Diffusion & Cell Size Lab

Diffusion & Cell Size Lab

Diffusion & Cell Size Lab


The absorption of nutrients, excretion of cellular wastes, and the exchange of respiratory gasses are life processes which depend upon the efficient transport of substances into, out of, and throughout living cells. The process of diffusion can be easily visualized by adding a drop of blue food coloring to a glass of water. Initially, the food coloring remains in a small area in the water, dying it a dark blue. Over time, the molecules of food coloring collide with each other, and with molecules of water, and the food coloring eventually disperses throughout the entire glass of water, resulting in a light blue color in the water. Much like the drop of blue dye diffuses through the glass of water, many important substances move into and out of cells by diffusion. Diffusion is the movement of a substance through a concentration gradient from high to low concentration. It is an example of passive transport because it requires no energy on the part of the cell. For this reason, diffusion is one of the most common and efficient means by which substances are transported between cells and their environment.

The cell membrane is the selectively permeable barrier whose total surface area is important in regulating the substances that diffuse into or out of the cell. However, as a cell grows in size, its volume increases at a greater rate than its surface area. Consequently, the surface area of the growing cell soon becomes inefficient for effective diffusion throughout the cell. This relationship between surface area and the volume of a cell can be expressed as a ratio (SA:V); and the need for an effectively large surface area to volume ratio is considered to be the most significant factor in triggering a cell to divide, and therefore, determining cell size.


  • Determine the extent and rate of diffusion into three different size agar cubes.
  • Calculate the surface area to volume ratio for each agar cube.
  • Observe the relationship between cell size and extent of diffusion in the agar cubes.
  • Understand the necessity for microscopic cell sizes.


3 cm x 3 cm x 6 cm phenolphthalein agar block

Plastic knifePlastic ruler

Plastic cupDiffusion medium


1.Obtain anagar block. Using a plastic knife, trim this piece to a cube 30mm x 30mm x 30mm. Repeat this procedure to make two more cubes 20mm3 and 10 mm3 (Figure 1).

2.Place the three cubes carefully into a plastic cup. Add diffusion medium until the cup is approximately half full. Be sure the cubes are completely submerged. Using a plastic spoon, keep the cubes submerged for 10 minutes, turning them occasionally. Be careful not to scratch any surface of the cubes.

3.As the cubes soak, calculate the surface area, volume, and surface area to volume ratio for each cube. Record these values in Data Table 1. Use the following formulas:

surface area =length x width x number of sidesvolume =length x width x height

4.After 10 minutes, use a spoon to remove the agar cubes and carefully blot them dry on a paper towel. Then, cut the cubes in half. Note the color change from clear to pink that indicates the diffusion of diffusion medium (NaOH) into the cube.

5.Using a metric ruler, measure the distance in millimeters that the diffusion medium diffused into each cube by measuring the width of the pink area (Figure 2). 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 millimeters per minute.

6.Calculate the percentage of cell reached by diffusion for each cube.

a.Measure one side of the clear area of each cube. Calculate the volume of the clear area for each cube and record in Data Table 3.

b.Determine the volume of the pink area of the cubes by subtracting the volume of the clear area from the total volume. Record in Data Table 3.

c.Finally, calculate the percentage of each cube into which the diffusion medium diffused. Divide the volume of the pinkarea by the total volume and multiply by 100. Record in Data Table 3.

7.Clean up your area. Dispose of agar cubes in the garbage. Return diffusion medium to a waste beaker designated by your instructor. Rinse and return cup, knife and ruler neatly to your lab table.

Diffusion & Cell Size Data Tables

Data Table 1: Surface Area to Volume Ratio

Cube Size (mm) / Surface Area (mm2) / Volume (mm3) / Surface Area : Volume

Data Table 2: Rate of Diffusion

Cube Size (mm) / Depth of Diffusion (mm) / Time (min) / Rate of Diffusion

Data Table 3: Efficiency of Diffusion

Cube Size(mm) / Total Volume of cube (mm3) / Volume of colored area (mm3) / Volume of clear area (mm3) / Percentage of cube reached by diffusion

NAME ______PERIOD ____


1.The agar you used to make your cubes contained phenolphthalein and had a pH of less than 4 . Explain how the use of a pH indicator allowed you to visualize the extent of diffusion into the cubes.

2.According to Data Table 2, into which cube did the diffusion medium diffuse the deepest?

3.Into which cube did the diffusion medium diffuse the most by volume?

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 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 the ratio between the two values as a cell grows larger.

6.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?

7.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.

8.Compare the surface area to volume ratios between one 27 cm cube and twenty-seven 1cm cubes.Show work on back of this sheet.

Cube Size / Surface Area / Volume / SA/V Ratio
1- 27cm cube
27 – 1cm cube