1. Chapter 29 & 30
Endosymbiotic Theory
“No great discovery was ever made
without a bold guess.”
--Isaac Newton

2. Atmospheric Oxygen
Most atmospheric O2 has been
produced by the water-splitting step of
photosynthesis.
– Cyanobacteria.

3. 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.

4. 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.

5. 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.
http://www.nature.com/nature/journal/v451/n7176/fig_tab/nature06587_F2.html

6. 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.

7. Atmospheric Oxygen
Some species of bacteria survived in
habitats that remained anaerobic, and
others adapted to the changing
atmosphere.

8. 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.

9. 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.

10. 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.

11. 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.

12. 3. Digestive Vesicles
The formation of digestive vesicles
allowed for membrane bound enzymes
to form.
– If unbound, these enzymes would destroy
the cell.

13. Endosymbiotic Theory
Where did the features of eukaryotic
cells come from?

14. 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.

15. 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.

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

17. 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.
http://en.wikipedia.org/wiki/File:Plastids_types_en.svg
http://www.tutorvista.com/biology/biology-cytoplasm

18. Serial Endosymbiosis
Thus, according to the
hypothesis of serial
endosymbiosis,
mitochondria evolved
before plastids.
– This was the result of
numerous symbiotic
events.

19. 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-replicate—via binary
fission—just like prokaryotes.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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 nucleus—called a
nucleomorph.
Nucleomorphic genes are still transcribed.
Their DNA sequences are very similar to
those of green algae—further supporting the
hypothesis that an ancestral eukaryote
engulfed a green algae.

26. 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 alga’s plasma membrane.
– The outermost membrane is derived from
the heterotrophic eukaryote’s food
vacuole.

28. Click here for a
video summary.
Secondary Endosymbiosis
Summary

29. 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.

30. 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.

31. 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 fermentation—
something mitochondria normally do.