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A Clever Idea for Teaching about Cell Membranes
The amount of time I spend searching for classroom resources is substantial. I’ve been doing it a long time. So, when I come across good ideas I’ve never seen before, I get a little excited. These intermittent jackpots keep me going. In psychology this is known as a “variable ratio schedule” of reinforcement. It’s the same reason people buy so many lottery tickets. The other day I was meandering the web looking for new ways to teach about membranes, the cellular variety. In my opinion, cell membranes can often be over looked. Open a student textbook and find a diagram of a cell. The membrane is often drawn as a simple line, unassuming and uneventful. In reality, the cell membrane is abuzz with a flurry of activity. It’s the interface between the internal and external cellular environments. Membrane interactions determine the cell’s destiny. Like many things “cellular,” membranes are hard to see; they’re tiny. In my search I was hoping to find something tangible for my students to experience that might reveal the dynamic and adaptive nature of cell membranes. To my pleasure, I came across something that does an excellent job modeling membranes, soap bubbles.
The Beauty of Bubbles
I like to rate instructional activities on three attributes: simplicity of design, instructional value, and engagement potential. The cell membrane bubble lab scores well across the board. First, blowing bubbles is pretty easy – ask any preschooler. Second, a bubble acts a lot like a cell membrane – you just have to know what to look for. Third, bubbles are fun – just ask my 2 yr old if he likes his “bubbly” bath. The beauty of the cell membrane soap bubble lab is that no matter your age, you’re going to learn something.
A Bubble is a lot like a Cell Membrane
I’m not sure who first came up with the idea of using bubbles as a stand in for membranes, but they work great! A major component of cell membranes is phospholipids. Phospholipids have a love-hate relationship with water. One end, the “head,” is attracted to water and the other end, the “tail,” is repelled by water. Place phospholipids in water, and they quickly form a double layer with the heads facing out. A soap molecule has the same split personality. The “head” of a soap molecule is charged (ionic) and attracts to water, which exhibits positive and negative regions of charge (polar). The hydrocarbon tail of the soap molecule is not charged and is repelled by water’s polar personality. This explains why we use soap to clean. The hydrocarbon tail of soap mixes with and dissolves in other hydrocarbons, like oils and fats, while the head region grabs a hold of passing water molecules and follows them down the drain. The surface of a bubble has three layers. The middle layer is a thin film of water. On both sides of this film is a layer of soap molecules with hydrophilic heads facing water and hydrophobic tails pointing out. The diagram below provides a nice illustration of this comparison.
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