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I seached for endosymbiotic theory in Wiki and I found this about endosymbiotic theory:

Symbiogenesis, or endosymbiotic theory, is an evolutionary theory which explains the origin of eukaryotic cells from prokaryotes. It states that several key organelles of eukaryotes originated as symbiosis between separate single-celled organisms. According to this theory, mitochondria and plastids (e.g. chloroplasts), and possibly other organelles, represent formerly free-living bacteria that were taken inside another cell as an endosymbiont, around 1.5 billion years ago.

But I couldn't find any answer for my question, so I decided to ask:

According to this theory and differences between inner and outer mitochondria and plastid membranes, which membranes (outer or inner membranes) of mitochondria and plastids (e.g. chloroplasts) relate to bacteria?

And why do mitochondria need to have a double membrane?

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The inner membrane is that of the engulfed bacteria. The bacterium would have been phagocytosed by a larger cell. Hopefully you can see in this image the smaller cell being engulfed in the membrane of the larger cell:

enter image description here

[ source ]

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  • $\begingroup$ How and why is the second membrane made? $\endgroup$
    – user12874
    Jan 25 '15 at 19:01
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    $\begingroup$ @Majid Look at the top left image. The outer membrane is from the larger cell. It surrounds the smaller cell and eventually pinches off. Read about phagocytosis for more information. $\endgroup$
    – canadianer
    Jan 25 '15 at 19:03
  • $\begingroup$ @Canadianer - yes but in the present situation (nowadays), is the outer membrane organelle-based or host-cell-based? I think that is Majid's question. +1 $\endgroup$
    – AliceD ♦
    Jan 26 '15 at 13:41
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    $\begingroup$ @ChrisStronks That is entirely unclear to me since the question is about embosymbiosis. In any event, the majority of lipids in the mitochondrial membrane are imported from the endoplasmic reticulum. Some, however, are synthesised by mitochondria in the inner membrane. Lipids can be transported between the two membranes. $\endgroup$
    – canadianer
    Jan 26 '15 at 17:00

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