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 beakersas 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 thebeaker. Use your thumb and pointer finger to rub the tubing betweenyour fingers; this will open the tubing. Close one end of thetubing by folding over 3.0 cm of one end (this will become thebottom). Fold it again and secure with a yellow rubber band(use

4. Tie a knot in the remaining dialysis tubing just above orjust below the rubber band. This will create a seal and ensuresthat solution will not leak out of the tube later in theexperiment.

5. To test that no solution can leak out, add a few drops ofwater to 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 theinstructions provided on the bottle?s label (your kit contains 60 gof sucrose in a chemical bottle) . This will create 200 mL of a 30%stock sucrose solution.

8. Use Table 2 to create additional sucrose solutions that are30%, 15% and 3% concentrated, respectively. Use the graduatedcylinder and waste beakers to create these solutions. Set thesesolutions aside.

Table 2: Serial Dilution Instructions

Sucrose Solution mL of Stock Sucrose Solution Needed mLof Water Needed

30% 10 0

15% 5 5

3% 1 9

3% 1 9

9. Pour 150 mL of the remaining stock sucrose solution intoBeaker 1.

10. Use some of the remaining stock sucrose solution to createan additional 200 mL of a 3% sucrose solution into Beaker2.

Hint: Use your knowledge of serial dilutions to create this final,3% sucrose solution.

11. Measure and pour 10 mL of the remaining 30% sucrosesolution into the dialysis bag with the yellow rubber band. Sealthe top of this tubing with the remaining yellow rubberband.

12. Measure and pour 10 mL of the 15% sucrose solution in thebag with the red rubber band, and seal the top of the dialysistubing with 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 eachbag in 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 eachdialysis bag. Include your hypotheses, along with supportingscientific reasoning in the Hypotheses section at the end of thisprocedure.

16. Allow the bags to sit for one hour. While waiting, pourout the water in the 250 mL beaker that was used to soak thedialysis tubing in Step 1. You will use the beaker in Step19.

17. After allowing the tubing to sit for one hour, remove themfrom the beakers.

18. Carefully open the tubing. The top of the tubing may needto be cut off/removed as they tend to dry out over the course of anhour. Measure the solution volumes of each dialysis bag using the100 mL graduated 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.

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