Why are Cells Small?
Does diffusion proceed fast enough to supply a cell with some of the materials it needs? What
about removing waste? In this investigation you will discover how the rate of diffusion and the
size of a cell are related.
Materials:
250 mL beaker
millimeter ruler
plastic knife
plastic spoon
paper towel
150 mL of vinegar
Block of agar with phenolphthalein
Procedure:
1. Using the plastic knife, trim the agar block to make 3 cubes with the following
dimensions—3 cm, 2 cm, and 1 cm on each side respectively.
2. Place the cubes in the beaker and add vinegar until the cubes are submerged. Record the
time. Use the plastic spoon to turn the cubes gently for the next 10 mintues.
3. Do the calculations necessary to complete Table 1 on your data sheet. The surface areato-volume ratio is calculated as follows:
Ratio of SA:V = Surface area/volume
This ratio should be expressed in its simplest form (ex: write 3:1 instead of 24:8)
4. Use the plastic spoon to remove the agar cube from the vinegar after 10 minutes. Blot
them dry. Avoid handling the cubes until they are blotted dry. Use the plastic knife to
slice each cube in half. Record your observations of the sliced surface. Measure the
depth of diffusion of the vinegar in each of the three cubes and record this in Table 1.
5. Clean up: Pour the vinegar down the sink with running water. Place the agar cubes and
paper towels in the trash when you are finished. Everything else is to be placed back
where you found it.
Key Question: Why have large organisms developed from more smaller cells rather than
a few larger cells?
Why are Cells Small Data and Questions Sheet
Table 1. Comparison of Agar Cubes
Cube
Surface area
Volume
2
dimension
(cm )
(cm3)
(s)
(sxsx6)
(sxsxs)
3 cm
2 cm
1 cm
0.01 cm
Surface Area
to Volume Ratio
(SA:V)
Observation of
sliced cube
Depth of
diffusion
Record your data from this lab in the table below and answer the following
questions in COMPLETE sentences.
Questions:
1. A) List the agar cubes in order of size, from largest to smallest. B) List them in order
of the ratios of surface area to volume, from the largest to the smallest ratio. C) How
do the lists compare?
A)
B)
C)
2. Which has the greater surface area, a cube 3 cm on a side or a microscopic cube the
size of an onion skin? (Assume the cell to be 0.01 cm on a side.)Which has the greater
surface area to volume ratio?
3. What evidence is there that vinegar diffuses into an agar cube? What evidence is
there that the rate of diffusion is about the same for each cube? Explain.
4. What happens to the surface area-to-volume ratio of cubes as they increase in size?
5. Most cells and microorganisms measure less than 0.01 cm on a side. What is the
relationship between rate of complete diffusion of the cell and cell size?
6. Based on the data from this experiment propose a hypothesis to explain one reason
why large organisms have developed from more smaller cells rather than fewer larger
cells.