Endosymbiotic Theory: Videos & Practice Problems

The endosymbiotic theory posits that mitochondria and chloroplasts in eukaryotic cells originated from independent aerobic and photosynthetic bacteria, respectively. Around 1.5 billion years ago, an anaerobic host cell engulfed these bacteria, forming a symbiotic relationship. Over time, the engulfed bacteria lost many of their genes and evolved into the organelles we see today. Mitochondria evolved from aerobic bacteria, while chloroplasts evolved from photosynthetic cyanobacteria. Supporting evidence includes similarities in DNA structure, ribosomes, and replication methods between these organelles and prokaryotes, as well as their double-membrane structure, indicating their engulfment.

Several lines of evidence support the endosymbiotic theory. First, mitochondria and chloroplasts have their own small circular DNA, similar to prokaryotic DNA. Second, they possess small 70S ribosomes, akin to those found in bacteria. Third, both organelles replicate via binary fission, a method used by prokaryotes. Additionally, mitochondria and chloroplasts have double membranes, consistent with the engulfment process. These similarities suggest that mitochondria and chloroplasts were once independently living bacteria that became integral parts of eukaryotic cells through a symbiotic relationship.

The engulfment of aerobic bacteria and cyanobacteria by an anaerobic host cell led to a symbiotic relationship where each organism benefited from the other's capabilities. The aerobic bacteria, which used oxygen for metabolism, provided the host cell with efficient energy production, eventually evolving into mitochondria. Similarly, the cyanobacteria, capable of photosynthesis, provided the host cell with the ability to convert sunlight into energy, evolving into chloroplasts. Over time, these engulfed bacteria lost many of their genes and functions, becoming specialized organelles within the host cell, contributing to the complexity of eukaryotic life.

Mitochondria and chloroplasts have double membranes as a result of the engulfment process described by the endosymbiotic theory. When an anaerobic host cell engulfed the aerobic bacteria and cyanobacteria, the engulfed bacteria retained their original membrane, and an additional membrane was formed from the host cell's membrane. This double-membrane structure is consistent with the idea of engulfment and serves as evidence supporting the endosymbiotic origin of these organelles. The inner membrane is derived from the original bacterial membrane, while the outer membrane comes from the host cell's engulfing vesicle.

Mitochondria, chloroplasts, and prokaryotes share several key similarities. First, they all have small circular DNA, unlike the linear DNA found in the nuclei of eukaryotic cells. Second, they possess small 70S ribosomes, which are characteristic of prokaryotes. Third, they replicate through binary fission, a process used by bacteria. Additionally, both mitochondria and chloroplasts have double membranes, which is consistent with the engulfment process described by the endosymbiotic theory. These similarities provide strong evidence that mitochondria and chloroplasts originated from independent prokaryotic organisms.