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Endosymbiosis
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Diagram of endosymbiotic theory. In primary endosymbiosis, a proto-eukaryote develops a nucleus and endoplasmic reticulum, engulfs aerobic bacteria that become mitochondria, then photosynthetic bacteria that become chloroplasts, forming a modern photosynthetic eukaryote. In secondary endosymbiosis, an ancestral host cell with mitochondria engulfs a photosynthetic eukaryote; its degenerated nucleus remains as a nucleomorph and its chloroplast gains four surrounding membranes. Color-enhanced electron micrograph showing yellow mitochondria and blue endoplasmic reticulum within magenta-stained hepatocyte liver cells at high magnification. Riftia pachyptila (giant tube worms) with bright red plumes and long white tubular bodies clustered around a deep-sea hydrothermal vent on the ocean floor. Close-up of a citrus mealybug (Planococcus citri), a small, soft-bodied insect covered in white, waxy filaments, clinging to a plant stem. The pest appears oval-shaped with fine, thread-like projections along its body, feeding on plant sap and commonly found on leaves, stems, or fruit of tropical and ornamental plants.
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Diagram of endosymbiotic theory. In primary endosymbiosis, a proto-eukaryote develops a nucleus and endoplasmic reticulum, engulfs aerobic bacteria that become mitochondria, then photosynthetic bacteria that become chloroplasts, forming a modern photosynthetic eukaryote. In secondary endosymbiosis, an ancestral host cell with mitochondria engulfs a photosynthetic eukaryote; its degenerated nucleus remains as a nucleomorph and its chloroplast gains four surrounding membranes.
Endosymbiotic theory Primary endosymbiosis is thought to have given rise to essential eukaryotic organelles, specifically mitochondria and chloroplasts, whereas secondary endosymbiosis has contributed to the diversity of complex eukaryotic life on Earth.

Endosymbiosis

biology
Also known as: endosymbiont
Written by
Manjishtha Bhattacharyya
Manjishtha Bhattacharyya is an editor at Encyclopaedia Britannica. She holds a PhD in zoology from Vidyasagar University, West Bengal, and has more than seven years of experience in science communication...
Manjishtha Bhattacharyya
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Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
Britannica Editors
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•History

Endosymbiosis, phenomenon in which one organism lives within the body of an unrelated organism, usually in a mutually beneficial relationship, although the relationship could also be parasitic. The term derives from the Greek endo- (meaning “within”) and symbiosis (meaning “living together”). The organism living inside its partner organism is called an endosymbiont, and the partner harboring the endosymbiont is called the host. Endosymbiosis has shaped biological evolution, and research suggests that it still plays a...

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