Why are cells so small? And why are we made of so many? It seems like it would be easier to be made of 100 or even 1,000 cells instead of trillions. One of the reasons we teach students that cells are small is because they need a large surface area to volume ratio. The larger the ratio, the more efficient the cell is at moving materials in and out of the cell.
I've seen cell size labs that use different sized agar cubes prepared with a pH indicator. The cubes start pink and lose their color as they soak. (Here is a free version from Flinn if you are feeling ambitious!) Frankly with 3 preps a day this year, I didn't have the time or energy to pour agar cubes. Instead I found a quick and easy way for students to see the same concept- using beets and bleach.
In this experiment, cut different sized beet cubes, a small, a medium, and a large. The students soak the cubes in bleach for roughly 30 minutes (I had them doing some practice SA:V calculations while they waited). Tip: if you use tupperware containers with lids you won't have to smell bleach fumes all day, or you can put parafilm over the beakers.
After 30 minutes of soaking, students remove the beets, cut them open, and measure the amount of red pigment remaining. It is an easy way to see that small cells are more efficient at moving materials in and out. If you are interested in seeing the lab write-up I wrote, you can view it here.
I hope your students enjoy it!