Cracking Osmosis

I can clearly remember the lesson from my own time at school. The teacher divided the blackboard (yes, I’m that old) in two with a dotted line, and we copied this, and the chalky molecules of water and sugar, and the arrows depicting the direction of water movement.

I also remember the faint feeling of bemusement. What was this all about? Mechanically copying an abstract drawing off the board enabled quality banter time with fellow class mates, but at no point did I have any concept of why this might be either interesting or important.

Since becoming a teacher myself, I’ve tried a dozen different approaches, and like to think that there has been some improvement over the years. I certainly think that this year I may even have cracked it.

So, first, why is it difficult to teach well? That’s the easy bit! It’s abstract, it’s difficult, it’s about plants, and it’s got funny words that don’t make sense. You learn a definition, maybe weigh some bits of potato, but it’s pretty dull, if we’re honest. And the osmometer? How is that relevant to anything? Yawn yuck bleurgh.

Let’s try again.

As always, I look for a narrative thread. We’ve just finished Diffusion with the homework on Mademoiselle Paramecium. meet-mr-paramecium Note the last question – it’s the bridge. We go through the other questions, and then introduce this idea that water molecules are also subject to the same rules – they, too, move.

I finished this single lesson by setting up a demo. I used to do this as a class practical, and may do again, but this year I simply gathered them round for a chat and some biological banter.

So, what’s this?

A potato!

Right. Is it living or non-living?

I love this bit. From Year 7 to Year 13, at an academically selective school, they hesitate. A potato. Is it alive? They’re really not sure! The class divides, fairly equally, into yes, no and not sure.

So what would happen if I buried it in the ground?

They twig (ho ho). Oh, right, yes, it would grow. It’s alive. Phew. They relax. I tell them about the Year 13 student I once taught who was a bit sceptical about the concept of plants as living things, but absolutely refused to countenance the possibility that they had sex.

OK, if it’s alive, what’s it made of?

They’re on firmer ground, now that the perfidy of my last question is forgotten.


What kind of cells?

Plant cells!

Tell me about plant cells?

They duly parrot the key features.

What makes up 70% or more of the contents?


Contained in?

The vacuole!

Lovely. Right, Persephone (or whoever), please can you cut me two chips of roughly equal size that will fit into a boiling tube.

Persephone dutifully sets about carving out a couple of chips. Much banter as she makes more or less of a mess of it. I get some of the other students to measure and record the mass and length of each chip, and then we put one into pure water and one into 1M sugar solution.

Before the next lesson, I set this out in a table and copy it ready for distribution.potato-tube-data-9g-november-2016 At the start of the lesson, we gather again around the boiling tubes and recap what we did. Then we retrieve the potato chips and measure them again. A volunteer records this new information on the board. We also pass around the chips – they are deeply amused by the contrast between the firm, swollen, rigid chip from the water and the floppy, wibbly, shrivelled chip from the sugar solution.

I get them to copy the data into their tables and write a conclusion. What MUST have happened?

Notice that I’m not interested in terminology or definitions. I’m just getting them to think about the results of an experiment and how they would interpret them. The bright ones get there quickly, the rest follow with a few prompts. The potato cells in water have gained mass. The only possible way that could have happened was if water entered the cells. The potato cells in sugar solution have lost mass. A lot of mass. The only way this could have happened was if water had moved out of the cells.

So all I want them thinking about at this stage is the idea that water moves in and out of cells.

I bring them back to the front around a sink where I have placed LOTS of newspaper. I have a balloon. I use the tap to put some water in the balloon and then hold it up. I advise proximate students to move their books and files away.

Why is a balloon a really good model of a cell?

A good discussion question this. It’s wibbly, it largely contains water, it has a membrane, and, important for KS4 students who usually think of a cell as a glorified 2D fried egg, it’s 3 dimensional.

Is this a plant cell or an animal cell?

Right. No cell wall makes it an animal cell. Like your cells.

I reattach the balloon to the tap.

Of course, I say, conversationally, water moving into a plant cell will eventually stop. Can anyone think why? Yes, exactly, the cell wall stops it.

The balloon is swelling up. Students near the sink are backing away nervously. There are lots of gasps and giggles.

But of course, you are made of animal cells. Animal cells have no structure to stop water coming in.

The balloon carries on swelling. It’s extraordinary how much water they can take on. Surely it will burst soon? Nope, it just gets bigger and bigger…. It’s great. The tension builds and builds and the challenge is to keep the merry biological banter going while the balloon inflates….

On and on it goes, until finally, with my right arm bearing several litres, and therefore several kilos of load, there is an almighty SPLOSH!!!!!! and a spectacular tsunami of water explodes EVERYWHERE, accompanied by an excited scream from the class. Brilliant. Unforgettable. Something they’ll mention at home and in their end of term evaluation and, it turns out, remember 6 years or more down the line…

Once the excitement has died down, and we’ve mopped up the water, there’s a few quick questions…

What happened to the cell? Why did it burst? Is that something you would like to happen to your cells?

This last is critical – it makes the point – water moving in and out of cells matters….

… which bring them to (continued next week…)




Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s