Eukaryotic Organelles and Prokaryotic Symbionts Symbiosis is the interactive association of two or more species living together. There are several types of symbiosis. Parasitism is a symbiosis where one organism causes harm to the other, its host. An example of this is a disease causing bacterium, such as Treponema pallidum , which causes the disease syphilis in humans. Commensalism is a symbiosis where one organism benefits and the other is not harmed or helped. The symbiotic relationship between alge and fungi in lichens is an example of this. Mutualism is a symbiosis where both organisms benefit. Mutualism examples are abundant: zooxanthellae are dinoflagellates that live within the body of coral; E. coli bacteria live in the human intestine; etc. The symbiotic model proposed by American biologist Lynn Margulis suggests possible symbiosis of bacteria within early eukaryotic cells. Margulis proposed the mechanism of endosymbiosis , shown in Figure 3, to explain the origin of mitochondria
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