Endosymbiotic theory explains that certain organelles in eukaryotic cells, especially mitochondria and chloroplasts, originated from free-living prokaryotic organisms. According to this theory, an ancestral eukaryotic cell engulfed aerobic bacteria (which became mitochondria) and, in some lineages, photosynthetic bacteria (which became chloroplasts). Instead of being digested, these organisms formed a mutually beneficial relationship with the host cell, wherein the engulfed cells provided energy, while the host provided protection and nutrients. Over time, they became permanently integrated as organelles. Evidence for this theory includes the fact that mitochondria and chloroplasts have their own circular DNA, bacteria-like ribosomes, and double membranes, all indicating a prokaryotic origin.
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