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.