Endosymbiosis Essay

2007 words - 9 pages

Every now and again, the scientific world makes discoveries or develops theories, which cause a complete change in scientific perspectives and thought, and this is referred to as a paradigm shift. This is exactly what happened in the 1960s when former Boston University biologist Dr. Lynn Margulis put forward her endosymbiotic theory, a theory that explains the evolutionary origin of organelles in modern day eukaryotic cells. While the theory may presently be viewed as a credible theory, this was not always the case. Both Margulis and her theory were highly questioned and even mocked by the scientific community throughout the 19th and 20th century as there was no way to support it with ...view middle of the document...

Ivan Wallin, an American biologist, expanded the idea of an endosymbiotic origin to mitochondria in the 1920s, however these theories were disregarded. Having said that, more detail electron microscopic comparisons amid cyanobacteria and chloroplasts in combination with the discovery of how plastids and mitochondria contain their own DNA generated a revival of the idea in the 1960s. It was Lynn Margulis who linked the past works together, in addition to her own research, which led to the endosymbiotic theory.

The mechanisms of the endosymbiotic theory are rather complex, particularly in reference to how endosymbiosis is divided into three distinct levels, primary, secondary, and tertiary. Primary symbiosis is the process in which a eukaryote engulfed another living prokaryote. Unable to digest the engulfed bacteria, the host was able to survive, and a symbiotic relationship developed over billions of years. This symbiotic relationship was beneficial because the host cell would have provided nutrients to the engulfed prokaryotic cell and in exchange the smaller prokaryotic cells used these nutrients to synthesize ATP molecules, and this ATP was used as an energy source by the host cell. Fu, the smaller prokaryotic cell was provided a safe environment as well as receiving nutrients from the largest host cell. As a result of the symbiotic relationship, the prokaryotes that were once unable to utilize oxygen began to acquire abilities to carry out aerobic respirations and eventually photosynthetic reactions by successive endosymbiosis events. As time progressed, the engulfed bacteria transferred some genes to the host cell’s genome, resulting in the permanent interdependence on one another’s metabolic activities. The smaller prokaryotic cell would eventually develop into some of the known organelles such as mitochondria and chloroplasts. Mitochondria would have been formed when a much larger cell ingested bacteria capable of aerobic respiration, while chloroplasts were formed when photosynthetic bacteria were ingested. Over time, they lost their cell wall and the majority of their DNA, as they were not of benefit within the host cell. Accordingly, mitochondria and chloroplasts cannot grow outside of their host cell in normal conditions because now they depend on the cell for protection and nutrients. The reason this type of symbiotic relationship is not seen today is because conditions are significantly different now then they were on earth millions of years ago when this symbiotic event initially took place.

Endosymbiosis can occur a second time, when a eukaryote cell engulfs another eukaryote cell that posses a plastid derived from a primary endosymbiosis. This is known as second endosymbiosis and has occurred very often through time and has lead to the great genetic diversity we find on earth. In actual fact, endosymbiosis can occur a third time, when a eukaryotic cell engulfs another eukaryotic cell that has a plastid derived from a...

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