A2.2.12 Origin of eukaryotic cells by endosymbiosis (HL) Notes

Eukaryotic Cells Originated through Endosymbiosis

1. All eukaryotes evolved from a common unicellular ancestor that had a nucleus and reproduced sexually.
2. Mitochondria evolved by endosymbiosis.
3. In some eukaryotes, chloroplasts subsequently also had an endosymbiotic origin.

What is Endosymbiosis?

1. In eukaryotic cells, this involved a larger host cell engulfing smaller prokaryotic cells.
2. These smaller prokaryotes became permanent residents and eventually evolved into mitochondria and chloroplasts.

Steps of Endosymbiosis

1. Engulfment
   1. A larger host cell engulfed a smaller prokaryote capable of aerobic respiration.
   2. Instead of digesting it, the host allowed the prokaryote to survive. This engulfed prokaryote became the ancestor of mitochondria.
2. Mutual Benefit
   1. The prokaryote provided energy through respiration, while the host cell provided protection
3. Integration
   1. Over time, the engulfed prokaryote lost independence and became a fully integrated organelle
4. Photosynthetic Endosymbiosis
   1. In some cases, a eukaryotic cell engulfed a photosynthetic prokaryote, which evolved into chloroplasts.

Evidence Supporting the Endosymbiotic Theory

The endosymbiotic theory is supported by a wealth of evidence.

1. Structural Evidence

1. Double membranes in mitochondria and chloroplasts.
2. Inner membrane resembles bacterial plasma membranes.
3. Outer membrane originates from host endocytosis.

2. Genetic Evidence

1. Circular naked DNA inside mitochondria and chloroplasts (like prokaryotes).
2. DNA codes for enzymes and proteins used internally.

3. Biochemical Evidence

1. 70S ribosomes (same as prokaryotes, different from 80S eukaryotic ribosomes).
2. Protein synthesis in mitochondria and chloroplasts resembles bacterial translation.

4. Reproductive Evidence

1. Both organelles replicate only by binary fission, like bacteria.
2. They are formed only from pre-existing organelles, not de novo.

5. Molecular & Genomic Evidence

1. Comparative sequencing:
   1. Mitochondria closely related to α-proteobacteria.
   2. Chloroplasts closely related to cyanobacteria.
2. Shared conserved genes suggest inheritance from common ancestors.