Thursday, November 18, 2010

Diffuson confusion

I used to be able to "teach" the concept of diffusion in less than two periods. I threw in the sucrose solution in dialysis tubing demo, its slowly increasing turgidity eliciting giggles from my larval humans. I even tossed in fart jokes.

Now it takes me a week, or longer.

And for some of my lambs, even two weeks would be too quick.

I may or may not be getting better at teaching, but I am getting better at recognizing ignorance, and it's put a huge dent in my schedule. Life was easier when I thought an exam truly measured understanding.

***

To understand diffusion, you have to know a tad about statistics, a little about kinetics, a thing or two about randomness, a smidgen about molecules, and a lot about energy. You also need a dollop of faith in the laws of thermodynamics.

I am a molecule, I cannot stop moving.





Molecules move. If two molecules are randomly moving across this page, they will mostly be far apart, with an occasional pass close enough to wave hello. If they bump into something (including the other molecule), they change directions.

Why do they move? Ask the cosmologists....they'll sputter some nonsense about singularities and expanding universes. Sometimes a shrug makes as much sense as anything.

I am a molecule, I cannot stop moving.


Molecules drift from a high concentration to a lower concentration. This is a purely statistical, random event. Molecules have no desires, no wants. They just keep bouncing around, have been for about 14 billion years, give or take.

It's why "whoever smelt it, dealt it." It's why chocolate syrup dissolving in milk fascinates me. It's why you don't drown in your own carbon dioxide.

I am a molecule, I cannot stop moving.

Unlike previous years, I did not do the dialysis tubing demo this year. I set out 1 M sucrose solution, beakers, and dialysis tubing, then asked the students to figure out the four possible scenarios using one or the other solution in the tubing and in the beaker.

I explained that the tubing acts like a net--its holes were big enough to let water molecules slip past, but not the much larger sucrose molecules. I reminded them that "sucrose water" held two separate types of molecules--this is not obvious to children who take biology before learning chemistry.

The students then picked one of these scenarios, predicted what would happen, and ran the experiment. A few of the students were angry that I would not tell them specifically what to do. (I did advise a bit--tie the tubing like a balloon, don't fill the tubing all the way to the top, don't punch your partner, the usual.)

After setting up the experiment, I asked the students to tell me which way the water molecules go. (Since they are bouncing around in both pure water and in sucrose solution, water molecules are entering and leaving the dialysis tubing simultaneously, just not at the same rates.)

The result? About 30% of my kids got it--some of them with yelps and huge smiles. I know they got it because I listened as they explained it to other students.

Getting 30% of a class to grasp how diffusion works after almost a week setting up the various facets needed to grasp the concept may seem like an incredible waste of instructional time, and perhaps it is.

Still, 30% getting it after 4 days trumps nobody getting it after two. And those who got it are now ready to tackle membrane physiology.




Stimpy fart pic from A Cartoon Christmas.

2 comments:

Angela said...

"I may or may not be getting better at teaching, but I am getting better at recognizing ignorance, and it's put a huge dent in my schedule. Life was easier when I thought an exam truly measured understanding."

Very profound and well-said. Thanks for sharing.

fnoschese said...

I'm trying to do the same thing in physics with Modeling Instruction. Concepts that used to take a few days now take weeks as students design their own experiments and reason about their results. Obviously I don't "cover" as much, but, like you, I think it's worth it. Keep fighting the good fight!