Endosymbiotic Theory

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Chapter 29 30

• Endosymbiotic Theory

No great discovery was ever made without a bold
guess. --Isaac Newton
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Atmospheric Oxygen

• Most atmospheric O2 has been produced by the
  water-splitting step of photosynthesis.
• Cyanobacteria.

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Atmospheric Oxygen

• When photosynthesis first evolved, the O2
  produced dissolved into the surrounding water.
• Eventually it reacted with dissolved iron and
  precipitated as iron ore.

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Atmospheric Oxygen

• After the iron had precipitated out, O2 continued
  to accumulate until the waterways became
  saturated and the remaining O2 then entered the
  atmosphere.

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Atmospheric Oxygen

• Atmospheric oxygen continued to accumulate
  gradually from about 2.7 bya until about 2.3 bya
  and then dramatically increased.
• The increase was likely due to the evolution of
  more oxygen producing organisms.

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Atmospheric Oxygen

• The increasing O2 levels on the planet likely led
  to the extinction of numerous prokaryotic groups.
• Oxygen is a highly reactive compound that damages
  cells and disrupts chemical bonds.

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Atmospheric Oxygen

• Some species of bacteria survived in habitats
  that remained anaerobic, and others adapted to
  the changing atmosphere.

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The First Eukaryotes

• About 2.1 bya, the first eukaryotic fossils began
  forming.
• Eukaryotic cells have a number of complex
  features.
• Three such evolutionary novelties came to define
  the early eukaryotes.

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A Change in Cell Structure and Function

• Three evolutionary novelties
• 1. The formation of ribosome studded internal
  membranes.
• 2. The appearance of a cytoskeleton.
• 3. The evolution of digestive vesicles.

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1. A Ribosome Studded Membrane

• The ribosome-studded membrane assisted in the
  movement of protein products throughout the
  internal portion of the cell without harm to
  other cytoplasmic factors.

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2. The Appearance of a Cytoskeleton

• The cytoskeleton is comprised of actin fibers and
  microtubules.
• Allows form movement of the cell and movement of
  the internal contents.
• The development allows for phagocytosis.

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3. Digestive Vesicles

• The formation of digestive vesicles allowed for
  membrane bound enzymes to form.
• If unbound, these enzymes would destroy the cell.
  

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Endosymbiotic Theory

• Where did the features of eukaryotic cells come
  from?

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Endosymbiotic Theory

• A wide variety of evidence supports the theory
  that small prokaryotes began living in larger
  (host) cells.
• These cells likely gained entry to the host as
  undigested prey, or internal parasites.
• This process has been observed by scientists in
  as little as 5 years.

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Endosymbiotic Theory

• The benefits of the relationship are easy to see.
• A photosynthetic endosymbiont would provide
  nutrients to the heterotrophic host.
• The host would provide shelter for the anaerobic
  prokaryote from the increasingly aerobic
  environment.

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Endosymbiotic Theory

• Over time, this relationship would result in a
  situation where to two parts would become
  inseperable giving rise to a single organism.

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Serial Endosymbiosis

• All eukaryotes have mitochondria (or remnants of
  them), but not all have plastids.
• Plastids are chloroplasts or any related
  organelle.
• Chloroplasts for photosyntheis
• Chromoplasts for pigment synthesis and storage.
• Gerontoplasts control the dismantling of the
  photynthetic apparatus.
• Leucoplasts for monoterpene (fragrance, etc.)
  synthesis.
• Amyloplasts for starch storage and
  gravitropism.
• Elaioplasts for storing fat.
• Proteionplasts for storing and modifying
  proteins.

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Serial Endosymbiosis

• Thus, according to the hypothesis of serial
  endosymbiosis, mitochondria evolved before
  plastids.
• This was the result of numerous symbiotic events.

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Evidence for Endosymbiosis

• The evidence is overwhelming
• Both organelles have circular chromosomes.
• These chromosomes lack histones.
• Both organelles have their own DNA.
• Both organelles can perform transcription and
  translation of their own DNA.
• Both organelles can self-replicatevia binary
  fissionjust like prokaryotes.

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Evidence for Endosymbiosis

• The evidence is overwhelming
• The inner membranes of both organelles have
  enzymes and transport systems that are homologous
  to those found in the plasma membranes of living
  prokaryotes.
• Both organelles are approximately the same size
  as typical bacterium.
• Both organelles use many bacteria-like enzymes.

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Evidence for Endosymbiosis

• The evidence is overwhelming
• Both organelles are sensitive to certain
  antibiotics.
• Some antibiotics interfere with mitochondrial
  protein synthesis.
• Rifampicin-binds to bacterial RNA polymerase
  preventing transcription.
• Can prevent mitochondrial RNA synthesis, but only
  at a very high concentration.

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Evidence for Endosymbiosis

• The evidence is overwhelming
• Both organelles contain ribosomes.
• These ribosomes are very similar to bacterial
  ribosomes.
• The ribosomes are nearly the same size, have very
  similar RNA sequences, and are sensitive to the
  same antibiotics as bacterial ribosomes.
• The ribosomes are more similar to bacterial
  ribosomes than they are to eukaryotic ribosomes.

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Secondary Endosymbiosis

• Secondary endosymbiosis is another step in
  eukaryotic evolution.
• In this process, a heterotrophic eukaryote
  engulfed an unrelated photosynthetic eukaryote
  (plastid).
• The plastids were likely ingested into the food
  vacuole, and over time formed a symbiotic
  relationship with the host.

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Secondary Endosymbiosis

• Studies of plastid bearing eukaryotes demonstrate
  how this process has taken place.
• Red and green algae, produced from primary
  endosymbiosis, provide a nice example of this
  process.
• Chlorarachinophytes are a specific example.
• Green algae engulfed by a heterotrophic eukaryote.

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Secondary Endosymbiosis

• Within the engulfed cell, we see lines of
  evidence for this process having taken place.
• Within the cell is remnants of an engulfed cell
  with a vestigal nucleuscalled a nucleomorph.
• Nucleomorphic genes are still transcribed.
• Their DNA sequences are very similar to those of
  green algaefurther supporting the hypothesis
  that an ancestral eukaryote engulfed a green
  algae.

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Secondary Endosymbiosis

• The plastids are surrounded by four membranes.
• The inner two membranes originated as an inner
  and an outer membrane of an ancient
  cyanobacterium.
• The third membrane is derived from the engulfed
  algas plasma membrane.
• The outermost membrane is derived from the
  heterotrophic eukaryotes food vacuole.

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Secondary Endosymbiosis Summary

• Click here for a video summary.

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Could it Really Occur?

• It is now
• Some eukaryotes live in low O2 environments and
  lack mitochondria.
• They have endosymbionts that live within them and
  generate energy for them.

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Could it Really Occur?

• Protists live symbiotically in the hindgut of
  termites.
• The protists, in turn, are colonized by symbiotic
  bacteria similar in size and distribution to
  mitochondria.
• These bacteria function well in low O2
  environments--unlike mitochondria.
• They oxidize food and create ATP for the protist.

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Could it Really Occur?

• A study of Pelomyxa palustris provides some
  interesting insight
• This ameoba lacks mitochondria.
• It contains at least 2 kinds of endosymbiotic
  bacteria.
• Killing the bacteria with antibiotics causes an
  increase in lactic acid.
• This suggests that the bacteria oxidize the end
  products of glucose fermentationsomething
  mitochondria normally do.