The endosymbiotic theory is the scientific explanation for the origin of certain organelles in eukaryotic cells, specifically mitochondria and chloroplasts. It proposes that these organelles were once free-living prokaryotic microbes that were engulfed by a larger host cell. Instead of being digested, these prokaryotes formed a symbiotic relationship with the host, eventually evolving into the organelles we see today inside eukaryotic cells
Key Points of the Endosymbiotic Theory:
- The first eukaryotic cell likely arose when an amoeba-like host cell engulfed prokaryotic cells capable of aerobic respiration (which became mitochondria) and photosynthesis (which became chloroplasts).
- These engulfed cells lost some of their independence, such as their cell walls and much of their DNA, and became permanent, semi-autonomous organelles within the host cell.
- Mitochondria and chloroplasts reproduce independently within the cell by binary fission, similar to bacteria.
- Both organelles have their own circular DNA and ribosomes that resemble those of prokaryotes, not the linear DNA and ribosomes typical of eukaryotic cells
Evidence Supporting the Theory:
- Size and Division: Mitochondria and chloroplasts are about the size of prokaryotic cells and divide by binary fission, like bacteria
- Genetic Material: They contain their own circular DNA, distinct from the nuclear DNA of the eukaryotic cell
- Ribosomes: Their ribosomes are similar to bacterial ribosomes (30S and 50S subunits) rather than eukaryotic ribosomes (40S and 60S subunits)
- Membrane Composition: The membranes of mitochondria and chloroplasts resemble those of bacteria, including specific proteins like porins and lipids such as cardiolipin found in bacterial membranes
- Phylogenetic Relationships: Mitochondria are closely related to a group of bacteria called Rickettsiales, and chloroplasts are related to cyanobacteria
In summary, the endosymbiotic theory explains how eukaryotic cells acquired mitochondria and chloroplasts through a symbiotic merger with prokaryotic cells, a key event in the evolution of complex life
