Year 12. Carbohydrates. Sugars and polysaccharides. And a real shock to the non-chemists who despite lots of advice and warnings to the contrary, still manage to look aggrieved when biological molecules start appearing in the lab. I try to keep the pace brisk, so after an introductory lesson on glucose, where they build them with molymods and work out the formation and structure of a glycosidic bond for themselves, they cover all the other sugars with a homework on Sugar Cards (Top Trumps? Dating Agency introductions? Collectable Cards? Their choice!)
But if sugar biochemistry can be unpleasant, it’s nothing to the sense of horror when starch and cellulose appear. As if plants themselves weren’t bad enough, now we have to learn about their polysaccharides! Aarrghhh!!!!!
So I start with Teddy. Everyone over here! (a key part of all my lessons is to keep the students on the move). Meet Teddy!
He’s not had the easiest of lives, starting off as a handy board rubber (note black markings). But as you can see, he’s gamely decided to try a bungee jump. Only thing is, he can’t decide what to use for his rope – should he use a length of string (on the right)? Or should he use a length of spaghetti (on the left)? It’s a tricky one!
Students puzzled, intrigued, deeply amused.
So, what do you lot think? Spaghetti or string? String or spaghetti? Any thoughts? The piece of string? Why? Because it’s stronger. Really? Well, it’s a good hypothesis, but how could we possibly find out?
So Teddy leaps from the chair safely secured to a piece of string. Tied to a piece of spaghetti, however (and good luck tying a realistic knot with spaghetti!), he plummets to the floor. Just in case they’re not convinced, I ask two students to have a quick tug of war with the string, and then the spaghetti. Yep, that was a good prediction – the string is much stronger.
But why on earth are we comparing string and spaghetti? Isn’t it like comparing warthogs with paint? What do they have in common? OK, they’re a similar shape, but surely that’s it. Anything else? No? OK, start with spaghetti – what is it? Yes, it’s food. What food group? What’s it largely made of? Starch. A carbohydrate. Excellent – great for carbo-blasting before a long race or a big match. Good. And where do we get starch from? Wheat.
All very straightforward. But what about string?
They usually get there pretty quickly. It’s cotton. Also a plant product. Made largely of plant cell walls. In other words, cellulose. Which is another? Carbohydrate.
We can now draw on what they learned from their sugar homework – they recall that starch is a polymer of alpha glucose, cellulose is a polymer of beta glucose – one makes fantastic food but isn’t something you would make a T-shirt out of, the other makes everything from jeans to string, but isn’t remotely appetising, even with bolognaise sauce. But they’re both made of glucose! How is such a thing possible?
Now they need to build it, to actually see how and why the position of the OH group on carbon 1 can make such an enormous difference. These cut out card monomers building starch and cellulose template (orange for starch, green for cellulose) to enable them to do this. They cut them out and stick them together, carbon 1 to carbon 4, using the pattern shown on the instructions building starch and cellulose instructions and questions. The alphas are easy. But the betas? Some see it instantly, others take a while, but eventually they all figure out that you have to flip every other glucose to make the glycosidic bond work.
So now they can extend their oligo-saccharides by joining them to other people’s. I encourage liberal use of glitter/decoration to lend a festive touch (we’re closing in on Xmas at this time of term) and to lighten up the subject – heaven knows that plant polysaccharides need all the glitter they can get –
and now you have a model to illustrate all the other key learning points. First the shape – the starch just spirals round and round, while the cellulose is a straight line. Now stack the cellulose molecules in parallel – why is this so strong? Lengthen the starch – why is this so great for storing energy? Oh, but there’s a problem with only having 2 available glucoses at any one time. What could we do? They suggest branches. Bingo! Add some 1’6 connections. Put it all up on a display board, cellulose flanking starch to represent a plant cell (-ish), and let them tackle the questions. Encourage them to take pictures of their molecules – good reference for revision.
Suddenly they’re excited. Are we going to build any more models? Oh yes. Just wait until we do proteins…