The term endosymbiont describes and organism, which lives inside another organism.

Examples of endosymbionts:Edit

Nitrogen-fixing bacteria:Edit

These bacteria, which are diazotrophic and live in root nodules of vegetables are also known as Rhizobia. Rhizobia are not capable of fixing nitrogen indipendently, therefore they require a plant host.

Algae inside reef-building corals:Edit

Most reef-building corals have a mutually beneficial symbiotic relationship with a microscopic unicellular algae called zooxanthellae. The coral provides the algae with a protected environment and the compounds necessary for photosynthesis (the metabolic waste products of the coral). The algae remove metabolites from the coral, help to produce oxigen and provide nutritients (carbohydrates)

Endosymbiotic theory:Edit

The endosymbiotic theory describes how a large host cell and ingested bacteria could easily become dependent on one another for survival, resulting in a permanent relationship. It argues that plastids and mitochondria each arose from a single endosymbiotic event. It is suspected that mitochondria and plastids were originally free living bacteria, which were swallowed and survived inside of another cell. It can be assumed that chloroplasts arose from cyanobacteria and that mitochondria were originally protobacteria (Rickettsiales). The theory was first proposed by Andreas Schimper in 1883, but was only acknowledged in the 1960s after the discovery of mitochondrial DNA.

Evidence of endosymbiosisEdit

Mitochondria nad chloroplasts contain several features, which underline the endosymbiotic theory. First of all, both mitochondria nad chloroplasts are surrounded by a double membrane. They also contain their own genome, which is similar to prokaryotic genomes (i.e. circular DNA), and their own proteosynthetic aparate. This proteosynthetic aparate is also similar to the one of prokaryotic organisms (70S). Mitochondria and chloroplast also reproduce like bacteria, they replicate their own DNA and direct their own division.


Chloroplasts are genetically closest to photosynthetic bacteria: cyanobacteria, they even share the same photosynthetic system (Photosystem I+II).


Mitochondrial DNA is unique for its maternal pattern of inheritance. It is passed down directly from mother to child and it accumulates mutations more slowly than other types of DNA. For this reason, mitochondrial DNA is often used for creating antient human migration patterns.

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