Osmosis is a special kind of diffusion; it is the diffusion of water through a selectively permeable membrane (a membrane that allows for diffusion of certain solutes and water) from an area of higher water potential to an area of lower water potential. Water potential is the measurement of free energy of water in a solution.
Diffusion and osmosis do not entirely explain the movement of ions or molecules into and out of cells. Active transport is another property of a living system. The process of active transport uses energy from ATP to move substances through a cell membrane. The substances are usually moves against a concentration gradient, from a region of low concentration of a certain substance to a region of higher concentration. Diffusion, osmosis, and active transport are of interest because they explain how living cells are able to survive with changing conditions. Through diffusion, osmosis, and active transport, cells are able to maintain a constant and stable environment and protect their own integrity. They are able to absorb needed nutrients and rid themselves of waste products.
Material and Methods
To measure diffusion of small molecules through dialysis tubing, a 30-cm piece of 2.5-cm dialysis tubing that had been soaking in water was obtained. One end of the bag was tied in a knot to form a bag. 15 mL of a 15% glucose/1% starch were placed in the bag. The other end of the bag was then tied, leaving sufficient space for the expansion of the contents in the bag. The color of the solution and the presence of glucose were then recorded. A 250-mL beaker was filled with distilled water. 4 mL of IKI were added to the distilled water and the color of the solution and the presence of glucose were recorded. Then the bag was immersed in the beaker of solution for 30 minutes. After 30 minutes, the color of the solution in both the bag and the beaker were noted. The liquid in the bag and beaker were then tested for the presence of glucose. The results were recorded.
To investigate osmosis, two 30-cm strips of presoaked dialysis tubing were obtained. One end of each tubing was tied. In one tubing, 25 mL of 0.4 M sucrose was poured. In the other tubing, 25 mL of 0.6 M sucrose was poured. The other end of the bags was tied, leaving sufficient space for the expansion of the contents. The initial mass of each bag, expressed in grams, was recorded. Each bag was then place in an empty 250-mL beaker and a label was placed on the beakers to indicate the molarity of the solution in the bag. The beakers were then filled two-thirds full with distilled water. They were left standing for 30 minutes. Following the 30 minutes, the bags were removed from the water and the mass of the bags were recorded. Data was obtained from other lab groups.
To determine the water potential of potato cells, 100 mL of 0.4 M and 0.6 M sucrose solution was poured into two labeled 250-mL beaker. Eight potato cylinders were then cut and the skin was removed. The mass of each of the four cylinders was recorded. Four cylinders were placed in each beaker. The beakers were covered and stood overnight. The next day, the cores were removed from the beaker and the mass of the cylinders was recorded. The class data was then pooled and recorded. The percentage change was calculated. The class data for the percentage change in mass was then graphed.
Results
Table 1.1 shows the initial and final solution color, as well as the initial and final presence of glucose in the bag with 15% glucose/1% starch and the beaker with H2O and IKI. Table 1.2 shows the initial and final mass of each of the solutions in the separate bags. Mass difference and the percent charge in mass (((final – initial) / initial) x 100) of the solutions was calculated and recorded. Table 1.3 shows the class data for percent change in mass of the dialysis bags for the experiment. Table 1.4 shows the initial and final mass of the potato cores in the different solution as well as the mass difference, percent change in mass, and class average percent change in mass. Table 1.5 shows the class data for the potato core results. Graph 1.1 shows the class average for the percent change in mass of the dialysis bags. The independent variable is the solution. The dependent variable is the percent change in mass of the dialysis bags. Graph 1.2 is the class average for the percent change in mass of the potato cores at different molarities of sucrose. The independent variable is the sucrose molarity within the beaker. The dependent variable is the percent increase or decrease in mass of the potato cores.
Table 1.1
Calculations
Table 1.2: Dialysis Bag Results – Individual Data
Table 1.3: Dialysis Bag Results - Group Data
Calculations of Percent Change in Mass:
Formula: ((Final Mass – Initial Mass) / Initial Mass) x 100
Work:
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M Distilled Water = ((21.84 – 21.87) / 21.87) x 100 = -.137
0.2 M Sucrose = ((17.40 – 16.58) / 16.58) x 100 = 4.946
0.4 M Sucrose = ((23.31 – 21.30) / 21.30) x 100 = 9.437
0.6 M Sucrose = ((25.18 – 21.65) / 21.65) x 100 = 16.305
0.8 M Sucrose = ((31.94 – 27.21) / 27.21) x 100 = 17.383
1.0 M Sucrose = ((37.32 – 30.81) / 6.51) x 100 = 21.130
Potato Core:
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M Distilled Water = ((9.59 – 8.17) / 8.17) x 100 = 17.381
0.2 M Sucrose = ((9.88 – 9.7) / 9.7) x 100 = 1.856
0.4 M Sucrose = ((6.26 – 6.73) / .673) x 100 = -6.984
0.6 M Sucrose = ((5.02 – 6.49) / 6.49) x 100 = -22.650
0.8 M Sucrose = ((3.86 – 5.94) / 5.94) x 100 = -35.017
1.0 M Sucrose = ((4.27 – 6.42) / 6.42) x 100 = -33.489
Table 1.4: Potato Core – Individual Results
Table 1.5: Potato Core Results – Class Data
Analysis of Results
P. 3/4
1) Water and IKI are entering the bag and 15% glucose/1% starch is leaving the bag to create equilibrium. Experimental evidence to support this evidence is the glucose tests. Initially, there was no glucose in the beaker. After the experiment, glucose was present. The change in color in the bag from cloudy clear to royal blue indicated that IKI had diffused into the bag.
2) The results showed that diffusion occurs between the bag and the beaker. Equilibrium was reached and glucose was now present in the beaker as well as the bag. The same amount of concentration was present in both the bag and beaker. Diffusion is a movement of molecules from an area of higher concentration to an area of lower concentration. The bag had a higher concentration of 15% glucose/1% starch than the beaker, so the glucose molecules move across the membrane to the beaker. The beaker had a higher concentration of H2O and IKI, so the water and IKI molecules moved through the membrane into the bag. Small solute molecules and water molecules can move freely through a selectively permeable membrane, but large molecules will pass through more slowly, or perhaps not at all. The size of the minute pores in the dialysis tubing determined which substances could pass through the membrane.
3) Finding the initial and final mass of the bag and calculating the difference in mass and the percent change in mass showed that water diffused into the dialysis bag. The percent change in mass should be positive.
4) Based on my observations, water molecules are the smallest. Followed by glucose molecules, IKI molecules, starch molecules, and last membrane pores.
5) Equilibrium would be reached with glucose and IKI crossing the membrane to go to the beaker and starch and water crossing the membrane to go into the bag.
P. 7/8
1) The change in mass increase as the molarity of sucrose within the dialysis bags increase.
2) Osmosis would occur. Molecules from the hypertonic solution would move to the hypotonic solution to create equilibrium. The percentage change in mass for 0.0 M distilled water in 0.4 M sucrose would be negative, 0.2 M sucrose would be a slightly higher but still negative, 0.4 M sucrose would be close to 0. 0.6 M sucrose would be slightly positive. 0.8 M sucrose would be a higher positive than 0.6 M sucrose, and 1.0 M sucrose would be the highest positive.
3) The change in mass only shows the difference in grams. The difference in percentage of the weight of the solution is more valuable.
4) ((18-20) / 20) x 100 = -10
5) The sucrose solution in the beaker would have been hypertonic to the distilled water in the bag.
P. 18
1) Plasmolysis is the shrinking of the cytoplasm of a plant cell in response to diffusion of water out of the cell and into a hypertonic solution (higher solute concentration) surrounding the cell.
2) The onion cells are hypotonic to the NaCl. Therefore, water diffuses out of the onion cells and the cells shrink.
3) The salt causes the grass cells to become hypertonic and the cells shrivel and die.
Discussion/Conclusion
The results from the lab helped explain what occurs during osmosis and diffusion. In exercise 1A, the change in the amount of glucose present in the beaker showed that diffusion had occurred (glucose had crossed the membrane from the bag into the beaker. In exercise 1B, we demonstrated how osmosis occurs. Water, from the distilled water in the beaker, moved across the selectively permeable membrane of the bag and increased the mass of the solution in the bags. The higher the molarity of the sucrose in the bag, the more water diffused across the membrane from the beaker into the bag. This explains the increase in the mass of the bag from the initial to the final measurement. In exercise 1C, the potato was put in a solution of increasing molarity. Since the potato was hypotonic to the solution, water left the potato by osmosis and the potato shrunk in size. This was reflected by the increasingly negative percent change in its mass. There are believed to be no great errors that occurred during the experiment except in the 0.8 M and 1.0 M sucrose in the potato core lab. The percent change does not follow the trend it is supposed to follow. Experimental error could have been incorrectly weighing the mass of the cylinders. One would be able to identify an error by seeing a great change between intervals in the percent change in mass of the two experiments. As the molarity increases in exercise 1B, the percent change in mass should increase. As the molarity increases in exercise 1C, the percent change in mass should decrease. Future experiments could include testing a variety of molarities not used during this experiment or a different solution than sucrose. The results of this experiment are very significant. They help explain how cells of an organism can survive under changing conditions. Cells need to be able to obtain nutrients and rid themselves of waste. By osmosis and diffusion, this can be accomplished.