Virgil, UV and shake it up baby

I think you can tell a lot about a teacher from the way they approach certain topics. Take the Water Cycle. With a KS4 class. How would you go about it? I must admit that my first response to these kind of topics is either to set it for homework or give them some interpretive exercise to complete in class as quickly as possible so that we can get on to something more interesting. There may be something in the Water Cycle that appeals to the deep in certain pupils, but if there is, I can’t see it….

And then there’s my friend Bill. Bill doesn’t think a dull topic should result in a dull lesson. Rather than shying away from it, he sees it as a challenge. So he takes the students on an imaginative journey to ancient Rome. He tells them of Dido and Aeneas. He describes the Punic Wars and the massive boat building programme that resulted in the deforestation of Sicily. They research the effects of this environmental catastrophe and then have to write a poem on the Water Cycle in the style of Virgil, which they read to the class for some peer assessment activity.

He was rather pleased that this lesson was observed. I certainly wish I could have seen it!

More prosaically, I mentioned Year 13s extracting geranium leaf photo-pigments to analyse the absorption spectrum.

IMG_1077

We returned to it this week. Here it is. What colour is it? They agree, cautiously, that it’s green. Are they sure? They pause, worried about the trick question, but eventually decide that whatever’s coming next, it is definitely green.

But why is it green? In other words, why do we see it as green?

They’re a bit more confident about this. Because it reflects green light.

Right. So what is it doing with other wavelengths?

They feel it’s just a revision exercise from the homework so are quick to explain that other wavelengths are absorbed.

Very good.

Now, what about this?

We put the boiling tube of the pigment into in a box with a black cloth over it – students can peek in….) and shine a UV light on it.

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If you don’t think that is seriously cool, then you shouldn’t be doing Biology (indeed, I nearly lost my temper with a Year 12 student who stated confidently that “chloroplasts are irrelevant”).

Luckily (for them), my Year 13s are impressed (I don’t quite get gasps of wonder, but the spontaneous reaction is nonetheless gratifying).

Indeed, they are absolutely intrigued by this – it’s a lovely awe and wonder moment. So, remind me. What’s the green stuff? Chlorophyll. Why is it green? It reflects green light. Why isn’t it green in UV light?

This takes them a little longer, but they’re generally happy that an absence of green light means that it has nothing to reflect, so can therefore not appear green. But which end of the spectrum is UV? So why doesn’t it appear blue? Or violet? Why does it appear red?

So I show them a molecule of chlorophyll. Why the long hydrocarbon tail? Good AS revision of the membrane SKU they’ll need to tackling thylakoids. Yes, it needs a hydrophobic portion to sit happily in the thylakoid membrane. It’s the head bit that’s interesting, a porphyrin ring with a Magnesium ion in the middle, and pair of electrons whizzing around it…

So what do electrons do when they absorb light energy? They move up to a higher energy level. And then? If nothing else happens, they fall back down, to a lower energy level, emitting lower energy light (red light, with its longer wavelength) as it does so.

We can then see what chlorophyll actually does with these electrons when it’s part of the photosystem in the thylakoid membrane.

And the last thing this week is just plain silly. But it worked.

I had introduced enzymes to the Year 12s with the Sugar Shake app game I mentioned a few burbles ago. They loved the game, but really struggled with the observations. Perhaps I need to make it more structured? Half of them literally did not notice that the enzyme changed shape when the substrate entered the active site!

Anyway, there were the usual questions about how an enzyme can lower activation energy. I haven’t decided how good the following analogy is – it was spur of the moment improvisation – but it certainly works at one level.

Take two large lego blocks and put them in a bin/box. These are two molecules in solution that could join together. They’re in solution – so what will they be doing? I gently shake the bin to provide kinetic energy and, lo, you can hear the molecules colliding! But these are not successful collisions! I stop shaking and pick out the blocks to show that they have not joined together. How could we increase the likelihood of a successful collision? More heat! More kinetic energy! I frantically shake the bin and the molecules collide more frequently and energetically! But still nothing changes.

And then I take the blocks out and line them up in exactly the right orientation and then my hands apply a little induced fit. Look! They pop together with hardly any effort at all. Lining molecules up in exactly the right positions is clearly a key enzyme feature….

Happy bin shaking.

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2 thoughts on “Virgil, UV and shake it up baby

  1. Bill Burnett

    It was Carthage, Paul, Carthage. Rome was what Aeneas’ bunch built after he’d sailed off to Sicily with the sky behind him lit by suicidal Dido’s funeral pyre. Honestly, sometimes it’s hard to believe we were at the same school. 😉

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    1. paulweeks2014 Post author

      But Punic Wars were Rome vs Carthage (or so Wikipedia would have me believe!). Anyway, first draft said “ancient Greece” until I had the bright idea of looking it up, so it could have been worse. First search for Dido and Aeneas just brought up the opera and I ran out of time… Besides, I never got beyond “I sing of arms and the man….”

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