Name:__________________________________

VCE Biology

Practical 1 - Dialysis: An investigation of movement of substances across membranes

Introduction: Diffusion and osmosis can be explored in the laboratory through the use of a semi-permeable membrane such as cellulose tubing which allows small molecules to pass

through pores in the membrane while large molecules are not able to diffuse through the

pores. This method, called dialysis, is applied in both laboratory science and medicine.

Aims: In this experiment you will explore the process of dialysis using cellulose tubing as the semi-permeable membrane and a solution (Solution A) containing the following solutes: sodium chloride (a salt), starch (a polysaccharide), glucose (a monosaccharide), and albumin (a protein). You will find out which molecules are able to diffuse across the membrane and investigate whether osmosis occurs. You will then apply your findings to discussion of laboratory and medical dialysis.

Prediction: For the four solutes found in solution A and listed in the table, predict which will cross the semi-permeable cellulose membrane and justify your answer:

Solute Prediction Justification

Starch (polysaccharide)

Sodium chloride (found in solution as the ions

Na+ and Cl-)

Glucose (monosaccharide)

Albumin (protein)

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Materials¨ 30cm of cellulose tubing (20mm wide)

¨ Electronic balance

¨ 50mL beaker

¨ 3 x 80mL beaker

¨ 100mL Distilled water (dH2O)

¨ Silver nitrate solution (indicator for chloride ions)

¨ Iodine reagent (indicator for starch)

¨ 4 x Diastix strip (indicator for glucose)

¨ 4 x Multistix strip (indicator for protein)

¨ Scissors

¨ 10mL measuring cylinder

¨ 100mL measuring cylinder

¨ Small rubber band

¨ Paper towel

¨ Test tube rack

¨ 4 x 2mL transfer pipettes

¨ 8 x micro test tubes

¨ Small funnel

¨ 28mL of Solution A (1% Starch, 2% NaCl, 2%, glucose, 0.5% Albumin protein) – 7mL of each of the 4 solutes

Starchandindicator

Glucoseandindicator Proteinandindicator

Saltandindicator

Note:Youneedtesttubesanytimeyouaretestingforsaltorstarch.Notforglucoseandprotein.

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Method

How to test for each solute • Transfer 2mL of the NaCl into a micro test tube.

Add 2 drops of AgNO3 into the NaCl. Record the observation based on the table below.

• Transfer 2mL of the starch into a micro test tube. Add 2 drops of iodine into the starch Record the observation based on the table below.

• Dip a Diastix strip into the glucose bottle Take it out and lay it on a paper towel Leave it for 30 seconds and record observation based on the table below.

• Dip a Multistix strip into the protein (albumin) bottle Take it out and lay it on a paper towel Leave it for 30 seconds and record observation based on the table below.

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Step 1: Test for each of the solute separately, as per above (Teacher may demonstrate)

Step 2: Create 28mL of Solution A in an 80mL beaker. Label the beaker Sol A. Using a clean transfer pipette for each solute, add in 7mL of each to the beaker.

Step 3: Fill the 100mL measuring cylinder with dH2O

Step 4: Before dialysis is performed, test the Solution A and dH2O for each solute (as per above). Label the test tubes as IB Before (Inside the Bag) for the solution A tests, and OB Before (Outside the Bag) for the dH2O tests. (Note: You will need 4 test tubes for this step – and 4 test tubes for Step 11). Record results in Table 1.

Step 5: Using a 50mL beaker, soak a 30cm piece of Cellulose tubing in dH2O for 2-5 minutes then remove the tubing and twist and double tie one end.

Step2

Step5

Step6Step6

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Step 6: Insert the funnel into the open end of the bag and pour all of Solution A into the bag.

Step 7: Gently press the sides of the bag to remove the air, leave 2cm space above the solution then twist and double tie the open end of the tubing.

Step 8: Over the sink, rinse the outside of the bag with dH2O to ensure no solution A is contaminating the outside of the bag. Blot off excess water and check to make sure there are no leaks in your bag.

Step 9: Use the electronic balance (be sure to tare it – press the tare button to reset the balance to 0.0g) to weigh the bag and its contents – record the mass in grams.

Step 10: Place the cellulose bag in the 100mL measuring cylinder of dH2O; use an elastic band to tie the top of the bag over the lip of the cylinder so the bag is suspended in the water. If necessary top up the cylinder with dH2O to fully cover the solution in the bag. Sit at room temperature for at least 30 minutes (it can be left overnight).

Step10 Step11b

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Step 11: After at least 30 min dialysis

a. Remove the dialysis bag from the container, blot off the excess fluid and weigh it. Record the mass.

b. Label a clean 80mL beaker IB After (inside the bag). Hold the bag over the beaker and chop off the bottom with scissors so that the IB solution falls into the beaker. Pour approx.. 20mL of OB solution in a clean 80mL beaker labeled OB After (outside the bag).

c. Test for all solutes for both IB and OB.

d. Record your results in Table 2.

Clean up

All solutions can be tipped down the sink. Wash all tubes and beakers thoroughly.

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