In this video, we're going to introduce the endosymbiotic theory. 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 long time ago, about 1,500,000,000 years ago, there was an aerobic bacterium or a bacteria that uses oxygen in its metabolism. 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 very long period of time, again about 1,500,000,000 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 endosymbiotic theory also suggests that a photosynthetic cyanobacterium, or a bacterium that's capable of performing photosynthesis, was also engulfed by a host cell, and, very similarly 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 is living separately and independently from the host cell that's over here. But then, eventually, this aerobic bacterium was engulfed by the host cell. The aerobic bacterium over a long period of time eventually developed into today's mitochondria that we find inside of animal cells. This process 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 this nice looking smile here. A long time ago there was a cyanobacterium, a photosynthetic cyanobacterium that was capable of performing photosynthesis, and this cyanobacterium was also engulfed. When it was 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. You can see that plant cells today, they have both chloroplasts and mitochondria as well, and that developed into today's plant.
This is once again 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 or do, they both have small circular DNA. Mitochondria, chloroplasts, and prokaryotes all have small circular DNA. They also all have small 70S ribosomes, and they all replicate via a process called binary fission. These are a lot of similarities that mitochondria and chloroplasts have with prokaryotes, and that suggests supporting evidence to suggest that mitochondria and chloroplasts, they were once independently living prokaryotes or bacteria. In addition to these similarities, mitochondria and chloroplasts 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 pretty consistent with this idea of being engulfed or engulfment, where a host cell engulfs them and that makes them acquire a second membrane. All of this here, these similarities and the fact that both mitochondria and chloroplasts have two membranes, is all consistent supporting evidence of the endosymbiotic theory.
This here concludes our introduction to the endosymbiotic theory, and how it suggests that today's mitochondria and chloroplasts were once independently living bacteria. 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.
