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Most websites and textbooks say that the double membrane of mitochondria and chloroplasts are a result of the endocytosis of ancient prokaryotes (the outer membrane is from the vesicle containing the prokaryote, the inner membrane is from the prokaryote itself). However, some sources say that this is not the case. The precursor to the mitochondrion (alphaproteobacteria) was a gram-negative bacteria, which has a double membrane. This site says that the reasoning for the double membrane that is often taught is a lie, and that the mitochondrial precursor had a double membrane already to generate ATP through the ETC.

So, if this is the case, why do mitochondria now have two membranes, and not three (two from the prokaryote and one from the vesicle)?

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  • $\begingroup$ This is a genuine question about your question; why would the Gram negative bacteria need to have a vesicle? Perhaps it the case of Gram positive and vesicle versus Gram negative only. $\endgroup$ – James Feb 16 '16 at 1:30
  • $\begingroup$ @James How would the Gram negative bacteria (or any bacteria) be endocytosed or enter the cytoplasm of the proto-eukaryotic cell if it's not surrounded by a vesicle? $\endgroup$ – Jason Yang Feb 16 '16 at 1:34
  • $\begingroup$ From the link you shared "It's simply not true that the double membranes of bacteria and chloroplasts were the result of endocytosis." After a brief bit of googling and reading some comments by internet folk I am convinced that the third membrane could be lost. I still can't answer the question though. Very interesting! $\endgroup$ – James Feb 16 '16 at 1:57
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The answer lies in the protein composition of mitochondrial membranes, which undoubtedly prove that the outer membrane (OM) is of alphaproteobacterial origin, and the phagosomal membrane (if there was any) is lost. Though the endosymbiotic origin is without question, bear in mind that the phagocytotic origin is still debated (cf. López-García & Moreira 2015). And if the symbiont has entered the host some other ways than phagocytosis (cf. papers of Tom Cavalier-Smith) or syntrophic engulfment (cf. papers by Bill Martin; Searcy; López-García & Moreira) (e.g. puncturing the plasma membrane as a pathogen), then there never was a third membrane.

From Symbiogenesis Wikipedia:

Porins (transport proteins) are found in the OM of mitochondria and chloroplasts, are also found in bacterial cell membrane (Fischer et al. 1994Zeth & Thein 2010Fairman et al. 2011). The membrane lipid cardiolipin is exclusively found in the inner membrane of mitochondria and bacterial cell membrane Mleykovskaya & Dowhan 2009.

According to many (Cavalier-Smith 2002, Cavalier-Smith 2006, Zeth 2010), the presence of β-barrel transport proteins proves that the OM evolved from the bacterial OM, not from the host food vacuole membrane. Both mitochondria and plastids contain in their OM β-barrel proteins (e.g. Tom40, Sam50 and porins in mitochondria, Toc75 homologue of Sam50 and their joint negibacterial ancestor Omp85 in chloroplasts) and these are only present in the OMs of gram-negative bacteria (cf. Schleiff & Soll 2005). All Alphaproteobacteria are gram-negative, hence the ancestor of mitochondria originating from the Rickettsiales or very close was also.

Note, that many secondary and tertiary plastids still retain their third, outermoest phagosomal membrane originating from the host's plasma membrane (cf. Keeling 2010).

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