Explain the concept of endosymbiosis and its relevance to eukaryotic cells.

The endosymbiotic theory proposes that mitochondria and chloroplasts, key organelles in eukaryotic cells, originated from ancient prokaryotic cells that were engulfed by a larger host cell. Instead of being digested, these prokaryotes formed a symbiotic relationship, where the host provided protection and nutrients, and the engulfed cells provided energy (mitochondria) or produced food (chloroplasts). Evidence supporting this includes their similar size to bacteria, circular DNA, 70S ribosomes, and double membranes.