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Chapter 29 & 30
Secondary Endosymbiosis
The Eukaryotic Lineage
Eukaryotes are believed to have arisen
as a result of symbiosis.
 All prokaryotes have cell walls

–
The first step is believed to be the origin of
a flexible cell surface.
– This increases the cell surface area.
– Bacterial chromosome is attached to the
membrane of the cell.
 Formation
of the nucleus.
A Change in Cell Structure and
Function

Three evolutionary novelties:
The formation of ribosome studded
internal membranes
2. The appearance of a cytoskeleton
3. The evolution of digestive vesicles
1.
1. A Ribosome Studded
Membrane

This 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

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 these allowed for
membrane bound enzymes.
 If unbound, these enzymes would
destroy the cell.

Increasing O2 Concentration
Result of cyanobacteria
 Many obligate anaerobes went extinct
 It is believe that a prokaryotic
heterotroph was taken up by a
phagocytotic, “pre-eukaryotic” cell.

The Prokaryotic Heterotroph
Escaped digestion.
 Could break down toxic oxygen
containing compounds.

–
These may have evolved into
peroxisomes.
– Was the first in a series of important
endosymbiotic relationships.
Protobacterium
It is believed that these were engulfed
next and gave rise to mitochondria.
 These use O2 in the production of
energy.
 Much research supports this.

Serial Endosymbiosis
Supposes that mitochondria evolved
before plastids.
 All eukaryotes have mitochondria, or
genetic remnants, but not all of them
have plastids.

Research in Support of
Mitochondrial Evolution

The nucleotide sequence of the
SSRNA.
–

Present in all organisms--early origin.
Comparative evidence of rRNA with
that of alpha protobacterium suggests
a close relationship
Research in Support of Plastid
Evolution
Plastids are believed to have arisen
from cyanobacteria.
 Evidence from comparative analysis of
rRNA supports this.
 Association was mutually beneficial.
 The plastid could use the O2, and the
predator could use the organic
products.

Research Supporting
Mitochodrial and Plastid
Evolution
Both divide by binary fission.
 Each has its own DNA, double
stranded, and circular.
 No association with chromatin or other
proteins.
 tRNAs, ribosomes, etc. are found
within these organelles.

Research Supporting
Mitochodrial and Plastid
Evolution

Ribosomes have many similarities:
–
Similar in size
– Nucleotide sequence
– Sensitivity to antibiotics
– Analysis of rRNA reveals striking
similarities:
 Mitochondria
and alpha protobacteria
 Plastids and cyanobacteria
Secondary Endosymbiosis

Red and green algae were ingested in
the food vacuole of a heterotrophic
eukaryote.
–

Gave rise to the chlorarachinophytes.
–

Became endosymbionts.
Green algae engulfed by a heterotrophic
eukaryote.
Carries out photosynthesis and
contains a small, vestigial nucleus.
Secondary Endosymbiosis

These plastids contain four membranes:
 1 and 2: The inner and outer membrane of
the ancient cyanobacterium
 3: The one derived from the engulfed alga’s
plasma membrane.
 4: The outermost membrane is derived from
the heterotrophic eukaryote’s food vacuole.
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.