Diffusion and Cell Size Lab
Biology
Name: ____________________________________________ Period: _________________
Diffusion and Cell Size Lab
Background: The absorption of nutrients, excretion of cellular wastes, and the exchange of respiratory gasses are life
processes, which depend upon the efficient transport of substances into, out of, and throughout living cells. Many
important substances move into and out of cells via diffusion. Diffusion is the movement of substances from areas of
high concentration to low concentration (with their concentration gradient). It is an example of passive transport
because it requires no energy. For this reason, diffusion is one of the most common and efficient means by which
substances are transported between cells and their environment. This occurs via the cell membrane, a selectively
permeable barrier whose total surface area is important in regulation of the substances that diffuse into or out of the
cell. As a cell grows in size, its volume increases at a greater rate than its surface area. Consequently, the surface area
of the growing cell soon becomes inefficient for effective diffusion throughout the cell. The relationship between
surface area and the volume of a cell can be expresses as a ratio (SA:V). The need for an effectively large surface area to
volume ratio is considered to be the most significant factor in triggering a cell to divide (and therefore, determining cell
size).
Materials:
3cm x 3cm x 6cm phenolphthalein (indicator) agar block
plastic knife
plastic cup
plastic ruler
diffusion medium
Procedure:
1. Obtain an agar block. Using a plastic knife, trim this piece into the following three cubes:
a. 30mm x 30mm x 30mm
b. 20mm x 20mm x 20mm
c. 10mm x 10mm x 10mm
2. Place the three cubes into a plastic cup. Add diffusion medium until the cubes are submerged. Keep the cubes
submerged in the diffusion medium. Be sure to keep the cubes submerged in the diffusion medium.
3. As the cubes soak, calculate the surface area, volume, and surface area to volume ratio for each cube. Record
these values in table 1. Use the following formulas:
a. SA = L x W x # of sides
b. V = L x W x H
Table 1: Surface area to volume ratio
Cube size
Surface area (mm2)
(SA= L x W x # of sides)
Volume (mm3)
(V= L x W x H)
Surface Area to Volume
Ratio
30mm x 30mm x 30mm
20mm x 20mm x 20mm
10mm x 10mm x 10mm
4.
After 10 minutes, remove the agar cubes and carefully blot dry on a paper towel. Then cut the cubes in half.
Note the color change from clear to red or pink indicates the diffusion of the diffusion medium into the cube.
Diffusion and Cell Size Lab
Biology
5.
Calculate the percentage of cell reached by diffusion for each cube.
a. Measure one side of the clear area of each cube.
i. 30x30x30: _______________
ii. 20x20x20: _______________
iii. 10x10x10: _______________
b. Calculate the volume of the clear area for each cube and record in table 2.
c. Determine the volume of the pink area of the cubes by subtracting the volume of the clear area from the
total volume. Record in table 2.
d. Finally, calculate the percentage of each cube into which the diffusion medium diffused. Divide the
volume of the pink area by the total volume and multiply by 100. Record in table 2.
Table 2: Efficiency of Diffusion
Cube size
Total volume of
cube (mm3)
(from table 1)
Volume of clear area (mm3)
(V=L x W x H)
Volume of pink area (mm3)
(total volume-volume of clear
area)
Percentage of cube
reached by diffusion
(vol. pink/total vol.) x100
30mm x 30mm x 30mm
20mm x 20mm x 20mm
10mm x 10mm x 10mm
Post-lab questions:
1. The agar used to make your cubes contained phenolphthalein (an indicator) and had a pH of greater
than 9. The diffusion medium had a pH of 3. Explain how the use of a pH indicator allowed you to
visualize the extent of diffusion into the agar cubes.
Diffusion and Cell Size Lab
Biology
2.
Which cube had the highest percentage of the cube reached by diffusion?
3.
Make a generalized statement about the relationship between cell size and the percentage reached by
diffusion. Make sure to support your statement with your data.
4.
If each cube represented a living cell and the diffusion medium represented a substance needed
within the cell, what problem might exist for the largest cell?
5.
Compare the surface area to volume ratio and the percentage of cell reached by diffusion. Is it more
efficient to have a high surface area to volume ratio, or a low surface are to volume ratio?
6. How do you think the percentage of cell reached by diffusion would compare for a thin, flat cell
(shaped like a pancake) compared to a cube-shaped cell? Why?
7. Describe a cell that would be most efficient in terms of volume and surface area.
Diffusion and Cell Size Lab
Biology