Endosymbiosis (College Board AP Biology)

The Origins of Cell Compartmentalization

• The evolution of compartmentalized cells with distinct, membrane-bound organelles is thought to have occurred via a process called endosymbiosis

Endosymbiosis

• Endosymbiosis is where one organism lives within another
• If the relationship is beneficial to both organisms the engulfed organism is not digested
• For endosymbiosis to occur one organism must have engulfed the other by the process of endocytosis

Endosymbiotic Theory

• The endosymbiotic theory is used to explain the origin of eukaryotic cells
• The evidence provided for this theory comes from the structure of the mitochondria and chloroplasts
• It is believed that all eukaryotic cells evolved from a common unicellular ancestor that had a nucleus and reproduced sexually
• Scientists have suggested that these ancestral cells evolved into ancestral heterotrophic and autotrophic cells through the following steps:
• Heterotrophic cells:
  • To overcome a small surface area-to-volume ratio, ancestral prokaryote cells developed folds in their membranes
  • From these infoldings, organelles such as the nucleus and rough endoplasmic reticulum formed
  • A larger, anaerobically respiring cell engulfed a smaller aerobically respiring prokaryote (which was not digested)
  • This gave the larger cell a competitive advantage as it had a ready supply of ATP and gradually the cell evolved into the heterotrophic eukaryotes with mitochondria that are present today
• Autotrophic cells:
  • At some stage in their evolution, the heterotrophic eukaryotic cell engulfed a smaller photosynthetic prokaryote
  • This cell provided a competitive advantage as it supplied the heterotrophic cell with an alternative source of energy, namely carbohydrates
  • Over time, the photosynthetic prokaryote evolved into chloroplasts and the heterotrophic cells into autotrophic eukaryotic cells

Diagram of Endosymbiotic Theory and the Evolution of Eukaryotic Cells

The endosymbiotic theory - an explanation for the evolution of eukaryotic cells

Evidence to support the endosymbiotic theory

• The evidence to support the endosymbiotic theory arises from the features that the mitochondria and chloroplasts have in common with prokaryotes:
  • Both replicate by binary fission
  • Both contain their own circular, non membrane bound DNA
  • They both transcribe mRNA from their DNA
  • They both have 70S ribosomes to synthesize their own proteins
  • They both have double membranes
  • A mitochondrion is in the same order of magnitude of size as many prokaryotic cells
    • Mitochondrion - approx 1-3 µm long
    • Bacterial cell - approx 2-8 µm long
  • A chloroplast is also in the same order of magnitude of size as many prokaryotic cells
    Chloroplast - approx 3-4 µm long
    Bacterial cell - approx 2-8 µm long