Name:_______________________________________________Date:____
_____Period:______
Diffusion In Agar Cubes
Is Bigger Better? OR Is Smaller Smarter?
Adapted from Flinn Scientific Publication No. 10110
Introduction:
Diffusion is one of the very important processes by which substances such as nutrients,
water, oxygen, and cellular waste 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 phenolphthalein has
been added. Phenolphthalein is an acid/base indicator that turns pink in the presence of
a base such as NaOH. Thus the surface of the agar cubes will turn pink immediately when
put into a NaOH solution. The NaOH will continue to diffuse through the cube and
gradually turn the inside of the cube pink. The guiding question for this lab is thus:
What determines the efficiency of diffusion throughout the model “cells”? Use this question
to help formulate a hypothesis:
Your hypothesis:
Materials:
Agar
A ruler
0.1M NaOH per group 200mL
Knife
Large beaker
Spoon/Tongs
Procedure:
1. Each group will cut three agar cubes: A 3cm cube, a 2cm cube, and a 1cm cube. Also –
cut a rectangular prism with dimensions of your choosing. CUT AS ACCURATELY AS
POSSIBLE.
2. Pour 200mL of 0.1M sodium hydroxide solution into your 400mL beaker.
3. Immerse your 3 cubes and rectangular prism in the sodium hydroxide solution, noting the
time. (WARNING: Sodium hydroxide is a strong base. If it spills, please contact teacher
immediately.)
4. Let the cubes soak for approximately 8 minutes.
5. Periodically, gently stir the solution, or turn the cubes over.
6. After 8 minutes, use a spoon or tongs to remove the cubes from the sodium hydroxide
solution.
7. Blot the agar with a paper towel.
8. Promptly cut each cube in half and measure the depth to which the pink color has
penetrated. Sketch each block’s cross-section. (Use colored pencils.)
9. Record your measurements and sketch each cube (or rectangular prism) in your data
table.
10. Do the following calculations for each cube (or rectangular prism) in your data table:
Data: Create a data table in your lab notebook.
Calculating % diffusion in each cube (note: your rectangular prism may differ):
 Calculate total volume of each cube (volume = L x W x H)
 Calculate volume that did not turn pink. (That is, calculate total volume of the small
portion of the cube that did not turn pink – use the same formula L x W x H)
 Calculate volume diffused = total volume – volume not pink.
 Calculate % diffusion = Volume diffused /total volume x 100
Calculate the surface area of each cube and the surface area to volume ratio (note: your
rectangular prism may differ):
 Calculate the surface area of a cube = L x W x # of sides
 Calculate surface area/volume ratio.
Conclusion Questions: In the conclusion section of your lab notebook, 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. 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. How does your body adapt surface area-to-volume ratios to help exchange gases?
6. Why can’t certain cells, like bacteria, get to be the size of a small fish?
7. What are the advantages of large organisms being multicellular?
DATA TABLE:
Cube
Size
Total
cube
volume
(cm3)
Diffusion In Agar Cubes
Total
volume
that
was
not
pink
(cm3)
Sketch of
each
Cube
Volume of the
diffused cube
Percent
Diffusion
(total volume –
volume that
was not pink)
Surface
area of
cube (cm2)
Surface
area to
volume
ratio
1cm
2cm
3cm
Diffusion In Agar Cubes
Name:___________
DATA TABLE:
Cube
Size
1cm
2cm
3cm
Total
cube
volume
(cm3)
Total
volume
that
was
not
pink
(cm3)
Sketch of
each
Cube
Volume of the
diffused cube
(total volume –
volume that
was not pink)
Percent
Diffusion
Surface
area of
cube (cm2)
Surface
area to
volume
ratio
Preparation of agar
1. Mix 20g of agar with one liter of distilled or deionized water. Recipe based on 15
groups of students working in pairs.
2. Heat almost to a boil. Stir frequently until solution is clear
3. Remove from heat. AS the agar mixture cools add 10 mL of 1% phenolphthalein
solution (1g phenolphthalein in 100 mL 95% ethyl alcohol) and stir. If mixture is pink, add
a few drops of dilute hydrochloric acid until the pink color disappears.
4. Pour agar into a shallow tray to a depth of 3cm and allow it to set (over night). A tray
measuring 12cm x 2cm that is at least 3cm deep will accommodate one liter of agar
mixture.