I’ve burbled on this topic before, but only briefly, tucking it at the end of a piece principally about World War 1 Shell Casings and the link to deadly diet drug, DNP…. I want to revisit it, primarily to describe in more detail the lesson, how I structure it, why it works, and use it as an example of how you can turn a non-practical lesson into a process of inquiry and discovery.

See what you think. Better still, let me know what you think!

Year 12. We’ve just about finished Cell Ultra-structure, they’re about to do a homework on Electron Microscopes. I show them this picture…


As ever, Powerpoint for me is a way of projecting thought-provoking images or interactive animations. I never use it to deliver notes – the phrase “Death by Powerpoint” exists for a reason…

So, what is this?

And immediately, they’re engaged. It’s a mystery! A puzzle! They’re all thinking and all have some kind of suggestion. I also use it as a way of encouraging involvement – have a go! what’s the worst thing that can happen? you might be wrong! but at least you tried…

After a round or two of interesting ideas, I start providing clues and someone usually guesses, correctly, that it’s a human embryo on the end of a pin.

I follow this up with another picture of a human embryo, just a bit closer in.

embryo 1

OK, key question, what did this start life as?

Yes, that’s right, a zygote. A single cell. Formed when a sperm and egg fuse…

But that egg, where did that come from?

From the mother. Of course.

And how did that individual start life? Yes, exactly, another zygote! Also formed from a sperm and an egg…

And that egg, where did that come from…?

We start a regress, back through the generation, through ma to grandma to great grandma and beyond.

If we keep going, I ask, where do we eventually get to?

To apes, someone say! Well, OK, yes, a common ancestor of us and apes to be strictly accurate, but no, much further… how far can you go?

They get it. The very first cell… This year, one of them can actually refer to LUCA (Last Universal Common Ancestor).

OK, all very cool and a bit mind boggling and stuff, but how would they describe that very very early cell, in just one word?

We try out a few suggestions before agreeing on “Simple”. This makes sense. Simple things evolve before complicated things. But what does that actually imply? What would this very early, very simple cell, have looked like?

We can’t know, of course, but as a way of stimulating discussion, I show them this:


What do they notice about this cell, particularly compared to the cells they’ve been studying?

That’s right – it’s got no nucleus, no mitochondria, no rough endoplasmic reticulum, none of the complicated internal structures of the rat liver cell that has cropped up so frequently on their interpretive electron micrographs.

So if this isn’t LUCA, what is it? What very simple cells are still among us? Again, they generally get this – it’s a bacterium.

I then contrast it with a more familiar A-level cell…

plant cell

… from which they can cheerfully pick out half a dozen different clearly visible structures.

Which brings us neatly to the key question of the lesson. How did life on earth go from this…


to this…

plant cell                           ?

At this point, I construct a time line on the board, starting with the origin of the Earth 4.5 billion years ago and sign posting it with the key relevant events along with the way (origin of life, 3.5 billion years ago, origin of complicated cells, 1.5 billion years ago, origin of modern humans, 200,000 years ago…). I then use this to add a few notes about the types of cell involved – the simple ones, the Prokaryotes, and what this means, and then the complicated ones, from which all multi-cellular life evolved, the Eukaryotes.

This allows us all to draw breath, before we return to the key question…


We bat a few ideas around. I drop a few visual hints (mainly be juxtaposing the two images and asking them what the bacterium resembles). And eventually someone has the necessary brainwave – did one of them start living inside the other?

Bingo! Endosymbiosis! Lynne Margulis’ brilliant idea that was, as things so often are, ridiculed and rejected….

Lynn Margulis and Endosymbiosis

And finally we can get on to the main exercise where I get them to figure it out for themselves (see attachment above). For if mitochondria used to be free-living bacteria, then perhaps we can make some predictions about what we might expect mitochondria to be like.

I put them into groups of 4 and get them to jot down all the things they already know about bacteria, without looking anything up in their books or their smartphones. Their list will eventually include at least some of the following…

  • they’re really small
  • they have plasmids
  • they reproduce through binary fission
  • they have ribosomes
  • we kill them with antibiotics

From this, I get them to come up with simple 5 predictions about mitochondria, again without looking anything up. Things like, “if mitochondria used to be bacteria, then they should have ribosomes…”

Once they have a list of 5, I let them check their predictions. And lo and behold, they’re right! There is evidence for endosymbiosis, even if it happened over a billion years ago. And they figured it out.

They’re convinced. They’re happy. It’s a successful lesson.

But now contrast it with an alternative lesson plan, perhaps put into a Powerpoint presentation… it might go something like…

“Today we are going to learn about endosymbiosis. Endosymbiosis is the theory that cell structures like mitochondria used to be free-living bacteria. Evidence for this theory comes in the similarities between the two. See the following list…”

It’s the self-evaluation every teacher should carry out before every single lesson plan – am I teaching? Or am I delivering information? The difference, at every level, could not be greater…


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