Name/Date/Period__________________________________________________________________
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. Wear gloves and 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. When you have finished cleaning up let Miss Egan know so that she
can stamp your paper.
Name/Date/Period__________________________________________________________________
Name:
Date & Period:
Why are Cells Small Data and Questions Sheet
Record your data from this lab in the table below and answer the following questions in
COMPLETE sentences.
Table 1. Comparison of Agar Cubes
Cube
Surface area
Volume
2
dimension
(cm )
(cm3)
(s)
(sxs)
(sxsxs)
Surface Area
to Volume Ratio
(SA:V)
Observation of sliced
cube
Depth of
diffusion
3 cm
2 cm
1 cm
0.01 cm
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. How do the lists compare?
A)
B)
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 HCl 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 diffusion and cell size?
6. Propose a hypothesis to explain one reason why large organisms have developed from more cells
rather than larger cells.