ENDOSYMBIOTIC THEORY

Karlie Jaeger

General Biology I

17 March 2014

Endosymbiosis

Endo- within

Symbiosis- ‘together’ and ‘living’

Endosymbiosis- when one organism lives within another

Ex.) Humans and intestinal bacteria relationship

Endosymbiotic Theory

•“States that the mitochondria and chloroplasts in eukaryotes represent formerly free-living bacteria that were taken inside another cell”

•Mitochondria- a membrane bound organelle found in most eukaryotic cells which generate most of the cell’s supply of ATP. Other tasks include cell signaling, cellular differentiation, cell death, and control of the cell cycle and cell growth.

•Chloroplast- organelle found in plant and algal cells which conduct photosynthesis to produce ATP. Other tasks include synthesizing fatty acids and amino acids.

It’s History

•1883, botanist Andreas Schimper observed that the division of chloroplasts in green plants closely resembled that of free-living cyanobacteria

•1910, botanist Konstantin Mereschkowski proposed that green plants had risen from a symbiotic union of two organisms

•1920s, Ivan Wallin proposed the idea of an endosymbiotic origin to mitochondria and proteobacteria

•1960s, the discovery that chloroplasts and mitochondria contain their own DNA resurrected these notions

The Process

Evidence Supporting the Theory• In some algae, the chloroplasts can be destroyed by certain chemicals or prolonged absence of light

without otherwise affecting the cell; the chloroplasts will not regenerate. (Shows that the chloroplast regeneration relies on an extracellular source)

• Transport proteins (porins) are found in the outer membranes of mitochondria and chloroplasts and also bacterial cell membrane

• A membrane lipid cardiolipin is exclusively found in the inner mitochondrial membrane and bacterial cell membrane

• Both mitochondria and chloroplasts contain single circular DNA that is different from that of the cell nucleus, but similar to that of bacteria

• Genome comparisons indicate that cyanobacteria contributed to the genetic origin of chloroplasts

• Mitochondria and chloroplasts’ ribosomes are like those found in bacteria

• Much of the internal structure and biochemistry of chloroplasts is similar to that of cyanobacteria

• Mitochondria have several enzymes and transport systems similar to those of bacteria.

• If a cell’s mitochondria or chloroplasts are removed, they do not have the means to create new ones

• A double membrane surrounds both mitochondria and chloroplasts, further evidence that each was ingested by a primitive host

References

• Genetic Science Learning Center. "The Evolution of the Cell." Learn.Genetics 16 March 2014 <http://learn.genetics.utah.edu/content/cell/organelles/> 

• Margulis, Lynn. "Endosymbiosis." Understanding Evolution. Berkeley University , n.d. Web. 17 Mar 2014. <http://evolution.berkeley.edu/evolibrary/article/history_23>.