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Endosymbiosis – Definition, Theory, Evidence, Examples

What is Endosymbiosis?

  • Endosymbiosis is a specialized form of symbiosis where one organism, termed the endosymbiont, resides intracellularly within another, the host organism. This intricate association is characterized by mutual benefits, with both entities deriving advantages from their coexistence.
  • The term “symbiosis” describes a relationship between two organisms that coexist in a manner where both benefit and utilize available resources for survival. The prefix “endo-” signifies an internal relationship, indicating that the symbiotic interaction occurs within the confines of the host organism. This contrasts with “ectosymbiosis,” where the symbiotic relationship is external.
  • Endosymbionts, by definition, are organisms that live within the body or cellular structures of another organism. This relationship can range from mutualistic, where both parties benefit, to other forms that may not necessarily be advantageous to both. Classical examples of endosymbiosis include the nitrogen-fixing bacteria, known as rhizobia, which inhabit the root nodules of leguminous plants, and single-celled algae that reside within corals, aiding in reef formation. Additionally, certain insects harbor bacterial endosymbionts that furnish them with vital nutrients.
  • The origins of endosymbiosis are deeply rooted in the annals of evolutionary biology. The endosymbiotic theory, or symbiogenesis, posits that certain organelles within eukaryotic cells, notably mitochondria and plastids (e.g., chloroplasts), evolved from free-living bacteria that were engulfed by ancestral eukaryotic cells. This theory provides a compelling explanation for the presence of these organelles in eukaryotes, underscoring the evolutionary significance of endosymbiotic events.
  • Transmission of symbionts can be categorized into two primary modes: horizontal and vertical. Horizontal transmission involves the acquisition of symbionts from the environment in each generation, exemplified by the relationship between nitrogen-fixing bacteria and some plants. In contrast, vertical transmission ensures the direct passage of symbionts from parent to progeny, as observed in pea aphid symbionts. Some systems exhibit mixed-mode transmission, where vertical transmission is predominant, but occasional horizontal acquisition occurs, introducing new symbionts.
  • Endosymbiotic relationships can be obligate or facultative. In obligate endosymbiosis, the survival of one or both partners is contingent upon the presence of the other. Mitochondria and chloroplasts in eukaryotic cells exemplify obligate endosymbiotic relationships. Conversely, facultative endosymbiosis does not necessitate the constant presence of the symbiont for the host’s survival.
  • In conclusion, endosymbiosis is a multifaceted biological phenomenon that underscores the intricate relationships and evolutionary dynamics between different organisms. Through mutualistic interactions, organisms have co-evolved, optimizing their survival in diverse environments. The study of endosymbiosis offers profound insights into the complexities of life and the myriad ways organisms interact and coexist.

Definition of Endosymbiosis

Endosymbiosis is a form of symbiosis in which one organism (the endosymbiont) lives inside the cells or body of another organism (the host), often resulting in mutual benefits for both entities.

Endosymbiotic Theory

The Endosymbiotic Theory offers a comprehensive explanation for the evolutionary transition from prokaryotic to eukaryotic cells. Central to this theory is the proposition that certain organelles within eukaryotic cells, specifically mitochondria and chloroplasts, originated from free-living prokaryotic cells. This groundbreaking hypothesis was first introduced by botanist Konstantin Mereschkowski between 1905 and 1910.

Overview of the Process of Endosymbiosis.
Overview of the Process of Endosymbiosis. Image Source:

Mechanism of the Endosymbiotic Theory:

  1. Initial Cellular Evolution: The foundational eukaryotic cell exhibited primitive features, including an early endoplasmic reticulum and a nascent nuclear envelope. These structures are believed to have arisen from the invagination or infolding of the plasma membrane.
  2. Engulfment of Aerobic Bacteria: At some point in evolutionary history, this primitive eukaryotic cell engulfed an aerobic bacterium through a process termed endophagocytosis. This mechanism involves the inward folding of the plasma membrane, forming vesicles that transport the engulfed bacterium into the cell’s interior.
  3. Formation of Mitochondria: Over countless generations, the engulfed aerobic bacteria and their descendants became increasingly integrated and reliant on the host cell. This symbiotic relationship eventually led to the transformation of these bacteria into what we now recognize as mitochondria, the powerhouse of eukaryotic cells.
  4. Incorporation of Photosynthetic Cyanobacteria: In a subsequent evolutionary event, a eukaryotic cell engulfed a photosynthetic cyanobacterium. Over time, this cyanobacterium, too, became an integral part of the host cell, evolving into the organelle known as the chloroplast, responsible for photosynthesis in plants.
  5. Serial Endosymbiosis: Given that these two symbiotic events occurred sequentially, the entire process is often referred to as serial endosymbiosis.

In essence, the Endosymbiotic Theory elucidates the intricate evolutionary pathways that gave rise to the complex eukaryotic cells from simpler prokaryotic predecessors. This theory underscores the adaptive nature of life, where cooperative relationships can drive significant evolutionary advancements.

Determining the Chronology of Mitochondrion and Chloroplast Formation

The Endosymbiotic Theory, which postulates the origin of certain eukaryotic organelles from engulfed prokaryotic cells, has been instrumental in understanding the evolutionary history of eukaryotic cells. A pivotal aspect of this theory is the sequential formation of mitochondria followed by chloroplasts within these cells. The determination of this order was achieved through a meticulous examination of phylogenetic data.

  1. Evidence from Prokaryotic Lineage: Bacteria, being prokaryotic organisms, inherently lack organelles like mitochondria and chloroplasts. This absence serves as a foundational reference point in tracing the evolutionary trajectory of eukaryotic organelles.
  2. Ubiquity of Mitochondria in Eukaryotes: Mitochondria are found in a vast array of eukaryotic organisms, encompassing protozoa, animals, fungi, plants, and algae. The widespread presence of mitochondria across these diverse groups indicates that the engulfment of an aerobic bacterium, leading to the formation of the mitochondrion, occurred early in the evolutionary timeline of eukaryotic cells.
  3. Restricted Distribution of Chloroplasts: In contrast to the ubiquity of mitochondria, chloroplasts are localized to specific eukaryotic lineages, primarily plants and algae. This limited distribution suggests that the engulfment of a photosynthetic cyanobacterium, culminating in the formation of the chloroplast, transpired after the divergence of the ancestral lineage leading to plants and algae from other eukaryotic lineages, such as those leading to protozoa, animals, and fungi.

In summary, by analyzing the distribution and presence of these organelles across various eukaryotic groups and leveraging the insights from phylogenetic studies, scientists deduced that mitochondria evolved prior to chloroplasts in the eukaryotic lineage. This methodical approach underscores the power of evolutionary biology in deciphering complex biological phenomena.

Endosymbiotic Evidence in Eukaryotic Cells

An overview of the endosymbiosis theory of eukaryote origin (symbiogenesis).
An overview of the endosymbiosis theory of eukaryote origin (symbiogenesis). | Image Credit: Eosmanov, CC BY-SA 4.0, via Wikimedia Commons

The Endosymbiotic Theory posits that eukaryotic cells evolved through a symbiotic relationship between primitive eukaryotic cells and certain prokaryotes. These prokaryotes, over time, became integrated as organelles within the eukaryotic cells, leading to the complex cellular structures observed today. Several lines of evidence support this theory, underscoring the endosymbiotic origin of organelles like mitochondria and plastids:

  1. Binary Fission in Organelles: Both mitochondria and chloroplasts undergo binary fission, a mode of division reminiscent of prokaryotic cells. This suggests that these organelles might have once existed as independent entities before becoming part of eukaryotic cells.
  2. Similarities in Size: The dimensions of mitochondria and chloroplasts closely resemble those of certain bacteria, hinting at a shared evolutionary lineage.
  3. Distinct Outer Membrane: The presence of an additional outer membrane in mitochondria and chloroplasts can be attributed to the process of endocytosis during their initial engulfment by ancestral eukaryotic cells.
  4. Presence of 70S Ribosomes: While eukaryotic cells typically contain 80S ribosomes, mitochondria and chloroplasts house 70S ribosomes, a type commonly found in prokaryotes. This ribosomal similarity further strengthens the endosymbiotic connection.
  5. Circular and Naked DNA: Both mitochondria and chloroplasts possess circular, non-histone-bound DNA, akin to the genetic material observed in prokaryotic cells. This feature diverges from the linear, histone-associated DNA typically found in eukaryotic nuclei.
  6. Antibiotic Susceptibility: Mitochondria and chloroplasts exhibit sensitivity to certain antibiotics like Chloramphenicol, a trait characteristic of bacterial cells. This suggests a bacterial ancestry for these organelles.

In light of these compelling pieces of evidence, the Endosymbiotic Theory provides a robust framework for understanding the evolutionary trajectory of eukaryotic cells. The integration of once-independent prokaryotic entities into eukaryotic cells underscores the dynamic and cooperative nature of cellular evolution.

Significances of Endosymbiosis in Cellular Biology and Evolution

Endosymbiosis, a process where one organism resides within another, has profound implications for the biology and evolution of both the host and the endosymbiont. The mutualistic interactions that arise from such relationships have led to several significant outcomes:

  1. Optimal Environment for Endosymbionts: Residing within a host provides the endosymbiont with a stable and protected environment, shielding it from external adversities and ensuring its survival.
  2. Nutrient Exchange: The host organism often assimilates a variety of nutrients, some of which are essential for the growth and proliferation of the endosymbiont. This symbiotic relationship ensures a consistent supply of these vital nutrients to the endosymbiont.
  3. Beneficial Compounds for Hosts: Endosymbionts often produce compounds that are advantageous for the host. For instance, E. coli, an endosymbiont in the human gut, produces a chemical called colicin. This compound has the capability to deter other potentially harmful pathogens, thereby offering a protective advantage to the human host.
  4. Evolutionary Advancements: Endosymbiosis has played a pivotal role in cellular evolution. The engulfment of one cell by another and their subsequent coexistence has led to the development of intricate cellular structures. This process has been instrumental in the evolution of eukaryotic cells from simpler prokaryotic predecessors.
  5. Diversification of Life: The endosymbiotic events have contributed to the vast diversity of life forms observed today. By facilitating the integration of different organisms and the subsequent evolution of new cellular structures, endosymbiosis has been a driving force behind the emergence of various organisms.

In summary, endosymbiosis is not merely a biological curiosity but a fundamental process that has shaped the course of cellular evolution and diversification. The mutual benefits derived from such relationships underscore the intricate and cooperative nature of life at the cellular level.

Examples of Endosymbionts

Endosymbiosis, a phenomenon where one organism resides within another, is pervasive across various biological taxa, ranging from plants and bacteria to protists and vertebrates. This intricate relationship often results in mutual benefits for both the host and the endosymbiont. Here are some illustrative examples of endosymbionts across different organisms:

  1. Rhizobium in Leguminous Plants: Rhizobium, a nitrogen-fixing bacterium, establishes a symbiotic relationship within the root nodules of leguminous plants. While the bacterium derives essential nutrients from the plant, it reciprocates by converting atmospheric nitrogen into nitrogenous compounds, which the plant can assimilate.
  2. Buchnera in Aphids: The aphid, Acyrthosiphon pisum, harbors an endosymbiotic bacterium known as Buchnera spp. This bacterium plays a crucial role in synthesizing essential amino acids that the aphid requires, showcasing a mutualistic relationship.
  3. Symbiodinium in Corals and Mollusks: Symbiodinium, a type of dinoflagellate, resides within certain mollusks and corals. These endosymbionts assist in capturing and storing sunlight, providing the energy necessary for carbonate deposition, a fundamental process in coral reef formation.
  4. Richelia in Diatoms: In marine environments, certain diatoms, such as Hemialus, rely on nitrogen fixed by an endosymbiotic bacterium, Richelia. This symbiotic relationship ensures a consistent supply of nitrogen, a vital nutrient for the diatom’s growth and metabolism.
  5. Oophila Algae in Salamanders: The algae of the genus Oophila form an endosymbiotic association with salamanders belonging to the Ambystoma genus. This unique relationship is characterized by the algae’s ability to provide oxygen and other benefits to the developing salamander embryos.

These examples underscore the diverse manifestations of endosymbiosis across the biological spectrum. Such relationships highlight the intricate interdependencies that have evolved over time, facilitating the survival and thriving of both the host and the endosymbiont.


Which term describes a relationship where one organism resides within another organism?
a) Ectosymbiosis
b) Parasitism
c) Endosymbiosis
d) Commensalism

Which organelle in eukaryotic cells is believed to have originated from an engulfed aerobic bacterium?
a) Lysosome
b) Golgi apparatus
c) Mitochondrion
d) Peroxisome

Which of the following is a nitrogen-fixing bacterium that forms an endosymbiotic relationship with leguminous plants?
a) Escherichia coli
b) Staphylococcus aureus
c) Rhizobium
d) Bacillus subtilis

The endosymbiotic theory suggests that chloroplasts in plant cells originated from which engulfed organism?
a) Aerobic bacterium
b) Photosynthetic cyanobacterium
c) Methanogenic archaea
d) Yeast

Which organelle in plant cells is responsible for photosynthesis and is believed to have an endosymbiotic origin?
a) Nucleus
b) Vacuole
c) Chloroplast
d) Endoplasmic reticulum

Which of the following is NOT a characteristic that supports the endosymbiotic origin of mitochondria and chloroplasts?
a) Presence of double membranes
b) Ability to undergo binary fission
c) Linear DNA structure
d) Presence of 70S ribosomes

Which insect harbors the endosymbiotic bacterium Buchnera spp.?
a) Honeybee
b) Aphid
c) Mosquito
d) Butterfly

Symbiodinium, a type of dinoflagellate, forms an endosymbiotic relationship primarily with:
a) Birds
b) Mammals
c) Corals and mollusks
d) Amphibians

Which of the following organelles does NOT have an endosymbiotic origin?
a) Mitochondrion
b) Chloroplast
c) Lysosome
d) Both a and b

The term “serial endosymbiosis” refers to:
a) Multiple endosymbiotic events occurring simultaneously
b) Multiple endosymbiotic events occurring sequentially
c) A single endosymbiotic event leading to the formation of multiple organelles
d) None of the above


What is endosymbiosis?

Endosymbiosis is a symbiotic relationship where one organism (the endosymbiont) lives inside the cells or body of another organism (the host).

How does endosymbiosis differ from ectosymbiosis?

While endosymbiosis involves one organism living inside another, ectosymbiosis refers to a symbiotic relationship where the organisms live on the external surface of each other.

Which organelles are believed to have originated from endosymbiosis?

The mitochondria and chloroplasts in eukaryotic cells are believed to have originated from endosymbiotic events.

Who proposed the endosymbiotic theory?

The endosymbiotic theory was first introduced by botanist Konstantin Mereschkowski in the early 20th century.

Why is the endosymbiotic theory important?

The endosymbiotic theory provides insights into the evolutionary origins of eukaryotic cells and explains the presence of certain organelles within these cells.

How does the DNA of mitochondria and chloroplasts support the endosymbiotic theory?

Both mitochondria and chloroplasts possess circular DNA, similar to prokaryotic cells, supporting the idea that they originated from free-living bacteria.

Are there any modern examples of endosymbiosis?

Yes, examples include the relationship between nitrogen-fixing bacteria (like Rhizobium) and leguminous plants, as well as certain bacteria living within insect cells.

How did endosymbiosis contribute to the evolution of life on Earth?

Endosymbiosis led to the development of complex eukaryotic cells from simpler prokaryotic cells, paving the way for the evolution of diverse multicellular organisms.

Can endosymbionts be harmful to their hosts?

While many endosymbiotic relationships are mutualistic, some can be parasitic, where the endosymbiont benefits at the expense of the host.

What evidence supports the idea that mitochondria originated from engulfed aerobic bacteria?

Evidence includes the presence of double membranes, the ability of mitochondria to undergo binary fission, and the presence of 70S ribosomes and circular DNA, all of which are characteristics of bacteria.


  1. BioNinja. (n.d.). Endosymbiosis.
  2. Wikipedia contributors. (n.d.). Endosymbiont. Wikipedia.
  3. University of California Museum of Paleontology. (n.d.). How endosymbiosis changed life on Earth: Evidence for endosymbiosis.
  4. Northern Arizona University. (n.d.). Endosymbiosis.
  5. Open Geology. (n.d.). Endosymbiosis.

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What is Karyotyping? What are the scope of Microbiology? What is DNA Library? What is Simple Staining? What is Negative Staining? What is Western Blot? What are Transgenic Plants? Breakthrough Discovery: Crystal Cells in Fruit Flies Key to Oxygen Transport What is Northern Blotting? What is Southern Blotting?
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