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( Prokaryote)
The prokaryotes (pronounced /pro?'kærio?ti?z/; singular prokaryote /pro?'kæri?t/) are a group of organisms that lack a cell nucleus (= karyon), or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but some prokaryotes are multicellular organisms. The word prokaryotes comes from the Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, + suffix -otos, pl. -otes; it is also spelled "procaryotes".[1] The prokaryotes are divided into two domains the bacteria and the archaea. Archaea are a newly appointed domain of life. These organisms were originally thought to live only in inhospitable conditions such as extremes of temperature, pH, and radiation but have since been found in all types of habitats. A distinction between prokaryotes and eukaryotes (meaning true kernel, also spelled "eucaryotes") is that eukaryotes do have "true" nuclei containing their DNA, whereas the genetic material in prokaryotes is not membrane-bound. Eukaryotic organisms may, as in the case of amoebae, be unicellular or, as in the case of humans, be multicellular. The difference between the structure of prokaryotes and eukaryotes is so great that it is considered to be the most important distinction among groups of organisms. In 1977, Carl Woese proposed dividing prokaryotes into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of the major differences in the structure and genetics between the two groups of organisms. This arrangement of Eukaryota (also called "Eukarya"), Bacteria, and Archaea is called the three-domain system replacing the traditional two-empire system. A criticism of this classification is that the word "prokaryote" itself is based on what these organisms are not (they are not eukaryotic), rather than what they are (either archaea or bacteria). The cell structure of prokaryotes differs greatly from that of eukaryotes. The defining characteristic is the absence of a nucleus. Instead, the genomes of prokaryotes are held within an irregular DNA/protein complex in the cytosol called the nucleoid, which lacks a nuclear envelope.[2] Prokaryotes generally lack membrane-bound cell compartments such as mitochondria and chloroplasts. Instead processes such as oxidative phosphorylation and photosynthesis take place across the prokaryotic plasma membrane.[3] However, prokaryotes do possess some internal structures, such as vacuole and cytoskeletons,[4][5] and the bacterial order Planctomycetes have a membrane around their nucleoid and contain other membrane-bound cellular structures.[6] Both eukaryotes and prokaryotes contain large RNA/protein structures called ribosomes, which produce protein. Prokaryotes are usually much smaller than eukaryotic cells.[1]
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