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Prokaryotes (
IPA: //) 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 (fruit) or kernel, referring to the cell nucleus, +
suffix -otos,
plural -otes; it is also spelled "procaryotes".Campbell, N. "Biology:Concepts & Connections". Pearson Education. San Francisco: 2003.
The prokaryotes are divided into two domains: the bacteria and the
archaea. Archaea or Archaebacteria are a newly appointed
Kingdom (biology). 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 is not 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 is considered to be the most important distinction among groups of organisms. Most prokaryotes are bacterium, 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 are (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
cell 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,
mitochondrion 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 do not 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."
Prokaryotes also differ from eukaryotes in that they contain only a single loop of stable chromosomal DNA stored in an area named the
nucleoid, while eukaryote DNA is found on tightly bound and organized chromosomes. Although some eukaryotes have satellite DNA structures called plasmids, these are generally regarded as a prokaryote feature and many important genes in prokaryotes are stored on plasmids."
Prokaryotes have a larger surface area to volume ratio giving them a higher metabolic rate, a higher growth rate and consequently a shorter generation time compared to Eukaryotes."
Genes
Colonies
While prokaryotes are nearly always
unicellular, some are capable of forming groups of cells called
colony (biology). Unlike many eukaryotic multicellular organisms, each member of the colony is undifferentiated and capable of free-living (but consider
cyanobacteria, a very successful prokaryotic group which does exhibit definite cell differentiation). Individuals that make up such bacterial colonies most often still act independent of one another. Colonies are formed by organisms that remain attached following cell division, sometimes through the help of a secreted slimy layer.
Structure
Recent research indicates that all prokaryotes actually do have
cytoskeletons albeit more primitive than those of eukaryotes. Besides homologues of actin and tubulin (MreB and
FtsZ) the helically arranged building block of flagellum,
flagellin, is one of the most significant cytoskeletal proteins of bacteria as it provides structural backgrounds of
chemotaxis, the basic cell physiological response of bacteria. At least some prokaryotes also contain intracellular structures which can be seen as primitive organelles. Membranous organelles (a. k .a. intracellular membranes) are known in some groups of prokaryotes, such as vacuoles or membrane systems devoted to special metabolic properties, e. g. photosynthesis or chemolithotrophy. Additionally, some species also contain protein-enclosed
microcompartments mostly associated with special physiological properties (e. .g. carboxysomes or gas vacuoles).
{]|-|
Cell membrane|-| [Cytoplasm|-| [Nucleoid|-| [Inclusions|}
Morphology of Prokaryotic cells
Prokaroyotic cells have various shapes, the three basic shapes are.Bauman, R. w. "Microbiology". Pearson Education. San Francisco: 2006.
- 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
origin of life 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. Scientific American, October 21, 1999
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,
archea can be found in the cold of Antarctica and in the hot
Yellowstone Hot spring.
References
See also
- Archaea and Bacteria, the two prokaryotic taxa
- Monera, an obsolete kingdom (biology) including both of the above
- Bacterial cell structure
- Nanobe
- Virus
- Prion
- Symbiogenesis
External links
- Prokaryote versus eukaryote, BioMineWiki
Prokaryotes (IPA: //) 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 (fruit) or
kernel, referring to the cell nucleus, +
suffix -otos,
plural -otes; it is also spelled "procaryotes".Campbell, N. "Biology:Concepts & Connections". Pearson Education. San Francisco: 2003.
The prokaryotes are divided into two domains: the bacteria and the archaea. Archaea or Archaebacteria are a newly appointed
Kingdom (biology). 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 is not 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 is considered to be the most important distinction among groups of organisms. Most prokaryotes are bacterium, 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 are (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
cell 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, mitochondrion 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 do not 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."
Prokaryotes also differ from eukaryotes in that they contain only a single loop of stable chromosomal DNA stored in an area named the nucleoid, while eukaryote DNA is found on tightly bound and organized
chromosomes. Although some eukaryotes have satellite DNA structures called
plasmids, these are generally regarded as a prokaryote feature and many important genes in prokaryotes are stored on plasmids."
Prokaryotes have a larger surface area to volume ratio giving them a higher
metabolic rate, a higher growth rate and consequently a shorter generation time compared to Eukaryotes."
Genes
Colonies
While prokaryotes are nearly always unicellular, some are capable of forming groups of cells called colony (biology). Unlike many eukaryotic multicellular organisms, each member of the colony is undifferentiated and capable of free-living (but consider
cyanobacteria, a very successful prokaryotic group which does exhibit definite cell differentiation). Individuals that make up such bacterial colonies most often still act independent of one another. Colonies are formed by organisms that remain attached following cell division, sometimes through the help of a secreted slimy layer.
Structure
Recent research indicates that all prokaryotes actually do have
cytoskeletons albeit more primitive than those of eukaryotes. Besides homologues of actin and tubulin (
MreB and
FtsZ) the helically arranged building block of flagellum, flagellin, is one of the most significant cytoskeletal proteins of bacteria as it provides structural backgrounds of
chemotaxis, the basic cell physiological response of bacteria. At least some prokaryotes also contain intracellular structures which can be seen as primitive organelles. Membranous organelles (a. k .a. intracellular membranes) are known in some groups of prokaryotes, such as vacuoles or membrane systems devoted to special metabolic properties, e. g. photosynthesis or chemolithotrophy. Additionally, some species also contain protein-enclosed
microcompartments mostly associated with special physiological properties (e. .g. carboxysomes or gas vacuoles).
{]|-|
Cell membrane|-| [Cytoplasm|-| [Nucleoid|-| [Inclusions|}
Morphology of Prokaryotic cells
Prokaroyotic cells have various shapes, the three basic shapes are.Bauman, R. w. "Microbiology". Pearson Education. San Francisco: 2006.
- 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
origin of life 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. Scientific American, October 21, 1999
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,
archea can be found in the cold of
Antarctica and in the hot Yellowstone
Hot spring.
References
See also
External links
- Prokaryote versus eukaryote, BioMineWiki
Prokaryote - Wikipedia, the free encyclopedia
The prokaryotes (pronounced /proʊˈkærioʊts/; singular prokaryote /proʊˈkæriət/) are a group of organisms that usually lack a cell nucleus (= karyon), or any other membrane ...
Image:Prokaryote cell diagram.svg - Wikipedia, the free encyclopedia
Prokaryote_cell_diagram.svg (SVG file, nominally 870 × 480 pixels, file size: 119 KB)
prokaryote - Wiktionary
Definition from Wiktionary, a free dictionary
Definition: prokaryote from Online Medical Dictionary
The Online Medical Dictionary is a searchable dictionary of definitions from medicine, science and technology.
prokaryote - definition of prokaryote in the Medical dictionary - by ...
prokaryote /pro·kary·ote/ (-kar´e-ot) a unicellular organism lacking a true nucleus and nuclear membrane, having genetic material composed of a single loop of naked double ...
prokaryote - Hutchinson encyclopedia article about prokaryote
prokaryote. In biology, an organism whose cells lack organelles (specialized segregated structures such as nuclei, mitochondria, and chloroplasts).
Evolution - A-Z - Prokaryote
A prokaryote is a cell without a distinct nucleus. Bacteria and some other simple organisms are prokaryotic. The genome of a prokaryote is in the form of a single DNA molecule ...
prokaryote -- Britannica Online Encyclopedia
Britannica online encyclopedia article on prokaryote:any organism that lacks a distinct nucleus and other organelles due to the absence of internal membranes. Bacteria are among ...
Image:Prokaryote cell diagram.svg - Wikimedia Commons
Description: a basic diagram on the elements conforming a prokaryote cell. in this case a bacteria. Source: i did the diagram myself using adobe illustrator , as source for the ...
Prokaryote - MSN Encarta
Prokaryote, relatively simple unicellular organism, such as a bacterium, characterized by the absence of a nucleus and other specialized cell...
