Lecture Notes: Eukaryotic Cell Structure and Origins of Mitochondria and Chloroplasts

Introduction

• Discussion on the origins of mitochondria and chloroplasts in eukaryotic cells.
• Evolutionary transition from prokaryotic to eukaryotic cells.

Lynn Margulis and Endosymbiotic Theory

• Lynn Margulis: Proposed the endosymbiotic theory.

  • Margulis was an evolutionary biologist.
  • Her theory is widely accepted today.
  • Personal anecdote: Married to Carl Sagan.

• Endosymbiotic Theory:

  • Larger bacteria engulfed smaller bacterial cells.
  • Smaller bacterial cells evolved into mitochondria and chloroplasts.
  • Dates back to 1.5 billion years ago.

Formation of Proto-Eukaryotes

• Process:
  • Plasma membrane of prokaryotes folded inward.
  • Formation of nucleus and endoplasmic reticulum.
  • Aerobic bacteria were absorbed, leading to the formation of mitochondria.
  • Photosynthetic bacteria were absorbed, leading to chloroplasts in photosynthetic eukaryotes.

Evidence Supporting Endosymbiotic Theory

• Chromosomes:

  • Mitochondria and chloroplasts have a single circular chromosome like prokaryotes.
  • Eukaryotes have multiple linear chromosomes.

• Cell Division:

  • Mitochondria and chloroplasts divide by binary fission, similar to prokaryotes.
  • Eukaryotic cells undergo mitosis (prophase, metaphase, anaphase, telophase).

• Ribosomes:

  • Mitochondria and chloroplasts have 70s ribosomes, similar to prokaryotes.
  • Eukaryotes have 80s ribosomes.

• Electron Transport Chain:

  • Found in the plasma membrane of prokaryotes and mitochondria/chloroplasts.
  • In eukaryotic cells, it is located only in mitochondria and chloroplasts.

• Size and Structure:

  • Similar size between prokaryotes (1-10 microns) and mitochondria/chloroplasts.
  • Eukaryotic cells are generally larger.

• Bacterial Similarities:

  • Chloroplasts resemble cyanobacteria.
  • Mitochondria resemble purple bacteria and Rickettsia.

Conclusion

• Evidence supports simultaneous formation of eukaryotic cells, mitochondria, and chloroplasts.
• Comparison of cyanobacteria with chloroplasts shows structural similarities.
• Upcoming topic: Viruses and viral life cycles.