Eukaryotic Cells Originated through Endosymbiosis
- All eukaryotes evolved from a common unicellular ancestor that had a nucleus and reproduced sexually.
- Mitochondria evolved by endosymbiosis.
- In some eukaryotes, chloroplasts subsequently also had an endosymbiotic origin.
What is Endosymbiosis?
Definition
Endosymbiosis
Endosymbiosis refers to one organism living inside another in a mutually beneficial relationship.
- In eukaryotic cells, this involved a larger host cell engulfing smaller prokaryotic cells.
- These smaller prokaryotes became permanent residents and eventually evolved into mitochondria and chloroplasts.
Steps of Endosymbiosis
- Engulfment
- A larger host cell engulfed a smaller prokaryote capable of aerobic respiration.
- Instead of digesting it, the host allowed the prokaryote to survive. This engulfed prokaryote became the ancestor of mitochondria.
- Mutual Benefit
- The prokaryote provided energy through respiration, while the host cell provided protection
- Integration
- Over time, the engulfed prokaryote lost independence and became a fully integrated organelle
- Photosynthetic Endosymbiosis
- In some cases, a eukaryotic cell engulfed a photosynthetic prokaryote, which evolved into chloroplasts.
- In some cases, a second endosymbiotic event occurred.
- Certain eukaryotic cells engulfed photosynthetic prokaryotes (like cyanobacteria), which eventually evolved into chloroplasts.
- This process gave rise to photosynthetic eukaryotes, such as plants and algae.
Evidence Supporting the Endosymbiotic Theory
The endosymbiotic theory is supported by a wealth of evidence.
