Endosymbiotic Theory (HL) (HL IB Biology)

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:volume 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 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, 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 synthesise their own proteins
  • They both have double membranes

NOS: Factors determining the strength of a theory - A wide range of observations are accounted for by the theory of endosymbiosis

• The strength of the theory comes from the observations the theory explains and the predictions it supports
• If new observations do not support a theory, it must either be adjusted or rejected
• The more observations and data that are predicted by a theory, the stronger the theory is
• A range of observations are accounted for by the theory of endosymbiosis
  • Membranes: Mitochondria and chloroplasts have their own cell membranes, just like a prokaryotic cell does
  • DNA: Each mitochondrion and chloroplast have their own circular DNA genome, like a bacteria's genome, but much smaller
  • Replication: Mitochondria multiply by pinching in half which is the same process used by bacteria