Cell Membranes
Activity #2
Diffusion Through a Membrane: Supplement or Alternative to Activity #1
Procedure for Activity #2
- Using two 20-cm lengths of cellulose tubing (premoistened), tie off one end of each of the
tubes, 1 cm from the end.
- Into one tube, add starch solution to within 5 cm of the top. Tie off the tube using enough
string to leave an excess of 10-15 cm, then rinse the tube under running water to remove any
starch on the outside. Place the tube in a 600-ml beaker filled with water with the excess
string hanging over the side for ease of removing the tube later.
- To the beaker of water, add the iodine solution until the water turns a definite yellow
color.
- For the second tube, add glucose solution to within 5 cm of the top. Again tie off the tube
with an excess of string (as in step #2). Rinse the tube under running water to remove excess
glucose on the outside.
- Place the tube in a second 600-ml beaker. DO NOT ADD IODINE!
- Allow the tubes to stand for 20 minutes. Observe for any changes that occur in the size
and/or shape of the tubes. Also note any changes in the water.
- For the beaker with the tube of glucose, test the water for the presence of the sugar, using
commercial Tes-tape or Clinitest tablets. Record any results.
- For the beaker with the tube of starch, note any color change in the water containing
iodine. Record any results.
- ALLOW ALL TUBES TO STAND OVERNIGHT IN THE WATER. OBSERVE and TEST THE WATER AGAIN. Are there any differences from the results of the previous day? If so, what might be the reasons? How do the results affect your responses to the idea of molecular size?
Data Analysis for Activity #2
- What do the results in the beaker with the starch tube tell you about the
relative (comparative) molecular size of the starch, the iodine, the water? What is the evidence? What
does this tell you about the molecular spacing in the wall of the cellulose tubing relative to
starch, iodine, and water molecules?
- What do the results in the beaker with the glucose tube tell you about the relative
molecular size of glucose, water, and the molecular spacing in the wall of the cellulose tube?
What is your evidence?
- Can you make a relative size comparison of iodine, sugar, water, and starch molecules based on the experimental data? Find the chemical formulas of each compound and revisit the question about relative size. What other factors might determine whether or not a molecule can pass through the molecular pores of a membrane?
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