The Endosymbiotic Theory

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Researchers comparing the structures of prokaryotes and cell organelles, as shown in Figue 2, came to the conclusion that organelles such as mitochondria and chloroplasts had originally been bacteria that were taken into larger bacteria by endocytosis and not digested. The cells would have had a mutually beneficial (symbiotic) relationship. The ingested cells developed into organelles, such as mitochondria and chloroplasts, which now cannot live outside the host cell.