According to the endosymbiotic theory of the origin of eukaryotic cells, how did mitochondria originate? ? from infoldings of the plasma membrane, coupled with mutations of genes for proteins in energy-transfer reactions ? by secondary endosymbiosis ? when a protoeukaryote engaged in a symbiotic relationship with a protobacteria ? from the nuclear envelope folding outward and forming mitochondrial membranes ? from engulfed, originally free-living protoeukaryote
The Correct Answer and Explanation is:
The correct answer is: when a protoeukaryote engaged in a symbiotic relationship with a protobacteria
Explanation:
The endosymbiotic theory provides a compelling explanation for the origin of mitochondria within eukaryotic cells. This transformative event in the history of life is thought to have occurred over a billion years ago. The theory proposes that mitochondria began as independent, free-living prokaryotic organisms.
Specifically, the ancestor of the mitochondrion was an aerobic alpha-proteobacterium. This type of bacterium was highly efficient at cellular respiration, a process that uses oxygen to generate large quantities of ATP, the main energy currency of cells. At some point, a larger, anaerobic ancestral host cell—a protoeukaryote or possibly an archaeon—engulfed one of these smaller bacteria. Instead of being digested, the bacterium survived and continued to live inside the host cell’s cytoplasm.
This arrangement developed into a mutually beneficial symbiotic relationship. The host cell provided the engulfed bacterium with protection from the external environment and a steady supply of nutrients. In return, the bacterium, functioning in an oxygen-rich internal environment, produced a surplus of ATP, which it shared with its much larger host. This gave the host a significant metabolic advantage, allowing it to thrive and outcompete other cells.
Over millions of years of coevolution, the two organisms became inseparable. The engulfed bacterium transferred much of its genetic material to the host cell’s nucleus, losing its ability to live independently and becoming a permanent, integrated organelle. This is why mitochondria today have a double membrane—the inner one corresponding to the original bacterial membrane and the outer one derived from the host cell’s membrane during the initial engulfment. Further evidence supporting this theory includes the fact that mitochondria contain their own circular DNA and ribosomes, similar to those found in prokaryotes, and they replicate by a process resembling bacterial binary fission.
