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Investigation: Why Are Cells So Small?

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    Essential Question: How does the size and shape of a cell influence the speed at which materials can move into and out of the the cell?

    Process: Create cell models using agar molds to compare rates of diffusion.


    • Agar Mold with BTB (made in advance)
    • Tweezers, Scalpel (or plastic knife)
    • Ruler, Beaker with white vinegar



    1. You will receive a small tray filled with an agar mold. *See below for directions* Avoid handling the agar with your bare hands and use a scalpel and tweezers to cut three agar cubes with the following approximate dimensions. Save your agar, you will need it later!

    • 1 cm x 1 cm x 1 cm (small)
    • 2 cm x 2 cm x 2 cm (medium)
    • 1 cm x 1 cm x 6 cm (large)

    2. Measure your cubes (the actual dimensions may not be perfect, depending on how you cut it) and determine the surface area, the volume, and the SA:V ratio. Record on data table.

    3. Drop each block into a separate beaker (or container) of vinegar. The agar has been infused with a chemical called bromothymol blue, the blue will turn to a yellow in the presence of acid. You will be able to observe this change with your cubes. Record the time it takes for the blue to completely disappear.



    Actual Dimensions

    Surface Area


    SA / V

    Time (Blue to Orange)

    Small Cube






    Medium Cube






    Large Cube







    Part 2: How Does Shape Influence Rates of Diffusion?

    With what remains of your agar, design a cell that maximizes volume and mass, but minimizes diffusion time.  Your "cell" will compete with other cells in the class to see which one has the fastest diffusion time.


    • No donut-like holes through the agar cell - cell membranes cannot sustain this shape

    • No poking or agitating the beaker when the cell is submerged

    • Instructor determines when 100% diffusion has occurred

    • Agar cell will be massed at the end of the race

    • Winner = highest ratio of mass divided by time

    Sketch your design.



    1. Which of the initial cubes had the fastest diffusion time? Which had the slowest?
    2. Which of the three variables you tested seemed to have the biggest impact on the rate of diffusion? Explain how you know this.
    3. How does the agar cube model the cell and the cell membrane?
    4. What designs (Part 2) seemed to have the fastest diffusion rate?
    5. How do these experiments model the cell and the cell membrane?

    Investigation: Why Are Cells So Small? is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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