Origins of Compartmentalization (College Board AP® Biology): Study Guide

Endosymbiosis

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

• Endosymbiosis is where one organism lives within another

• If the relationship is beneficial to both organisms the engulfed organism is not digested

Endosymbiotic theory

• For endosymbiosis to occur one organism must have engulfed the other by the process of endocytosis

• 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

Evidence to support the endosymbiotic theory

• The evidence to support the endosymbiotic theory arises from the features that 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 proteins

  • They both have double membranes

  • Mitochondria/chloroplasts and prokaryotic cells are a similar size

    • Mitochondrion = 1-3 µm long

    • Chloroplast = 3-4 µm long

    • Bacterial cell = 2-8 µm long