Tag Archives: enzymes

Blindness, insanity and death

Tell them stories…

I love stories. They provide context. They make things memorable. They are ideal learning tools.

I also love a bit of theatre. Teaching is, at least in part, a performance art.

This lesson combines the two. The content isn’t terribly original, but even the weaker students remember every single detail… So listen up! I shall be testing you later…

So, I say, did any of you go on the Duke of Edinburgh expedition?

An excited babble of reminiscence follows.

Did you stay in a tent? Was it really cold and uncomfortable?

They compete for the most uncomfortable story.

Did you cook your own food? Yes? What did you have? Pasta? Most of them will have had pasta. What did you cook it on?

A pause to remember the name, and then, a Trangia!

Right, a Trangia. What’s the fuel for a Trangia? That purple stuff? Anyone remember the name? Methylated spirits. Did any of you spill it in the pasta? What did it taste like? Vile!!!!

Why does it make such a great fuel? We talk about energy content and flammability and I light a little evaporating basin full of meths.What’s the flammable component? Yes, that’s right. It’s methanol. An alcohol. A clear, colourless liquid that tastes like its close cousin, ethanol.

So why are methylated spirits bright purple and taste vile?

Do they add stuff to it? Indeed they do. It’s not a great idea to drink it.

I direct them back to the burning meths.

Note the flame, I say. Orange tips. Very useful.

Why?

Ah. So we talk about methanol and ethanol and I tell them of my time in Uganda where a jerrycan of the local hooch -waraji, or banana gin – would set you back about 50p (50p for 30 litres of neat spirits!). But before we started making our waraji and passion juice cocktails, we always poured a little on the floor and lit it. If it burned with a clean blue flame, all was well. But orange tips to the flames? We used it in the paraffin stoves (it was much cheaper than paraffin).

Why?

Well, a clear blue flame indicates that it’s ethanol, the stuff we call alcohol, and which our livers can break down to carbon dioxide and water.

But an orange flame indicates methanol. Our livers break down methanol too, but they break it down to something else.

I produce my next prop, a mouse pickled in a jar of formaldehyde.

Lots of good reactions to this, mainly sympathy for the mouse.

Look at this mouse, I say. It’s perfectly preserved! It will never rot! It will swim in its little bath of formaldehyde for all eternity. But how good is it at being a mouse? What do I mean? You know, mousey things. The squeaks and the scurrying and the nibbling cheese stuff that mice do. How mousey is it? Not at all. It’s bloody rubbish at being a mouse.

And this is the problem with drinking methanol. Your liver cheerfully metabolises it, but instead of producing water and carbon dioxide, which your body can deal with, it produces formaldehyde. And this travels round the body. And starts to pickle it. Starting with your retina. Your retina becomes perfectly preserved! It’ll last forever! It’s just not very good at being a retina any more. Then your brain. Blindness, insanity… it’s not a good way to go. If you’re ever in Uganda, I say, or, indeed, anywhere where people distil their own hooch (i.e. pretty  much everywhere), remember the orange flame test!

They like the story, they love the mouse, they enjoy the anecdotes, they are happy to remember character building DoE expeditions. But where’s this going…?

I often digress at this point to discuss the power of addiction. Who drinks methylated spirits? Er, no-one? But they’re wrong. And they shake their heads in astonishment at the thought of someone needing an alcohol high so badly that they’ll over come the repulsive taste and the gruesome side effects…. But then it’s time to get back to the plot…

So, I say, here’s the punchline. The enzyme that breaks down ethanol to water and carbon dioxide is called alcohol dehydrogenase – and this same enzyme is the one that breaks down methanol to formaldehyde. Which means, I say, increasing the volume and emphasis for dramatic (some might say melodramatic) effect, every DoE leader must guard against possible methanol poisoning, and carry in their medical kit a large bottle of…. what?

And they see it. They really do. They’re also delighted. Every DoE leader should include in their risk assessment and medical supplies, a large bottle of vodka! They think this is hilarious. Voddie! On DoE!!! But they also get the science. Swamp the alcohol dehydrogenase with ethanol and the active sites will be too busy to get round to metabolising the methanol (which can be safely excreted on the breath, or in sweat, or in urine….).

Time for a few notes and diagrams – as Feynmann used to say, once you’ve understood it, you can write it down.

To my delight, this year, a couple of Year 12s compared the formula of the two alcohols and wanted more rigour on the enzyme (what does the name tell you about its job?), and were able to work out that, yes, if you strip a couple of hydrogens off methanol, CH3OH, you do get formaldehyde, CH2O. It meant we had to explore the ethanol pathway in more detail, but that was fine too.

Stories…. tell them stories.

Enzymatic Virgins

After concluding Osmosis with the now traditional paper fight across a line of chairs, it’s time to launch the Year 9s into their next topic. As always, the method of launch is as critical as the actual launch itself.

This is the classic example of what Bill calls the “flipped practical”, one where the students have no idea what’s going on, but are forced into asking questions from their experimental observations. The subsequent explanations then slot neatly into the pre-formed holes in their neural networks (well, that’s how I like to imagine it). It’s all about the “That’s Funny…” reaction, the most exciting words in science according to Isaac Asimov (who he?). I like this because it really is how science works and it’s what makes science uniquely exciting.

  • those galaxies have a red shift, that’s funny….
  • none of the first generation are dwarf, that’s funny…
  • there’s no bacteria round that fungal contamination, that’s funny….

(insert your own favourites).

I also like it because, for me, it all snowballed from this example. We’ve found that just about everything in our lessons works, and works better, with the “flipped” approach. Students are more engaged, more excited, more curious when they don’t know the answer in advance. At this point, you may be in Basil Fawlty mode, “Can’t we get you on Mastermind? Next contestant, Dr Paul, special subject, the bleeding obvious…”

So maybe you already do stuff like this – but I know from the Training Days that I run that it’s a novel approach for many teachers, so if it is new to you, give it a go. After all, what’s the worst thing that could happen…? If nothing else, you can be sure that no student will say, “Oh, look, it’s doing what it’s supposed to do….”

Anyway, to my wonderful Year 9s. What am I saying? All Year 9s are wonderful. But wonderful or not, it’s time to start enzymes. Except that I don’t want to get bogged down in terminology and definitions and labelling active sites just yet. I want to open up some receptive gaps in their brains. So they do this. Catalase interpretation sheet

The lesson plan is pretty self-explanatory, but here are a few notes to help you negotiate around any pitfalls.

I introduce hydrogen peroxide. Do they know anything? Not a lot. It bleaches hair? offers someone. We chat a bit about its various uses,  but the key point I want to get over is the fact that it goes “flat”. After a year, it’s all turned into water and oxygen and you have to buy a fresh batch.

They take their protocols and collect the apparatus and then it’s all hands on deck getting round to the pairs to help them over the first hurdle. They’re not quite sure what to write. Rate of peroxide break down…? Yes, how quickly is it happening? Huh…? Well, what would you expect to see when it breaks down? Um…. what does it break down to? Water and oxygen. Right. And oxygen is a…? Oh, bubbles. Right! Have another look. How quickly is it breaking down? Very fast – it’s already finished! Think about it – how long did we say it took to go flat? Oh, very slowly? Right! Too slow to see, unless it’s a particularly perky batch in which case you might see a few bubbles around the edge of the boiling tube.

Then it gets exciting. Add potato – whoosh! Add yeast – fizz! Add liver – yee hah! Ideally, their tubes overflow. Get them to test it for oxygen, while they’re at it. What on earth has happened? What do these things have in common?

This invariably goes well, but, for me, it’s not the most effective part of the lesson. After all, they’ve seen similar stuff in Chemistry (at least, you hope they have!), and while it’s fun, it’s not completely novel. It’s when they heat it all up, boiling the yeast and cooking the potato and frying the liver. If they already know about active sites and denaturing this is simply confirming what you told them –but if they’re enzymatic virgins, the reaction is, “huh? That’s funny….”, because heat usually speeds things up. Here it has done quite the opposite. And they want to know why.

The pH foaming towers activity can also be squeezed into a double period with a competent and fast working group.

And so you end with a series of questions…

  • What is it about cells that speeds up this process?
  • Why does extreme heat stop them from doing it?
  • Why is it also affected by pH?

… which they will remember for the next lesson, when you can then introduce the word enzyme and, if you feel like it, use this Powerpoint animation. enzyme animation showing lock and key specificity and denaturing

Have a good week!

Paul

Health and Safety note

Make sure they’re heating the yeast/potato/liver safely – yeast in a test tube has a tendency to shoot violently out the end when boiled!