Cell Size Lab (Agar)

Question:

What is the effect of cube size on the ability of the cell to interact with their external environments?

Background:

  1. We want to know if the size of a cell has an impact on the ability of the cell to survive. In this lab we are using the agar blocks as models for cells.
  2. Remember that all nutrients, water and gases must go through the cell membrane. If the entirety of the cell can not get rid of wastes or get nutrients, water and gases then the cell will not survive.
  3. The agar block is our cell. If the agar cell can access the external environment through the cell membrane then the external environment (NaOH) will turn that part of the cell pink. Any natural agar color left in our agar cell shows that the cell is too big to completely access its external environment.
  4. In your background, summarize what you know about the size of cells, surface area and volume, and the interaction of cells with their external environments through the cell membrane.

Hypothesis:

Predict what will happen to the interaction of cells with their external environments when the size of the cubes are increased. We measure that interaction with the pink color. You are changing the cube size, the percentage of pink agar will be the result.

Materials:

(Students may use this listing from Flinn Scientific Publication No. 10110 "Diffusion in Agar Cells".)

Agar powder, 20 g Heat source Metric ruler
Phenolphthalein indicator solution, 1%, 10 mL Tray for casting gel Spoon or tongs
Sodium hydroxide solution, 0.1 M, NaOH, Plastic knife Beaker, 400-mL
Water, distilled 200 mL Safety goggles

Procedure:

(Students may use this listing from Flinn Scientific Publication No. 10110 "Diffusion in Agar Cells".)

agar cube
  1. Safety goggles will be worn at all times.
  2. Each group will cut three agar cubes. A 3-cm cube, a 2-cm cube, and a 1-cm cube. Cut as accurately as possible.
  3. Pour 200 mL of 0.1 M sodium hydroxide solution into the 400-mL beaker. Note the time and immerse the three blocks in the sodium hydroxide solution. Let them soak for 3 minutes with periodic gentle stirring and turning.
  4. After 3 minutes, use a spoon or tongs to remove the blocks and blot dry with a paper towel.
  5. Promptly cut each block in half and measure the depth to which the pink color has penetrated.
  6. Sketch each block's cross section, include the dimensions!
 

Data:

Table 1: Data

Length of a cube side (cm) Length of a cube side (cm) Surface area (cm2) Volume (cm3) Penetration distance (cm) Surface area / volume ratio
Ideal Measured = 6 * (length)2 = length 3 Measured = Surface area / volume ratio
1          
2          
3          
Example cube 5 5.0 = 6 * 52 = 150 = 53 = 125 1.0 = 150 / 125 = 1.2

Analysis

  1. We want to know how much of the agar cell was able to access its external environment i.e. turned pink. This can be expressed as the percentage of access.
  2. The percentage of access is the ratio of the total cell volume divided by the pink volume. It is difficult and tedious to cut away the pink parts of the agar cell and measure that volume.
  3. Instead we will find the pink volume by subtracting the non-pink volume from the total volume. Look at your cubes. You have little non-pink cube hiding in the big cube!
  4. Use a scatter plot to show S/V Ratio vs. percent of access.

Note: You must show sample calculations!!

Table 2: Analysis

Length of the total cube side (cm) Length of non-pink cube side (cm) Non-pink volume (cm3) Pink volume (cm3) Percentage of access
Measured (from Table 1) Measured = length 3 = total volume - non-pink volume = 100 * (volume of color change / total volume)
1        
2        
3        
Example cube 5 3.0 = 33 = 27 = 125 - 27 = 98 = 100 * (98 / 125) = 78%

Conclusion:

(Write your conclusion in this section. Be sure to follow all guidelines for writing conclusions to experiments and include data and an explanation of how the data supports your conclusion)

 

Teacher Preparation

Flinn Scientific Publication No. 10110 "Diffusion in Agar Cells".)

  1. Mix 20 g of agar with one liter of distilled or deionized water. Note: Recipe based on 15 groups of students working in pairs.
  2. Heat almost to a boil. Stir frequently until solution is clear.
  3. Remove from heat. As the agar mixture cools add 10 mL of 1% phenolphthalein solution (1 g phenolphthalein in 100 mL 95% ethyl alcohol) and stir. Note: If the mixture is pink, add a few drops of dilute hydrochloric acid until the pink color disappears.
  4. Pour agar into a shallow tray to a depth of 3 cm and allow it to set (overnight). A tray measuring 12 cm × 25 cm that is at least 3 cm deep will accommodate one liter of agar mixture. Volume adjustments may be necessary depending on the tray used.
  5. Cut the agar into 3 cm × 3 cm × 5 cm blocks, one per lab group.