Endosymbionti

Endosymbionti refers to a symbiotic relationship where one organism lives inside another. This can occur between two eukaryotes, a prokaryote and a eukaryote, or two prokaryotes. The host organism provides shelter and nutrients for the endosymbiont, while the endosymbiont often offers metabolic advantages to the host. A classic and highly significant example is the endosymbiotic theory for the origin of mitochondria and chloroplasts.

According to this theory, mitochondria, the powerhouses of eukaryotic cells, and chloroplasts, responsible for photosynthesis in plant and algal cells, were once free-living prokaryotes. These prokaryotes were engulfed by an ancestral eukaryotic cell, but instead of being digested, they established a mutually beneficial relationship. The engulfed prokaryotes provided energy (through respiration for mitochondria and photosynthesis for chloroplasts) to the host cell, and in return, they received protection and resources. Over evolutionary time, these endosymbionts became integrated into the host cell, losing many of their independent genes and relying on the host for essential functions, while the host gained crucial metabolic capabilities. Evidence supporting this theory includes the fact that mitochondria and chloroplasts have their own DNA, which is circular like bacterial DNA, and they replicate independently of the cell's nucleus through a process similar to binary fission.