Oh. My. Goodness. This is one of my favorite activities for introducing cell membranes and diffusion! The original idea is from Flinn Scientific, but the “lab” provided only the barest outlines of what you could do with this activity. This activity is FUN but definitely needed a little supplementing to make it more educational. I used the activity as a standalone for several years, but a couple of years ago I beefed up the analysis with a worksheet.
So first, let me tell you the fun part. You take latex balloons and fill each one with a little bit of flavoring extracts. Students try to identify what each smell is – it’s always entertaining to watch them sniff the balloon and argue with each other what it smells like.
I have a stockpile of four or five different flavor extracts so I can change them up. Try to use ones that smell distinct from each other – lemon and lime smell very similar. Word of caution, though – I used maple extract one time. Do not recommend . . . unless you want your classroom to smell like pancakes for a few days. Whew, that smell LINGERS! I use four balloons per class and tape them at different places around the room.
You can reuse balloons if you have back-to-back classes, because the smell will still be strong enough. However, I like to make fresh balloons in front of the students so they can see that I’m putting the extract inside of the balloon. It leads to a great discussion – “You saw me put the liquid INSIDE, so explain how you can smell it outside of the balloon!” – that they would miss out on if you just gave them a pre-prepared balloon. And occasionally we get the unintentional comedy when I let go of the blown-up balloon before I’ve tied it in a knot.
I added a second procedure that rounds out the idea of semi-permeability of cell membranes. I add two or three stations with a scale and some water balloons. I’ve recorded the mass of the water balloons ahead of time, and students have to weigh the water balloon and record the initial and final mass. (OK, full disclosure here: sometimes I have to fudge this a little bit. I tape a weigh boat to the scale, because if the balloon is sitting at a different location on the weigh plate, the mass will be different. And to be honest, if I forgot to do this ahead of time, I’ll weigh the balloon right before class and pretend I did it much earlier.)
After students have complete both of those activities, we have a discussion about why the extract molecules “escaped” but the water molecules didn’t. If they’re having trouble with an explanation, I project a photo of latex under an electron microscope so they can see that what seems like a solid sheet of material actually has spaces between the molecules. That leads into a discussion of the relative sizes of the extract molecules and water molecules.
The final step is for students to complete an analogy map. They have to explicitly compare the balloon to a cell membrane, the flavor extract to small molecules, and the water to large molecules.
This activity is a great way to introduce students to cell membranes and permeability, as a lead-in to discussing osmosis and diffusion. It doesn’t take much time to set up, and it’s enough of a discrepant event to get students thinking. Plus, it’s very memorable to students – you can refer back to it later when they have to think through diffusion, facilitated diffusion and osmosis, and even active transport.