NAME:______PERIOD:____DATE:______
Lab: Cell Size
Introduction:
The absorption of nutrients, excretion of cellular wastes, and the exchange of respiratory gases 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 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.
Most human cells are .01 to .03mm. The following activity is designed to demonstrate that, while an increase in size and cell membrane surface area may increase the rate of diffusion, cells must remain small to maximize the efficiency of diffusion.
Objectives:
J Determine the extent and rate of diffusion into three different size agar cubes
J Calculate the surface area to volume ratio for each agar cube
J Observe the relationship between cell size and extent of diffusion in the agar cubes
J Understand the necessity for microscopic cell sizes.
Materials:
J 1 – 3cm X 3cm X 6cm phenolphthalein agar block
J 1 plastic knife
J 1 plastic cup
J 1 plastic ruler
J Diffusion medium (vinegar)
Procedure:
1. Obtain a 3cm X 3cm X 6cm agar block from your teacher. Using a plastic knife, trim this piece to a cube 3cm X 3cm X 3cm. Repeat this procedure to make two more cubes 2cm X 2cm X 2cm and 1cm X 1cm X 1cm.
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 into the cube.
5. Using a metric ruler, measure the distance in centimeters that the diffusion medium diffused into each cube (figure 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.
Data:
Data Table 1: Agar Cubes (5 pts)
Cube Size / Surface Area(cm2) / Volume
(cm3) / Surface Area/ Volume
(smallest ratio)
1cm X 1cm X 1cm
2cm X 2cm X 2cm
3cm X 3cm X 3cm
Data Table 2: Rate of Diffusion (5 pts)
Cube Size / Depth of Diffusion(cm) / Time
(min) / Rate of Diffusion
(cm/min)
1cm X 1cm X 1cm
2cm X 2cm X 2cm
3cm X 3cm X 3cm
Data Table 3: % of Cube Diffused (5 pts)
Cube Size / Depth of Diffusion(cm) / Total Depth of Diffusion
per side (cm) / % Diffusion
1cm X 1cm X 1cm
2cm X 2cm X 2cm
3cm X 3cm X 3cm
To get Total Depth of Diffusion per side you multiply the Depth of Diffusion by 2.
To get % Diffusion you divide the Total Depth of Diffusion per side by the volume of the cube and then multiply by 100.
Analysis:
1. According to Data Table 2, into which cube did the diffusion medium diffuse the deepest?
2. 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.
3. 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.
4. 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 and why?
Extra Credit (2pts)
5. In the Artic the Artic Hare and the Artic Fox are much smaller than their southern relatives. Explain how surface area to volume ratio plays a part in their survival in this harsh environment.
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