Endosymbiotic Theory: Videos & Practice Problems

The endosymbiotic theory posits that mitochondria and chloroplasts in eukaryotic cells originated from independent aerobic and photosynthetic bacteria, respectively. Approximately 1.5 billion years ago, an anaerobic host cell engulfed these bacteria, leading to a symbiotic relationship. Over time, the engulfed bacteria lost many genes, evolving into organelles. Supporting evidence includes similarities in DNA structure, ribosomes, and replication methods between these organelles and prokaryotes. Both organelles possess double membranes, consistent with their engulfment. This theory underscores the evolutionary significance of symbiosis in complex life forms.

Several lines of evidence support the endosymbiotic theory. Mitochondria and chloroplasts share similarities with prokaryotes, such as having small circular DNA, 70S ribosomes, and replicating via binary fission. Additionally, both organelles possess double membranes, which is consistent with the engulfment process. These similarities suggest that mitochondria and chloroplasts were once independently living bacteria. The presence of these features in both organelles and prokaryotes provides strong evidence for the endosymbiotic origin of these eukaryotic cell components.

According to the endosymbiotic theory, mitochondria evolved from an aerobic bacterium that was engulfed by an anaerobic host cell around 1.5 billion years ago. This symbiotic relationship allowed the host cell to utilize oxygen for metabolism. Over time, the engulfed bacterium lost many of its genes and evolved into the mitochondria we see in eukaryotic cells today. Similarly, chloroplasts evolved from a photosynthetic cyanobacterium that was engulfed by a host cell. This relationship enabled the host cell to perform photosynthesis, and over time, the cyanobacterium evolved into the chloroplasts found in plant cells.

Mitochondria, chloroplasts, and prokaryotes share several key similarities. All three have small circular DNA, which is distinct from the linear DNA found in the nuclei of eukaryotic cells. They also possess 70S ribosomes, which are smaller than the 80S ribosomes found in the cytoplasm of eukaryotic cells. Additionally, mitochondria, chloroplasts, and prokaryotes replicate through binary fission, a process different from the mitosis seen in eukaryotic cells. These similarities provide strong evidence for the endosymbiotic theory, suggesting that mitochondria and chloroplasts originated from prokaryotic ancestors.

Mitochondria and chloroplasts have double membranes as a result of the engulfment process described by the endosymbiotic theory. When an anaerobic host cell engulfed an aerobic bacterium (which evolved into mitochondria) or a photosynthetic cyanobacterium (which evolved into chloroplasts), 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 an engulfment event and provides further evidence supporting the endosymbiotic origin of these organelles.