Mysteries of Science Revealed: The Endosymbiotic Theory

This summer I’ve been taking a Biology class at my community college, and I learned a Thing™.


PD image by NASA via Wikimedia Commons

Organisms as we typically think of them are classified into two domains: Bacteria and Eukarya.  There’s also a third domain–Archaea–but you probably wouldn’t know any of those guys unless you have microscopic vision and hang out in extreme environments like deep-ocean vents, or spend a lot of time looking into your own navel.

Either scenario presents its own special danger.

Domain Bacteria contains microorganisms such as germs and cooties.  They are prokaryotes, with cells that lack the nuclei and organelles found in more complex organisms.

Plants and animals fall within Domain Eukarya.  As eurkaryotes, their cells contain both membrane-bound nuclei and organelles that perform specialized functions, such as energy production and protein replication.


Plant Cell Schematic
PD image by Mariana Ruiz via Wikimedia Commons

Scientists have theorized that prokaryotes appeared first, followed by eukaryotes.  In order to explain how such a thing could be, they developed the Endosymbiotic Theory.

This theory proposes that the first eukaryotic cell was originally a prokaryotic organism that happened to have a nucleus, which then consumed another prokaryote in a scene that probably looked something like this:

“I happen to have a nucleus and I sure am hungry.  Oh, hello, fellow prokaryote…nice to eat you…I mean MEET you.  Aw, screw it…GET IN MAH BELLEH!”


But then something strange happened that altered the course of the world…

“Good gravy!  Something strange is happening that will alter the course of the world…what is wrong with mah belleh?”



Recalcitrant Cheeseburger
PD image via Pixabay

Rather than being digested like a recalitrant cheeseburger, the engulfed prokaryote incorporated itself into the structure of its devourer, and together they became the first eukaryotic organism.

“Whew, I feel better now.  And I’ll be darned if I don’t suddenly have improved powers of energy production…”

Scientists have offered evidence to support the Endosymbiotic Theory, pointing to two organelles in particular–the chloroplast and the mitochondrion, which function as energy-production structures in plant cells:

1.  The DNA of a eukaryotic cell is stored in the nucleus, but chloroplasts and mitochondria contain their own DNA and can even manufacture proteins, such as the microscopic equivalent of beef jerky.

2.  Chloroplasts and mitochondria are roughly the same size as bacteria, which, of course, they would be if they were once organisms that were minding their own business until something came along and ate them.

3.  Chloroplasts and mitochondria have double membranes–an inner one similar to that found in prokaryotic cells, as well as an outer one like those of eukaryotic cells.  I’ve seen this kind of thing before in bags of candy.  When globs of chocolate devour almonds, well…the almonds are still inside.

4.  Like prokaryotes, chloroplasts and mitochondria reproduce through binary fission, a process I shall now demonstrate through the medium of interpretive dance:

“Hi, I’m a chloroplast.  I’m green and pretty awesome.  You know what would be more awesome?  Two of me!”

I thought this whole idea was pretty interesting until I realized something.  Animals don’t have chloroplasts–in much the same way that Trix is for kids and not for rabbits–but they do have mitochondria.

Humans have mitochondria too.

If endosymbiosis happened before, what if it happens again?  I just ate a sandwich.  Rather than dissolving into questionable nutrients like it’s supposed to, is it even now incorporating itself into my structure as some strange new internal organ made of whole wheat and turkey?

This is the kind of thing that keeps me awake at night…

*     *     *

What cool Thing™ have you learned that sparked your imagination?  And what did you have for lunch today?  😉

18 thoughts on “Mysteries of Science Revealed: The Endosymbiotic Theory

  1. sheilapierson

    You’re so sexy when you speak biology 🙂 …. Let’s see, what cool Thing has sparked my imagination … lately? I discovered while actually watching World War Z what the “Z” stood for and my mind immediately raced to your last post … I was in real need of those tips… too bad I didn’t have a stinkin’ Smart Phone to look it up!!! (lol) And for lunch… it was a sad selection today… leftover pizza bread stuffed with feta cheese and spinach, but homemade guacamole tonight!

    1. Mike Schulenberg Post author

      I still need to see that movie, even though I’ve heard it doesn’t compare favorably to the book, but we’ll see. Your lunch sounds fancier that what I had, which I’ve already described in some detail 😉

  2. Tami Clayton

    Can this Thing you learned and that is now keeping you awake at night be summed up in the good by the classic phrase, “you are what you eat”? If so, I fear I will turn into some combination of a berry smoothie (today’s lunch) and a Mediterranean spinach/quinoa salad with a side of watermelon (tonight’s dinner). It’s going to make heading into the office tomorrow a bit weird.

      1. corajramos

        Now THAT I understand, I’m a cup of coffee and toast. For a second there I thought you were an alien, speaking alien-speak, and ready to devour my brain. I don’t know how I evaded it but I never had biology–I think I hung out in the stars instead-astronomy.

        1. Mike Schulenberg Post author

          Astronomy sounds really cool. I still need 8 hours of natural science to get my associate degree…I might have to see if my school has astronomy, and if it qualifies.

          Thanks for stopping by 🙂

  3. Prue

    Love it!!! 😀 😀 😀
    You could do a lot for teaching science and making it fun!

    Maths leaves me cold (real maths I mean), physics I can do but just don’t get it (except for understanding that you do not mix electricity and water!), chemistry I find boring, but biology is something which has always fascinated me.

    I worked in science for 30 years so I had heard of the weirdo prokaryotes which hang out in some of the most extreme habitats on earth. I think they’re awesome, and their discovery relatively late on makes me wonder what else there is to discover out there.

    Just why the prokaryote that got nommed wasn’t also digested is interesting – perhaps faulty enzymes? A sort of bacterial indigestion 😀

    Great to hear from you again Mike, and good to know you’re still hanging on in there, working your way through the syllabus.

    Prue x

    1. Mike Schulenberg Post author

      Thanks for dropping by, Prue…it’s great to see you 🙂 I enjoyed the challenge of trying to write something sciencey that’s fun to read, so hopefully I was at least modestly successful.

      Yeah, this Biology class has been pretty fascinating. I kinda regret taking it as a summer class because the speed at which everything moves makes it harder to enjoy, but still managed to get a lot out of the experience, I think.

      I too wonder why that prokaryote was spared so long ago. He must’ve somehow persuaded his devourer that he had a better idea than digestion 🙂

      How are things with you these days?

      1. Prue

        It was very readable!

        I’m ok enough – thanks for asking. Just coming up to the 15th which is the first anniversary but the year won’t be over until 9th August which was the date of the service of thanksgiving for Mr Prue’s life. I’m staying at home, pottering. Every time I go out, things seem to go wrong (brain probably not plugged in correctly :)) – and actually I’m enjoying being home. This last year has been busy with me being out more than in thanks to all the friends who’ve included me in all sorts of things.
        Hope to get back to my blog after the 9th – and my manuscript is printed out waiting to be read.

        Enjoy the summer!
        Prue x

        1. Mike Schulenberg Post author

          There’s certainly nothing wrong with spending time at home if that’s where you like being. Home is one of my favorite places. Sounds like you have some pretty good friends there to do some stuff with, which is always a good thing 🙂

  4. Kirsten

    Well this sure takes me back to my biology major days! I had forgotten about the endoplasmic reticulum (both smooth and rough!) Great job bringing biology to the masses. After reading this post, I honestly don’t think there’s anything that wouldn’t be entertaining here at the Mike Schulenberg worlds of perilous wonder blog. 🙂
    I have a cool Thing for you though. At the lab we have a calendar where the microbe of the month is an immortal strain of … well, I don’t remember off the top of my head, but the concept of an immortal bacterium is ripe for story telling of the sci-fi/fantasy sort, don’t you think?
    And it’s nice to see you up there, Prue. 😀 I hope you’re doing well!

    1. Mike Schulenberg Post author

      Thanks, Kirsten. I wasn’t sure if I’d be able to extract a sense of whimsical fun from the Endosymbiotic Theory, but it was sure fun to try. And maybe my instructor will enjoy that I took something from his class and ran with it 🙂

      I think I remember seeing something about such an immortal stain of bacteria, but I can’t remember its name. I might be confusing it with the so-called “resurrection plant.” There are some really interesting organisms out there, that’s for sure.

  5. Ellen Gregory

    I LOVE learning about cool Thing™s… It’s one of the best things about my job (when I have one). Today and yesterday I have been learning about an environmentally friendly alternative to traditional Portland Cement — called geopolymer — and how it behaves under triaxial stresses (compression while radially confined). Not sure there’s enough fodder for a blog post though — or whether I’m even allowed to. 🙂

    I like your biology story.

    1. Mike Schulenberg Post author

      Thanks, Ellen…that means a lot 🙂

      Geopolymer sounds awesome, like some sort of futuristic sci-fi material or something. I’ll have to see what Wikipedia says about this, since it’s the repository of all human knowledge and all…

      1. Ellen Gregory

        It’s made from fly ash, which is a waste product from power stations, and doesn’t emit (much?) CO2 when being made – as normal cement does. So it’s win-win on the GREEN front. (Wonder if this reply will get spammed too…)


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