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It’s pretty fun to watch organisms eat in the microcosmos. There’s a whole range of methods
to enjoy. You’ve got single-celled organisms slowly absorbing prey into their bodies through
phagocytosis, rotifers whirling the crown of cilia on their head to get food into their mouths,
and so many other techniques that shape the life, and death, of our invisible little friends.
And at the core of all this is a simple, universal need: energy, energy stored chemically
as adenosine triphosphate—or ATP—that’s made from the breakdown of sugars and fats.
In bacteria, the production of ATP happens at the cell surface membrane, with protons
and electrons traveling at the borders of the organism to drive the creation of this fuel.
And the process is similar for eukaryotes. Except unlike prokaryotes, eukaryotes have organelles,
specialized bodies inside of them to carry out special chemical reactions.
And there is even an organelle dedicated to making ATP. Perhaps you have
heard about it? The mitochondrion? Better known as the powerhouse of the cell.
Yes the powerhouse of the cell.
The origins of that phrase seems to come from a 1957 Scientific American article written by
Philip Siekevitz that was titled “Powerhouse of the Cell.” It’s gone on to become
such a ubiquitous way to describe the organelle that it even has an entry on knowyourmeme.com,
which says, quote, “the phrase is typically mocked as an example of impractical
information taught in public schools.” Obviously, we at Journey to the Microcosmos
would never mock an organelle, especially one that is making it possible for us to do
just about every single thing we are doing at this moment. But we would like to take
a moment to maybe, lightly question some things. Because is the mitochondria always the powerhouse
of the cell? No. Not for all eukaryotes. And this matters because it shapes the way we
understand the evolution of eukaryotes. There’s a story about giant amoebas that
we’ve shared before but we want to talk about it in more detail today because when you’re talking
about exceptions and nuances to a commonly stated scientific fact, those details are fascinating.
We also want to talk about it because we want to talk about diplomonads
and to share, again, one of our favorite historical observations of the microcosmos.
In the 17th century, the original master of microscopes Antoni Von Leeuwenhoeck
woke up with some, less than ideal bowel movements. But Leeuwenhoek knew a good
opportunity when he saw one, and he took his watery sample to the microscope. He wrote:
I have at times seen very prettily moving animalcules. Their bodies
were somewhat longer than broad, and their belly, which was flattened,
provided with several feet, with which they made such a movement through the clear medium
and the globules that we might fancy we saw a pissabed running up against a wall.
Clifford Dobell, Leeuwenhoek’s biographer, took this description and matched it to the medical
knowledge available to him in the early 20th century to diagnose Leeuwenhoek with giardiasis,
a disease borne out a parasitic diplomonad that colonizes the human intestine.
Now not all diplomonads are parasitic, but they are all just a little bit weird. For one,
they have two nuclei. They also have eight flagella trailing behind and around them
as they spin through the microcosmos. What makes diplomonads really weird
is that they do not have: mitochondria. This puts diplomonads into a small and exclusive
club with a handful of other eukaryotes. These organisms, which scientists grouped
together into what they called the Archezoa, are anaerobic, meaning they don’t need oxygen and
might even prefer to live away from it. And that would explain why they might not
need mitochondria, which require oxygen to drive their reactions.
But it does not explain how they got that way. So scientists worked with the information they
had at the time to come up with an explanation, an explanation that would later be proven wrong.
They reasoned that diplomonads and their fellow Archezoans got that way because they just appeared
a little too early. Mitochondria are thought to have been the result of an endosymbiotic event,
where at one point in eukaryotic history, one cell engulfed a prokaryote.
But instead of getting digested and consumed, this little prokaryote inside of another cell
became useful in a different way. It became an organelle. And the diplomonads, scientists
thought, just missed out on that big event. Except, it turned out that the Archezoa might
have gotten in on the mitochondria party, they just also left it, losing their mitochondria
to evolution’s inventive ways. In the process, the Archezoa were left with
genetic mementos from their former organelle. One of the unique facets of mitochondria is
that they have their own DNA, separate from the DNA of a cell’s nucleus but still able to
instruct the production of proteins necessary to the mitochondria. But what became clear is
that there wasn’t nearly enough DNA to encode for all of those proteins.
In fact, the mitochondria only produce a small fraction of the proteins they need,
around 15% according to one estimate. The rest are encoded by DNA in the nucleus, transferred
there at some point by the endosymbiotic prokaryote that would become the mitochondria.
This fact became important when scientists realized that some of these genes could be
found in diplomonads and their fellow Archezoa. Genes that encoded proteins usually associated
with the mitochondria existed in organisms that were thought to have missed out on mitochondria
entirely. Which meant that perhaps they had not missed out on mitochondria. Maybe
they had had mitochondria and then simply traded them for a different way of life.
Now while there was always the possibility that these genes could have come from some other source,
that alternative seemed increasingly unlikely as it became clear that Archezoa have organelles
similar to mitochondria, just rewired to suit the chemical needs of these anaerobic organisms.
In the diplomonads, the organelle is called the mitosome. But curiously, despite the resemblance
to mitochondria, the mitosome doesn’t actually make ATP. It’s mostly known to make clusters
of iron and sulfur used in many cellular functions, including metabolism. Instead,
diplomonads make ATP in their cytosol, that soupy mix that surrounds their organelles.
Other members of what was once called the Archezoa have their own mitochondria-like organelles,
capable of making ATP. And the result has been a challenge to the story previously told
about them, that these were early progenitors of eukaryotes, simpler than what would come after.
If anything, it seems to be the opposite. These organisms likely evolved with mitochondria like
their fellow eukaryotes, but they also evolved out of them. Which means that
there are so many other questions to explore. Questions about how organisms found their way
and survived in anaerobic conditions. Questions about how mitochondria have
evolved. And questions about what the origins of eukaryotic life would have looked like.
And as for the diplomonad, I suppose we should say that the cytosol is the powerhouse of the
cell. And the point is not to poke holes in a popular biology aphorism taught to students.
The mitochondria is the powerhouse of the cell for many, many organisms, including you.
But when it comes to evolution, the exceptions to the rules we think define
us are the source of incredible futures written in mysterious and varied pasts.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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