Endosymbiosis is a mutually beneficial relationship between two species where one species lives within another.

  1. An ancestral prokaryote developed abilities for cellular respiration or photosynthesis

  2. The ancestral prokaryote was taken into an ancestral eukaryote cell by endocytosis but survives.

  3. The prokaryote remained functional inside its host cell as a symbiotic organelle: a mitochondrion or chloroplast.