In this video, we're going to introduce the endosymbiotic theory. And so the endosymbiotic theory is a theory that suggests that today's mitochondria and chloroplast organelles that we find inside of eukaryotic cells were once independently living bacteria. The endosymbiotic theory suggests that a really, really long time ago, about 1.5 billion years ago, there was an aerobic bacterium or a bacteria that uses oxygen in its metabolism. And this aerobic bacterium was engulfed by an anaerobic host cell or a host cell that does not use oxygen in its metabolism. When this aerobic bacterium was engulfed by the anaerobic host cell, it created a symbiotic relationship or a beneficial relationship between the two organisms, since each of them had abilities that the other did not have. Over a really, really long period of time, again about 1.5 billion years, the aerobic bacterium that was engulfed by the host cell lost many of its genes and abilities, and it developed and evolved into today's mitochondria. The theory of the endosymbiotic theory suggests that the mitochondria, it was once an independently living aerobic bacteria. Similarly, the endosymbiotic theory suggests that a photosynthetic cyanobacterium or a bacterium that's capable of performing photosynthesis was also engulfed by a host cell, and, over a long period of time, it evolved into today's chloroplast.
Let's take a look at our image down below to get a better understanding of this endosymbiotic theory. Notice over here on the far left, we're showing you in blue a host cell, and this host cell is an anaerobic host cell, which means that it does not use oxygen in its metabolism. And then over here, what we have is an aerobic bacterium or a bacteria that does use oxygen in its metabolism. This aerobic bacterium, living separately, eventually was engulfed by the host cell. You can see that the aerobic bacterium over a long period eventually developed into today's mitochondria that we find inside of animal cells. And so that is what allowed for complex living organisms to arise, such as this animal that you see over here, and this Rafiki-looking monkey that you see with a nice looking smile. Very similarly, a long time ago there was a cyanobacterium, a photosynthetic cyanobacterium that was capable of performing photosynthesis, and this cyanobacterium was also engulfed. Over time, it developed into today's chloroplast.
The host cell that had both mitochondria and chloroplast ended up developing into today's plant cell. And so you could see that plant cells today have both chloroplasts and mitochondria as well, and that developed into today's plant. Once again, this is a theory, and theories have lots of supporting evidence. Some of the supporting evidence that supports the endosymbiotic theory includes the vast amount of similarities between mitochondria, chloroplasts, and prokaryotes, like the aerobic bacterium and the photosynthetic cyanobacterium. The similarities between mitochondria, chloroplasts, and prokaryotes include that they both have small circular DNA. Mitochondria, chloroplast, and prokaryotes all have small circular DNA:
- 70S ribosomes
- They all replicate via a process called binary fission
These are some of the similarities that mitochondria and chloroplasts have with prokaryotes, thus supporting evidence to suggest that maybe mitochondria and chloroplasts were once independently living prokaryotes or bacteria. In addition to these similarities, mitochondria and chloroplasts, they both have two membranes. They both have an outer membrane, and they both have an inner membrane as well. Having an outer membrane and an inner membrane is consistent with the idea of being engulfed or engulfment, where a host cell engulfs them and that makes them acquire a second membrane. All of these similarities and the fact that both mitochondria and chloroplasts have two membranes are consistent supporting evidence of the endosymbiotic theory.
This concludes our introduction to the endosymbiotic theory, which suggests that today's mitochondria and chloroplasts were once independently living bacteria. And so we'll be able to get some practice applying the concepts that we've learned here in our next few videos. I'll see you guys in our next one.
