Observe osmosis with a thistle tube osmometer. This device consists of a hollow bell attached to a long tube. The tube is filled with molasses or sugar solution, and the large opening of the tube is covered with dialysis tubing and secured with a rubber band. The thistle tube osmometer is placed in a beaker of water and clamped to a ring stand (see picture below). The level of the molasses or sugar solution is indicated with a wax mark from a china marker.

Examine the setup throughout the lab period. You may find that the liquid in the thistle tube eventually stops rising. This occurs when the gravitational pressure equals the force exerted by the process of osmosis. This amount of force required to balance or equilibrate osmosis is the osmotic pressure.

 **The above text in italics is for reference in the future. For now this has changed slightly. Please refer to the text below these two figures to find out what changes in this excercise we have made and what you need to do**

In class you will see 4 osmometer setups (similar but slightly different than the figures above). The solutions in the dialysis bags (the 'bell' of the osmometer) that are attached to the graduated pipettes contain a Dextrose solution mixed with a red indicator dye so as to identify the level of the dextrose in the pipette more readily. The first osmometer labeled "100%" contains a saturated solution of Dextrose and is set as the standard. The second osmometer contains a solution that is diluted to 50% of the concentration of the standard and labeled "50%". The third osmometer is labeled "5%" and contains a solution that has a concentration of Dextrose that is only 5% as concentrated as the standard. The fourth osmometer contains distilled water and is labeled "0%".

The instructor will place the 'bells' in the beakers of distilled water and set a timer. You are to mark the level of the fluid in the graduated pipet at time 0. Every five minutes record the level of the fluids in the table below. Do this every 5 minutes for 30 minutes. You will use this data in your lab report. A graph of this data will be necessary to fully understand the data.

 Diffusion and osmosis can be demonstrated simultaneously in one setup. Obtain a 15 cm section of dialysis tubing that has been soaked in distilled water. Tie or fold and clip one end of the tubing to form a leakproof bag. Half fill the bag with 15ml of a 1% protein (albumin) solution and 15ml of a 10% NaCl solution (The resulting percent solutions change when they are combined... see diagram below). Also add a 3 ml sample of the same solution into each of four test tubes labeled "Inside Start."

 Now tie the bag closed with a leakproof seal. Wash the bag with distilled water.

Place the bag in a 250 ml beaker containing 40ml of a 10% starch solution and 160ml of a 40% glucose solution (again the resulting percentages change because of the combination of the solutions). Place 3 ml samples of the fluid from the beaker in each of four test tubes labeled "Outside Start." The starting conditions are summariezed in the figure below. This experiment will run for approximately 1 hours. Go on to the other experiments while this experiment runs in the background.

At 15 minute intervals, swirl the beaker containing the bag or place the beaker on a slowly turning magnetic stirrer. After one hour or longer, take four 3 ml samples from the beaker and place them in four test tubes labeled "Outside End." Now remove the bag, rinse it with distilled water. Empty the contents into a beaker, take 4 3 ml samples, and place them in four test tubes labeled "Inside End."

Now assay the inside and outside samples from the start and end for the presence of the compounds added at the beginning of the experiment. Record the results of your analysis in the table below, using plus and minus symbols to indicate the presence or absence of material both before and after incubation. The following are specific, easy-to-perform indicator tests:

Test for Chloride Ion

Add a few drops of 1 M AgNO3 (silver nitrate) to one inside and one ouside tube for both start and end samples. A milky white precipitate of AgCL indicates the presence of Cl-.

 

Test for Glucose

Dip Albustix reagent strips (usually used in urinalysis) into a start and end sample for both inside and outside solutions. Compare the color change results to the label on the bottle to determine if glucose is present or not. If the result is unclear then use the Benedict's solution test. Place 8 drops of a benedict solution in a test tube with 5ml of the test solution. Place the test tube in a beaker on a hot plate and heat for 5 minutes. The presence of an orange precipitate is a positive test for glucose.

 

Test for Protein

Dip Albustix reagent strips (usually used in urinalysis) into a start and end sample for both inside and outside solutions. The paper will turn green to blue-green if albumin is present.

 

Test for Starch

Add a few drops of I2KI to each remaining tube. If a blue color appears before mixing, it indicates the presence of starch.

 

 

Table: Dialysis Bag Experiment Results 

Assay	Inside Start	Outside Start	Inside End	Outside End
Cl-
Glucose
Protein
Starch

The following questions will help you summarize what this experiment demonstrated:

Which test tubes served as a control in this experiment?

Describe which ions were able to move through the dialysis tubing wall. Which Direction did they move? Why?

Were starch and protein able to move through the dialysis membrane? Why?

Did water move through the dialysis membrane? Why?

What evidence do you have that dialysis tubing is a differentially permeable membrane?