Endosymbiosis is an ecological association in which one organism, the endosymbiont, lives inside another. The definition specifies location, not that both partners benefit, nor that the internal partner has become an organelle. An intracellular association can remain an association between distinct organisms. Endosymbiotic theory addresses a different question: how particular ancestral bacteria became inherited, functionally integrated organelles during evolution.
The striking consequence is that a eukaryotic biological cell can combine histories: its nuclear lineage and its mitochondria need not have the same ancestry. Mitochondria have bacterial ancestry related to Alphaproteobacteria; the plastids of plants and many algae, including chloroplasts, have ancestry within cyanobacteria. These are ancestral lineages, not a claim that a named living bacterial species is the literal ancestor. Present-day bacteria and organelles have both evolved since their shared ancestors. [ref]
Origin and Evolution of Plastids and Photosynthesis in Eukaryotes by Geoffrey I. McFadden. Opening paragraphs and “The endosymbiont: when and who?”: bacterial origins and uncertainty about closest relatives
From an internal partner to an integrated organelle
. Original conceptual schematic, not an observed historical sequence. Persistence, metabolic exchange, endosymbiotic gene transfer, organelle protein import and coordination of division illustrate integration; they are not dated stages in a compulsory order. The illustrated partner is generic, and the host outline does not specify the ancestral host of mitochondria. The protein-delivery arrow abbreviates nuclear gene expression, biological translation in the cytosol and organelle protein import; it does not place protein synthesis inside the cell nucleus. Details.
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