Describe how some cells came to have chloroplasts, according to the theory of endosymbiosis.

The endosymbiotic theory concerns the mitochondria, plastids (e. g. chloroplasts), and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales, the SAR11 clade,[1][2] or close relatives) and chloroplasts from cyanobacteria. Enjoy!