Name _________________________-Period __________- Date_______________________
Diffusion and Cell Size
Student study and analysis sheet
Introduction
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 ourselves, 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?
Diffusion is a fairly slow process and a cell that relies primarily on diffusion to transport essential molecules into and
throughout its interior—and to carry waste products out—could conceivably grow too large for this process to work
efficiently. 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. To observe diffusion, we will use a special type of agar called phenolphthalein agar.
Phenolphthalein agar contains both phenolphthalein and NaOH (sodium hydroxide). In the presence of sodium
hydroxide, this agar turns pink. When diffusion medium is absorbed into the agar, the pink color will become colorless
(where have we seen this before????...remember acid breath….?)
Objective:
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.
Materials – Each group should find the following at their lab table:
-
1 plastic cup
1 pair of plastic gloves
calculator
1 metric ruler
1 paper towel
1 plastic knife
1 plastic spoon
1 block of phenolphthalein agar
200 ml diffusion media (vinegar)
Name _________________________-Period __________- Date_______________________
Procedure:
1. No one should handle the agar blocks without gloves. All students should wash their hands after completing the
lab. Take the measurements and draw scaled pictures of the agar blocks carefully and make sure you have all
measurements required BEFORE doing the math. All measurements should be made to the nearest 0.1 cm.
2. You have a 3cm x 3cm x 6cm block of agar. You will need to cut three different size cubes out of your block (see
below) , so PLAN first BEFORE cutting. Measuring what you have (the block will not be “exact”. Using the plastic
knife, to carefully cut the block .
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 in the plastic cup. Add enough diffusion medium to cover all three cubes. Use the spoon to
gently turn the cubes periodically (this is important as the diffusion media will not be able to penetrate surfaces that
are “on top of “ each other). Keep all cubes submerged for 10 minutes. USE A TIMER!
4. As the cubes soak, use the following equations to complete Data Table 1.
a. Surface area = length x width x number of sides (use units!)
b. Volume = length x width x height
5. 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 red/pink to
clear/white (okay, the agar will probably be a “brown” color) that indicates the diffusion of the diffusion medium
into the cube.
6. Using a metric ruler, measure the distance in centimeters that the diffusion medium diffused into each cube.
Measure to 0.1 cm! (Measurement C in the diagram below) Record the data in Data table 2. Also record
measurement A in table 2 (note that A + 2C = B) Next, record the total time of diffusion. Finally, calculate and
record the rate of diffusion for each cube as centimeters per minute (C/time).
We are making the assumption that C will be the same depth around the entire cube. Should
Any of your cubes be “fully diffused”, use ½ the width of the cube as measurement C.
7. Calculate the extent of diffusion into each cube as a percent of the total volume. Use the
diagram to the right to help you.
Total cube volume (B3) - volume of cube that did NOT change (A3) x 100 = extent of diffusion
Total cube volume (B3
Name _________________________-Period __________- Date_______________________
Results:
Data Table 1: Agar Cubes
Cube size
Surface area
(cm2)
Volume
(cm3)
Size of undiffused
center (A)
Depth of diffusion
(cm)
(measurement C
from diagram)
Surface area/volume
(smallest ratio)
3cm
2cm
1cm
Data Table 2: Rate of diffusion
Cube size
Time
(min. )
Rate of diffusion
(cm/min)
3cm
2cm
1cm
Data Table 3: Extent of diffusion
Total volume of cube
(cm3)
Volume of cube which has
NOT changed color
(measure a, then cube it)
Volume of diffused agar
(Subtract column 2 from
column)
Percent volume of cube
which has changed color
(extent of diffusion)
27
8
1
Draw the cross-section of each of your cubes “to scale” showing the colors of each area
Name _________________________-Period __________- Date_______________________
Questions:
1. Explain why the diffusion of the diffusion medium into the agar cube caused the observed color change (hint:
look at the type of agar and the diffusion media. Where have we seen these before?).
2. According to Data Table 3, which cube showed the highest extent of diffusion – the largest or smallest?
3. If each cube represented a living cell and the diffusion medium a substance needed within the cell, what
problem might exist for the largest cell?
4. Examine your data in Data Table 2 for a relationship between cube size and the rate of diffusion into the cube.
Make a generalized statement about the relationship between the cell size and the rate of diffusion.
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 ration which best meet the diffusion needs of living cells.
Name _________________________-Period __________- Date_______________________
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 (how far in) and rate of diffusion
(how quickly) into this living cell as compared to the smallest agar cube. Explain how you arrived at your
answers.
8. Intestinal cells (columnar epithelial cells) and neurons (nerve cells) are cells that have modified structures to
increase the amount of surface area they have to support their cytoplasm. Research each cell type using the
links below or other links of your choosing. 1) Draw a picture of each of the cells below. 2) Identify the
structure that is responsible for the increase surface area of the cell. For each type of cell what advantage
does the increased surface area of the structure provide?
http://www.mc.vanderbilt.edu/histology/labmanual2002/labsection3/Intestines03.htm (diagram 85)
http://psychology.about.com/od/biopsychology/ss/neuronanat_2.htm