Prokaryote

Prokaryotes, also called prokaryotes or Monera ( Procaryota / Prokaryonta / Procarya, to ancient Greek πρό pró, before (her ) ' and κάρυον Karyon, nut '), are cellular organisms that lack a cell nucleus. Your cell type is referred to as Protocyte.

Since all cellular organisms are divided into three domains that comprise the domains Bacteria ( Bacteria) and Archaea ( Archaea) all prokaryotes.

Characteristic properties, differences eukaryotes

Eukaryotes possess in their cells ( eucytes ) a "real " by a double membrane from the surrounding cytoplasm delimited nucleus, where the DNA is organized into chromosomes. In prokaryotic cells ( Protocyten ), the DNA is, however, free in the cytoplasm. Usually it consists of a single double stranded DNA molecule tightly structured, which is closed in itself, that is having no ends and is referred to as the bacterial chromosome. The molecule is arranged in a small area and is therefore also referred to as core or nucleoid equivalent. It is not associated with histones, as is the case with the DNA of the chromosomes in eukaryotic organisms. Only some prokaryotes contain linear, ie provided with two ends of DNA duplexes (eg Borrelia ). Escherichia coli and other bacteria tested then the double-stranded DNA is about 1 mm long, self-contained molecule, which is arranged in a small area. The DNA is free in the cytoplasm, such as the polymerase, and the transcription and translation of protein find place in the cytoplasm.

In some bacteria also even smaller double, closed or linear DNA molecules are present, which are referred to as plasmids.

Prokaryotic have compared to the eukaryotes smaller ribosomes: 70 S ribosomes ( in eukaryotes: 80 S ribosome ).

Prokaryotes contain unlike eukaryotes no membrane-bound organelles such as plastids, chloroplasts and mitochondria, and no dictyosomes, centrioles and mitotic spindles. Likewise, they have no vacuoles and no endoplasmic reticulum.

The size of prokaryotes ( for elongated diameter) is 0.2 to 700 microns ( Thiomargarita namibiensis about 700 microns ).

The classification of organisms in prokaryotes and eukaryotes was first pointed out clearly by Edouard Chatton for protists and published in 1925.

Other properties

In many prokaryotes can be found among others the following properties:

• Complex structure of cell envelopes, some with a second cell membrane; is found only in the cell walls of bacteria cells murein ( composed of sugars and amino acids connection)
• Not very complex shapes, except in myxobacteria
• Simple, asexual reproduction, usually by binary fission
• A variety of metabolic processes: energy gain due to respiration, fermentation, phototrophy, Chemotrophie, Organotrophie, Lithotrophie
• High physiological flexibility
• Some are viable under extreme conditions: temperature range up to about 100 ° C; oxic or anoxic environment; acidic environment ( pH 1-4); high hydrostatic pressures ( 1000 bar )

Metabolic activities

Detection of prokaryotic species

About the number of all prokaryotic species there are very different opinions: some assume that only about 1 % of the prokaryote species have been discovered and described ( taxonomically distinct types), about 99 % are still unknown. Others estimate the number of existing on Earth prokaryote species to 108-1017, which would amount to only a fraction of a percent of known species. But the fact is that approximately 500 to 800 prokaryote species are discovered and described every year.

The respective type strain ( the cultivated prokaryotes strain, which was the redescription based ) must be deposited into at least two strain collections and thus made ​​accessible to other scientists. In Germany is the at the German Collection of Microorganisms and Cell Cultures (DSMZ ) possible at the Institut Pasteur in France, in Belgium at the BCCM (Belgian coordinated collections of microorganisms ).

The reason that so far only a small proportion as was recorded in species, is that traditionally, the organisms are described in terms of culture, but that so far are known culture methods only for a small fraction of prokaryotic species. Habitats from the nucleic acids therein, prokaryotes can be recovered and characterized. Based on the data obtained in the present therein prokaryote society can be characterized and the number of species it contains are estimated. From the results of these studies compared with the results of cultural studies, it includes a very large proportion of previously unculturable prokaryote species in prokaryotes companies natural habitats. In addition, many habitats are hardly available for study: prokaryotes were detected in up to 77 km height in the atmosphere and up to 4 km deep in the Earth's crust, far below the ground. Yet more than half of all prokaryotes live in the area between 10 and 100 m depth in the soil ( also undersea ), this depth is difficult to access already.

Total amount and distribution on Earth

The amount of prokaryotes on Earth can only be estimated using a variety of data on various aspects of life on earth. By Whitman et al. (1998) it is 4 - give 6 · 1030 prokaryotic cells on Earth, the 3,5 - 5,5 · 1017 g of carbon, 0.85 to 1.3 · 1017 g of nitrogen and from 0.09 to 0.14 · 1017 g phosphorus. By comparison, the carbon content of all plants on Earth is 1.0 to 1.7 times, the nitrogen and phosphorus content, only about one-tenth. The number of prokaryotic cells spread given to four major areas of life on earth as in the table.

In prokaryotes, the number of animals, or on plants, in the polar and in the atmosphere is so low that it can be ignored for the total number on earth. So the vast majority of prokaryotes is located in the aquatic and terrestrial subsurface.

It is estimated that each year about 1.7 · 1030 cells are newly formed. This value is displayed in relation to the total number of cells small, it can be explained by the fact that in terrestrial depths of the "turnover " time to 1000 - is estimated 2000 years.

Because dry masses of prokaryotes contains about 50 % carbon in the middle, it can be assumed that the 3,5 - 5,5 · 1017 g of carbon throughout the prokaryotes about 7 - equivalent to 11 · 1017 g dry mass. At a water content of microorganisms of about 80 %, this results in a total mass of prokaryotes from 3.5 to 5.5 · 1018 g ( 3,5 - 5,5 billion tons ).