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It’s fun to watch organisms eat in the microcosmos. There’s a whole range of methods to enjoy. And at the core of all this is a simple, universal need: energy, stored chemically as adenosine triphosphate—or ATP—that’s made from the breakdown of sugars and fats.

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PIA   provides a secure, reliable VPN connection.  Click the link in the description below to   receive 82% off the first two years,  plus four free months of free service.  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,   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.  And thanks again to Private Internet Access for  sponsoring today’s episode. PIA provides a secure,   reliable VPN connection.

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If you want to see   more from our Master of Microscopes, James Weiss,  you can check out Jam and Germs on Instagram. And if you want to see more from us, there's  always a subscribe button somewhere nearby.