Smelly Balloons – introducing cell membranes and permeability

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.

Before-During-After Drawings – Helping students differentiate between diffusion, osmosis, and active transport

Students seem to have difficulty sorting through the different types of cell transport, because it’s such an abstract concept. Even after reading the textbook, taking notes, and doing different activities, my students didn’t understand the difference between diffusion, osmosis, and active transport. I created a handout for my students to use as part of our review for the test that had them visually explain what was happening during each process.

The basic format is one of the formative assessments Paige Keeley sets out in her book, Science Formative Assessment (volume 2). She calls them “B-D-A Drawings”. Rather than do each process separately, I created one document to compare the three processes. It was a spur-of-the-moment creation, so I hand-drew the drawings for the “before” panels. One benefit of giving students the “before” drawings is that they’re all using the same basic shapes for solute and water, and the same number of molecules, and I can set it up to guide them toward what their “during” and “after” drawings will contain.

Students got this basic template, with the “before” drawings

I did a jigsaw activity for this handout – I counted off students as 1, 2, or 3, then put each number at a separate table. Each table was assigned one drawing to complete. As each table worked on their drawings, I circulated through the room to answer questions they had, or to ask groups questions to prompt them to think about what would happen for their assigned process. When the group working on diffusion seemed stuck, I did a quick demo with a beaker of water and some food coloring. The osmosis group had the right idea about water moving (instead of solutes), but when I saw that their drawings did not change the water level, I asked them what would happen to the water level on each side.

Students used their science notebooks to help them think through their processes. It took them about 5-10 minutes to discuss what they thought would happen and draw the “during” and “after” diagrams. It also prompted a good discussion about equilibrium – how it would be different for each process, and how the active transport process wouldn’t reach equilibrium.

After each group had completed their set of diagrams, I regrouped students so there was one person with each diagram at a table group. Each student had to explain their diagrams to their tablemates and answer any questions. After each student had explained their process, students had to complete the diagrams for the two processes they didn’t have.

My answer key – oops, forgot to take pictures of student samples!

Overall, this activity gave students a visual explanation of the differences between diffusion, osmosis, and active transport. With larger classes, you could create multiple groups for each process to keep group size small enough to keep students focused on the work.

Quick Takes: Reviewing with Tarsia puzzles

If you haven’t heard of Tarsia puzzles, they are kind of like jigsaw puzzles and kind of like a matching game. They remind me a lot of Triominoes – that game with triangular pieces with numbers on each side, and you have to match numbers to create a big triangle.

Tarsia is the brainchild of Hermitech Laboratory, and is software that will create customized card sort activities. I think it was originally created for math teachers, but it can easily be adapted (with some workarounds) for any content area. (One caveat: there is no Mac option for the program – I’m lucky to have access to both Mac and Windows computers, so if I want to make a Tarsia, I make it on my home computer and save it as a PDF to print at work.)

The Tarsia software has a variety of geometric shapes to use – triangles, hexagons, rectangles, etc. – and make a puzzle with between 17 and 30 paired expressions. There’s a standard version, where the outside edges are left blank so students can easily find the borders, and an extended version, where there are unpaired phrases on the outside edges so it’s more challenging to figure out the borders.

To make a Tarsia puzzle, you create a list of paired words or expressions. (The software also allows you to insert images, but I haven’t tried that yet.) The major limitation for the paired words is space – since the software was originally created for math, there’s only room for a few characters. Once you get more than about 20 characters, the print size and spacing between words gets so small it’s difficult to read. Not as much of a problem if you are creating tabletop-sized pieces for students to use. However, if you want to use them for student notebook activities, you’re reducing the size of the puzzle by a decent amount, making it very hard for students to read the text. I’ve used a blank template and handwritten the paired phrases if I’m worried about legibility (or if I’m at work and want to make the puzzle).

Handwritten phrases in a blank template

Once you created your paired phrases, the software creates pages with scrambled puzzle pieces. There are usually 2 or 3 pages of shapes, so what I do is print them out and reduce the size of the images until I can fit all of the pieces onto one sheet of copy paper.

A one-page Tarsia handout for student notebooks. (See what I mean about legibility?)

When I give students this one-page handout, they cut apart the pieces and then have to create the completed puzzle, matching words and phrases. A word of caution: it takes students a loooooong time to complete the puzzle. I usually have them work in pairs, with the understanding that each student needs to have a completed puzzle in their notebook. I have a completed puzzle in my notebook to use as an answer key, so I can easily check student work. I also have a photo of a completed puzzle that I can project on the board so everyone can check their own puzzle.

The answer key – a completed puzzle!

This activity is great for reviewing vocabulary and basic factual information. Obviously with the size limitation, it isn’t great for more in-depth information. I plan to experiment with using images in a puzzle – I think that would be great for a beginning-of-the-year practice for lab tools, especially the different kinds of glassware.