In this investigation, students explore the relationship between the surface area to volume ratio and cell division. Students are expected to calculate the ratio and make predictions about how that ratio affects a cell’s need to divide. This investigation uses phenolphthalein agar cubes to represent cells. The phenolphthalein in the agar cubes reacts with the NaOH, changing the color of the cube to pink. After the cubes are exposed to NaOH, you will be able to see how far the NaOH diffused based upon the change in color which it caused. This will allow students to determine the relationship between diffusion and the surface area and volume of the cubes. At the conclusion of this investigation, students will understand cells are limited in size because the outside cell membrane must transport the food, oxygen, and other molecules to the parts inside. As a cell gets bigger, the outside (cell membrane) surface area is unable to keep up with the inside volume, because the inside volume grows at a faster rate than the outside surface area.
• Agar powder
• Hot plate
• Phenolphthalein indicator solution
• Metric ruler
• Sodium hydroxide solution
• Plastic knife
• Water, distilled
• Spoon or tongs
• Beaker, 1000-mL
• Clear plastic cups or 400mL beakers
• Tray/cookie sheets/pans for casting gel
Prepping Agar Cubes:
1. Mix 20 g of agar powder with one liter of distilled water.
2. Heat solution. Stir frequently until solution is clear.
3. Remove from heat. As the agar mixture cools add....
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Marianne Dobrovolny - www.biologylessons.org