Prokaryotae

(biology) A superkingdom of predominantly unicellular microorganisms lacking a membrane-bound nucleus containing chromosomes and having asexual reproduction by binary fission; it includes the kingdom Monera, and viruses, which are acellular, are included by some.

A group of predominantly unicellular microorganisms or infectious agents of cells (the viruses), lacking nuclei, and having asexual and chromonemal reproduction and unidirectional recombination. The Prokaryotae may be considered to include a kingdom for viruses, although such “organisms” are considered acellular or noncellular (even nonliving) by many authorities, and one for the typical moneran forms, the many kinds of bacteria plus the cyanobacteria (the blue-green algae) and the Prochlorophycota. See also Virus.

Bacteria, viruses, and blue-green algae possess little in common, besides such superficial characters as microscopic (or ultramicroscopic) size and frequent involvement in causing diseases in other organisms, including human beings, and such negative characters as not being eukaryotic, not possessing any mouth opening, and not being multicellular (or, in the case of viruses, even cellular) in their organization. In the smallest dimension, prokaryotes measure from 0.2 to 10 micrometers; viruses show a diameter of 10 to 300 nanometers, the largest, therefore, just barely overlapping in width with the smallest bacterium.

Monerans (above the virus level) are generally solitary, unicellular forms; but some species are filamentous, colonial, or mycelial. Some are also motile, either by gliding or by the action of bacterial flagella containing the protein flagellin. Modes of nutrition are diverse: absorptive, chemosynthetic, photoheterotrophic, and photoautotrophic. Respiration is anaerobic or aerobic, or facultatively either one. Respiratory and photosynthetic functions are both generally associated with the plasma membrane system: there are no specialized organelles such as mitochondria or plastids, although thylakoids are present in cyanobacteria and in Prochlorophycota.

Virus particles can survive in a dried, crystalline, metabolically inert state. Bacteria may produce endospores of great resistance to a variety of environmental stresses; the trophic forms occur ubiquitously in aquatic or moist habitats, including cells and tissues of hosts belonging to all other groups of organisms. Complex viruses have an envelope surrounding the nucleocapsid; many bacteria possess rigid cell walls, and some produce outer sheaths. See also Algae; Bacteria; Cyanobacteria; Eukaryotae; Prochlorophyceae.