Eukarotyes - Chapter 6

- Hello, I'm Lindsay Turnbull, and I teach biology at the University of Oxford. In this episode, I want to describe how one group of cells became monsters, combining bacterial brilliance with some pretty good tricks of their own. These cells are called eukaryotes and they're what you and I are made from.

(bird chirping) (frog croaking)

Once the microscope was invented, biologists quickly realised that there appeared to be two different types of cell. On the one hand, there were the bacteria, pretty difficult to see, even with a light microscope, because they're very small and they just have simple rigid shapes, like rods or cylinders. On the other hand, there were the eukaryotes.

These cells were much bigger, much easier to see under a light microscope, like these twirling paramecia, and inside they appeared to be much more complicated as well. So if you looked under a higher magnification, you could see inside them, and they have this kind of grainy appearance, and that's because inside eukaryotic cells there are internal membranes that make internal structures called organelles, and we'll look at those a bit later.

So that seemed to make sense to scientists, two kinds of cells, bacteria and eukaryotes. Then in about the 1970s, biologists started to study cells in a different kind of way, so they started to sequence their genomes. That means reading every letter in the book of instructions that all organisms carry, and then you can compare those letters to find out just how similar different organisms are.

And when they did that for lots of different cell types, they found that there seemed to be a third type of cell, hiding in plain sight.

Now scientists called this third group of cells archaea. They weren't new to science, people had found them already, but originally scientists thought that they were just another type of bacteria because they also had simple rigid shapes, just like bacteria, and they were similar size, very small cells, hard to see under a microscope.

But what was unusual when looking at the genomes is it was very clear that they actually were very different, and now we know that their ribosomes are a bit different, their membranes are a bit different.

They were called Archaea because most of them at that time were known from what are thought of as quite extreme environments, like these sulphurous vents around a volcano, and so people had the idea that maybe these were like the original cells that populated the early Earth, which was a a very difficult place to live. We don't think that anymore. In fact, bacteria and archaea are equally ancient and they probably split away from each other more than 3 billion years ago.

But now, scientists were thinking, "Okay, so we have three different cell types. We have bacteria, we have archaea, and we have the eukaryotes." Now we'll return to that story in a minute because it has yet another twist to it, but if we're going to understand it fully, we first need to look inside a eukaryotic cell.

So let's take a closer look inside a eukaryotic cell. Now it is more complicated, but we can see things that are very familiar. All cells have to have a genome and eukaryotic cells are no exception. What's unusual, though, is that the eukaryotic cell keeps its genome enclosed in a membrane. So when you look at a eukaryotic cell underneath a microscope, you can often see this sort of darkish blob that's called the nucleus, and that's what that is, it's the genome encased in a membrane.

Now, of course, the genome's no use if messages can't fly out of it, so that membrane has to have little holes in it and those are called pores, so you've got the nuclear pores that the messages fly out of.

The rest of the cell is often called the cytoplasm and that includes other organelles, which are membrane-bound compartments, and also the sort of liquid, and floating around in the liquid part, there are of course the ribosomes, all cells have to have those, and they trap messages and they make the tools and the machinery and the spare parts that the cell needs, exactly the same as what's happening in the bacterial cell, but there are some structures that are unique to eukaryotes.

So next to the nucleus there's this sort of labyrinthine arrangement of membranes. It looks a bit like a maze and it's studded with dots and those dots are actually ribosomes. And then close to that, you've got a series of long sausage-shaped compartments. So the first thing is the endoplasmic reticulum and the second thing is the Golgi apparatus, and those things work together.

The ribosomes feed in some of the proteins that they're making into the endoplasmic reticulum and the proteins are finished there. Some proteins that eukaryotic cells make are very large and it's difficult to get them to fold into the right shape, so that takes place inside the warehouse, and they also distribute these proteins to different parts of the cell. Some of them are targeted to different destinations and they're carried in tiny little membrane-bound vesicles.

Now there are other organelles in the cell and one of the prominent types of organelle is also a bit like a sort of sausage-shaped thing, and they're called mitochondria, and those are the powerhouses of the eukaryotic cell. They have two membranes, an outer and an inner one, and the inner one is covered in these little turbines that recharge the ATP batteries that the cell needs. In a bacterial cell, those are in the bacterial outer membrane.

So we have little structures called mitochondria that are recharging ATP batteries. They're about the same size and shape as a bacterium and inside a mitochondria, it turns out, it also has its own small genome and it also has a few ribosomes of its own. So this is really quite a strange thing to have inside another cell.

Well, one day, Lynn Margulis, a biological scientist, was peering at some cells and she had a bit of a brainwave. She thought maybe those mitochondria were bacteria once upon a time. In other words, a eukaryotic cell at some time in the past has engulfed a bacterial cell and then kind of domesticated it and kept it, and that's why it's about the same size and shape as a bacteria, that's why it has its own genome, that's why it has its own ribosomes.

There wasn't really any other explanation that made sense. But then, hang on a minute, what kind of cell did the engulfing? In other words, what kind of cell was the ancestor of the eukaryotic cell? Well, once again, genome sequencing came to our rescue and it turns out that the ancestor of the eukaryotic cell was an archaeal cell, and this meant that really we were back to only having two types of cell.

There were bacterial cells and there were archaeal cells, and eukaryotes are just a special type of archaeal cell, one that has engulfed a bacterium. So eukaryotes are this kind of unlikely mashup between bacteria and archaea. But it still leaves us with a problem, because we just have to understand how this event took place. How is it that a simple rigid cell with a rigid shape, i.e. an archaeal cell, was somehow able to engulf a bacterium millions of years ago?

Well, to understand this problem a bit better, let's have a look at this enormous eukaryotic cell called amoeba, and we can see it moving around. Now no bacterial or archaeal cell that we know about can move like this, they just have simple rigid shapes, and that's because they have an outer cell wall, but most eukaryotic cells ditch that wall, and instead they're supported by an internal spider web of filaments called the cytoskeleton, and this allows them to do this kind of dynamic crawling.

And an amoeba can engulf other cells, no problem. It can extend two of those kind of blobby arms and engulf it. But the problem is, the ancestor of the eukaryotic cell can't have looked like this, this is a eukaryotic cell, so how could it have engulfed a bacteria? Well, the answer to this problem lay in a rather surprising place.

Once scientists realised that archaeal cells were our ancestors, they became a lot more interested in them and they started looking harder for them, and one of the places they looked were these hydrothermal vents at the bottom of the ocean, and sure enough, there were archaeal cells there, but they were a different kind of archaeal cell that nobody knew anything about.

Unfortunately, it turned out that they were very difficult to culture, so scientists couldn't see them properly, But a Japanese team worked really hard at that, and in 2020, they showed some pictures that shocked the world. They showed an archaeal cell that looked like this, with extendable spaghetti-like arms.

Now we don't know whether that cell can actually capture a bacteria, but it looks like at least there's a possibility that it might, and so this theory of Lynn Margulis that started off as frankly seeming slightly crackpot, is now mainstream science. Just about everybody believes that this is how eukaryotic cells got started, an ancient archaeal cell engulfed a bacteria and put it to work.

So one of the most surprising things about eukaryotic cells is that they're able to gang up to form multicellular organisms like you and me. How did that start? Well, that's a complicated business, but we can see a very simple multicellular organism right here. This is just made from a collection of amoebae, and it's called a slime mould.

Amoebae normally live alone in soil, hunting cells, but if food becomes hard to find, they start to collect together and coalesce to form these structures, and people are quite fascinated by them as they are pretty amazing. But to form a proper multicellular being, like you or I, which is a permanent collection of cells working together, you have to form in a different way. You can't just get together with your mates.

You have to start as a single cell, and you and I are made from 37 trillion cells, but every single one of those is a product of cell division. We all started our lives as a single cell and that's what you need to get the cooperation that you must have to get a multicellular being working properly. Now one really major group of multicellular beings is the animals, and we're gonna find out more about them in the next episode.

Well, I hope you enjoyed that episode and found it useful, and if you did, do please share the link with friends and colleagues. There's a lot more information about eukaryotic cells in my book, "Biology: The Whole Story," in particular there's a lot more about eukaryotic genomes and how different they are from bacterial genomes, there's some fun stories there. Also, if you wanna buy the book, there is a link below.

Otherwise, join me for the next episode, which is covering chapter 7, which is all about the animals. (bird chirping)