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 membranes 2. The appearance of a cytoskeleton 3. 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 O 2 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 O 2 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 O 2, 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. – 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 O 2 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 O 2 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.