Prokaryotic cell structure

PROKARYOTIC CELL STRUCTURE

Living organisms are differentiated from nonliving matter by their (1) ability to reproduce (2) ability to ingest or assimilate food and metabolize them for energy and growth (3) ability to excrete waste products (4) ability to react to changes in their environment (irritability) and (5) Susceptibility to mutation. The living organisms include a variety of micro and macro organisms of differing size , shape morphology, and behaviour. They include tiny bacteria, protozoans, worms, plants and animals like man,

whale and elephants.

Carlous Linnaeus (1707-1778), the Swedish botanist was the first to introduce nomenclature for plants and animals. Until 18^thcentury only plant and animal kingdoms were recognized. However some organisms are predominately plant like, some animal like and some do not fall in both the groups. Therefore it was felt a third kingdom was necessary. Haeckel (1866), a German zoologist suggested a third kingdom Protista to include those organisms that are not typically plants and animals. Bacteria, algae, fungi and protozoa are cellular organisms placed under protista. Viruses are not cellular organisms and therefore not classified as protists. Bacteria were lower protists while algae, fungi and protozoa were higher protists. A satisfactory criteria to differentiate bacteria, fungi and algae could not be made until the development of electron microscope, which depicted the internal structure of these organisms. The absence of membrane bound internal structures in bacteria and their presence in fungi, algae, protozoa, plants and animal cells was taken as criterion to differentiate prokaryote and eukaryote.

Whittakar (1969) proposed five kingdoms based on three levels of cellular organization and three principal modes of nutrition, photosynthesis, absorption and ingestion. The prokaryotes lacking ingestive mode of nutrition are included in the kingdom. Monera. In the kingdom protista unicellular eukaryotic microorganisms representing all the three modes of nutrition are included. The multicellular green plants and higher algae were placed in the kingdom plantae while multinucleate higher fungi in the kingdom fungi and the multicellular animals in the kingdom Animalea.

Bacteria and cyanobacteria (the blue green algae) of monera, microalgae and protozoa of protists and yeasts molds and fungi represent the microorganisms. Most of them are invisible to the naked eye and requires magnification. The oratically a black dot of 4mμ in diameter on a white background can be perceived by retina of human eye but in reality an object of above 30mμ in size only will be visible to the eyes and objects lesser than that requires magnification.

Prokaryotes are organisms with primitive type of nucleus lacking a well-defined membrane a less complex nuclear division than mitosis. The nuclear material is a DNA molecule in prokaryotes compared to chromosomes of higher organisms. Eukaryotes

are organisms with cells having true nuclei enclosed in a nuclear membrane and are structurally more complex them prokaryotes. A varying degree of localization of cellular functions in distinct membrane bound intracellular organelles like nuclei, mitochondria

chloroplasts etc. The cells of living organisms are either prokaryotic or eukaryotic in nature and there is not any intermediate condition. The size, shape, morphology and the internal cellular organizations are different in these two groups.

The size of the microorganisms varies from unicellular tiny bacteria to large brown algae and mushroom. Bacteria are unicellular, small 0.5-1.0mm in diameter, which multiply by binary fission. The algae are photosynthetic simple organism withunicellular primitive types to aggregates of similar cells and to large brown algae with complex structure. Protozoa are unicellu-lar, most of them living freely in soil and water while a few cause disease of man and animals.

The rigid cell wall of the bacterium confers shape. Bacteria vary in shape from spherical (Coccus) rods (Bacillus) and heli-cally curved rods (Spirillum). Most bacteria possess a constant shape but some exhibit polymorphism (variety of shape).

Bacterial cells are arranged in a characteristic manner in a particular species. In cocci the arrangement is known as diplo-cocci when cells divide in one plane and remain attached in pairs, streptococci when divide in one plane and remain attached to form chains; tetrococci, when they divide in two planes and form group of four cells (tetrads), staphylococci when they divide in three planes and form bundles, sarcinae when they divide in three planes in a regular manner and form a cutridal arrangements.

Bacilli are not arranged in such complex form as in cocci. Most of them occur singly or in pairs (diplobacilli), form chains (streptobacilli) form trichomes, similar to chains but with a larger area of contact between cells and lined side by side like match sticks (palisade arrangement) at angles to one another.

Some others form long branched multinucleate filaments called hypha as in fungi. Hyphae ramify and collectively form mycelium. The curved bacteria are vibrioid with less than one twist or turn of helical with one or more complete turns. Rigid helical shape is in Spirilla and is flexible in spirochete.

Cell wall is a very rigid structure that confers shape to the cell. This prevents expansion of cells and bursting due to uptake of water as most bacteria live in environments of higher osmotic pressure than that exists in cells (hypotonic environments). A cell wall is common to almost all bacteria except in mycoplasma that lacks typical cell wall and L-forms of bacteria like Streptobacillus that are having walls but loose them when grown in media con-taining sub lethal levels of cell wall synthesis inhibiting antibiot-ics like penicillin. Mycoplasma lack cell wall permanently and hence pleomorphic while L-forms of bacteria can revert back to walled forms. The isolated cell walls without cellular constituents retain the original contour of cells from which they are derived indicating that cell wall confers shape. This is further strength-ened as the protoplast derived from any type of cell cocci or bacilli show a spherical shape. Both eubacteria and archaebacteria are grouped as Gram positive and Gram negative based on the wall thickness. As the chemical composition of both eubacteria and archaeobacteria differ it is only the thickness rather than the chemi-cal composition is the key factor for Gram reaction.

Cell wall constitutes 10-40% of cell. It is essential for growth and division. Cells without walls (protoplasts) cannot grow and divide.

The cell wall in eubacteria consists largely of an insoluble porous, cross-linked polymer of enormous strength and rigidity viz., peptidoglycan (also called murein).

This is a bag shaped macromolecule and surrounds the cyto-plasmic membrane and found only in prokaryotes. Although it is tough but in a dynamic state. It is a polymer of N-acetyl glu-cosamine, N acetyl muramic acid, L-alanine, D-alanine, D-glutamatic and a diamino acid (LL or meso diaminopimelic acid, L-lysine, L-orthinine or L-diaminobutryic acid).

The cell wall composition of archaeobacteria is different from eubacteria. Their walls are composed of proteins, glycoproteins or polysaccharides. But in some genera as Methanobacterium the cell walls composed of pseudosuriein that have some superfecial re-semblance to peptidoglycan but differs in chemical composition.

The peptidoglycan constitutes about more than 50% of the dry weight of cells in gram-positive eubacteria but only 10% in gram-negative bacteria. In addition to peptidiglycan other sub-stances like polysaccharides in Streptococcus pyrogenes teichoic acids in Staphylococcus aureus, lipids as mycolic acids in Coryne-bacterium and Mycobacterium. The acid fast cord factor, a my-colic acid derivative is toxic and plays a role in diseases due to Corynebacterium diphtheriae and M. tuberculosis.

The wall of Gram negative consists of a thin peptidoglycan layer surrounded by an outer membrane rich in lipids. The lipids in the wall constitute 11-12% of the dry weight of the cells, The outer membrane is an impermeable barrier preventing the escape of important enzymes from the periplasmic space between the cy-toplasmic membrane and outer membrane. The outer membrane also prevents external chemicals and enzymes that can destroy cells. Lysozyme, which dissolves selectively the peptidoglycan can dam-age gram positive bacteria.

The outer membrane, a bilayered structure consisting many of phospholipids, proteins, and polysaccharides is anchored to the peptidoglycan layer by means of Braun’s lipoprotein. The li-popolysaccharide (LPS) layer has toxic properties and known as endotoxin. This occurs only in outer membrane and is composed of lipid A, core polysaccharide and O antigen. The outer mem-brane is impermeable to large molecules like protein but allow smaller molecules like monosaccharides peptides and amino ac-ids through channels called porins. Porins span the membrane and are specific for different kinds of small molecules.

There are several structures external to cell wall in bacteria, which vary in structure and composition depending upon the type of bacteria. They are flagella, pili or fimbriae, capsules, sheath, prosthecae and stalk. Flagella are locomotory organs in bacteria, which vary in number and arrangement. Some bacteria do not have flagella.

Flagella are hair like helical appendages 0.01 – 0.02 nm in diameter the flagellar arrangements vary with the organisms. It may be polar if the flagella are at one or both the ends or lateral if they are arranged on sides. They protrude through the cell wall. A flagellum is composed of a basal body a short hook and a helical filament longer than the cell. The basal body is associated with cytoplasmic membrane and cell wall.

Bacteria swim by rotating their helical flagella similar to cork screw. Bacteria with polar flagella swim in a back and forth fash-ion. Those with lateral flagella swim in a more complicated man-ner. Removal of flagella from a flagellate bacterium will not result in death of bacterium and only motility will be affected Spiro-chetes, the helical bacteria, swim even in viscous media, without any external flagella. They have flagella like structure within the cell located just beneath the cell envelope. They are known as periplasmic flagella (also called endoflagella, axial filament). Spiroplasmas are also helical in shape and swim in viscous media without even periplasmic flagella.

Some bacteria like Cytophaga exhibit a gliding motility, which is a slow sinuous flexing motion. This occurs when the cells come in contact with solid surface.

Pili are short, hollow, non helical and filamentous append-ages. They are thinner than flagella but more in number than fla-gella. They are found in both motile and non motile bacteria and hence not involved in motility.

F pilus (sex pilus), a type of pilus serves as port of entry for genetic material during bacterial mating. Some pili in pathogenic bacteria serve as attachment with host cells in human beings fa-cilitating infection without being washed off easily by mucous.

Capsules

A viscous substance forming a covering layer around the cell is found in some bacteria and is known as capsule. If it is too thin it is called as microcapsule. If it is loose and many cells are embedded in a matrix it is known as slime. The capsular material is not water soluble in many bacteria but in some it is highly water soluble, thus making the medium in which they grow more viscous. Capsular material is primarily polysaccharide in most bacteria. It may be a homopolysaccharide, made up of a single kind of sugar, synthesized outside the cell from disaccharides. The capsule of S.mutons is a glucan (a glucose polymer) synthesized from sucrose. Capsules composing of several kinds of sugars are termed

heteropolysaccharides. These are synthesized from sugars within the cell, transported and polymerized outside the cell. The capsule of Klebsiella pneumonia is a heteropolysaccharide. The capsule of some bacteria is polypeptides. The capsule of anthrax organism Bacillus anthracis is a polymer of D-glutamic acid.

Sheath is a hollow tube that encloses cells in the form of chains or trichomes. This is present in some bacteria living in fresh water and marine environment. The cells some times move out of sheath. In a few cases the sheath is strengthened by deposition of ferric and manganese hydroxides.

Aerobic bacteria in fresh water and marine environment possess prosthecae, which increases the surface area of cells for nutrient absorption from the dilute aquatic environment. They are semirigid extension of cell wall and cytoplasm membrane and smaller than the cell. Some bacteria have single prostheca (Caulobacter) and others have more than one (Stellar and Ancalomicrobium).

Stalks are also found in some bacteria like Gallionella or Planctomyces. They are non-living ribbon like or tubular append-ages that are excreted by cell. These stalks aid in attachments of cells to surface.

The structures internal to cell are cytoplasmic membrane, protoplast, intracellular membranes, the cytoplasm, cytoplasmic inclusions and nuclear material, the DNA.

The cytoplasmic membrane is immediately beneath the cell wall and is about 7.5nm thick. It is made up of phospholipids (about20-30 percent) forming a bilayer to which both integral pro-teins and peripheral proteins are held. The membrane has fluidity owing due to its lipid matrix and this allows components to move laterally.

The phospholipids of eubacteria and archaeobacteria differ in composition. The phospholipids of eubacteria are phosphoglycerides. In this straight chain fatty acids are linked to glycerol by ester linkage. In archaeobacteria, the lipids are polyisoprenoid branched chain lipids. In this phytanols, (long chain branched alcohols) are ether linked glycerols.

The cytoplasmic membrane is a barrier for penetration of most of water soluble molecules. But the small molecules like nutrients and waste products are transported across the membrane by specific proteins. The membrane also contains various enzymes of respiratory metabolism and synthesis of cell wall components and capsule. It is also the site of generation of proton motive force that drives ATP synthesis, nutrition, transport system and flagellar motility. The damage to membrane by physical or chemical agent lead to death of cells.

The cytoplasmic membrane and the cell material bounded by it are called protoplast. The bacterial cell minus the cell wall is the protoplast. Protoplasts of gram positive bacteria can be pre-pared by dissolving the cell wall by lysozyme or by growing the bacteria in penicillin containing media. Penicillin prevents the syn-thesis and formation of cell wall. Protoplasts thus prepared have to be suspended in an isotonic medium, other wise the bacteria living in hypotonic environments tend to absorb water and burst.

In Gram-negative bacteria lysozyme treatment may destroy the cell wall. The outer membrane remains with the cytoplasmic membrane enclosing the cell content. Such type of protoplasts with the outer membrane is called as spheroplast.

The bacteria that lack cell wall like mycoplasma are similar to protoplasts but they are parasites of animals, plants or arthropods and hence live in osmotically favourable or isotonic environments.

Bacteria are prokaryotes that do not contain any membrane bound organelles inside the cells. But bacteria have specialized invagination of cytoplasmic membrane that increase the surface area for certain functions. Mesosomes are convoluted tubules or vesicles formed by membranous invagination in bacteria. Central mesosomes are located near the middle of the cell and penetrates deep into the cytoplasm. It seem to be attached to the nuclear ma-terial. Peripheral mesosomes shallowly penerate into the cytoplasm and seen to be invalid in export of exocellular enzymes.

The intracellular membrane is extensive in all phototrophic bacteria, chemoautotrophs and in methane oxidizing bacteria. In phototrophic bacteria they are the sites of photosynthesis as the increased surface area increase the light absorbing pigments.

Thylakoids are special intracellular membranes that occur in cyanobacteria but they are separate from cytoplasmic membrane.

The cytoplasmic membrane bound the cytoplasm. the cyto-plasm consists of a cytoplasmic area, a chromatinic area and con-sists of Ribosome. Ribosomes are macromolecular RNA protein bodies and are the sites of protein synthesis. The chromatinic area is rich in DNA. The fluid proteins contain the dissolved substances.

Ribosomes of prokaryotes have a sedimentation coef of 70 Svederberg units (70S) and are composed of two subunits of 50S and 30S. On the other hand ribosomes of eukaryotes have a sedimentation coefficient of 80S and are composed of 60S and 40S subunits.

Cytoplasmic inclusions are concentrated deposits of certain substances. Volutin granules or metachromatic granules are polyphosphates deposits. It is a reserve of phosphate. Poly-B-hy- droxy butyrate is a chloroform soluble lipid like material and serve as carbonTheyandareenergyfoundsourceina.robic bacteria.

Polysaccharide granules viz glycogen is present as inclusion. El-emental sulfur accumulates in certain bacteria growing in envi-ronments rich in hydrogen sulphide.

Bacteria in aquatic habitats have gas vacuoles to provide buoyancy. Gas vacuoles have water impermeable boundary but permeable to dissolved gases which fill the cavity. Bacteria do not have a nucleus with a nuclear membrane. The nuclear material is only a single circular DNA molecule. This is called as nucleoid, the chromatin body, the nuclear equivalent or functional chromosome.