Eukaryotes endosymbiotic theory

Cholesterol is found almost exclusively in eukaryotic cells. Animal membranes contain substantially more cholesterol than plant membranes, in which cholesterol is usually replaced by other sterols. There is no cholesterol at all in prokaryotes (with a few exceptions). The inner mitochondrial membrane of eukaryotes is also low in cholesterol, while it is the only membrane that contains large amounts of cardiolipin. These facts both support the endosymbiotic theory of the development of mitochondria (see p. 210). 

It is not improbable that further investigation of PolyP metabolism in mitochondria and chloroplasts would reveal novel features of similarity with eubacteria in favour of the endosymbiotic theory of the origin of eukaryotes. 

The Endosymbiotic Theory states that mitochondria and chloroplasts were once free living and possibly evolved from prokaryotic cells. At some point in our evolutionary history, they entered the eukaryotic cell and maintained a symbiotic relationship with the cell, with both the cell and organelle benefiting from the relationship. The fact that they both have their own DNA, RNA, ribosomes, and are capable of reproduction helps to confirm this theory. 

The endosymbiotic theory of the origin of eukaryotes states that eukaryotes arose from symbiotic groups of prokaryotic cells. According to this theory, smaller prokaryotes lived within larger prokaryotic cells, eventually evolving into chloroplasts and mitochondria. Chloroplasts are the descendant of photosynthetic prokaryotes and mitochondria are likely to be the descendants of bacteria that were aerobic heterotrophs. Serial endosymbiosis is a sequence of endosymbiotic events. Serial endosymbiosis may also play a role in the progression of life forms to become eukaryotes. 

Margulis, Lynn (bom 1938) American biologist she is best known for her theory on the origin of eukaryotic organelles, and her contributions to the endosymbiotic theory. 

Endosymbiotic theory Holds that the organelles of eukaryotic cells arose from prokaryotes that came to live, in a symbiotic relationship, inside the eukayote-to-be cell. 

In line with the endosymbiotic theory of Lynn Margulis > and others, the eukaryotic cell is the result of the association of several prokaryotic cells, which while still retaining some of their ancestral features, have given up their independence to work for the eukaryotic... 

Overmann J, van Gemerden H. Microbial interactions involving sulfur bacteria implications for the ecology and evolution of bacterial communities. FEMS Microbiol. Rev. 2000 24 591-599. Margulis L. Symbiotic theory of the origin of eukaryotic organelles criteria for proof. Symp. Soc. Exp. Biol. 1975 29 21-38. Smith DC, Bernays FA. Why do so few animals form endosymbiotic associations with photosynthetic microbes . Phil. Trans. R. Soc. Lond. B 1991 333 225-230. 

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