What is a prokaryotic cell in biology?
Introduction
A prokaryote (Gk. Pro before, Karyons = nucleus) is a unicellular organism having a simple structure that lacks a membrane-bound nucleus and other membrane-bound organelles like mitochondria, Golgi complex, etc. Prokaryotes have great economic and environmental importance. They also greatly affect human health and are largely used in research and biotechnology.
6.1 Taxonomy of Prokaryotes
The A. V. Leeuwenhoek (Dutch scientist) first discovered bacteria in 1674 and called them animalcules. Ehrenberg introduced the name bacterium in 1828 (Gk: bacterial means small staff or rod). The taxonomic position of prokaryotes has witnessed continuous changes since their disco bacteria and others.
6.1.1 Taxonomic position of Prokaryotes as a kingdom(Monera)
According to the kingdom system of classification all microorganisms were included in the kingdom Plantae. In 1861 John Hog proposed a separate kingdom Protista for all microorganisms including bacteria. In 1866 Ernst Haeckel made a separate group the Monera for Prokaryotes within the same kingdom Protista.
In 1938 Herbert Copeland separated the group Monera from Protista and formed the kingdom Monera in which he had placed only prokaryotic organisms. Robert H. Whittaker an American biologist in 1969 proposed a five-kingdom system of classification for living things.
Lynn Margulis and Karlene Shwartz in 1988 modified the kingdom classification. They distinguished between kingdoms according to cellular organization and mode of nutrition. They had placed all prokaryotes in kingdom monera, whereas eukaryotes were classified into four kingdoms viz. Protista, Plantae, Fungi and Animalia.
6.1.2 Taxonom "Domain Archaea"
Earlier the term prokaryotes was "Domain Bacteria" and was used for all microscopic unicellular prokaryotes but later molecular systematics studies exhibit that prokaryotic life consists of two separate domains. Thus, both these domains have superseded the kingdom as the broadest taxonomic group.
Bacteria and Archaea evolved independently from an ancient common ancestor. These two domains, along with Eukarya, are the basis of the three-domain system, which is currently the most widely used classification system in bacteriology.
6.1.3 Phylogenic position of prokaryotes
Phylogeny is the evolutionary relationship among various groups of organisms (e.g., Species or populations). The study of phylogenic evolutionary history of a species or group of related species is called systematics. The bacterial phylogeny was reconstructed in 1977.
The new phylogenetic taxonomy is based on the discovery of genes encoding ribosomal RNA because there is little or no change in ribosomal RNA generation after generation. Thus ribosomal RNA is commonly recommended.
6.2 Archaea
The microorganisms belonging to the domain archaea are unicellular prokaryotes, previously known as archaeobacteria. Archaea exhibit similarities both with bacteria as well as eukaryotes.
They also differ from bacteria and possess unifying features thus placed in separate domains.
The unifying archaeal features are:
Their plasma membrane contains different kinds of lipids than bacteria which allows them to function at high temperatures.
The cell wall in bacteria is made up of a carbohydrate-protein complex called peptidoglycan but the cell wall of archaea lacks this complex. Their cell wall is largely composed of polysaccharides or pure protein.
The rRNA of archaea is unique, i.e., different from that of bacteria. The lipids of bacteria contain glycerol with fatty acids while lipids of archaea contain glycerol linked to the branched chain of hydrocarbons. A unique ability of methanogenic archaea is the formation of methane. Archaea are mostly autotrophs.
6.2.1 Habitat of Archaea
Most live in extreme environments. There are three types of archaea:
(i) Methanogenic archaea
(ii) Halophiles
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