Prokaryote Totally Explained

Prokaryotes (IPA: /prəʊˈkæriəʊtiz/) are a group of organisms that lack a cell nucleus (= karyon), or any other membrane-bound organelles. 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's also spelled "procaryotes".
The prokaryotes are divided into two domains: the bacteria and the archaea. Archaea or Archaebacteria are a newly appointed kingdom 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.

Relationship to Eukaryotes

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 isn't membrane-bound. Eukaryotic organisms, such as humans, may be unicellular or multicellular. The difference between the structure of prokaryotes and eukaryotes is so great that it's considered to be the most important distinction among groups of organisms. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. In 1977, Carl Woese proposed dividing prokaryotes into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of the significant genetic differences between the two. 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're (either archea or bacteria).
In light of the two-empire system, the cell structure of prokaryotes differs greatly from eukaryotes. Thus using the three-domain system, the cell structure of archea are to a great extent (and bacteria to some lesser extent) differ from the cell structure of eukaryotes. The defining characteristic is the absence of a nucleus or nuclear envelope. Prokaryotes were also previously considered to lack cytoskeletons and to lack membrane-bound cell compartments such as vacuoles, endoplasmic reticulum/endoplasmic reticula, Golgi apparatus, mitochondria and chloroplasts. In eukaryotes, the latter two perform various metabolic processes and are believed to have been derived from endosymbiotic bacteria. In prokaryotes similar processes occur across the cell membrane; endosymbionts are extremely rare. The cell walls of prokaryotes are generally formed of a different molecule (peptidoglycan) to those of eukaryotes (many eukaryotes don't have a cell wall at all). Both eukaryotes and prokaryotes have structures called ribosomes, which produce protein. Prokaryotes are usually much smaller than eukaryotic cells."

Cocci - spherical
Bacilli - rod shaped
Spiral - curve

Environment

Prokaryotes are found in nearly all environments on earth. Archaea in particular seem to thrive in harsh conditions, such as high temperatures, thermophiles, or salinity, halophiles. Organisms such as these are referred to as extremophiles. Many prokaryotes live in or on the bodies of other organisms, including humans.

Evolution of prokaryotes

It is generally accepted that the first living cells were some form of prokaryote and may have developed out of protobionts. Fossilized prokaryotes approximately 3.5 billion years old have been discovered (less than 1 billion years after the formation of the earth's crust), and prokaryotes are perhaps the most successful and abundant organism even today. Eukaryotes only formed later, from symbiosis of multiple prokaryote ancestors; their first evidence in the fossil record appears approximately 1.7 billion years ago, although genetic evidence suggests they could have formed as early as 3 billion years ago.
   While Earth is the only known place in the universe where life exists, some have suggested evidence of life on Mars in the form of fossil or living prokaryotes; this is open to considerable debate and skepticism.
   Prokaryotes diversified greatly throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes, leading to many highly distinct types of prokaryotes. For example, in addition to using photosynthesis or organic compounds for energy like eukaryotes do, prokaryotes may obtain energy from inorganic chemicals such as hydrogen sulfide. This has enabled the bacteria to thrive and reproduce. Today, archaebacteria can be found in the cold of Antarctica and in the hot Yellowstone springs.

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