This is a remix of https://goopenva.org/courses/agar-cell-diffusion from the Exploratorium Teacher Institute. The original uses agar blocks to demonstrate the effect of cell size on cell transport., and it includes great instructions on how to incorporate the lab into your classroom However, plain gelatin is often easier to obtain than agarose and can be used instead. Standard gelatin recipes are too soft to carve though, so my coworkers and I have tweaked the recipe over the years until we found one that is fool proof and comes out perfectly every time. This recipe will yield gelatin that cuts easily and has almost the consistency of the agar. It is firmer and easier to carve than standard gelatin. If you can spare the expense, agar is still the best option. However, this gelatin recipe is a great substitute. I hope other teachers will find the gelatin variation useful! Simply make the gelatin as noted below and then follow the instructions on the original resource found here https://www.exploratorium.edu/snacks/agar-cell-diffusion. 

Gelatin Cell Blocks Recipe

Heat 1.75 cups (414 mL) of water in a microwave or hot plate until boiling or close to boiling. Carefully pour the hot water into a glass baking dish. Sprinkle three (3) packs of plain gelatin on top of the hot water and allow it to bloom for a few seconds. Whisk the gelatin into the water until dissolved. Allow the mixture to cool for just a bit. You want it to be still liquid, but not so hot that it evaporates your base. Once the mixture is cooler, add a few drops of phenolphthalein indicator and just enough weak base to turn the gelatin a dark pink/purple. We don't have an exact measurement for how much base we add. Generally we just add splashes of base and stir until we get the correct color. It's quite a bit of base to add (probably around 1/2 cup total), but go slow and monitor the color to decide when to stop. Be aware that the gelatin will most likely lighten up over night a bit. Also using too strong of a base or too much base can denature the gelatin and prevent it from setting up. We have used weak ammonia and 0.1M NaOH successfully in the past. Other bases may work as well, but you want something that will be safe for the students and not overly strong. Just strong enough to turn your indicator a nice dark pink. Cover and place your dark pink gelatin into a fridge overnight to gel. If the gelatin lightens up too much the next day, you can add more base on top and give it about an hour to soak back in. It will deepen the color more. The gelatin can be carved up and soaked in white vinegar for the experiment the next day. The vinegar will turn the gelatin clear as it diffuses into the blocks. 

Safety concerns: some of these chemicals can be irritating. Students must wear goggles and could even use gloves if available. If gloves are not available, have them wash their hands very well after they are done carving the gelatin blocks.