endosymbiotic theory

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endosymbiotic theory

n.
A theory stating that the eukaryotes evolved through a process whereby different types of free-living prokaryotes became incorporated inside larger prokaryotic cells and eventually developed into mitochondria, chloroplasts, and possibly other organelles.
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The origin of the more complicated eukaryotic cell type has been partially elucidated by the now widely-accepted endosymbiont theory, which posits that two of the major membrane-bound compartments of eukaryotic cells--mitochondria, the energy-making organelles, and chloroplasts, the organelles where photosynthesis occurs in plant cells--were formed from ancient bacteria that invaded the cytoplasm of an ancient proto-eukaryotic cell and eventually took up residence in that cell.
While the endosymbiont theory is the best present answer to the question of eukaryotic cell origins, there is still widespread uncertainty regarding the identity of the original cell that hosted the initial endosymbiosis.
Such archezoa - which by their very existence bolstered faith in this part of the endosymbiont theory - should permit further tests of ideas about the endosymbiotic host.
In this century, biologists' perspectives on the endosymbiont theory of the origin of the mitochondrion have changed dramatically.