∞ generated and posted on 2016.08.26 ∞
Cell that lives inside of another cell, typically in a long-term relationship that is not damaging to the larger cell.
These different cells are distinct species from that of the host and the endosymbiont's "host" cell (the outer or bigger one) will have acquired the endosymbiont, often sometime in the distant past.
The most common endosymbionts are mitochondria and chloroplasts, both of which retain both the chromosome and protein-making machinery including ribosomes of their once free-living ancestors. As a result, eukaryotic cells can possess three or more distinct genomes, the nuclear genome, the mitochondrial genome, and even one or more plastid genome (plastid is a more general term than the more familiar chloroplast).
Some endosymbionts are descendants of eukaryotic organisms, in some cases complete with nucleus, though most are descendants instead of prokaryotic organisms. There are a few cases where the host cell itself is a prokaryotic organisms (bacteria living within bacteria), though this is much rarer than the case of eukaryotes as the host organism.
The following is a great video on the origin and to some degree the diversity of cellular life, with great visuals; though a bit old, © 1981, it is surprisingly accurate and takes a refreshingly phenotypic rather than genotypic perspective: