Monthly Archives: November 2016

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…)



Random mutations

A few new ideas are starting to bubble to the surface – which is a relief, not quite burned out yet – though the main constraint on sharing them with anyone remains time. So it’s another quickie this week.

It’s Year 10. It’s gas exchange and smoking.

I like to start this topic by showing them a picture of Humphrey Bogart and Lauren Bacall.


Who are these? There are some wonderfully entertaining guesses, but no-one ever gets it right. Never mind. I then show a clip from The Big Sleep. Don’t worry too much about the plot or the dialogue – though notice the on-screen chemistry between the two – but what do you notice?

Yep, everyone is smoking. I talk about how smoking used to be allowed in pubs – going out for a drink when I was at university meant coming home with your clothes reeking of cigarette smoke. Just horrible. My mother can remember when you could smoke on the London Underground. Imagine that!

But why?

This leads into the rest of the slide show where I show these extraordinary adverts that see smoking promoted by dentists, doctors and babies. Look at the tag-lines – “throat protection.” It almost beggars belief. We discuss the effects of advertising, of peer pressure, of glamorous Hollywood stars. So why did it change?

Time for a bit of data interpretation, with a simplified version of Richard Doll’s data followed by getting them to design an experiment to test the link. This generally leads to heated arguments about whether we should experiment on humans or animals, but is great for thinking about controls and measurements and sample size. The pictures of the beagles and the mice are sobering for everyone.

But we also need to stay up to date. The recent publication of research showing the actual genetic effect of smoke on lung cells is extraordinary and fascinating, but how to make it accessible to Year 10s who haven’t covered DNA and have but the shakiest idea of what a gene might be.

Try this. random-mutations-in-dna-interactive

An interactive Powerpoint which introduces the chromosomes – brief introduction – and then has 20 or so locations on the chromosomes (disclaimer – these are not the precise location of specific genes!). The locations are hyperlinked. I hand the remote mouse to the students and tell them take it in turns to “smoke a cigarette” and thereby induce a mutation by clicking on a blue circle. All but one of these “mutations” will go to Slide 3 where there is no effect. But one of them hyperlinks to Slide 4, Onco-gene mutation and its consequences.

It conveys the idea that mutations are random, and that smoking is like Russian Roulette. You might dodge the bullet for a while, but sooner or later…

Let me know what you think. Ideas for improvement or adjustment particularly welcome.