Endosymbiont

The serial endosymbiont theory proposes that mitochondrion and plastids such as chloroplasts of the modern day eukaryotic cell evolved from different types of bacteria that were engulfed by prokaryotes through endophagocytosis. An endosymbiont is any organism that lives within another cell or organism.

It is believed that the mitochondrion of today’s organisms was first an aerobic form of bacteria that was ingested by an anaerobic prokaryote but the aerobic bacteria wasn’t digested. Inside the anaerobic cell the aerobic bacterium would have been able to flourish in the cytoplasm of the host cell feeding on the partially digested food particles. The aerobic bacterium would have been able build up huge stores of energy using oxygen to breakdown the particles. With the abundance of energy inside the aerobic bacterium it is believed that some of it might have leaked from the bacterium into the anaerobic prokaryote in the form of ATP. This would have allowed the anaerobic cell to break down food using the aerobic pathway with the help of the bacterium. Eventually the aerobic bacterium wouldn’t be able to sustain life outside the anaerobic prokaryote and would have become a mitochondrion.

Plastids such as chloroplasts were developed in much the same way as mitochondrion. The one exception is chloroplasts started from cyanobactrium,