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:1. The formation of ribosome studded
internal membranes2. The appearance of a cytoskeleton3. The evolution of digestive vesicles
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 cyanobacteriaMany obligate anaerobes went extinctIt 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
EvolutionBoth 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
EvolutionRibosomes have many similarities:
– Similar in size– Nucleotide sequence– Sensitivity to antibiotics– Analysis of rRNA reveals striking
similarities:Mitochondria and alpha protobacteriaPlastids and cyanobacteria
Secondary Endosymbiosis
Red and green algae were ingested in the food vacuole of a heterotrophic eukaryote.– Became endosymbionts.
Gave rise to the chlorarachinophytes.– 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.