Chapt 27 - CHAPTER 27 - Prokaryotes and the Origins of...

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CHAPTER 27 - Prokaryotes and the Origins of Metabolic Diversity A. The World of Prokaryotes 1. They’re (almost) everywhere! An overview of prokaryotic life Prokaryotes were the earliest organisms on Earth and evolved alone for 1.5 billion years. Today, prokaryotes still dominate the biosphere. Their collective biomass outweighs all eukaryotes combined by at least tenfold. More prokaryotes inhabit a handful of fertile soil or the mouth or skin of a human than the total number of people who have ever lived. Prokarytes are wherever there is life and they thrive in habitats that are too cold, too hot, too salty, too acidic, or too alkaline for any eukaryote. The vivid reds, oranges, and yellows that paint these rocks are colonies of prokaryotes. We hear most about the minority of prokaryote species that cause serious illness. During the 14th century, a bacterial disease known as bubonic plague spread across Europe and killed about 25% of the human population. Other types of diseases caused by bacteria include tuberculosis, cholera, many sexually transmissible diseases, and certain types of food poisoning. However, more bacteria are benign or beneficial. Bacteria in our intestines produce important vitamins. Prokaryotes recycle carbon and other chemical elements between organic matter and the soil and atmosphere. Prokaryotes often live in close association among themselves and with eukaryotes in symbiotic relationships. Mitochondria and chloroplasts evolved from prokaryotes that became residents in larger host cells. Modern prokaryotes are diverse in structure and in metabolism. About 5,000 species of prokaryotes are known, but estimates of actual prokaryotic diversity range from about 400,000 to 4 million species. 2. Bacteria and archaea are the two main branches of prokaryote evolution Molecular evidence accumulated over the last two decades has led to the conclusion that there are two major branches of prokaryote evolution, not a single kingdom as in the five-kingdom system. These two branches are the bacteria and the archaea. The archaea inhabit extreme environments and differ from bacteria in many key structural, biochemical, and physiological characteristics. Current taxonomy recognizes two prokaryotic domains : domain Bacteria and domain Archaea. A domain is a taxonomic level above kingdom. The rationale for this decision is that bacteria and archaea diverged so early in the history of life and are so fundamentally different. At the same time, they are both structurally organized at the prokaryotic level. B. The Structure, Function, and Reproduction of Prokaryotes Most prokaryotes are unicellular. Some species may aggregate transiently or form true colonies, even extending to division of labor between
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This note was uploaded on 12/14/2009 for the course BIOCHEM bIO taught by Professor Professor during the Spring '09 term at École Normale Supérieure.

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Chapt 27 - CHAPTER 27 - Prokaryotes and the Origins of...

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