There are a lot of amazing theories in science.
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General theory of Relativity …Atomic theory…The cell theory.
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And one thing we want to point out about a scientific theory is that it’s a lot different
What is a Scientific Theory?
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from how you might use the word ‘theory’ in your daily life.
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A scientific theory is not an educated guess.
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A scientific theory is an explanation of a scientific event supported by scientific evidence---it
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must be testable and tested over and over and over again.
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And while theories can be changed or even disproven,
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you do want to realize there are a lot of
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facts behind them.
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One of our favorite theories of all time---is the endosymbiotic theory.
Endosymbiotic Theory Defined
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We love it, because we just find the events in this theory to be amazing.
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The theory gives an explanation for how eukaryote cells could have evolved from prokaryotic
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cells that lived in symbiosis.
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Prokaryote symbiosis means these organisms lived together.
Prokaryotes vs. Eukaryotes
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So just a recap from our intro to cells video: when you think of prokaryote, think of ‘pro’
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rhyming with NO.
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No nucleus.
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No membrane-bound organelles.
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Eukaryotes on the other hand---remember “eu” rhymes with “do” and they DO have a nucleus
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and other membrane-bound organelles.
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Both prokaryotes and eukaryotes ARE cells though---and therefore they do have a few
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things in common that cells have including a cell membrane, cytoplasm, ribosomes, and
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genetic material.
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But just look at these two cell types and you will see that one is WAY more simple.
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So what explanation does the endosymbiotic theory provide for how eukaryote cells
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could have evolved from prokaryote cells?
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A long, long time ago…seriously, a long time---over 2 billion years ago
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there were prokaryotes.
Prokaryote Diversity (focusing on select ones in theory)
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Much like there are today.
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They had been around for a long time.
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Just as there are in many organisms, there was variety in prokaryotes.
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Some of these prokaryotes had photosynthetic abilities, which means, they could make their
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food using sunlight energy.
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These were photosynthetic bacteria.
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Some of these prokaryotes were bacteria that had the ability to use oxygen to produce ATP
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energy.
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And some prokaryotes were larger and could consume others.
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Engulfing them.
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So obviously, they had to be larger.
Endosymbiotic Theory Shown
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The endosymbiotic theory is that some of these large cells engulfed some of these
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small bacteria---but instead of those small bacteria getting digested, some of them remained
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intact and they actually began to live as symbionts.
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That means, they lived together!
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This is believed to be the ancestor heterotroph eukaryote cell.
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Eventually SOME of these cells engulfed the small bacteria that could do photosynthesis
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in addition and there was more living as symbionts.
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This is believed to be the ancestor autotroph eukaryote.
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Now it’s time to do a little reveal.
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We consider the bacteria that used oxygen to produce their own energy to have evolved
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into what is now mitochondria.
Revealing Organelle Development
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And the bacteria that use sunlight energy to produce their own food?
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Well we consider those to have evolved into what
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are now chloroplasts.
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It is likely that these ancient eukaryotes had an advantage in their endosymbiosis.
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It is also likely that the mitochondria developed first before chloroplasts, because if you
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remember from our intro to cells video, nearly all
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eukaryote cells have mitochondria.
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It’s just that eukaryote cells that can do photosynthesis can have both mitochondria
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and chloroplasts.
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Now if you remember our beginning about theories, you may be wondering, where are the facts
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for this?
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We’ll give you a few of the facts.
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First, mitochondria and chloroplasts have their OWN DNA!
Facts that Support Endosymbiotic Theory
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Yes!
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Separate DNA from what is found in the nucleus.
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Not to mention that their DNA is arranged in a similar way to prokaryote DNA---specifically,
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bacterial DNA.
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The size of mitochondria and chloroplasts tends to be similar to the size of bacteria
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and when mitochondria and chloroplasts divide---which they can divide all on their own, independently---they
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divide in a way that is similar to how bacteria divide.
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So the endosymbiotic theory provides an explanation for how modern eukaryotes evolved from prokaryotes.
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There’s obviously more questions you may want to explore---for example, what about
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some of the other structures and organelles in eukaryotes?
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Well keep exploring---secondary endosymbiosis is
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a great place to start.
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One last thing we want to make sure to emphasize.
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Endosymbiosis isn’t just reserved for a theory that explains a past event in ancient
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history.
Endosymbiosis Example in Termites
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No because endosymybiosis is actually happening today
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with many other kinds of organisms.
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One of our favorite examples?
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The termite!
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Yes, termites can have prokaryotes that live in
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their gut and help them digest wood.
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And without them?
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Well let’s just say that digesting wood won’t happen
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so well…
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Well that’s it for the Amoeba Sisters, and we remind you to stay curious!
In your own words, describe the endosymbiotic theory.