Name: _____________________________________
TOC#____
Background
Diffusion is the process in which substances move from areas of high concentration to areas of low
concentration. This is particularly important in regulating a cells equilibrium. Diffusion allows
substances such as nutrients, water, oxygen, and cellular wasted are transported between living cells and
their environment. This activity will help you explore the relationship between diffusion and cell size
by experimenting with model “cells.”
In this experiment, you will use agar cubes to which the indicator bromthymol blue has been added.
Bromthymol blue is an acid/base indicator that turns yellow in the presence of an acid such as vinegar.
Thus the surface of the agar cubes will turn yellow immediately when put into a vinegar solution. The
agar cubes are semi-permiable like a cell membrane and the vinegar will diffuse through the cube and
gradually turn the inside of the cube yellow.
The guiding question for this lab is: What determines the efficiency of diffusion throughout the model
“cells”? Does size matter? Will the smaller cells or the larger cells be yellower in the middle?
Focus Questions
By the end of this lab, you should be able to answer the following questions.
What is diffusion?
How does cell size relate to diffusion?
Pre-Lab Questions
1. Define Diffusion
2. Define osmosis
3. Define hypotonic, hypertonic, isotonic
4. Define Passive transport
5. Define Active transport
6. What is the cell membrane made of?
Materials
Agar infused with bromthymol blue
A ruler
200mL vinegar per group
Knife
Spoon/Tongs
Large beaker
Procedure
1. Each group will cut three agar cubes: A 3cm cube, Flow Chart:
a 2cm cube, and a 1cm cube. CUT AS
ACCURATELY AS POSSIBLE. (This may be
already completed for you.)
2. Pour 200mL of vinegar solution into your 400mL
beaker.
3. Immerse your 3 cubes in the vinegar solution,
noting the time.
4. Let the cubes soak for approximately 10 minutes.
5. Periodically, gently stir the solution, or turn the
cubes over.
6. After 10 minutes, use a spoon or tongs to remove
the cubes from the vinegar solution.
7. Blot the cube with a paper towel.
8. Promptly cut each cube in half and measure the
depth to which the yellow color has penetrated the
cube. Sketch each block’s cross-section. (colored
pencils are available)
9. Record your measurements and sketch each cube
in the table found on the next page.
10. Do the following calculations for each cube and
complete the data table.
Experimental Design Guideline
Based on these directions and the questions complete the experimental design guideline below.
Title:
Hypothesis: What determines the efficiency of diffusion throughout the model “cells”? Does size matter? Will
the smaller cells or the larger cells be yellower in the middle? Create a hypothesis.
Independent Variable I.V:
Levels of I.V.
(2 or more plus the control,which must be identified)
Number of trials you will conduct for each I.V. level
Dependent Variable:
Constants:
Data Table
Use the calculations below to help you fill out the data table
Diffusion In Agar Cubes
A.
B. Total C. Total
Cube cube
volume
Size
volume
that was
3
(cm )
not yellow
(cm3)
D.
Sketch of
each
Cube (cross
section)
E.
Volume of the diffused
cube (total volume –
volume that was not
yellow): B-C (cm3)
F.
Percent
Diffusion
E/B
x 100(%)
G.
Surface
area of
cube
(cm2)
H.
Surface
area to
volume
ratio G/B
1cm
2cm
3cm
Calculating % diffusion in each cube:
 Calculate total volume of each cube (volume = L x W x H) (See B in table)
 Calculate volume that did not turn yellow. (Calculate total volume of the small portion of the
cube that did not turn yellow – use the same formula L x W x H) (See C in table)
 Calculate volume diffused = total volume – volume not yellow. (See E in table)
 Calculate % diffusion = Volume diffused /total volume x 100 (See F in table)
Calculate the surface area of each cube and the surface area to volume ratio:
 Calculate the surface area of a cube = L x W x # of sides (always 6 in a cube) (See G in table)
 Calculate surface area/volume ratio for each cube (See H in table)
Conclusion
Please answer the following questions.
1. In terms of maximizing diffusion, what was the most effective size cube that you tested?
2. Why was that size most effective at maximizing diffusion? What are the important factors that affect
how materials diffuse into cells or tissues?
3. If a large surface area is helpful to cells, why do cells not grow to be very large?
4. Let’s say you have three cubes, A, B, and C. They have surface to volume ratios of 3:1, 5:2, and 4:1
respectively. Which of these cubes is going to be the most effective at maximizing diffusion, how do
you know this?
5. Why do you think certain cells, like bacteria, can’t get to be the size of a small fish?
6. What do you think are the advantages of large organisms being multicellular?