Diffusion: Why Must Cells Stay Small?
Name____________________
Purpose
The purpose of this laboratory experiment is to measure the rate of diffusion in agar and to
calculate the surface area to volume ratios of varying cell sizes.
Background Information (NOTE: do some research on diffusion and cell size and diffusion rate
to supplement the INTRODUCTION section of your lab report – do not just copy the
information below into your lab report)
In this lab you will construct a simple model that will allow you to examine the relationship between
cell structure and cell function. The structure of a cell relates to its function. The difference in the
structure of a nerve cell and a skin cell, for example, reflects the difference in their functions.
Increasing cell size affects passive diffusion-the movement of small molecules across the cell
membrane from regions of higher concentrations to regions of lower concentration. How effectively
a cell can exchange substances in, out, and throughout the cell depends on the surface area-tovolume ratio (surface area divided by volume). In this lab you will investigate how cell size affects
the diffusion of substances into a cell. This will be accomplished by modeling the relationship
between diffusion and the ratio of a cell's surface area to its volume.
Phenolphthalein is an indicator that changes color in the presence of acid or base. We will use
agar cubes of various sizes to simulate cells. The vinegar they are placed in will diffuse into the
cells, just as water would diffuse into or out of cells in any aqueous solution.
Materials
Phenolphthalein agar, 100 ml vinegar, 250 ml beaker, mm ruler, scalpel, stopwatch
Procedure
1. Calculate the surface area to volume ratio for each cube in table 1.1 below. Show all work
below. Fill in the first three columns of data table 1.1. SA = 6l2 and Vol = l x w x h or l3 if all sides
equal.
2. Propose a hypothesis and write it here. How will cell size affect the rate of diffusion throughout a
cell?
3. Prepare agar cubes that are 0.4, 0.6, 0.8, 1.0 and 1.2 cm per side. Make all sides as equal as
possible, but don’t make yourself crazy! Cubes must be cubes (not rectangular prisms) and without
nicks or damage.
4. Pour 100 mls of vinegar in a beaker.
5. Get your stopwatch ready.
6. Add the cubes to the vinegar at the same time you press the timer start button.
7. Record the time as each agar cube losses its color.
8. When all cubes lose color or after 30 minutes, whichever comes first, wrap the agar in paper
towel and dispose of in trash (NOT SINK).
9. Fill in your data table.
DATA TABLE 1.1
Cube Size,
Surface Area
cm
Volume
Ratio
(fraction)
Ratio
(whole
num)
Diffusion Rate
(minutes:secon
ds)
0.4
0.6
0.8
1.0
1.2
10. Graph the size vs. time data on a separate piece of GRAPH paper.
For this graph the independent variable is ___________________
For this graph the dependent variable is _____________________
Questions: (answer on this paper, these answers will be integrated into the Discussion and/or
Conclusion of your Formal Lab Report).
1. Why is it necessary that none of the blocks are nicked or damaged? How could this skew your
results?
2. According to your data, how does the surface area to volume ratio relate to the rate of diffusion?
3. If cells continued to grow instead of dividing, how would this affect cell function? Use examples.
This will be our first formal FULL lab write-up. Refer to the handout “Format for Biology:M Lab
Write-Ups” and follow its guidelines closely. Attach the lab handout with your calculations, notes,
etc. to the back of your Lab Write-Up when turning it in.
DUE DATE (5 school days from today): ___________________________________