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Diffusion and Cell Size
Why are cells so small? Most cells grow, but upon reaching a certain size, a cell will divide becoming two smaller cells.
This is how multi-cellular organisms, like us, grow. But why do cells stop growing when they reach a certain size? Why
does a cell divide and multiply rather than simply continue to grow bigger? One possible answer can be found in the
relationship between cell size and the diffusion of substances across the cell membrane.
Diffusion is the spontaneous movement of a substance from high to low concentration. It is how many substances
naturally move from where there is more to where there less: such as the smell of perfume moving across the room.
Diffusion is one of the very important processes by which substance such as nutrients, water, oxygen, and cellular
wastes are transported between living cells and their environment.
The cell membrane is the selectively permeable barrier whose total surface area is an important factor in regulating the
substances that diffuse into or out of the cell. In fact, cells that specialize in absorption or secretion often have relatively
large surface areas to increase the efficiency of diffusion (e.g. intestinal cells contain villi and microvilli). So again, if a
large surface area is helpful, why do cells not grow to be very large?
Scientific evidence suggests that once a cell grows to a certain size it becomes too large for the complete diffusion of
needed substances throughout its cytoplasm. As a cell grows, its volume increases faster than its surface area; as a
result, the surface area of the cell membrane gradually becomes less efficient for the diffusion of substances throughout
the entire volume of the cell. The relationship between the surface area and volume of a cell can be expressed as a
ratio, and this ratio is believed to be a significant factor in triggering a cell to divide, preventing cells from becoming too
large, and maintain efficient diffusion throughout the cell.
In this activity, you will investigate, using model cells, the relationship between cell size and diffusion which will help to
explain why cells remain small.
In this activity you will determine the extent and rate of diffusion of a substance into three cubes of different sizes. In
addition, you will calculate the surface area to volume ratio for each cube.
1. READ through the entire lab.
2. Complete this table. If you need to look up the calculations for surface area and volume then please do so.
Data table 1:
Cube size
Surface area (cm2)
3cm x 3cm x 3cm
2cm x 2cm x 2cm
1cm x 1cm x 1cm
(pre-lab continued on next page)
Volume (cm3)
Surface area/volume (smallest ratio)
Pre-lab continued
3. Write a scientific purpose for this lab.
4. Identify your dependent and independent variables for this lab.
Materials – Each group should pick up all of the following
1 plastic cup
1 metric ruler
1 pair of plastic gloves
1 plastic knife
1 paper towel
1 block of bromothymol blue agar
1 plastic spoon
Groups will also SHARE the diffusion medium (Dilute Hydrochloric acid)
1. No one should handle the agar blocks without gloves. All students should wash their hands after completing the
2. Obtain a 3cm x 3cm x 6cm block of agar. Using the plastic knife, cut the block into three cubes with the following
measurements. PLAN AHEAD, this is the only block you are getting!
a. Cube 1: 3cm x 3cm x 3cm
b. Cube 2: 2cm x 2cm x 2cm
c. Cube 3: 1cm x 1cm x 1cm
3. Place the three cubes into the plastic cup. Add enough diffusion medium to cover all three cubes. Use the knife to
turn the cubes periodically but keep them submerged for 10 minutes.
4. After 10 minutes, use the spoon to remove the agar cubes and carefully blot them dry on a paper towel. Then, cut
the cubes in half, rinsing and drying the knife between each cut. Note the color change from bluish to yellowish that
indicates the diffusion of the diffusion medium into the cube. The diffusion medium did not reach any part of the
cube that remains blue.
5. Using a metric ruler, measure the distance in centimeters that the diffusion medium diffused into each cube.
(Measurement C in the diagram below) Record the data in Data table 2. Next, record the total time of diffusion.
Finally, calculate and record the rate of diffusion for each cube as centimeters per minute.
6. Calculate the extent of diffusion into each cube as a percent of the total volume. Use the
diagram to the right AND the formula below to help you.
Total cube volume (B3) - volume of cube that did NOT change (A3) x 100 = extent of diffusion
Total cube volume (B3)
Data Table 2: Rate of diffusion
Cube size
Depth of diffusion
(measurement C from
(min. )
Rate of diffusion
Data Table 3: Extent of diffusion
Total volume of cube
Volume of cube which has
NOT changed color
Percent volume of cube which
has changed color
(extent of diffusion)
1. Explain why the diffusion of the diffusion medium into the agar cube caused the observed color change.
2. Examine your data in Data Table 2 for a relationship between cube size and the rate of diffusion into the cube.
Does the size of the cell affect the rate of diffusion? Explain (Hint: what is a rate? And what is moving?)
3. Which cube had the greatest extent of diffusion? Support your answer with data from table 3.
4. If each cube represented a living cell and the diffusion medium a nutrient needed within the cell, what problem
might exist for the largest cell?
5. Examine your data in Data Table 1. Describe what happens to the surface area, the volume and their ratio as a
cell grows larger.
6. According to the results of your investigation, describe the characteristics of cell size, surface area, and surface
area to volume ratio which best meet the diffusion needs of living cells.
7. The size of some human cells is .01mm. Using the formulas in this activity, calculate the surface area to volume
ratio of such a cell (assume the cell is a .01mm cube). Describe the extent and rate of diffusion into this living
cell as compared to the smallest agar cube. Explain.
8. Intestinal cells and neurons are cells that have modified structures to increase the amount of surface area they
have to support their cytoplasm. Google images of each and sketch a picture of these cells below. Then
describe how their special shapes solve the problem of surface area to volume.