In this video, we're going to begin our lesson on the Endosymbiotic Theory. The Endosymbiotic Theory suggests that today's mitochondria and chloroplast organelles, found in many eukaryotic cells, were once independently living bacteria. This would have occurred a very long time ago. In fact, the endosymbiotic theory suggests that about 1,500,000,000 years ago, an aerobic bacterium (a bacteria that utilizes oxygen in its metabolism) was engulfed by an anaerobic host cell (a host cell that does not utilize oxygen in its metabolism). When this aerobic bacterium was engulfed by the anaerobic host cell, it created a symbiotic relationship between the two organisms where both benefit. The aerobic bacterium would have benefited from this engulfment by receiving extra protection, and the anaerobic host cell would have benefited by acquiring a metabolism that utilizes oxygen. Over time, after engulfing the aerobic bacterium, the bacterium would have altered and lost many of its genes and abilities. Over an extremely long period of 1,000,000,000 years, it would have developed into today's Mitochondria, an organelle found in eukaryotic cells. The mitochondria today is not considered a bacterium. However, the endosymbiotic theory suggests that mitochondria were once an independently living aerobic bacteria.
In a similar manner, a photosynthetic cyanobacterium (a bacterium capable of performing photosynthesis) would have also been engulfed by the host cell. Over an extremely long period of time, 1,000,000,000 years, this photosynthetic cyanobacterium would have evolved into today's Chloroplast organelles found in many plant cells. If we look at our image down below, we can clarify the idea of the endosymbiotic theory. On the far left-hand side, we show a host cell that is an anaerobic host cell that does not utilize oxygen in its metabolism and an aerobic bacterium that does utilize oxygen. Notice this aerobic bacterium is independently living, surviving without the host cell. About 1,500,000,000 years ago, this aerobic bacterium would have been engulfed by the host cell. You can see the engulfment process beginning here, and over time, it is completely engulfed. Over 1,000,000,000 years, this aerobic bacterium that was engulfed by the host cell would have altered many of its genes and lost many of its abilities, developing into today's mitochondria organelle. The development of the mitochondria ultimately allowed for complex organisms and animals to arise, such as the monkeys shown here.
Similarly, a prokaryotic cell here, a cyanobacterium that is photosynthetic, would have also been engulfed. A host cell that engulfed both the Cyanobacterium and the aerobic bacterium would have become a plant cell. The cyanobacterium would have evolved over an extremely long period, lost and changed many of its genes and abilities to become today's chloroplast. Cells that had both the Chloroplasts and the mitochondria would have developed into today's plant cells. Recall that plant cells have both chloroplasts, as seen here in this image, these little green structures, and mitochondria, which is important to keep in mind.
The endosymbiotic theory is supported by solid evidence. One supporting aspect is the similarities between today's mitochondria and chloroplasts and prokaryotic cells. Both mitochondria and chloroplasts have small circular DNA, 70s ribosomes (characteristic of prokaryotic cells), and both replicate via a process known as binary fission. These are all similarities between the mitochondria and chloroplasts and prokaryotes, suggesting that these organelles may have once been prokaryotes that were independently living 1,000,000,000 years ago. Also, mitochondria and chloroplasts both have two membranes, an outer and an inner membrane. Having an outer and inner membrane is consistent with the idea of engulfment where the second membrane, the outer membrane, would have been acquired through the engulfment process. These ideas support the Endosymbiotic Theory.
This concludes our brief lesson on the Endosymbiotic Theory and how it suggests that today's mitochondria and chloroplasts were once independently living bacteria billions of years ago. I'll see you all in our next video where we can apply some of these concepts.
