There are a lot of amazing theories in science. The general theory of relativity, atomic theory, the cell theory. And one thing we want to point out about a scientific theory is that it's a lot different from how you might use the word theory in your daily life.
A scientific theory is not an educated guess. A scientific theory is an explanation of a scientific event. Supported by scientific evidence.
It must be testable and tested over and over and over again. And while theories can be changed or even disproven, you do want to realize there are a lot of facts behind them. One of our favorite theories of all time is the endosymbiotic theory.
We love it because we just find the events in this theory to be amazing. The theory gives an explanation for how eukaryote cells could have evolved from prokaryote cells that lived. symbiosis.
Prokaryote symbiosis means these organisms live together. So just to recap from our intro to cells video, when you think of prokaryote, think of pro rhyming with no. No nucleus. No membrane bound organelles.
Eukaryotes, on the other hand, remember you rhymes with do and they do have a nucleus. And other membrane bound organelles. Both prokaryotes and eukaryotes are cells though. And therefore, they do have a few things in common that cells have, including a cell membrane, cytoplasm, ribosomes, and genetic material. But just look at these two cell types and you will see that one is way more simple.
So what explanation does the endosymbiotic theory provide for how eukaryote cells could have evolved from prokaryote cells? A long, long time ago, seriously a long time, over 2 billion years ago, there were prokaryotes, much like there are today. They've been around for a long time. Well just as there are in many organisms, there was variety in prokaryotes.
Some of the prokaryotes had photosynthetic abilities, which means they could make their own food using sunlight energy. And those were the photosynthetic bacteria. Some of these prokaryotes were bacteria that had the ability to use oxygen to produce ATP energy.
And some prokaryotes were larger, they could consume others, engulfing them. So obviously they had to be larger. The endosymbiotic theory is that some of these large cells engulfed some of these small bacteria.
But instead of those small bacteria getting digested, some of them remained intact. And they actually began to live as symbionts. That means they lived together. This is believed to be the ancestor heterotroph eukaryote cell.
And eventually some of these cells engulfed the small bacteria that could do photosynthesis in addition. And there were some more living as symbionts. This is believed to be the ancestor autotroph, eukaryote. Now it's time to do a little reveal. We consider the bacteria that used oxygen to produce their own energy to have evolved into what is now mitochondria.
And the bacteria that used sunlight energy to produce their own food? Well, we consider those to have evolved into what are now chloroplasts. It's likely that these ancient eukaryotes had some kind of advantage in their endosymbiosis.
And it's also likely that the mitochondria developed first, before the chloroplasts, because if you remember from our intro to cells video, nearly all eukaryote cells have mitochondria. It's just that eukaryote cells that can do photosynthesis can have both mitochondria and chloroplasts. Now if you remember our beginning about theories, you may be wondering, where are the facts for this?
We'll give you a few of the facts. First, mitochondria and chloroplasts have their own DNA. Yes. separate DNA from what is found in the nucleus.
Not to mention that their DNA is arranged in a similar way to prokaryote DNA, specifically bacterial DNA. The size of mitochondria and chloroplasts tend to be similar to the size of bacteria, and when mitochondria and chloroplasts divide, which they can divide all on their own independently, they divide in a way that is similar to how bacteria divide. So the endosymbiotic theory provides an explanation for how modern eukaryotes evolved from prokaryotes. There's obviously more questions you might want to explore.
For example, what about some of the other structures and organelles in eukaryotes? Well, keep exploring. Secondary endosymbiosis is a great place to start.
One last thing we want to make sure to emphasize. Endosymbiosis isn't just reserved for a theory that explains a past event in ancient history. No, because endosymbiosis is actually happening today, with many other kinds of organisms.
One of our favorite examples? The termite. Yes, termites can have prokaryotes that live in their gut and help them digest wood.
And without them? Well, let's just say digesting wood wouldn't happen so well. Well, that's it for the Miba Sisters, and we remind you to stay curious.