Endosymbiotic Theory and the Evolution of Eukaryotic Cells

Introduction

• Mitochondria and Chloroplasts: Cannot survive outside a cell today, but were once independently living cells.
• Endosymbiotic Theory: Suggests these organelles originated from free-living prokaryotic cells.

Origin of Cells

• First Cells: Appeared about 3.8 billion years ago.
  • These were prokaryotes, lacking organelles and membrane-bound structures.
• Eukaryotic Cells: Appeared around 2.7 billion years ago.
  • Evolved from prokaryotes.

Evolution of Eukaryotes

• Prokaryote Growth: Increased in size leading to a decreased surface area to volume ratio.
  • Developed membrane infoldings to solve nutrient gathering problem.
  • Infoldings eventually formed the early endomembrane system and nucleus.
  • This marked the emergence of the first eukaryotic cell.

Symbiotic Relationship

• Endocytosis:
  • Eukaryotic cells engulfed aerobic prokaryotic cells (likely an alpha Proteobacteria).
  • These cells avoided digestion and became endosymbionts (living inside another cell).
• Benefits:
  • Aerobic prokaryotes utilized oxygen for energy (respiration).
  • Benefitted from host cytoplasm full of digested food molecules.
  • Shared ATP with host cell.

Mitochondria and Chloroplasts

• Mitochondria:
  • Became an obligate endosymbiont, unable to survive independently.
• Chloroplasts:
  • Formed later through similar endosymbiotic process with cyanobacterium.
  • Ancestor to plants and algae.
  • Evidence: Plant cells have both mitochondria and chloroplasts, while animal cells have only mitochondria.

Evidence for Endosymbiosis

• Replication: Mitochondria and chloroplasts replicate similarly to binary fission.
• Genome: Similar to prokaryotic DNA; single circular DNA molecule.
• Proteins and Lipids:
  • Porins and cardiolipin are unique to mitochondria, chloroplasts, and bacteria.

Inheritance

• Mitochondrial Inheritance: Mitochondria are inherited maternally.

Conclusion

• The endosymbiotic theory provides a compelling explanation for the origin of complex eukaryotic cells and their organelles.

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