Endosymbiotic Theory

Certain organelles originated when free-living bacteria were taken into larger cells as endosymbionts.

…recall that all eukaryotic cells contain organelles.

For example:

nucleusmitochondriachloroplasts

How did organelles originate?

Endosymbiotic Theory

• proposed by some scientists from 1883-1920s.

• chloroplasts in plant cells resemble free-living cyanobacteria and proteobacteria .

The Cyanobacteria Eucapsis

Photo by: Jason Oyadomari http://plantphys.info/organismal/lechtml/cyanobacteria.shtml

Chloroplasts (Elodea)

The proteobacteria Escherichia coli

http://yours-health.info/2011/e-coli-attacking-europe-e-coli-can-make-dead

Mitochondrion

Endosymbiont

• endo (Gr.) “inside”

• syn (Gr.) = “with”

• biosis (Gr.) = “living”

Symbiosis

close, often long-term interaction between different biological species.

Evidence • New mitochondria and plastids are formed only through a

process similar to binary fission.

• In some algae, such as Euglena, the plastids can be destroyed by certain chemicals or prolonged absence of light without otherwise affecting the cell. In such a case, the plastids will not regenerate.

• They are surrounded by two or more membranes, and the innermost of these shows differences in composition from the other membranes of the cell. They are composed of a peptidoglycan cell wall characteristic of a bacterial cell.

more evidence

• Both mitochondria and plastids contain DNA that is different from that of the cell nucleus and that is similar to that of bacteria (in being circular in shape and in its size).

• Much of the internal structure and biochemistry of plastids is very similar to that of cyanobacteria.

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

• Mitochondria and plastids are similar in size to bacteria.

Vocabulary

• organelle• cyanobacteria• mitochondria• plastid• chloroplast• eukaryote• symbiosis• phagocytosis

Resources

http://zoowiki.wikispaces.com/

http://zoologyblog.blogspot.com/

McElroy, Kerensa. 2010. Nature's best examples of symbiosis. Cosmos Magazine, Tuesday, 28 December 2010