Desiccation Tolerance in Prokaryotes Term Paper

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Desiccation Tolerance in Prokaryotes

Prokaryotes or eukaryote is the organism that makes up the microbial world. Prokaryotes are deficient of internal unit membranes and are self-sufficient cells or organisms. The best-known prokaryotic organisms are the bacteria. The cell membrane in prokaryotes makes up the cell's primary osmotic barrier and consists of a phsopholipids unit membrane. The ribosome carries out translation and protein synthesis and is present in the cytoplasm. Normally, the nuclear regions consist of circular, double-stranded deoxyribonucleic acid.

Plasmids, the accessory self-replicating genetic structure is present in many prokaryotes with extra not necessary cell functions like encoding proteins to inactivate antibiotics. On the other hand, the eukaryotic cells have a nuclear membrane, well-defined chromosomes, mitochondria, a sector device, an endoplasmic reticulum and digestive system with many cell types. The prokaryotes are deficient of structural multiplicity and consist of millions of genetically distinct unicellular organism, which is well-known among eukaryotes and they make up for in their physiological diversity. A particular group of prokaryotes is often distinguished and combined by a particular physiological characteristic. [Major groups of prokaryotes]

The groups of prokaryotes are formed on the basis of easily observed traits like morphology, gram stain, motility, and structural features and on distinguishing physiological features, in Bergey's Manual. But the right method to classify prokaryotes is on a genetic basis i.e., by comparison of the nucleotide sequences of the small subunit ribosomal RNA that is contained in all cellular organisms. Groups of prokaryotes can be positioned under unimportant headings, based on common structural, biochemical or ecological properties. Some prokaryotes are in more than one group and some groups consist of both Archaea and Bacteria. Based on the RNA analysis, the Archaea consists of three phylogenetically distinct groups: Crenarchaeota, Euryarchaeota and Korarchaeota. Archaea can be arranged into three types, based on their physiology: Methanogens-prokaryotes producing methane, extreme halophiles-prokaryotes that exist at very high concentrations of salt and severe thermophiles-prokaryotes that exist at very high temperatures. [Major groups of prokaryotes]

The prokaryotes display unique structural or biochemical attributes, which adjusts them to their particular habitats, in addition to the unifying archael features that differentiate them from bacteria. It has been confirmed by the phylogenetic analysis of the Bacteria that eleven distinct groups exist but most of them consist of members that are phenotypically and physiologically irrelevant. The major group of Bacteria are Photosynthetic purple and green bacteria, Purple and green sulfur bacteria, Spriochetes, Cyanobacteria, Myxobacteria, Lithotrophs, Pseudomonads, Enterics Vibrios, Nitrogen fixing organisms, Pyogenci cocci, Lactic acid bacteria, Endospore forming bacteria, Actinomycetes and related bacteria, Rickettsias and Chlamydiae, Mycoplasms, Plant pathogenic bacteria etc. [Major groups of prokaryotes]

The conventional categorization of prokaryotic envelopes as either gram positive or gram negative is used in the modern reviews about bacterial cell wall properties. When tainted with crystal violet, both envelope types show a characteristic difference mainly based on the difference of their peptidoglycan architecture. Normally the multi-layered peptidoglycan of gram-positive bacteria forms a physical barricade for the dye. But in gram-negative bacteria the tint can be easily washed out because of their comparatively thin sacculus. The peptidoglycan architecture largely decides the different properties of the two cell wall types in terms of mechanical stability, permeability and resistance toward chemical substances along with other traits like the occurrence of accessory cell wall polymers or the presence of an outer membrane. [Major groups of prokaryotes]

The cell bound water is evaporated through air-drying and subsequent addition of water to air-dried cells is the process through which the prokaryotes have been formed. This procedure of air-drying is called desiccation. Some bacterial cells which have been subjected to air-drying, the removal of free cytoplasm water can be fast. In such cases the preferred balance between cell-bound water and the environmental water potential is got immediately. This state of the endurance of the organism in response to such air-dried state is known as desiccation tolerance. For most of the cells, even a shortfall of a small fraction or intracellular water is deadly. Some cells subsists extreme desiccation for a long time. While some can withstand desiccation for thousands and millions of years, there are some bacteria, which stay alive only for a few seconds in such a dried state. [Desiccation tolerance of prokaryotes.]

The term used to specify the amazing capability of certain organism to withstand almost total dehydration is Anhydrobiosis. It is a condition representing life without water. The anhydrobiotic cell is typified by its singular lack of water with contents as low as 0.02g of H2O g-1. The monolayer coverage by water of macromolecules, including DNA and proteins is troubled at these levels. As a result the method that bestows desiccation tolerance upon air-dried bacteria is clearly different from those, such as the mechanism of preferential exclusion of compatible solutes that preserve the integrity of salt, osmotic ally and freeze-thaw-stressed cells. An intricate range of interactions at the structural, physiological and molecular level is shown by the desiccation tolerance. It is clear that they involve interactions such as those between proteins and co-solvents that get from the exclusive properties of the water molecule, though most of the methods remain cryptic. How the non-reducing disaccharides trehalose and sucrose maintain the integrity of membranes and proteins is given by the water replacement hypothesis. Amongst the prokaryotes, the cyanobacteria have a distinct ability to live as anhydorbiotic cells. [Desiccation tolerance of prokaryotes.]

An essential branch in cell biology is the study of the method by which some organisms tolerate complete desiccation. To outlive severe water shortage, desiccation tolerant cells employ structural, physiological and molecular mechanism. By postponing fusion between neighboring membrane vesicles during drying, and also by preserving membrane lipids in the fluid phase in the absence of water, some sugars mainly trehalose prevent damage from dehydration. Water molecules supply to the steadiness of proteins, DNA and lipids and are significant components of the reaction mechanisms. In the derivation and development of the genetic code, water might have played a determinative role. The water replacement hypotheses explain how the non-reducing disaccharides sucrose and trehalose defends membranes and proteins in vitro from dehydration damage. [Role of Lipids and fatty acids in stress tolerance]

The sensitivity to drying can increase survival, is connected with the endurance of dehydration damage in a variety of organisms with intracellular accumulation of one of these disaccharides and even the addition of exogenous trehalose or sucrose to cells. The capacity to live on desiccation may be givenby transfection of desiccation-sensitive cells with genes, which allow synthesis of trehalose or sucrose. Since fusion of either disaccharide requires only two steps, engages only two gene products, a synthase and phosphatase and requires substrates found in all cells, this seems practical.

Latest proof show that the phase transition temperatures and vibration frequencies (P=O) stretch of the phospholipid rises in frequency by about 30 cm-1 when the protein is dried without trehalose but is reduced to or below the frequency of hydrated P=O when the protein is dried with trehalose. Trehalose can be installed between the phosphates of neighboring phospholipids, is shown by the molecular modeling. At low trehalose/lipid ratios trehalose is not available to bind water thereby showing a direct interaction between the sugar and lipid. Free radicals begin fatty acids deesterication from phospholipids, when the membranes are secluded. [Role of Lipids and fatty acids in stress tolerance]

During aging, free fatty acids normally collect in desiccation sensitive cells and are a reason for reduced membrane integrity. The number of free radicals in the dry state associates well with the respiratory rate earlier to desiccation, which suggests that curbing of the respiratory metabolism earlier to dehydration, may be necessary for the maintenance of membrane integrity and desiccation tolerance. As dehydration may involve a reverse phase change of membrane lipids from the gel to liquid crystal phase, which takes place in the presence of water, the imibition of viable, dry cells may lead to extensive leakage and death mainly when it occurs at low temperatures.

In reacting to environmental stresses, alteration in the lipid content of membranes of an organism is of main significance. A central method of desiccation tolerance is symbolized by the maintenance of membrane integrity in anhydrobiotic organisms. The existence of bacteria at severe temperatures, salinity and drying has been observed by the role of membrane fluidity and lipid composition. There was a minor increase in the amount of total lipids in the rehydration of dry mats of Scytonema geitleri. Membranes can be made steady by trehalose. Upon consequent rehydration, membranes dried without trehalose experience vesicle fusion, change in morphology and loss of calcium transport activity. [Role of Lipids and fatty acids in stress tolerance]

To explain how the non-reducing sugar trahalose defends cells, membranes, proteins and nucleic acids when they are dried, is explained by the water replacement hypothesis developed by Oliver et al. Trehalose after desiccation appears to lower the phase transition temperature of the dry lipids and maintains them in the liquid-crystal state. Highly pigmented sheaths are present in a number of desiccation tolerant cyanobacteria. The scytonemin,…[continue]

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