Chapter 29 & 30
Endosymbiotic Theory
“No great discovery was ever made
without a bold guess.”
--Isaac Newton
Atmospheric Oxygen

Most atmospheric O2 has been
produced by the water-splitting step of
photosynthesis.
–
Cyanobacteria.
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.

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.
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
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.

Atmospheric Oxygen

Some species of bacteria survived in
habitats that remained anaerobic, and
others adapted to the changing
atmosphere.
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.

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.
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.
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.
3. Digestive Vesicles

The formation of digestive vesicles
allowed for membrane bound enzymes
to form.
–
If unbound, these enzymes would destroy
the cell.
Endosymbiotic Theory

Where did the features of eukaryotic
cells come from?
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.
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.
Endosymbiotic Theory

Over time, this relationship would result
in a situation where to two parts would
become inseperable giving rise to a
single organism.
Serial Endosymbiosis

All eukaryotes have
mitochondria (or
remnants of them), but
not all have plastids.
–




http://www.tutorvista.com/biology/biology-cytoplasm
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
Serial Endosymbiosis

Thus, according to the
hypothesis of serial
endosymbiosis,
mitochondria evolved
before plastids.
–
This was the result of
numerous symbiotic
events.
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.
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.
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.
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.
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.
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.
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.
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.
Secondary Endosymbiosis
Summary

Click here for a
video summary.
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.
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.
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.