1. (Make sure speakers are on)
NY State Biology Lab
Diffusion Through a
Membrane By
Hilary Beirne Eastchester UFSD
Eastchester NY.
(Make sure speakers are on)

2. WHAT IS DIFFUSION? BACKGROUND WHAT IS DIFFUSION?
Diffusion is the movement of a substance (liquid or gas) from an area of higher concentration to one of lower concentration.

3. What does semi-permeable mean?
A membrane is said to be semi-permeable or selectively permeable when only certain molecules diffuse through it

4. So what determines which molecules will diffuse across a cell membrane and which ones will not? ANSWER Molecular size and the pore size in the “cell” membrane

5. How to fill the dialysis tubing “cell” with the starch and glucose solution. (Clip on image to view hints to filling dialysis tubing)

6. Molecules in this lab ranked largest to smallest
Starch
Glucose
Water
Pores in dialysis tubing
Lugol’s solution Iodine. (Starch turns blue-black in the presence of iodine solution).
Smallest

7. Predict what will happen to the
Page 2 of handout
Students should be able to
predict that glucose will move
out of the “cell” and that starch
is too large a molecule to
move out of the cell.
Predict what will happen to the
substances inside and outside of the “Cell”

8. Initial State
The dialysis tubing is cloudy white (glucose and starch).
The beaker solution is amber (iodine and water).
The dialysis tubing solution should test positive for glucose.
The beaker solution should test negative for glucose.

9. Final State
The iodine diffuses into the tubing “cell” because the liquid inside the tubing “cell” was a bluish black color and the color in the beaker became less amber.
The solution in the beaker should test positive for glucose indicating that glucose diffused out of the tubing

10. From the “cell” results we can concluded.
The iodine diffused from a region of high concentration in the beaker to a region of low concentration in the dialysis tubing.
The glucose diffused from a region of high concentration in the “cell” to a region of low concentration in the beaker.
Starch molecules which are a lot larger than glucose, water, and iodine molecules could not diffuse through the dialysis tubing membrane.

11. Test to see if Glucose diffused out of the “cell” into the beaker (Page 4)
Remove 10 drops from the beaker and place in a test tube containing Benedicts solution (glucose indicator-blue color ) and then place into a beaker of hot water.
If the solution changes from blue to a brick red color, this will indicate that glucose molecules diffuse through the pores in the dialysis tubing into the beaker.
Before heating
After heating
Blue
Brick red

12. Chemical Testing for Starch & Glucose page 3 Handout
Obtain six test tubes and place 10 drops of water in 2, 10 drops of starch in another 2 and 10 drops of glucose in the remaining 2 test tubes.
Added 10 drops of Benedict's solution to three of the six test tubes (as per the image on the left below. Place the three test tubes in a hot water bath and a brick red color will indicated a positive reaction (test) for glucose. Test results in center image below.
Add 10 drops of Iodine to the remaining three test tubes to obtain the test results in the iodine image below on the right
Iodine Results
Water-Starch-Glucose
Benedict’s
Before heating
Water-Starch-Glucose
Benedict’s Results
After heating
Water-Starch-Glucose

13. Chemical Test Results (Page 3)
Material Tested
Indicator Solution
Used
Distilled Water
Starch
Glucose
Blue-colored Glucose Indicator (Benedicts Sol)
Blue (no change)
Negative result
Orange (turned in heat)
Positive result
Amber-colored Starch Indicator (Iodine Sol)
Dark Amber/Brown (no change)
Blue/ Black
Dark Amber/Brown (no change)

14. Questions (page 4)
Carefully examine the “cell” and the beaker you put aside earlier.
Record any changes, including color changes, you observe in the “cell” and the beaker

15. Label the contents and note color changes (page 4)

16. 2. Did any starch diffuse out of the “cell”? Explain how you can tell.
Question 1 & 2 (page 4)
1. What is the best explanation for the color change that occurred inside the “cell”?
Answer 1) Iodine diffuses into the “cells” causing the “cell” to change color to a blue/black color
2. Did any starch diffuse out of the “cell”? Explain how you can tell.
Answer 2) The contents of the beaker did not change color (to blue/black)

17. Question 3 (page 4)
3. Did any glucose diffuse out of the “cell”? Explain how you can tell.

18. 4. Which substance(s) diffused through the membrane?
Question 4 (page 5)
4. Which substance(s) diffused through the membrane?

19. 5. Which substance(s) did not diffuse through the membrane?
Question 5 (page 5)
5. Which substance(s) did not diffuse through the membrane?

20. Question 7 (page 5)
6. Explain why some substances were able to pass through the membrane while others were not able to.

21. Question 7 (page 5)
7. In the “initial State” diagram below; Starch Indicator Solution is indicated with the letter “I” because it contains iodine. Using the letters “S” for starch and “G” for glucose, indicate the areas where each of these molecules are located in both
diagrams. Be sure you indicate the location of iodine molecules in the “Final State” diagram too.