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