Endosymbiosis (College Board AP® Biology)

How Prokaryotes Evolved & The Origin of Eukaryotes

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