Modelling kidney dialysis
Aims
In a dialysis machine, a person’s blood flows through partially permeable membranes and the dialysis fluid contains the same concentration of useful substances as the blood. In this activity, you are going to show how a partially permeable membrane can allow the passage of some molecules but not others.
Safety
- Wear eye protection
- Care should be taken when handling iodine solution and using Benedict’s reagent (see CLEAPPS).
- Wash your hands if you get any chemicals on them.
Equipment and materials
- three 15cm lengths of Visking tubing which have been soaked in water
- string
- dropping pipettes
- 1% starch solution
- 10% glucose solution
- test tubes and rack
- three beakers
- distilled water
- iodine solution
- spotting tile for iodine test
- Benedict’s reagent
- access to a water bath if using Benedict’s reagent
- labels
- marker pen
- eye protection
Method
1Knot the end of one piece of the Visking tubing securely and use a dropping pipette to fill the tubing three-quarters full with the 1% starch solution. Tie the end of the tubing securely with string, forming a bag.
2Rinse the outside of the Visking tubing with distilled water.
3Place the Visking tubing bag in a beaker of distilled water, label the beaker as A and note the time.
4Test the distilled water in beaker A for the presence of starch, using iodine solution. To do this, use a dropping pipette to take some of the water from beaker A. Transfer this water to another beaker and add iodine solution. If there is starch present, the iodine solution will turn a blue-black colour.
5Repeat the procedure with the other lengths of Visking tubing, filling one with the 10% glucose solution (beaker B) and the other with a mixture of the glucose and the starch solutions (beaker C). Make sure each piece of tubing is rinsed before it is placed in its beaker of distilled water.
6Test the distilled water in beaker B for the presence of glucose, using Benedict’s reagent. To do this, use a dropping pipette to take some of the water from beaker B. Transfer this water to a test tube in a water bath and add Benedict’s reagent. After heating in the water bath for a few minutes, the Benedict’s reagent will turn yellow or red if glucose is present.
7Test the distilled water in beaker C for the presence of glucose and starch.
8Each Visking tubing bag should be left in its beaker of water for 30 minutes.
9Test the water in beaker A for the presence of starch.
10Test the water in beaker B for the presence of glucose.
11 Test the water in beaker C for the presence of starch and glucose.
12Record all the results. Remove the Visking tubing bags from their beakers. Feel the texture of each one and record your observations.
Advice on collection of data
It is advisable to draw up a table of the different tests and the solutions that need testing so that you do not forget which ones you need to do.
Results
You should record all your observations in a table.
Make it clear which tests were carried out on each Visking tubing bag and the contents of its beaker.
Questions
1What was the purpose of rinsing each Visking tubing bag in distilled water before it was placed in its beaker?
2Why did you test the water in each beaker immediately after you had placed the Visking tubing bag in it?
3What were the results of the tests in each of the beakers after 30 minutes?
4What conclusions can you draw about the passage of molecules through the Visking tubing?
5Was there any difference in the texture of the bags?
6This activity demonstrates the partial permeability of the Visking tubing membrane. How could you modify it to make it resemble kidney dialysis more closely?
AQA B3.3 Controlling internal conditions – Joe Ewen, Rebecca Timms and Rebecca Bartlett (Cheney School)