The Endosymbiosis Theory
Taxonomy ultimately comes down to questions of origins. Some of the beginning of certain groups is surprisingly straightforward. Some involved steady adoptions to solely changing environment and conditions. In other situations dramatic changed in the conditions seem to force sudden and noteworthy changes to the evolution. And sometimes, a gigantic event makes a big splash in evolution. The origin of eukaryotes appears to have been a big splash in the form of endosymbiosis, a condition in which different organisms live together. The idea that endosymbiosis has been around since 1905 when a Russian biologist, C.
Mereschkowsky, hypothesized that plastids, like chloroplasts, are decedents of bacteria like organisms. Another idea came up in 1927 when I. Wallin straightforwardly predicted that the mitochondria, discovered in the late 1800s, had evolved from a type of bacteria. The Endosymbiotic Theory was first proposed by former Boston University Biologist Lynn Margulis in 1967 and was later published in her book. With these findings the theory that life arose billions of years ago from a simple microbe similar to a prokaryote cell has been thought right.
The theory basically states that some organelles in a eukaryotic cell were once prokaryote molecules. The idea is that the first eukaryote was probably an amoeba-like cell with a nucleus formed from a piece of the cytoplasm membrane pinched off around the chromosomes. It was thought that some of the amoebic-like organisms ate absorbed a prokaryotic cell that survived within the organism. The mitochondria were formed when bacteria able of aerobic respiration were absorbed; chloroplasts formed when photosynthetic bacteria were ingested.
In time they lost their cell walls and most of their DNA because they were not of use in the host cell. Evidence has been found to support the idea. Mitochondria are the same size as a prokaryotic cell. It can also divide by binary fission. The chloroplast can also do this. The mitochondria and the chloroplast have their own DNA that is circular not linear. The biologist Lynn Margulis brought the theory into the light. In the beginning, however, she was ridiculed by most important biologists at the time. Became of her persistence scientists now had a plausible explanation for the evolution of eukaryotes.
But what does Endosymbiosis really mean? Endo means within, just like endothermic. Cyto means cell so endocytosis is a process of cell eating, or a cell engulfed but usually digested. Endosymbiosis can then be concluded as being a cell engulfed, but not digested. It became a way for cells to live together in a relationship that they could both benefit from, symbiosis Her hypothesis originally said that mitochondria are the result of endocytosis of aerobic bacteria and chloroplasts are the result of endocytosis of photosynthetic bacteria.
In both cases an anaerobic bacteria that would not usually be able to exist in an aerobic environment could survive. This was how they both benefited each other in a symbiotic way. Her idea was that the the aerobic bacteria would have handled the toxic oxygen for the anaerobic bacteria; the anaerobic bacteria would ingest food. In the diagram we see the host cell and the bacterium enter an open space of the cell membrane. The bacterium is taken in a vesicle with its own cell membrane. Eventually this led to the idea that the bacterium is now those mitochondria with its own inner and outer mitochondrial membrane.
This is the idea behind the theory: there are two membranes within the cell itself. The success of eukaryotic cells, however, has been that they mutually support each other with the two major organelles. The mitochondrion takes in oxygen to release energy from carbon dioxide sources like glucose; it produces carbon dioxide and water in the process. The chloroplast, on the other hand, takes in water and carbon dioxide as it gets energy from light in the stroma. It takes the energy and breaks it down the chemical energy of glucose; this releases oxygen in the process.
As the environments in the early earth changed, so did the survival of most prokaryotes. Be getting absorbed into a eukaryotic cell the prokaryote became a prisoner. Being a prisoner came with benefits, however. The prokaryote was protected and in a steady environment in exchange for providing nutrients—chloroplast—or exploiting the new oxygen in the environment—mitochondria. As mentioned earlier, there is evidence to prove the theory of endosymbiosis. It has been observed in other parts of biology.
The Mitochondria and chloroplasts have interesting similarities in structure, reproduction, biochemistry, and genetic makeup to prokaryotes. It comes down to the fact that the mitochondria and the chloroplast have genetic information of their own. This alone argues in favor of the simple theory. Citations Lake, James A. “Evidence for an Early Prokaryotic Endosymbiosis. ” Macmillion Publishers Limited, 20 Aug. 2009. Web. 22 Aug. 2010. “Endosymbiosis and the Origin of Eukaryotes. ” 22 Aug. 2007. Web. 22 Oct. 2010. <http://users. rcn. com/jkimball. ma. ultranet/BiologyPages/E/Endosymbiosis. html>.
