Name: _________________________________
Date: __________________ Period
DIFFUSION AND CELL SIZE
PROBLEM: How does the relationship between the surface area and the volume of a cell affect the rate of
diffusion of materials in and out of a cell?
BACKGROUND:
Diffusion is the net movement of molecules from an area of HIGH concentration to an area
of LOW concentration. Diffusion is an important process for living cells. Materials that can pass easily
through the cell membrane, such as water, oxygen, and carbon dioxide, enter and leave cells by diffusion.
Water enters by osmosis and may other molecules enter through active transport methods.
The size of cells depends in part on how efficiently materials can move in and out of the cells by diffusion.
When cells grow to a certain size, their rate of growth slows until they stop growing entirely. They have
reached their size limit. When one of these larger cells divides into two smaller cells, the rate of growth again
increases.
In this lab, you will use cubes (representing cells) of agar that have an indicator (phenolphthalein) dissolved in
them. Think back – we used phenolphthalein as an indicator in the pH lab. In a basic (alkaline) solution, such
as sodium hydroxide (NaOH), phenolphthalein turns pink or red. It remains colorless in an acidic solution. In
this lab, you will determine diffusion in a period of time by measuring how far the color develops in the agar
cubes that were immersed in a basic solution. The diffusion corresponds to the efficiency with which materials
can enter and leave the cell.
MATERIALS:
3 cubes of 3% agar-phenolphthalein (1 cm, 2cm, and 3cm on a side)
4% sodium hydroxide solution (NaOH)
Beaker
ruler
plastic spoon
plastic knife
paper towel
PROCEDURE:
1. Calculate the surface area, volume, and surface area to volume ratio for each of the cubes. These cubes
represent different cell sizes.
2. In the last column of table 1, indicate the increasing and decreasing surface area to volume ratio as related
to cell size. As the cell grows the surface area to volume ratio
and as the cell divides
(becomes smaller) the surface area to volume ratio
.
3. Put on safety glasses and aprons.
4. Measure each agar block and trim if to make the sides the correct sizes: 3 cm, 2 cm, and 1 cm
5. Place the agar cubes in the solution of NaOH until the blocks are submerged (covered) for 5 minutes.
6. Turn the blocks frequently and carefully for the next 5 minutes.
7. After 5 minutes, remove the agar blocks from the NaOH and blot them dry. Avoid handling the blocks until
they are blotted dry.
8. Use the plastic knife or scalpel to slice each block in half. Measure the depth of diffusion of the NaOH in
centimeters (how far in did the color change).
9. Record in centimeters in table 2.
10. Wrap the agar cubes in the paper towels provided and throw them in the garbage. Any unused agar can be
reused. Place cubes that are large enough at the area assigned by your teacher.
11. Wash your hands, put safety glasses and aprons AWAY!
12. Wipe off the table using a paper towel and cleaning spray.
13. Answer the questions below – remember to be thorough, thoughtful, RSQ, and not lazy
Table 1: Surface Area to Volume Ratios of Cube Size
CUBE
SURFACE AREA (CM2)
VOLUME (CM3)
DIMENSIONS
6 (L x W)
LxWxH
SURFACE TO
Arrows
VOLUME RATIO
3 cm / side
2 cm / side
1 cm / side
0.01 cm / side
RESULTS- Diffusion of NaOH into cells
CUBE DIMENSION DEPTH OF DIFFUSION (cm)
Observations- cubes showing the most pink
3 cm / side
2 cm / side
1 cm / side
1. Which cube has the greatest surface area in proportion to its volume of the two cubes?
2. Does the surface area change in the same proportion as the volume? Use table 1 to answer and look at
the ratios.
3. What visible evidence is there that NaOH diffuses INTO an agar block? What caused the reaction to
happen? (2 to 3 sentences) Use information in background information.
4. Is it necessary for cells to diffuse materials outside the cell? What happens to this capability as the cell
grows. Explain.
5. Which cube had the most pink throughout? Therefore, which cell would most efficiently and
completely get materials necessary for cell growth to all parts? Why (relate to efficiency and SA:V
ratio?
6. What happens to the surface area to volume ratio of a cell as the cell grows? Use data from above to
support the answer. (2 to 3 sentences).
7. Why does the growth rate of a cell slow down as it gets larger? (include the terms diffusion and surface
area to volume ratio and efficiency- 2 to 3 sentences)
8.
How does cell division affect the cell's ability to absorb material for growth? Explain using how the
surface area to volume ratio increases and why this is important.