Eukaryotic Diversity

Serial Endosymbiosis: A Radical Hypothesis

• Question: How did the organelles in eukaryotes evolve?
• Answer: Serial Endosymbiosis
• What in the world is this?
  • Remember that eukaryotes are 10 to 100 × the size of prokaryotes.
  • It is believed that billions of years ago prokaryotes invaded eukaryotes and began living inside them. They developed a “symbiotic” relationship which was mutually beneficial to each. This is called mutualism.
    • It was called “endosymbiosis” because the prokaryotes lived inside the eukaryotes (the prefix endo means “inside”).
    • It was called “serial” because it happened several times during the evolutionary development of eukaryotes.

Two Prokaryotes Become Two Organelles

Origin of the two most important organelles

• Mitochondrion
  • An aerobic heterotrophic prokaryote invaded a larger cell and “evolved” to become the mitochondrion.
  • This organelle uses oxygen to convert chemical energy from one form into another (glucose to ATP).
• Chloroplast
  • A photosynthetic prokaryote invaded a larger cell and “evolved” to become the chloroplast.
  • This organelle converts sunlight energy into chemical energy (glucose).

Figure 28.2(a), page 523, Campbell's Biology 5th Edition

Evidence for the Serial Endosymbiotic Hypothesis

• Similarities between prokaryotes and the two organelles (mitochondria and chloroplasts):
  • Physical size is similar.
  • Both have circular DNA.
  • There are many molecular similarities.
    • Ribosomal RNA (rRNA) of mitochondria and chloroplasts is more similar to that of prokaryotes than it is to the rRNA used in the cell it is part of!
• Lynn Margulis came up with this hypothesis.
  • Trivia: At one time she was married to Carl Sagan, the astronomer who became popular through the famous “Cosmos” documentary on PBS.