Agar cubes infused with a pH indicator model how diffusion occurs in cells. When the cubes are placed in vinegar, hydrogen ions diffuse inward, changing the cube’s color. By comparing cubes of different sizes, students see how surface area-to-volume ratio impacts diffusion efficiency and why larger cells face transport challenges.

1. Prepare agar solution by mixing agar-agar powder with water, heating in a microwave until it boils, then adding a pH indicator (bromothymol blue or phenolphthalein) and ammonia to achieve a strong color.
2. Pour the agar into molds or a dish to solidify into blocks at least 3 cm deep.
3. Cut the solid agar into cubes of three sizes: 1 cm³, 2 cm³, and 3 cm³. Prepare one untreated set and one set for vinegar soaking.
4. Place one cube of each size into vinegar in a clear container, ensuring full submersion. Keep a matching untreated cube for comparison.
5. After 5 minutes, remove the cubes and compare the depth of color change with untreated cubes. Estimate how far vinegar has penetrated.
6. Return cubes to vinegar and repeat observations every 5 minutes until diffusion is complete.
7. Calculate surface area, volume, and surface-area-to-volume ratio for each cube and relate to observed diffusion rates.

Agar Cube Surface Area - Southern Biological:

Surface Area to Volume Ratio Diffusion Practical with Agar - Science4Breakfast:

📄 Agar Cell Diffusion - Science Snack: https://www.exploratorium.edu/snacks/agar-cell-diffusion

• Try different shapes (spheres, rods) using molds to see how cell shape affects diffusion.
• Use red cabbage juice as a low-cost alternative to pH indicators.

• Use oven mitts or hot pads when handling hot agar solution.
• Handle knives carefully when cutting agar cubes.
• Work on a heatproof surface.
• Clean spills promptly to avoid slipping hazards.

• Which cube size allowed the fastest penetration of vinegar? Why?
• How does surface-area-to-volume ratio explain the differences in diffusion times?
• Why do larger biological cells require additional structures (e.g., endoplasmic reticulum) to move materials efficiently?
• What would happen if cells continued to grow larger without adaptations?
• How might different shapes of cells (not just cubes) affect diffusion rates?