Endosymbiotic Theory Overview

Aug 29, 2025

Overview

This lecture explains the endosymbiotic theory, describing how complex eukaryotic cells may have evolved from simpler prokaryotic cells living together in symbiosis.

Scientific Theories

  • A scientific theory is an explanation supported by extensive evidence and repeated testing.
  • Theories are different from casual guesses and are backed by many facts but can be updated or disproven.

Prokaryotes vs. Eukaryotes

  • Prokaryotes have no nucleus and no membrane-bound organelles ("pro" rhymes with "no").
  • Eukaryotes have a nucleus and membrane-bound organelles ("eu" rhymes with "do").
  • Both cell types have a cell membrane, cytoplasm, ribosomes, and genetic material, but prokaryotes are simpler.

Endosymbiotic Theory

  • The endosymbiotic theory explains how eukaryotic cells evolved from prokaryotic cells that lived in symbiosis.
  • Over 2 billion years ago, some large prokaryotes engulfed smaller bacteria without digesting them.
  • Engulfed bacteria lived inside the host cell and became symbionts, leading to ancestor heterotroph eukaryotes.
  • Some eukaryotes later engulfed photosynthetic bacteria, creating ancestor autotroph eukaryotes.
  • Bacteria that used oxygen evolved into mitochondria; photosynthetic bacteria evolved into chloroplasts.
  • Mitochondria likely developed before chloroplasts, as nearly all eukaryotes have mitochondria, but only some have chloroplasts.

Evidence Supporting Endosymbiotic Theory

  • Mitochondria and chloroplasts have their own DNA, separate from nuclear DNA.
  • The DNA of these organelles is arranged similarly to bacterial DNA.
  • Mitochondria and chloroplasts are similar in size to bacteria.
  • These organelles can divide independently in a way similar to bacterial cell division.

Modern Examples of Endosymbiosis

  • Endosymbiosis still occurs today, such as termites having prokaryotes in their gut to help digest wood.

Key Terms & Definitions

  • Scientific Theory — an extensively tested explanation for scientific events, supported by evidence.
  • Prokaryote — a cell without a nucleus or membrane-bound organelles.
  • Eukaryote — a cell with a nucleus and membrane-bound organelles.
  • Symbiosis — a relationship where different organisms live closely together, often for mutual benefit.
  • Endosymbiosis — one organism living inside another in a symbiotic relationship.
  • Mitochondria — organelles responsible for energy production in eukaryotic cells, thought to have evolved from prokaryotes.
  • Chloroplasts — organelles in plants/algae for photosynthesis, thought to have evolved from photosynthetic bacteria.

Action Items / Next Steps

  • Explore secondary endosymbiosis and other eukaryotic cell structures.
  • Review the differences between prokaryotic and eukaryotic cells.