Three Domians of Life

There are Three Distinct Domains of Life

All living organisms fall into one of three large groups (kingdoms, or domains) that define three branches of evolution from a common progenitor. Two large groups of prokaryotes can be distinguished on biochemical grounds:  archaebacteria (Greek  arche-, “origin”) and  eubacteria (again, from Greek  eu, “true”). Eubacteria inhabit soils, surface waters, and the tissues of other living or decaying organisms. Most of the well studied bacteria, including  Escherichia coli, are eubacteria. The archaebacteria, more recently discovered, are less well characterized biochemically; most inhabit extreme environments—salt lakes, hot springs, highly acidic bogs, and the ocean depths. The available evidence suggests that the archaebacteria and eubacteria diverged early in evolution and constitute two separate domains, sometimes called Archaea and Bacteria. All eukaryotic organisms, which make up the third domain, Eukarya, evolved from the same branch that gave rise to the Archaea; archaebacteria are therefore more closely related to eukaryotes than to eubacteria. Within the domains of Archaea and Bacteria are subgroups distinguished by the habitats in which they live. In  aerobic habitats with a plentiful supply of oxygen, some resident organisms derive energy from the transfer of electrons from fuel molecules to oxygen. Other environments are  anaerobic, virtually devoid of oxygen, and microorganisms adapted to these environments obtain energy by transferring electrons to nitrate (form(energy from organic compounds) Examples: •Most prokaryotes •All nonphototrophic eukaryotes atoms exclusively from CO2 (that is, no chemotrophs are autotrophs), but the chemotrophs may be further classified according to a different criterion: whether the fuels they oxidize are inorganic  (lithotrophs) or organic  (organotrophs). Most known organisms fall within one of these four broad categories—autotrophs or heterotrophs among the photosynthesizers, lithotrophs or organotrophs among the chemical oxidizers. The prokaryotes have several gening N2), sulfate (forming H2S), or CO2 (forming CH4). Many organisms that have evolved in anaerobic environments are  obligate anaerobes: they die when exposed to oxygen. We can classify organisms according to how they obtain the energy and carbon they need for synthesizing cellular material. There are two broad categories based on energy sources:  phototrophs (Greek  trophe-, “nourishment”) trap and use sunlight, and  chemotrophs derive their energy from oxidation of a fuel. All chemotrophs require a source of organic nutrients; they cannot fix CO2 into organic compounds. The phototrophs can be further divided into those that can obtain all needed carbon from CO2 (autotrophs) and those that require organic nutrients (heterotrophs). No chemotroph can get its carbon eral modes of obtaining carbon and energy.  Escherichia coli, for example, is a chemoorganoheterotroph; it requires organic compounds from its environment as fuel and as a source of carbon. Cyanobacteria are photolithoautotrophs; they use sunlight as an energy source and convert CO2 into biomolecules. We humans, like  E. coli, are chemoorganoheterotrophs.

From the Auther- Azeem Farooqui