According to the endosymbiosis hypothesis (also referred to as the endosymbiosis theory), how did mitochondria originate? a. They were direct results of genetic mutations in eukaryotic cells that allowed for oxygen-using metabolism. b. They were eukaryotes that became engulfed by a progenote. c. They were prokaryotes that became engulfed by eukaryotes. d. They resulted from infoldings of the plasma membrane.
The Correct Answer and Explanation is:
Correct Answer: c. They were prokaryotes that became engulfed by eukaryotes.
The endosymbiosis theory proposes that mitochondria originated from free-living prokaryotic cells, specifically aerobic bacteria, that were engulfed by an ancestral eukaryotic cell. Rather than being digested or destroyed, these engulfed prokaryotes formed a symbiotic relationship with the host cell. Over time, the engulfed cells and the host evolved together, with the prokaryotes becoming permanent residents inside the host cell, eventually transforming into mitochondria.
Evidence supporting this theory includes structural and genetic similarities between mitochondria and certain bacteria. For example, mitochondria possess a double membrane, consistent with what would be expected if one cell engulfed another. Additionally, mitochondria have their own circular DNA, similar to bacterial DNA, and they replicate independently of the host cell’s nucleus through a process resembling binary fission, just like bacteria.
Furthermore, mitochondrial ribosomes are more similar in size and structure to those found in prokaryotes than to the eukaryotic ribosomes found in the cytoplasm. Mitochondria also have enzymes and transport systems in their inner membranes that resemble those found in modern prokaryotes, especially the alpha-proteobacteria group, which are believed to be the closest relatives of the ancestral mitochondrion.
This endosymbiotic event allowed the host cell to use oxygen efficiently to produce ATP through aerobic respiration, offering a significant evolutionary advantage. Over time, most of the genes of the engulfed prokaryote were either lost or transferred to the nucleus of the host cell, making the mitochondrion a semi-autonomous organelle. The endosymbiosis theory not only explains the origin of mitochondria but also sheds light on how complex eukaryotic cells evolved from simpler prokaryotic ancestors.
