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 mutually beneficial relationship. Over time, these bacteria lost many genes and evolved into organelles. Supporting evidence includes similarities in DNA structure, ribosome type, and replication methods between these organelles and prokaryotes, reinforcing the idea of their bacterial origins.

Endosymbiotic Theory

The Endosymbiotic Theory proposes that mitochondria and chloroplasts, essential organelles in eukaryotic cells, originated from independently living bacteria billions of years ago. Approximately 1.5 billion years ago, an aerobic bacterium, which utilizes oxygen, was engulfed by an anaerobic host cell that does not use oxygen. This event initiated a symbiotic relationship where both organisms benefited: the aerobic bacterium gained protection, while the host cell acquired the ability to metabolize oxygen.

Over an extensive period, the engulfed aerobic bacterium underwent significant genetic changes, losing many of its original genes and functions, ultimately evolving into the mitochondria we recognize today. Mitochondria are crucial for energy production in eukaryotic cells, enabling the development of complex organisms, including animals.

Similarly, a photosynthetic cyanobacterium was also engulfed by a host cell, leading to the formation of chloroplasts, which are vital for photosynthesis in plant cells. This process mirrors that of the mitochondria, with the cyanobacterium evolving over billions of years to become the chloroplast organelle.

Supporting evidence for the Endosymbiotic Theory includes several similarities between mitochondria, chloroplasts, and prokaryotic cells. Both organelles possess small circular DNA, 70S ribosomes characteristic of prokaryotes, and replicate through binary fission. Additionally, mitochondria and chloroplasts have two membranes—an outer and an inner membrane—consistent with the engulfment process that would have led to their formation.

In summary, the Endosymbiotic Theory provides a compelling explanation for the origin of mitochondria and chloroplasts, suggesting that these organelles were once free-living bacteria that formed symbiotic relationships with host cells, leading to the complexity of life as we know it today.

Endosymbiotic theory is supported by the discovery of non-nuclear DNA in the ______ and ________ organelles.

• A. Golgi apparatus and lysosomes.
• B. Mitochondria and lysosomes.
• C. Chloroplast and Golgi apparatus.
• D. Chloroplast and mitochondria.

Answer: (D)

According to the endosymbiotic theory, which of the following is likely the ancestor of the mitochondria?

• A. Aerobic eukaryotes.
• B. Aerobic bacteria.
• C. Anaerobic bacteria.
• D. Cyanobacteria.
• E. Chloroplasts.

Answer: (B)

Here’s what students ask on this topic:

What is the Endosymbiotic Theory?

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 mutually beneficial relationship. Over time, these bacteria lost many genes, evolving into organelles. Supporting evidence includes similarities in DNA structure, ribosome type, and replication methods between these organelles and prokaryotes, reinforcing the idea of their bacterial origins.

What evidence supports the Endosymbiotic Theory?

Several pieces of evidence support the Endosymbiotic Theory. Both mitochondria and chloroplasts have small circular DNA, similar to prokaryotic cells. They also possess 70S ribosomes, characteristic of prokaryotes, and replicate via binary fission. Additionally, these organelles have two membranes, an inner and an outer membrane, which is consistent with the engulfment process. These similarities suggest that mitochondria and chloroplasts were once independently living bacteria.

How did mitochondria and chloroplasts evolve according to the Endosymbiotic Theory?

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 relationship was mutually beneficial, with the host cell gaining the ability to utilize oxygen. Over time, the engulfed bacterium lost many of its genes and abilities, evolving into the mitochondria. Similarly, chloroplasts evolved from a photosynthetic cyanobacterium that was engulfed by a host cell, eventually losing many genes and becoming the chloroplasts found in plant cells today.

What are the similarities between mitochondria, chloroplasts, and prokaryotes?

Mitochondria, chloroplasts, and prokaryotes share several similarities. They all have small circular DNA, 70S ribosomes, and replicate through binary fission. Additionally, mitochondria and chloroplasts have two membranes, an inner and an outer membrane, which is consistent with the engulfment process described by the Endosymbiotic Theory. These similarities support the idea that mitochondria and chloroplasts were once independently living prokaryotic cells.

Why is the Endosymbiotic Theory important for understanding the evolution of eukaryotic cells?

The Endosymbiotic Theory is crucial for understanding the evolution of eukaryotic cells because it explains the origin of mitochondria and chloroplasts, key organelles in these cells. By elucidating how these organelles evolved from independent bacteria through a symbiotic relationship, the theory provides insights into the complexity and functionality of eukaryotic cells. This understanding helps explain the development of complex life forms and the evolutionary transition from simple prokaryotic cells to more complex eukaryotic cells.