Monthly Archives: May 2015

Gob-smacked

A few burbles ago, I mentioned that I’d be reporting back on my Year 10 Biotechnology Club, and how they had fared in their attempts to work out Mendelian laws of inheritance from first principle. I hadn’t seen them for a couple of weeks because of various other commitments, but now I have. And, well, wow.

So, just to recap, this is what they did.

I got in some flies. Fruit flies. This is the place: http://blades-bio.co.uk/

We already use Drosophila in Year 13 to investigate sex linkage, but I thought it would be worth trying them on a talented and motivated bunch of Year 10s to look at basic autosomal inheritance. I was initially just going to get some vestigial winged flies and cross them with wild types, but looking at the catalogue I started to see the possibilities, and ended up ordering vestigial winged males and females, and ebony males and females. Then I got the girls to set up vestigial/ebony crosses.

You can’t really go wrong with this as an activity. It’s just brilliant fun – different, interesting, challenging, in every way.  Knocking out the flies, setting up the little breeding tubes, making sure the unconscious flies don’t get stuck in the blue food goo or get smeared across the sorting paper by heavy handed use of a paintbrush, checking the flies have come round, looking first for larvae and the little tunnels through the food, and then for pupae, and then finally for the alarming clouds of offspring…

But the excitement really starts when they knock them out again and look at the phenotypes.

Remember, these are students who haven’t done any genetics. So there’s no pre-conceived theory or received ideas to help them explain or predict. They just see for themselves, from their own crosses, that ebony bodies and vestigial wings have disappeared. Vanished. Every single one of those first generation flies are 100% wild-type.

It’s the kind of thrill that Mendel himself must have had when the pea dwarfiness disappeared. That’s funny…. what’s going on?

And so they cross the first generation flies. This raises the skill bar considerably, as they have to distinguish males from females based on a tiny little black bristle on their front legs.

Slightly stressful, too, as they don’t give them quite enough ether and are still trying to identify males under the binocular microscopes when the knocked out flies start their little break dances, and then start to escape…. It’s a race to set up the new tubes and fly mortality is high…

But a couple of tubes are successful, which was probably a good thing because there are LOTS of F2 generation offspring to count. The students are brilliant, sharing out the work, and dutifully tallying up the 4 different phenotypes. Because, lo and behold, ebony and vestigial is back. In the rather wonderfully perfect ratios of 95 normal wing, normal body, 34 normal winged ebony body, 24 vestigial winged normal body, and 5 of the fly they’ve not yet seen, those with vestigial wings and ebony bodies.

Go on, I say, work it out.

They’re good. Oh, golly, they’re good. By the end of the session, working together, with absolutely no input from me whatsoever, they report that the expected ratio should be 9:3:3:1

I’ll admit it, I’m shocked. I thought they might get the basic idea of dominant and recessive alleles and how a characteristic could disappear for a generation. I thought they might even figure out the ratio of a simple monohybrid cross. But they’ve only gone and worked out the predicted ratio of a dihybrid cross. And seen that their results closely matches their prediction.

Make your students feel brilliant and they will do brilliant things.

I’m now wondering whether we could roll this out as an investigation for the Year 11s when they actually do genetics in the SoW. Cost is a consideration – virgin females are not cheap – and you’d have to be very tight on health and safety – ether is nasty stuff – but it’s got to be more interesting than pea plants….

Go to run. OCR moderation sample request has just come in…

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Kirsty tries not to pop

Year 9. A new idea for introducing Active Transport.

I started by showing them one of my favourite props – a small water balloon.

Why is this quite a good model of a cell, I ask.

  • Because cell’s are mainly full of water.
  • And have a squishy, flexible membrane.
  • And are 3D.
  • And are surprisingly tough (bounce them on the floor – they won’t burst!).

What kind of cell would a water balloon be?

An animal cell (you can always turn it into a plant cell by inserting it into a large beaker).

Can anyone remember what happens if too much water goes into an animal cell?

Indeed they can. It highlights the importance of memorable activities and demonstrations. Not only can they recall the investigation where they exploded red blood cells, they can vividly recollect the demo of the balloon attached to the tap, getting bigger and bigger and bigger, impossibly bigger, until it finally burst with a satisfyingly spectacular volume of water, most of it going over me.

Ah, yes, Osmosis.

So what’s the danger for a single celled organism, say, an Amoeba, living in a pond?

Oh no, Osmosis! The poor exploding Amoeba!

At this point I pull out the props and ask Kirsty to volunteer.

The props are pretty simple.

  • A large container of water (I used the paper recycling bin – paper carefully removed).
  • A large empty tub.
  • A large beaker.
  • A small beaker.

They’re intrigued. It catches their attention. What’s he up to now, they wonder.

So, I say, let’s imagine that Kirsty is an Amoeba. And she’s living in a fresh water pond. And she is much afflicted by Osmosis.

How is she going to survive? How is she going to avoid… exploding…?

I explain that the large container of water represents the pond, the external environment with the high water potential. Kirsty is the empty tub.

I hand Kirsty the small beaker. Ready?

She nods, uncertain. What’s going to happen?

I suddenly start transferring water from the bin to the tub with the large beaker. Look ! Water is moving in to Kirsty by Osmosis! Help! Quick! Kirsty, what are you going to do???

Everyone shouts suggestions and after half a second’s hesitation, Kirsty springs into action, bailing the water out of the tub and back into the bin. But my large beaker is out-competing her small beaker – the water level is inexorably rising! If it reaches the top, she’s a goner!

Kirsty bails quicker and quicker, more and more frantically, desperately shovelling the water out of her tub and back into the “pond”…. What a star!

We finally reach a kind of equilibrium and I call a halt. A truce. Amoeba vs Osmosis = a score draw. And the Amoeba lives!

Phew!

So, how did Kirsty survive?

She had to move the water out as fast as it moved in.

Is it Osmosis?

No.

Why not? It’s water moving across a selectively permeable membrane. That’s Osmosis, isn’t it?

No! It’s going against the gradient.

Ah, right. Pushing it up hill. What does that require? What was Kirsty using lots of?

Energy.

Exactly. It’s called Active Transport.

Time to draw a cell with some little pumps in the membrane and show them some video clips of contractile vacuoles.

And to tantalise them with the idea that, at rest, 30% of their energy is powering active transport of sodium ions to keep their nervous system running. Seeing, feeling, hearing, imagining, falling in love… all dependent on active transport.

How can anyone not love Biology?