1.2.2 Endosymbiotic Theory

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
• Scientists have suggested that ancestral prokaryote cells evolved into ancestral heterotrophic and autotrophic cells through the following steps:
• Heterotrophic cells:
  • To overcome a small SA:V ratio ancestral prokaryote cells developed folds in their membrane. From these infoldings organelles such as the nucleus and rough endoplasmic reticulum formed
  • A larger anaerobically respiring prokaryote engulfed a smaller aerobically respiring prokaryote (which is not digested)
    • This gave the larger prokaryote 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 heterotropic cell with an alternative source of energy, carbohydrates
  • Over time the photosynthetic prokaryote evolved into chloroplasts and the heterotrophic cells into autotrophic 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 reproduce 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 synthesise their own proteins
  • They both have double membranes