endosymbiotic theory assignments high school

Endosymbiotic Theory

The Endosymbiotic Theory is a theory about the origin of mitochondria and chloroplasts. It is the hypothesis that the ancestors of mitochondria organelles are a type of genus Rickettsia, gram-negative bacteria, which is symbiotic with the host after being phagocytized by the original eukaryote. Cyanobacteria are thought to be the progenitors of the chloroplasts for the same reason (ASU). This is a symbiotic relationship with the bacteria and the host cell: They have evolved together for a long time. Today, the bacteria that had been symbiotic in prokaryotic cells are now an indispensable organelle of the cell.

In 1905, when Konstantin Mereschkowski first outlined the Endosymbiotic Hypothesis, it was not accepted by the public. The evidence at the time was very inadequate, only some observed results, such as “the dividing process of plastid organelles looked very similar to the way some free-living bacteria divided” (Andreas Schimper). The hypothesis was revived and first published in the late 1960s by Lynn Margulis after the scientists discovered the presence of mitochondrial DNA and plastid DNA in the plant cell (ASU). Eventually, it became a popular theory, and more and more evidence has confirmed the authenticity of the Endosymbiotic Hypothesis. The evidence includes the following:

1. Both chloroplast and mitochondria contain their own undiscovered DNA, not fully regulated by the nucleus. The DNA of mitochondria and chloroplasts is very distinct from that of the nucleus, but more closely resembles the single circular DNA molecules of bacteria (T. Jeremy et al).
2. Both mitochondria and chloroplasts have their own special protein synthesis systems, which indicates that they could survive and reproduce on their own. Both chloroplast ribosome and mitochondrial ribosome have the sedimentation coefficient of 70S, as compared to 80S in the eukaryotic cells. The settlement coefficient of bacteria is 70S(30S&50S), not surprisingly, it is similar to the chloroplast and mitochondria (L. Manuell et al.).
3. Like gram-negative bacteria, both mitochondria and chloroplasts feature two enveloping lipid bilayers, called the inner and outer membranes. Research has revealed that the inner and outer membranes have varying chemical structures. The outer membrane, which is made up of lipopolysaccharide, is consistent with the membrane of the host, especially similar to the endoplasmic reticulum membrane. The inner membrane, which is a peptide polylayer, is similar to the membrane of gram-negative bacteria and cyanobacteria, respectively (K.Zeth)

Besides all the evidence above, there are also some similarities, such as the way of division, cell size, and organelle size, etc. But all theories have their flaw somewhere: no mitochondria and chloroplasts are, and can be, exactly like bacteria. Such as the presence of introns in genes from the mitochondria or chloroplasts, but not in bacteria. One example is that bacterial (Gram-negative) and cyanobacteria do not possess an actin-myosin system in the mitochondria, while other bacteria do. In summary, an increasing amount of data is available supporting the validity of the Endosymbiotic Theory.

The Endosymbiotic Theory is like one of the challenges to Darwin’s theory of evolution: “Is competition the creator of the world or cooperation the creator of the world?” Human beings can extend the depth and breadth of life in an infinite way based on the spirit of exploration in a limited life. I feel like in the future, better yet, life can be understood by man.

WorkCite:

Manuell, AL. Quispe, J. Mayfield, SP (2007) “Structure of the Chloroplast Ribosome: Novel Domains for Translation Regulation.” PLoS Biol 5(8): e209. https://doi.org/10.1371/journal.pbio.0050209

Timmis, Jeremy N. Ayliffe, Michael A. Huang, Chun Y. Martin, William. “Endosymbiotic gene transfer: organelle genomes forge eukaryotic chromosomes”. Nature Reviews Genetics. 2004/02/01/online. vl 5. Nature Publishing Group. https://doi.org/10.1038/nrg1271 10.1038/nrg1271

Warring, Sally. “Cells living in Cells”. ASU – Ask A Biologist. February 24, 2016 https://askabiologist.asu.edu/explore/cells-living-in-cells

Zeth, Kornelius. Biochim Biophys Acta. “Structure and evolution of mitochondrial outer membrane proteins of beta-barrel topology.”NCBI. 2010 Jun-Jul;1797(6-7):1292-9. https://doi.org/10.1016/j.bbabio.2010.04.019