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The Endosymbiotic Theory The exact origin of Eukaryotes is still under investigation, but one of the most popular theories involves a symbiotic relationship between prokaryotes and a pre-eukaryotic cell. This is known as the Endosymbiotic Theory, which originated in 1883 with Andreas Schimper. Schimper hypothesized that cells had an endosymbiotic nature. In 1905, Konstantin Merezhkovsky proposed the argument that the plastids were endosymbionts, further suggesting that symbiosis is the driving force behind evolution.1 In 1926, Merezhkovsky collaborated with Ivan Walin and used the Endosymbiotic Theory to explain the origin of the mitochondria in all eukaryotes through their book publication titled, Symbiogenesis and the Origin of Species. A more recent proponent of this theory is Lynn Margulis, who became famous through her research career that focused on this concept.2 Figure 1. The plant cell (eukaryote). The most important mechanism behind the Endosymbiotic Theory involves the process of phagocytosis. Phagocytosis involves the ability of one cell to engulf another cell, which likely initially evolved as a feeding mechanism. Before this process evolved, material (e.g., food) was transported through the cell membrane molecule by molecule. The evolution of phagocytosis gave cells the ability to engulf entire cells, setting the foundation for the endosymbiotic theory. According to Margulis, the pre-eukaryotic cell engulfed an aerobic bacterium (through phagocytosis), but rather than digest and destroy the bacterium, a symbiotic relationship was born. In this relationship, the aerobic bacterium Saylor URL: www.saylor.org/BIO102 subunit 6.1.2 The Saylor Foundation Saylor.org Page 1 of 3 provided energy through ATP (adenosine triphosphate) and the eukaryotic cell provided an environment to live while protecting the new symbiont from harmful environmental factors such as oxygen. Because almost all living eukaryotes have a mitochondria, it is safe to assume that this event happened before plants and animals split in the evolutionary lineage.3 After this first evolutionary leap came a second that would separate the plant and animal lineages forever. Through a second symbiotic event that involved the eukaryotic cell engulfing a cyanobacteria, the plants would gain the ability to photosynthesize and make their own food. This would categorize them into the autotrophs and secure their position at the bottom of the food chain, regardless of how many evolutionary events would take place from that point on.4 Figure 2. The chloroplast. Evidence to support Margulis’s Endosymbiotic Theory has grown over the years. This evidence includes (but is not limited to) the following:5,6,7 1. A double membrane surrounding the organelles with an inner layer that retains the bacteria’s characteristics and an outer layer that retains characteristics of the cell that engulfed it. 2. Mitochondria and chloroplasts are similar in size to prokaryotes. 3. Mitochondria and chloroplasts have their own DNA and lack histone proteins, the DNA is circular, and it is attached to the inner membrane just like in prokaryotes. Figure 3. The mitochondria. 4. Mitochondria and chloroplasts divide by fission, not mitosis. 5. The mitochondria, chloroplasts, and prokaryotes make proteins by similar biochemical pathways that differ from those in eukaryotes. Saylor URL: www.saylor.org/BIO102 subunit 6.1.2 The Saylor Foundation Saylor.org Page 2 of 3 6. The mitochondria and chloroplasts are susceptible to certain antibiotics just like prokaryotes. Eukaryotes are unaffected by these same drugs.2 1 G. S. Fliney, G. S. Hine, and M. Almanza. The Endosymbiotic Hypothesis. n.d. http://endosymbiotichypothesis.wordpress.com/. 2 Ibid. 3 University of California. Understanding Evolution. n.d. http://evolution.berkeley.edu/evolibrary/article/_0_0/endosymbiosis_04. 4 Ibid. 5 Indiana University. The Endosymbiotic Theory. n.d. http://www.biology.iupui.edu/biocourses/N100/2k2endosymb.html. 6 G. S. Fliney, G. S. Hine, and M. Almanza. The Endosymbiotic Hypothesis. n.d. http://endosymbiotichypothesis.wordpress.com/. 7 University of California. Understanding Evolution. n.d. http://evolution.berkeley.edu/evolibrary/article/_0_0/endosymbiosis_04. Pictures: Chloroplast: http://commons.wikimedia.org/wiki/File:Chloroplast_in_leaf_of_Anemone_sp_TE M_30000x.png Plant cell: http://commons.wikimedia.org/wiki/File:Plant_cell_structure.png Mitochondria: http://commons.wikimedia.org/wiki/File:Mitochondria_-_TEM.jpg Saylor URL: www.saylor.org/BIO102 subunit 6.1.2 The Saylor Foundation Saylor.org Page 3 of 3