Diffusion Laboratory
PSI Biology
Name____________________________________
Problem
What materials diffuse across a cell membrane?
Materials
Each student needs: apron, gloves, and goggles
Each group needs:
1 clear cup
1 plastic funnel
15 cm Soaked dialysis tubing
15 ml 15% Glucose/1% Starch solution
in a medicine cup
1 plastic pipet
2 Glucose test strips
2 10 cm pieces of string
Distilled Water
30 ml Starch indicator solution (IKI)
Time Requirements:
Pre-lab prep (For teachers): approx. 30min
Lab activity: 45 min.
Procedure
1. Fill the plastic cup to within 2 cm of the top with distilled water.
2. Dip a glucose test strip into the water in the cup for 1-2 seconds. Wait about 2-3 minutes and
observe any color change on the test strip. A greenish color on the test strip indicates a positive
test (glucose is present). No color change indicates a negative test (no glucose is present).
Record your results in Table 1.
3. Caution: IKI is an irritant that affects the skin and eyes. Avoid any contact with skin.
Make sure you are wearing proper safety equipment such as goggles, aprons, and
gloves.
Add 20-25 drops of IKI solution to the water in the plastic cup by using a plastic pipet. Observe
the color of the solution and record the color of the water in Table 1.
4. Take a piece of dialysis tubing that has been soaking in water. Rub it in between your fingers to
open it. Using a piece of string, tie off one end of the tubing. Make sure to tie it tightly.
5. Using the funnel, pour 15 ml of the 15% glucose/1% Starch solution into the open end of the
dialysis tubing. Try to squeeze out any air bubbles and tie the open end of the tubing with
another piece of string. Observe the color of the glucose-starch solution in the dialysis tubing
and record this in Table 1. Rinse the outside of the bag under running water.
6. Completely submerge the dialysis tube into the cup of water with the IKI solution and let it stay
undisturbed for 30 minutes.
7. After 30 minutes, use another glucose test strip to test the water in the cup for glucose. (See
step 2). Note any color changes in the dialysis tubing and/or in the cup and record this in Table
1. The presence of starch is indicated by a blue-black color.
Data
Table 1
Color
Initial
Glucose
Test (+/-)
Solutes
Present
Color
Final
Glucose
Test (+/-)
Solutes
Present
Water in
Cup
Solution in
Tubing
Analysis
1. What is the definition of simple diffusion? Give an example of diffusion occurring in this lab.
2. What two characteristics of certain substance would prevent them from diffusing across a
semipermeable membrane?
3.
What served as a semipermeable membrane in this experiment?
4.
Which molecules passed through the membrane? How do you know?
5.
Which molecules did not pass through the membrane? How do you know?
Teacher notes:
Pre-Lab Prep:
1. Cut enough 15cm lengths of dialysis tubing to accommodate all lab groups.
2. Cut enough10cm lengths of string to accommodate all lab groups.
3. Prepare a 15% glucose/1% starch solution. If you are using a lab kit, the materials will be given
to you.
4. Just before the lab, put the cut dialysis tubes into a cup of water.