Shape, Arrangement, and Size
One might expect that tiny , relatively simple organisms like prokaryotes would be uniform in shape and size. This is often not the case, because the microbial world offers almost endless variety in terms of morphology. However, the 2 commonest shapes are cocci and rods. Cocci (s., coccus ) are roughly spherical cells. they will exist singly or are often associated in characteristic arrangements that are frequently useful in their identification. Diplococci (s., diplococcus ) arise when cocci divide and remain together to make pairs. Long chains of cocci result when cells adhere after repeated divisions in one plane; this pattern is seen within the genera Streptococcus, Enterococcus, and Lactococcal. Divisions in two or three planes can produce symmetrical clusters of cocci. Members of the Micrococcus often divide into two planes to make square groups of 4 cells called tetrads. within the genus Sarina, cocci divide into three planes, producing cubical packets of eight cells.
Bacillus megatherium is an example of a bacterium with a rod shape . Rods, sometimes called bacilli (s., bacillus ), differ considerably in their length-to-width ratio, the coccobacilli being so short and wide that they resemble cocci. the form of the rod’s end often varies between species and should be flat, rounded, cigar-shaped, or bifurcated. Although many rods occur singly, some remain together after division to make pairs or chains (e.g., Bacillus megatherium is found in long chains).
Actinomycetes typically form long filaments called hyphae which will branch to supply a network called a mycelium. In this sense, they're almost like filamentous fungi, a gaggle of eukaryotic microbes. The oval- to pear-shaped Hyphomicrobium produces a bud at the top of an extended hypha. A couple of prokaryotes actually are flat. For instance, square archaea living in salt ponds are shaped like flat, square to rectangular boxes about 2 μm by 2 to 4 μm and only 0.25 μm thick. The myxobacteria are of particular note. These bacteria sometimes aggregate to make complex structures called fruiting bodies. Finally, some prokaryotes are variable in shape and lack one, characteristic form. These are called pleomorphic.
Nanobacteria and nanoarchaea range from around 0.2 μm to but 0.05 μm in diameter. Their discovery was quite surprising because theoretical calculations predicted that the littlest cells would be about 0.14 to 0.2 μm in diameter. At the opposite end of the continuum are bacteria like the spirochetes, which may reach 500 μm long, and therefore the photosynthetic bacterium Oscillatoria, which is about 7 μm in diameter (the same diameter as a red blood cell). the large bacterium Epulopiscium fishelsoni lives within the intestine of the brown surgeonfish, Acanthurus nigrofuscus. E. fihelsoni grows as large as 600 by 80 μm, a touch smaller than a printed hyphen. a good larger bacterium, Thiomargarita namibiensis, has been discovered in ocean sediment. Thus a couple of bacteria are much larger than the typical eukaryotic cell (typical plant and animal cells are around 10 to 50 μm in diameter).
Prokaryotic Cell Organization
Prokaryotic cells are morphologically simpler than eukaryotic cells, but they're not just simpler versions of eukaryotes. Although many structures are common to both cell types, some are unique to prokaryotes. Note that no single prokaryote possesses all of those structures in the least time. Some are found only in certain cells in certain conditions or in certain phases of the life cycle. Despite these variations, prokaryotes are consistent in their fundamental structure and most vital components.
Prokaryotic cells usually are bounded by a chemically complex cell membrane, which covers the cell wall. The cell wall successively surrounds the cytoplasm and its contents. Because most prokaryotic cells don't contain internal, membrane-bound organelles, their interior appears morphologically simple. The genetic material is localized during a discrete region, the nucleoid, and typically isn't separated from the encompassing cytoplasm by membranes. Ribosomes and bigger masses called inclusion bodies are scattered about the cytoplasm. Many prokaryotes use flagella for locomotion. Additionally, many are surrounded by a capsule or slime layer external to the cell membrane.
Prokaryotes vary in size the maximum amount as in shape. Escherichia coli may be a rod of about average size, 1.1 to 1.5 μm wide by 2.0 to 6.0 μm long. Near the tiny end of the dimensions, the continuum is members of the genus Mycoplasma, a stimulating group of bacteria that lack cell walls. For several years, it had been thought that they were the littlest prokaryotes at about 0.3 μm in diameter, approximately the dimensions of the poxviruses. However, even smaller prokaryotes are discovered. Nanobacteria and nanoarchaea range from around 0.2 μm to but 0.05 μm in diameter. Their discovery was quite surprising because theoretical calculations predicted that the littlest cells would be about 0.14 to 0.2 μm in diameter. At the opposite end of the continuum are bacteria like the spirochetes, which may reach 500 μm long, and therefore the photosynthetic bacterium.
Prokaryotic Structures and Their Functions
Plasma membrane = Selectively permeable barrier, mechanical boundary of cell, nutrient and waste transport, location of the many metabolic processes (respiration, photosynthesis), detection of environmental cues for chemotaxis.
Gas vacuole = Buoyancy for floating in aquatic environments.
Ribosomes = Protein synthesis.
Inclusion bodies = Storage of carbon, phosphate, and other substances.
Nucleoid = Localization of genetic material (DNA).
Periplasmic space = Contains hydrolytic enzymes and binding proteins for nutrient processing and uptake.
Cell wall = Provides shape and protection from osmotic stress.
Capsules and slime layers = Resistance to phagocytosis, adherence to surfaces.
Endospore = Survival under harsh environmental conditions.
Flagella =Swimming motility.
Endospore =Survival under harsh environmental conditions.
References :
Prescott's Principles of Microbiology by Joanne M. Willey, Linda M. Sherwood and Christopher J. Woolverton.
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