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Mitochondria and plastids have their own DNA, their own membranes, and their reproduction is not tied to the reproductive cycle of the host cell. However, they are considered to be organelles rather than a separate species in symbiosis with eukaryotes. Granted, mitochondria and plastids are incapable of living outside their parent cells, and likewise eukaryotes are incapable of surviving without the help of mitochondria and plastids. But this is also true of many other symbiotic pairs in Eukarya. So where does the distinction lie? What makes mitochondria and plastids organelles rather than separate organisms?

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Something to do with the lifecycle maybe? It is an interesting question. Perhaps it is because mitochondria are inseparably tied to the host cells mitotic cycle? A bacteria in a symbiotic relationship with a plant would have a distinct lifecycle... although I'm sure there would be examples where the distinction is less clear... –  Luke Aug 14 '12 at 13:31
    
In my naivety I think that Richard Dawkins has given the perfect answer in the last chapter of The Selfish Gene. It’s indeed (solely, he argues) a function of their life cycle. Mitochondria may be able to reproduce independently, but their proliferation is bound to their hosts. –  Konrad Rudolph Aug 15 '12 at 21:10

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I would say it has to do with the amount of mitochondrial or sequence that has been transferred to the host genome. As a consequence of all this information stored in the host genome, mitochondria cannot reproduce without the host. In this way, they are not their own organisms, but rather organelles.

Over evolutionary time, the line between organelle and intracellular endosymbiont gets blurry, but in their current state, mitochondria are organelles.

The same goes for for plastids, in general.

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I agree with this answer. Symbionts are distinguished as separate complete organisms in some stage, despite their need of other to survive. –  Martin Beracochea Aug 14 '12 at 17:29
    
Ninety percent of a mitochondrium's proteins are coded in the nucleus. –  rwst Aug 15 '12 at 6:48

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