AP Biology Diffusion Lab

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Name:
Date:
Diffusion Through a Nonliving Membrane
Introduction: Dialysis tubing is a membrane made of regenerated cellulose fibers formed into a flat
tube. Cellulose is the stringy fibrous material that makes up the cell wall of plants. In this lab, the dialysis
tubing will be used to simulate the actions of a cell membrane. If two solutions containing different
solutes and different solute concentrations are separated by this membrane, some substances may
readily pass through the pores of the membrane, while others are excluded.
Purpose:
1. To observe the movement of molecules across a membrane.
2. To determine which solutes are capable of crossing the membrane.
3. To make accurate observations of the size and molecular weights of solutes based upon their
movement across the membrane.
Materials: Dialysis Tubing
Glucose Solution
Beakers
Iodine
String
Glucose Test Strips
Starch Solution
Procedure for Set-up A:
Take a piece of cellulose tubing and tie a piece of string very tightly around one end. Fill the tubing to
within 1 cm from the top with glucose solution. Tie another piece of string around the top very tightly
after it is filled with glucose solution. Rinse the tubing under tap water. Fill a beaker 3/4's full of water
and place the tubing in the water. Label your beaker with a piece of tape giving: (1) the set-up A and (2)
your name and class.
Procedure for Set-up B:
Take a piece of cellulose tubing and tie a piece of string very tightly around one end. Fill the tubing to
within 1 cm of the top with starch solution. Tie another knot in the top of the tubing and rinse the tubing
under tap water. Fill a beaker 3/4's full of water and place the tubing in the water. Label your beaker
with a piece of masking tape giving: (1) the Set-up B, and (2) your name and class. Additionally, add
enough iodine solution to give the water in the beaker a distinct yellowish color.
Allow the set-ups to stand about 20 minutes. While you are waiting, read the procedure below
and begin to fill in your data table. Make a simple sketch of each set up in your data table. Label
the solutions found inside and outside of the tubing.
After 20 minutes: Set-up A
Test for glucose in the water in the beaker using a glucose test strip. If the strip changes color, glucose is
present.
After 20 minutes: Set-up B
Observe any color change in the beaker or in the cellulose tube.
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*** Note *** Be sure to record the results of both set-ups in your data table. After 24 hours, record
your observations again.
Data Table
Set Up
Draw and Label Each Set Up
Results After 20
minutes
Results After 24
hours
A
B
OBSERVATIONS:
1. On the chemical basis for the test for starch, what must have happened to the iodine molecules in
Set-up B?
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2. On the basis for the chemical test for glucose, what must have happened to the glucose molecules in
Set-up A?
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3. From the evidence obtained by allowing set-up A to stand overnight, what other substance must have
passed through the membrane?
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4. Which substance used in the experiment did not pass through the membrane? How do you know?
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5. From your results, predict the size of iodine, glucose and starch molecules. Put them in order from
smallest to largest.
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6. Suggest a hypothesis to account for the speed with which molecules pass through membranes.
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7. What assumption can you make about the structure of the membrane? ___________________________________
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8. What would you expect to happen if the iodine was placed inside the bag, and starch was placed in the
beaker?
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9. Was active or passive transport involved in this lab? How do you know? __________________________________
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10. If a 10% sugar solution is separated from a 20% sugar solution by a permeable membrane, in which
direction will osmosis take place? Why?
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Copyright © Amy Brown Science
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