotonic control group, contained water and was placed in a water environment. In order to test this prediction, five trials cell composed of dialysis tubing were filled with solutions containing varying solute concentrations. The trials were then placed into either hypertonic, hypotonic, or isotonic environments and allowed to osmosize a total of 60 minutes.
Procedure
In this experiment, five dialysis tube bags approximately five inches long were filled with 3 mL of the indicated solution, then folded on the ends and tied off using floss. Each bag was weighed using a triple beam balance, then placed into a beaker filled with just enough solution to cover the dialysis bag. At fifteen minute intervals, the bags were removed from the beakers, blotted dry, and weighed on the triple beam balance. Cells were allowed to undergo osmosis for a total of 60 minutes.
The standardized variables in this experiment included temperature of solutions, amount of solution inside the cell, and method of constructing the cell. All solutions used in the experiment were room temperature, for both inside the cell as well as the cellular environment. Each cell contained 3 mL of solution. Each dialysis bag was folded over and then tied with floss that was knotted three times on each end of the tube.
Results
Table 1: The Effect of Solute Concentration on Rate of Osmosis
Group
|
Bag
|
Beaker
|
0 min (g)
|
15 min
(g)
|
30 min
(g)
|
45 min
(g)
|
60 min
(g)
|
Total change
(g)
|
Rate of Osmosis (g/min)
|
A
|
Water
|
20% sucrose
|
3.4
|
3.7
|
3.1
|
2.9
|
2.6
|
0.80
|
-0.013
|
B
|
Water
|
60% sucrose
|
3.4
|
2.9
|
2.5
|
2.0
|
1.6
|
1.8
|
-0.030
|
C
|
20% sucrose
|
Water
|
3.6
|
4.6
|
4.7
|
5.2
|
8.1
|
4.5
|
+0.075
|
D
|
60% sucrose
|
Water
|
3.8
|
5.5
|
5.7
|
7.6
|
7.6
|
3.8
|
+0.063
|
E
|
Water
|
Water
|
3.3
|
3.3
|
3.3
|
3.3
|
3.3
|
0
|
0
|
The table above describes the bag weights for four treatment groups composed of artificial cells exposed to varying environments over sixty minutes. Treatment groups A and B were in hypertonic conditions, while treatment groups C and D in hypotonic conditions. Group E, the control, was exposed to isotonic conditions.
Figure 1: The Effect of Concentration on Change in Cell Volume
The figure above is a graphical analysis of the effect of changing the solute concentration gradient on the rate of osmosis. Treatment group A was a cell composed of pure water in a 20% sucrose solution environment. Treatment group B was a cell composed of pure water in a 60% sucrose solution environment. Treatment group C was a cell composed of 20% sucrose solution exposed to a pure water environment. Treatment group D was composed of 60% sucrose solution exposed to a pure water environment. Control group E was composed of pure water and exposed to isotonic conditions.
Figure 2: The Effect of Solute Concentration on Rate of Osmosis
The figure above compares the rate of osmosis for all five trial cells over a period of sixty minutes. Treatment group A’s cell was water in a 20% sucrose solution environment, while Treatment group B’s cell was water in a 60% sucrose solution environment. Treatment group C’s cell was 20% sucrose solution in a water environment. Treatment group D’s cell was 60% sucrose solution in a water environment. Control group E was a cell filled with water in a water environment.
The results of this experiment prove that cells placed in a hypertonic solution lost weight, while cells placed in hypotonic solutions gained weight (see Figure 1). The greatest loss of cell volume occurred in treatment group B of a 60% sucrose environment with a 1.8 gram total decrease (see Table 1). Treatment group A exhibited also exhibited a loss of volume, though lower at only 0.80 grams lost (see Table 1). The greatest gain in cell cell volume occurred in treatment group C of a 20% sucrose cell solution, with a 0.9 gram total increase (see Table 1). Treatment group D gained volume, but not as much, with only a 3.8 gram increase (see Table 1). Treatment groups B and C showed the greatest rates of increase, rate of osmosis at -0.030 g/min and 0.075 g/min respectively (see Figure 2). There was no change in weight or rate of osmosis is control group E (see Figure 2). As seen in Figure 1, the cells continued to exhibit a change in volume even as the final measurements were taken.
Discussion
The results of this experiment were inconclusive for the hypothesis that an increased difference in solute concentration across the concentration gradient would result in an increased rate of osmosis. The outcome of the experiment did not fully support the predictions that were made. It is correct that the cells in hypertonic solutions lost weight, while the cells in hypotonic solutions gained weight (see Figure 1). The cell placed in an isotonic environment exhibited no osmosis. However, the trend of a greater solute concentration yielding a greater rate of osmosis was exhibited only in Cells A and B in hypertonic solutions, but not Cells C and D in hypotonic solutions (see Figure 2). The cell placed is a more hypertonic environment did show a greater rate of osmosis (see Table 1). Cell B in 60% sucrose solution showed a -0.017 g/min greater rate of osmosis than Cell A in 20% sucrose solution (see Table 1). However, Cell D containing a concentration of 60% sucrose only had a rate of osmosis of 0.063 g/ min, while Cell C containing a concentration of 20% sucrose had a much greater rate of osmosis of at 0.075 g/ min (see Figure 1). This only partially proves the hypothesis, which makes the overall results invalid.
A cell changes its intracellular weight due to the movement of water in or out of the cell; this process is called osmosis (Laboratory Manual for General Biology I BSC 2010L., page 27). The difference in concentration between the inside of the cell and the outer membrane surroundings of the cell create a difference in potential energy known as a concentration gradient (Urry et al, page 132). This concentration gradient allows cells to osmoregulate as water travels to the more concentrated area either inside or outside the cell through osmosis (Urry et al, page 132). As more water enters the area of higher concentration, the water molecules bond to the solute and the concentration goes down. This regulation process is vital to the survival of cells. If too much water is released from a cell in a hypertonic environment, the cells could shrivel and die. If too much water is taken into a cell in a hypotonic environment, it could burst.
Cells exist in either hypertonic, hypotonic, or isotonic cellular environments. For cells in hypertonic solutions, water travels out of the cell and into the environment, leading to a negative change in weight. Cells in hypotonic environments take in water from the environment, which creates a positive change in weight. Isotonic cells contain an equal concentration of solute to that of the environment, which generates no osmotic rate of change. However, some environments are more hypertonic or more hypotonic than others. A greater difference in concentration gradient allows for a greater rate of osmotic change because there is a greater difference in potential energy.
According to biological theory, the experiment should have gone very differently. Comparison of these results to other research showed overall greater changes in Sample B and Sample D. It was predicted that if the hypothesis was correct, then Sample B would decrease more in mass than Sample A, while Sample D would increase more in mass than Sample C. This was because the increased solute concentrations generated an increased concentration gradient in Samples B and D. Sample B did decrease more in mass than Sample A. However, Sample D did not increase more in mass than Sample C. As shown in Figure 2, the graphical comparison of treatment group C in comparison to treatment group D shows that treatment group C was behaving as expected until the last measurement was taken, which was significantly higher than any other previous points. This unpredicted result could have a variety of explanations; however, the most likely one is there was added weight in last measurement that should not have been there. When weighing the cells on the triple beam balance, some extra liquid clung to the outside of the dialysis tubing after removing it from the environment, despite gently blotting it dry. This would have increased the masses of the cells inconsistently from measurement to measurement, allowing Cell C to appear to weigh more than Cell D and show an incorrect rate of change.
The possible explanation for the unexpected experimental results discussed above is also a weaknesses in the experimental design and execution. Also, water that clung to the outside of the cell when removed would pool at the bottom of the container used for weighing the cells. By adding a step in the procedure to remind scientists to dry out the container could allow for more accuracy in further experiments. Also, a better tool to mass the weight of the cells, like a digital scale, could allow for more accuracy when taking measurements. One way this experiment could be adapted would be to add dye to the sucrose solution, so the change is the cell as it went through osmosis would be more apparent. This experiment could be extended by increasing the volumes in the cell and in the environment to see if there is proportionally a greater ratio for the rate of change.
Conclusion
The results of this experiment were inconclusive for the effect of solute concentration on the rate of osmosis. While cells releasing water to a more concentrated hypertonic environment exhibited a greater rate of change, cells absorbing water from a hypotonic environment did not show a greater rate of change for a more concentrated intracellular environment.
Literature Cited
Urry, Lisa A., Michael Lee Cain, Steven A. Wasserman, Peter V. Minorsky, Jane B. Reece, and Robert B. Jackson. Campbell Biology. New York: Pearson, 2017. Print.
Laboratory Manual for General Biology I BSC 2010L. Hillsborough
Community College. 2016. Print
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