In this experiment you will use bags made of dialysis tubing to demonstrate membrane selectivity. Dialysis tubing is a synthetic membrane that allows small molecules to pass through but prevents large molecules from changing sides of the tubing. You will use two indicators for this experiment: phenolphthalein and iodine. Beaker A is filled with water and NaOH and the dialysis tube is filled with phenolphthalein. Initially the beaker filled with water and NaOH and was the dialysis tube filled with phenolphthalein was both colorless. After 30 minutes the beaker filled with water and NaOH turned into purple color and the dialysis tube filled with phenolphthalein was colorless. In Beaker B, the beaker is filled with water and iodine and the dialysis tube was filled with starch. Initially the beaker filled with water and iodine was yellow-orange and the dialysis tube filled with starch was both colorless. After 30 minutes, the beaker filled with water and iodine was still yellow-orange and the dialysis tube with starch turned dark blue. Which molecules and ions (phenolphthalein, iodine, starch, Na+, OH-, water) does your experiment give evidence for passage through the semipermeable membrane? Also, in which direction (into or out of the bag) are the molecules moving?
In this experiment you will use bags made of dialysis tubing to demonstrate membrane selectivity. Dialysis tubing is a synthetic membrane that allows small molecules to pass through but prevents large molecules from changing sides of the tubing. You will use two indicators for this experiment: phenolphthalein and iodine. Beaker A is filled with water and NaOH and the dialysis tube is filled with phenolphthalein. Initially the beaker filled with water and NaOH and was the dialysis tube filled with phenolphthalein was both colorless. After 30 minutes the beaker filled with water and NaOH turned into purple color and the dialysis tube filled with phenolphthalein was colorless. In Beaker B, the beaker is filled with water and iodine and the dialysis tube was filled with starch. Initially the beaker filled with water and iodine was yellow-orange and the dialysis tube filled with starch was both colorless. After 30 minutes, the beaker filled with water and iodine was still yellow-orange and the dialysis tube with starch turned dark blue. Which molecules and ions (phenolphthalein, iodine, starch, Na+, OH-, water) does your experiment give evidence for passage through the semipermeable membrane? Also, in which direction (into or out of the bag) are the molecules moving?
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Experiment 2: Osmosis - Direction and ConcentrationGradients
In this experiment, we will investigate the effect of soluteconcentration on osmosis. A semi-permeable membrane (dialysistubing) and sucrose will create an osmotic environment similar tothat of a cell. This selective permeability allows us to examinethe net movement of water across the membrane. You will begin theexperiment with a 30% sucrose solution, and perform a set of serialdilutions to create lower concentration solutions. Some of thesucrose concentrations will be membrane permeable; while otherswill not be permeable (can you determine why this is?).
Materials
(3) 250 mL Beakers
(1) 10 mL Graduated Cylinder
(1) 100 mL Graduated Cylinder
Permanent Marker
*8 Rubber Bands (2 blue, 2 green, 2 red, and 2 yellow)
60 g Sucrose (Sugar) Powder, C12H22O11
4 Waste Beakers (any volume)
*Paper Towels
*Scissors
*Stopwatch
*Water
*(4) 15 cm. Pieces of Dialysis Tubing
*Contains latex. Please handle wearing safety gloves if you have alatex allergy.
*You Must Provide
*Be sure to measure and cut only the length you need for thisexperiment. Reserve the remainder for later experiments.
Procedure
1. Use the permanent marker to label the three 250 mL beakers as 1,2, and 3.
2. Cut four strips of dialysis tubing, each 15.0 cm long. FillBeaker 3 with 100 mL of water and submerge the four pieces ofdialysis tubing in the water for at least 10 minutes.
3. After 10 minutes, remove one piece of tubing from the beaker.Use your thumb and pointer finger to rub the tubing between yourfingers; this will open the tubing. Close one end of the tubing byfolding over 3.0 cm of one end (this will become the bottom). Foldit again and secure with a yellow rubber band (use
4. Tie a knot in the remaining dialysis tubing just above or justbelow the rubber band. This will create a seal and ensures thatsolution will not leak out of the tube later in theexperiment.
5. To test that no solution can leak out, add a few drops of waterto the tubing and look for water leakage. If any water leaks,tighten the rubber band and/or the knot in the tubing. Make sureyou pour the water out of the tubing before continuing to the nextstep.
6. Repeat Steps 4 - 5 with the three remaining dialysis tubes,using each of the three remaining rubber band colors.
7. Reconstitute the sucrose powder according to the instructionsprovided on the bottle�s label (your kit contains 60 g of sucrosein a chemical bottle) . This will create 200 mL of a 30% stocksucrose solution.
8. Use Table 2 to create additional sucrose solutions that are 30%,15% and 3% concentrated, respectively. Use the graduated cylinderand waste beakers to create these solutions. Set these solutionsaside.
Table 2: Serial Dilution Instructions
Sucrose Solution mL of Stock Sucrose Solution Needed mL of WaterNeeded
30% 10 0
15% 5 5
3% 1 9
3% 1 9
9. Pour 150 mL of the remaining stock sucrose solution into Beaker1.
10. Use some of the remaining stock sucrose solution to create anadditional 200 mL of a 3% sucrose solution into Beaker 2.
Hint: Use your knowledge of serial dilutions to create this final,3% sucrose solution.
11. Measure and pour 10 mL of the remaining 30% sucrose solutioninto the dialysis bag with the yellow rubber band. Seal the top ofthis tubing with the remaining yellow rubber band.
12. Measure and pour 10 mL of the 15% sucrose solution in the bagwith the red rubber band, and seal the top of the dialysis tubingwith the remaining red rubber band. 10 mL of the 3% sucrosesolution in the bag with the blue rubber band, and seal thedialysis tubing with the remaining blue rubber band. The final 10mL of 3% sucrose solution in the bag with the green rubber band.Seal the dialysis tubing with the remaining green rubberband.
13. Verify and record the initial volume of solution from each bagin Table 3.
Figure 8: The dialysis bags are filled with varying concentrationsof sucrose solution and placed in one of two beakers.
14. Place the yellow, red, and blue banded tubing in Beaker 2.Place the green banded tubing in Beaker 1 (Figure 8).
15. Hypothesize whether water will flow in or out of each dialysisbag. Include your hypotheses, along with supporting scientificreasoning in the Hypotheses section at the end of thisprocedure.
16. Allow the bags to sit for one hour. While waiting, pour out thewater in the 250 mL beaker that was used to soak the dialysistubing in Step 1. You will use the beaker in Step 19.
17. After allowing the tubing to sit for one hour, remove them fromthe beakers.
18. Carefully open the tubing. The top of the tubing may need to becut off/removed as they tend to dry out over the course of an hour.Measure the solution volumes of each dialysis bag using the 100 mLgraduated cylinder. Make sure to empty and dry the cylindercompletely between each sample.
19. Record your data in Table 3.
Data Tables and Post-Lab Assessment
Table 3: Sucrose Concentration vs. TubingPermeability
Table 3: Sucrose Concentration vs. TubingPermeability | |||||
Band Color | % Sucrose in Beaker | % Sucrose in Bag | Initial Volume (mL) | Final Volume (mL) | Net Displacement (mL) |
Yellow | |||||
Red | |||||
Blue | |||||
Green | |||||
Hypothesis:
For each of the tubing pieces, identify whether the solutioninside was hypotonic, hypertonic, or isotonic in comparison to thebeaker solution in which it was placed.
Which tubing increased the most in volume? Explain why thishappened.
What do the results of this experiment this tell you about therelative tonicity between the contents of the tubing and thesolution in the beaker?
What would happen if the tubing with the yellow band was placedin a beaker of distilled water?
How are excess salts that accumulate in cells transferred to theblood stream so they can be removed from the body? Be sure toexplain how this process works in terms of tonicity.
If you wanted water to flow out of a tubing piece filled with a50% solution, what would the minimum concentration of the beakersolution need to be? Explain your answer using scientificevidence.
How is this experiment similar to the way a cell membrane worksin the body? How is it different? Be specific with yourresponse.
Cell Membranes Lab Report
- Diffusion
- Compare the following terms: solvent, solute, andsolution.
- Go to Lab, Section I, Exercise 1 to explore the factorsinfluencing the rate of diffusion and complete the following:
- Predict how molecular mass and temperature may affect theoutcome of this experiment.
- Record the data from information in the Lab in the tablebelow.
| Time (min) | Diameter (mm) | |||
5°C MB | 5°C PP | 23°CMB | 23°C PP | |
0 | ||||
15 | ||||
30 | ||||
45 | ||||
- Construct a line graph of the data from the table above. Yourgraph should include labeled axes, units, and a legend indicatingwhich treatment is presented. Sign, date and prepare an image ofyour graph and include it with this lab report.
- Osmosis
- Go to Lab, Section II, Exercise 2 to view a demonstration ofosmosis and answer the following questions:
- Describe the net movement of water in osmometer 1.
- How is the movement of water molecules related to theconcentration gradient of the solution?
- Go to Lab, Section II, Exercise 3 to observe the effect ofsolute concentration on the rate of osmosis. Answer the followingquestions:
- Was the net movement of water in bags 1 to 4 into or out of thebags?
- Explain the results from bags 4 and 5.
- Selective Permeability of Membranes
- Go to Lab, Section III, Exercise 4 to learn about theimportance of selectively permeable membranes, then complete thetable and answer the following questions:
Contents of Beaker | ||
0 Min | 30 Min | |
Starch | ||
Chloride ion | ||
- Which substances diffused through the dialysis membrane?
- How does dialysis tubing model the selective permeability of aplasma membrane?
- Tonicity
- Go to Lab, Section IV, Exercise 5 and 6 to observe plasmolysisin Elodea cells and tonicity in red blood cells. Summarizethe concept of tonicity using blood and Elodea cells asexamples. Your description should incorporate these terms: turgid,plasmolysis, hemolysis, and crenation.
Cells | Isotonic Solution | Hypertonic Solution | Hypotonic Solution |
Elodea | |||
RBC |
- Based on your previous work, reproduce the table below and drawan illustration of a single cell in each box. Include arrows toshow the direction of water flow relative to each cell and dots tosymbolize solutes. Sign, date and prepare an image of your drawingand include it with this lab report.
Cells | Isotonic Solution | Hypertonic Solution | Hypotonic Solution |
Elodea | |||
RBC |
Summary Questions
- Compare diffusion and osmosis. Give an example of each.
2. In which direction will osmosisoccur if a 15% sugar solution is separated from a 25% sugarsolution by a selectively permeable membrane?
3. Why did osmosis, but not diffusionof sucrose molecules, occur across the dialysis membrane containing20% sucrose solution?
4. You are having a party and thecelery is limp. What might you do to make the celery crisp (turgid)again? What will occur in the cells?
5. A small amount of fertilizer(mineral salts) will stimulate plant growth, but over fertilizationcan kill plants? Why?
6. Michael adds sugar to his coffee.Explain what in this drink is the solvent, solute, and solution.
7. You add a cube of sugar to yourdrink. How could you speed up the diffusion of sugar moleculeswithout stirring?
8. How is dynamic equilibriumestablished within a solution?
