Background

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; 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. OBJECTIVES

• 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. MATERIALS

3 cm x 3 cm x 6 cm phenolphthalein agar block

Plastic knifePlastic ruler

Plastic cupDiffusion medium

PROCEDURE

1. Obtain a 3 cm x 3 cm x 6 cm agar block. Using a plastic knife, trim this piece to a cube 3 cm x 3 cm x 3 cm. Repeat this procedure to make two more cubes 2 cm3 and 1 cm3 (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 sides volume = 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 red or pink to clear that indicates the diffusion of diffusion medium (vinegar) 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 clear 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 colored area of each cube. Calculate the volume of the colored area for each cube and record in Data Table 3. b. Determine the volume of the cleared area of the cubes by subtracting the...