4.2: Why Are Cells Small? (Activity)
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Introduction
- Take 3 blocks of agar of different size (1cm, 2cm, 3cm) → these are our cell models.
- Measure the length, width, and height of each cube using a ruler.
- Calculate the area of each face of the cubes and add all the areas together for a single cube.
- A cube has 6 faces → the total surface area is the same as the area of one side multiplied by 6.
- Calculate the volume of each cube.
- Report the surface area-to-volume in the table below.
Data Table: Calculating Surface Area-to-Volume Ratio
Cell Model (Cube) |
Length |
Width |
Height |
Total Surface Area |
Volume of cell |
Surface Area: Volume |
1 |
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2 |
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3 |
Stop and Think
- Which cube has the greatest surface area:volume ratio?
- Which cube has the smallest surface area:volume ratio?
- Hypothesize: In an osmosis or diffusion experiment, which cube size would have the greatest diffusion rate?
Procedures
- Each group will acquire three agar cubes: A 3cm cube, a 2cm cube, and a 1cm cube. CUT AS ACCURATELY AS POSSIBLE. (This may be already completed for you.)
- Place cubes into a beaker and submerge with 200 ml NaOH.
- Let the cubes soak for approximately 10 minutes.
- Periodically, gently stir the solution, or turn the cubes over.
- After 10 minutes, remove the NaOH solution.
- Blot the cubes with a paper towel.
- Promptly cut each cube in half and measure the depth to which the pink color has penetrated. Sketch each block’s cross-section.
- Record the volume that has remained white in color.
- Do the following calculations for each cube and complete the following data table:
Data Table: Calculation of Diffusion Area-to-Volume
Cube Size |
Cube volume (cm3) (Vtotal) |
Volume white (cm3) (Vwhite) |
Sketch of each Cube |
Volume of the diffused cube ( Vtotal – Vwhite ) = (Vdiffused) |
Percent Diffusion (Vdiffused/Vtotal) |
Surface Area: Volume (from the previous table) |
1 cm. | ||||||
2 cm. | ||||||
3 cm. |
Conclude
1. Which cube had the greatest percentage of diffusion?
2. Did this meet your expectations with your hypothesis?
3. If you designed a large cell, would it be a large sphere or something long and flat?