Nitrogen Cycle

disclaimer: this lesson has not yet been tested on school children

the quality of your lesson can go down as well as up

So there are some topics which just bring a cold chill to the heart when you contemplate planning the relevant lesson. A classic example of this is the Nitrogen Cycle. I’ve never taught it well, and I’ve seen several other people teach it to no useful effect. I’m bored, the students are bored – and we’re both slightly cross that any exam board would decide to include it in a specification. Covering important ideas is one thing, but we’re trying to get young people excited by science!

So is it just something you write off as a bad lot, hold your nose, teach the lesson as quickly as possible, and race on to something with more obvious appeal?

In a spirit of confession and openness, here is my Nitrogen Cycle lesson of recent years.

Nitrogen Cycle Exercise plus The Nitrogen Cycle

The idea is that they try to figure it out using the pictures as clues. Please note that I am at least trying to provoke active learning. I don’t expect them to get all of the blanks, but they should surely be able to figure out the middle bit with wee and poo and death and food. Shouldn’t they?

But no – they look blank, they look helpless, they look pathetic. No two ways about it, the lesson doesn’t work.

Back to the drawing board. To come up with something more successful, we need to determine the problem. I think a large part of it is that The Nitrogen Cycle comes from nowhere. It’s just this random topic that we insert into some convenient gap in the time table. There’s no reference point, no indication as to why it matters, and it’s just not interesting when all you’re expected to do is learn the various pathways and the unpronounceable and virtually unspellable micro-organisms.

But how to make it interesting? I wonder if we need to start with the atom itself….

So here are some ideas I’ll be trying next time the Nitrogen Cycle has to be taught. I’m thinking of A-level students. And I think I’ll cover it at the end of Photosynthesis in Year 13….

Opening question:

Name 5 biological molecules that contain a nitrogen atom…

With a bit of discussion between them I’d expect at least amino acids/proteins, nucleic acids, ATP, chlorophyll, NAD…

What does nitrogen bring to the party?

At this point I want them to think about why the properties of nitrogen make it so important to the biological molecules they’ve named. They already know about carbon and oxygen, so why does life also require nitrogen?

I’d expect this to take quite a lot of thought, discussion and prompting. I would want to remind them of carbon’s 4 bonds and the ability to form long chains. We would revise oxygen’s electro-negativity and the central importance of hydrogen bonds. Eventually, I’d hope to arrive at a point where they see that nitrogen is a bit like carbon and a bit like oxygen. It can form multiple bonds, but it doesn’t share its electrons nicely.

Where exactly are these properties important? Give specific examples.

This is great thinking fodder. Remember, the top universities are looking for students who both understand ideas and can apply them. Remember, the GCSE and A-level assessment is changing – factual recall is down (represents a max. 15% at iGCSE), using your brain is up. Will a question come up on the properties of nitrogen? Who knows! But it’s not a relevant query. This is training them in thinking, in not being afraid of the unfamiliar but tackling it head on.

Come on, describe some specific examples that you’ve met as part of the A-level course.

That generally puts the cat among the pigeons. An accusation that they’ve done this and should know it.

Tell me about proteins. What’s the role of nitrogen in building a protein?

Or maybe I’ll put the questions into an exercise and get them to work together in groups. Perhaps with some helpful diagrams that they can work with.

For DNA, proteins and chlorophyll, explain the importance of nitrogen in their structure and function.

So I would expect them to track down the nitrogen in the organic bases – maybe look at their own DNA model

and realise that the electronegativity of nitrogen is vital for the hydrogen bonds that connect the two stands.

And for protein, again referring to their models….

….they can trace it back to the peptide bond and the hydrogen bonds that stabilise the secondary structure.

As for chlorophyll, it needs that porphyrin ring to hold the magnesium ion. That ring is made possible by a combination of carbon and nitrogen…

So it’s key! We need to know more about it. The scene is set….

Question: where is most of the nitrogen on earth?

Right, now it’s time to consider diatomic nitrogen and why it’s so difficult to insert nitrogen atoms into large molecules. Let’s check out some enthalpy data.

N to N = 945.4 kJ/mol-1

Compare water (464 x 2) or carbon dioxide (805 x 2) kJ/mol-1

Basically, this is a molecule that’s less reactive than water! Demo trying to burn water for comedy value.

At this point I’d get them to research, in outline, the Haber process. What do we have to do when we want to rip nitrogen out of the atmosphere and combine it with other atoms?

450’C at 200atm with an iron catalyst. Yikes!

What is the challenge for any living organism wanting to go it alone and DIY its own nitrogen containing molecules?

Time for them to do some more research. What organisms can do this, what conditions do they need in order to do it, and where are these conditions found?

So, finally, after all this preamble, we’re dipping our toes tentatively into the nitrogen cycle. But this is seriously amazing and worth lingering over a bit. I like the contrast with our clumsy, clunky human chemistry, using sheer brute force to break the triple bond, and the beautiful elegance of biology, able to achieve the same ends at atmospheric pressure and the temperature of the soil. Primo Levi says this about photosynthesis:

“if the elaboration of carbon were not a common daily occurrence, on the scale of billions of tons a week, wherever the green of a leaf appears, it would by full right deserve to be called a miracle.”

I think the same sense of wonder is appropriate here.

There are some great images of the Rhizobium inside the legume nodules. What a cool relationship!

Rhizobium and nodules

This feels like the end of the lesson to me, so homework would be writing the diary of a nitrogen atom, detailing its adventures as it passes through every point of the nitrogen cycle…

As I say, I’ve not taught this, so caveat emptor. What do you think?