Friends, Romans, countrymen, lend me urea

A few weeks ago I burbled on how I use Molymods to enable Year 10 students to work out the balanced equation of aerobic respiration. Today I did something similar with Year 13, as we start to explore the liver.

As with all such activities, I encourage them to take photographs with their phones as they go along, which they can use for reference/revision.

I started by asking each student to make an amino acid from basic Molymod atoms and bonds. Once the initial panic/hilarity subsided (“what? remember something from AS?!?!!?), they actually did OK, perhaps needing a reminder on what to use for the C=O bond. And although most, understandably, went for glycine, there was an alanine and a serine for variety.

So, a load of amino acids swirling around. They’ve been absorbed in the small intestine and taken to the liver via the hepatic portal vein for processing. They’re being plucked from the sinusoid by the hepatocytes – what could the liver cells do with them?

OK, so they’re the raw ingredients for proteins, perhaps they’ll get converted into enzymes or muscle or membrane proteins or electron carriers or whatever.

But what if there’s too many? The liver can’t store excess amino acids – and the amine group is toxic. What’s going to happen? We talk about deamination. I tell them it requires an oxygen atom and produces ammonia.

They then use their amino acid and an oxygen atom to build the products, and use that to write out the equation.

What have they made? Along with the ammonia molecule, I tell them they’ve made a keto acid. Keto acids can be respired, but if the body is forced to use protein for respiration (in diabetes, anorexia, Atkins diet…) then too much keto acid goes into the blood. Why is this a problem?

We talk about keto-acidosis, bad breath and other interesting distractions.

But what about the ammonia?

We pass round a dilute ammonia bottle and take a sniff. Even the dilute stuff is pretty eye-watering (the concentrated version takes the back of your head off!). And you’re making this in your liver! What do you do with it?

Yes, it’s got to be excreted. So I ask them to react two of their ammonia molecules with one carbon dioxide to form a water molecule and one other molecule…

They dutifully work through the reaction with the models and figure out the equation.

Hmmmm, what’s this? They’ve built a brand new molecule! What could it be?

The chemists gamely tried to name it – di-amine-carbonyl? But eventually a light bulb goes on in someone’s brain and they realise what they’ve made.

What I like about this is that they build a molecule of urea without knowing that that’s what they’re doing or what the structure of urea actually is. It transforms the dry ornithine cycle into something immediately interesting and relevant and hands on, and they can use their photos for reference when they eventually sit down to learn it.

Next week, blood!

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