How does the endosymbiotic theory explain the origin of eukaryotic cells?

Sourav PanNovember 7, 2024

Answered

The endosymbiotic theory provides a compelling explanation for the origin of eukaryotic cells, proposing that these complex cells evolved through a symbiotic relationship between early prokaryotic organisms. Here’s a detailed overview of the theory and the evidence supporting it:

Overview of the Endosymbiotic Theory

The endosymbiotic theory suggests that eukaryotic cells originated when an ancestral prokaryotic cell (likely an archaea) engulfed another prokaryote (a bacterium) that was capable of aerobic respiration. Instead of being digested, this engulfed bacterium established a mutualistic relationship with the host cell, eventually evolving into organelles such as mitochondria and chloroplasts. This process likely involved several key steps:

1. Engulfment: An ancestral archaean cell engulfed an aerobic bacterium through a process similar to phagocytosis, allowing the bacterium to survive within the host cell.
2. Mutualism: The engulfed bacterium provided the host cell with additional energy through aerobic respiration, while the host offered protection and nutrients to the bacterium.
3. Genetic Integration: Over generations, the relationship became so interdependent that the engulfed bacterium transferred many of its genes to the host’s genome, leading to a loss of its independent living capabilities.
4. Organelle Formation: The engulfed bacteria evolved into mitochondria (in all eukaryotes) and chloroplasts (in plants and some protists), which are now essential for energy production and photosynthesis, respectively.

Evidence Supporting the Endosymbiotic Theory

Several lines of evidence support the endosymbiotic theory:

1. Mitochondrial and Chloroplast Characteristics:
   • Both mitochondria and chloroplasts are similar in size to prokaryotic cells and reproduce independently through binary fission, akin to bacterial division.
   • They contain their own circular DNA, which resembles bacterial genomes rather than linear eukaryotic DNA . This DNA is distinct from the nuclear DNA of the host cell.
2. Genetic Similarities:
   • The genetic sequences of mitochondrial and chloroplast DNA show significant similarities to those of certain bacteria (e.g., mitochondria are closely related to Rickettsiales bacteria, while chloroplasts are related to cyanobacteria) .
   • The ribosomes within mitochondria and chloroplasts are more similar to those found in prokaryotes (30S and 50S subunits) than those in eukaryotic cytoplasm (40S and 60S subunits).
3. Biochemical Evidence:
   • Mitochondria and chloroplasts possess unique membrane structures and transport proteins that are also found in bacterial membranes, supporting their prokaryotic ancestry.
4. Evolutionary Implications:
   • The theory explains why all eukaryotes possess mitochondria, indicating that this endosymbiotic event occurred early in evolutionary history. Chloroplasts were acquired later in specific lineages leading to plants.

Alternative Hypotheses

While the endosymbiotic theory is widely accepted, alternative hypotheses exist regarding the origin of eukaryotic cells, such as the nucleus-first hypothesis (where a nucleus evolved before endosymbiosis) or the eukaryote-first hypothesis (suggesting that eukaryotes evolved from simpler forms). However, these theories do not account for the extensive genetic and biochemical evidence supporting endosymbiosis as a key mechanism in eukaryotic evolution