The prokaryota are one of the two major groups of biological organisms. The other is the eukaryota . Prokaryotes consist of two kingdoms: the archaebacteria and the eubacteria. Prokaryotes contribute the greatest biomass (amount of living matter) of any biological group and inhabit virtually all known earthly environments.
Characteristics of Prokaryotic Cells
Prokaryotes exhibit a comparatively simple cellular organization compared to eukaryotes. For example, they lack membrane-bound organelles such as chloroplasts and mitochondria. They also lack a nucleus, and their DNA exists in the form of a single, circular chromosome that floats freely in the cytoplasm . Sometimes additional DNA is present in the cell in the form of smaller loops called plasmids. All prokaryotes are characterized by a cell membrane and a cell wall. Prokaryotic cell walls are composed of peptidoglycans, which are composed of amino acids and sugar. While some eukaryotes also possess cell walls, theirs are composed of different compounds.
Prokaryotes are sometimes described by their shape. Cocci are round, baccilli are rod-shaped, and spirochetes are helical. Prokaryotes are largely asexual and reproduce by fission (splitting up). However, some exchange of genetic material between individuals does occur via a process called conjugation.
The prokaryota are an extremely varied group. Some prokaryotes are heterotrophs (which obtain nutrients from other biological organisms), while others are autotrophs (which create their own nutrients from inorganic matter and energy sources). Some prokaryotic autotrophs photosynthesize. Others use resources not exploited by any known eukaryotes, such as hydrogen, ammonia, and compounds of sulfur or iron.
Major Groups of Prokaryotes
Because of the comparative simplicity of the prokaryotes' morphology (shape and structure), much of the classification within the group relies on features of chemistry, metabolism, and physiology, in addition to shape, motility (ablity to move), and structural features.
The archaebacteria, or archaea , are prokaryotes that inhabit some of the harshest environments that exist. They are extremely primitive, and retain some of the features of the earliest living cells. They differ from other prokaryotes, and from all living organisms, in the unusual lipids that are found in their cell membranes. Many aspects of their biochemistry are similarly distinctive.
There are three groups of archaebacteria. Species have been classified by their physiological characteristics and ecology (habitat, food resources used, etc.), rather than by their phylogenetic relationships (the sequence of branching events in evolutionary history which have resulted in the production of divergent species), which remain unclear. There is some evidence that archaebacteria may be more closely related to the eukaryotes than to the other group of prokaryotes, the eubacteria.
The methanogens are obligate anaerobes that can only survive in oxygen-free environments. Methanogens use hydrogen gas as an energy source. The symbiotic bacteria that live in the guts of species such as cows and sheep and assist in the digestion of plant material are methanogens. They are so named because methane gas is a by-product of their metabolism.
The second group of archaebacteria, the extreme halophiles, inhabit very salty environments such as the Dead Sea and the Great Salt Lake. Finally, the extreme thermophiles, which represent several distinct lineages of archaebacteria, live in very hot environments, near hot springs or in deep ocean thermal vents. It is believed that the extreme thermophiles were the first living creatures on earth.
The eubacteria form an extremely diverse group. Some major subgroups of eubacteria are discussed below.
The cyanobacteria are also known as blue-green algae. They perform photosynthesis , and many species can fix nitrogen (take in atmospheric nitrogen and incorporate it into the body of the organism) as well. Cyanobacteria are found in aerobic environments in which light and water are available.
Some cyanobacteria engage in mutualistic relationships with fungi to form lichens. They also form a major component of oceanic plankton. Stromatolites are impressive chalk deposits that result from the binding of calcium-rich sediments by large colonies of cyanobacteria.
Spirochetes are eubacteria with a unique spiral-shaped morphology. Spirochetes move using structures called undoflagella or axial filaments, which are similar to bacterial flagella but allow spirochetes to move by rotating the way a corkscrew rotates. Most species are free living, and some are pathogens ; spirochetes are responsible for diseases such as syphilis and Lyme disease.
The enterics are a group of rod-shaped eubacteria that live in the intestinal tracts of other organisms. This group includes the well-known Escherichia coli and its relatives. Enterics all ferment glucose. They are part of the normal gut flora of humans and have been extensively studied.
The myxobacteria are significant for having the most complex life cycles among prokaryotes. Myxobacteria aggregate, or come together, to form multicellular "fruiting bodies" that give rise to spores. They live in the soil.
Vibrios are eubacteria characterized by a curved rod shape. They are found primarily in aquatic habitats. Cholera is caused by a vibrio.
Rickettsias and chlamydiae are two groups of obligate parasites or pathogens. Rickettsias require hosts in order to obtain nutrients, while chlamydiae obtain ATP (adenosine-triphosphate, the organic molecule which forms the basis of energy in all living organisms) from host cells.
see also Eukaryota; Kingdoms of Life.
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