Prokaryotic Cell: Cell Biology
Structure of Prokaryotes :
The organisms in which nucleus is not bounded by a definite nuclear membrane are called prokaryotes.
Among the prokaryotes are the viruses. Pleuropnemonia like organisms (PPLO), bacteria and blue-green algae.
Prokaryotes are very small (generally from 0.5m to 3m) and exhibit no differentiation into nucleus and cytoplasm.
In the eukaryotic organisms the nucleus is bounded by a definite nuclear membrane.
Prokaryotic and eukaryotic cells differ fundamentally in following points:
1. Nuclear membrane:
As mentioned previously, prokaryotic cells exhibit no differentiation into nucleus and cytoplasm i.e. cytoplasmic and nuclear material remain intermingled.
Eukaryotic cells have definite nuclear membrane.
2. Chromosomes:
In prokaryotes the genetic information is located in a single chromosome which is a circular DNA, DNA of prokaryote lacks the basic proteins called histones.
In eukaryotic cells DNA of chromosome is complexed with histones.
3. Absence of Mitotic apparatus and nucleoli:
The cell division in prokaryotes is not typically mitotic nor does meiosis take place in the real sense.
4. Cytoplasmic organelles:
Prokaryotic cells lack in clearly defined membrane limited organelles such as endoplasmic reticulum, golgi complex, mitochondria, lysosomes, centrioles and chloroplasts.
The enzymatic func- tions of mitochondria are carried out by plasma membrane which becomes infolded at several points.
In eukaryotic cells, definite membrane bound structures like the endoplasmic reticulum, golgi complex, mitochondria, chloroplasts, lysosomes and centrioles are present.
5. Cell wall:
The cell of prokaryotes contains amino sugars, muramic acid and other carbohydrates. In eukaryotes the cell wall, when present, does not contain these substances.
6. Plasma membrane:
Plasma membrane in prokaryotes forms intrusions in the cytoplasm called mesosomes. In eukaryotes mesosomes do not exist.
7. Flagella:
Flagella or cilia found in some prokaryotes do not have 9 peripheral and 2 central fibrils (9+2 configuration).
In eukaryotes flagella and cilia exhibit a definite structural plan, having 9 peripheral fibrils and two central fibrils.
8. Photosynthetic apparatus:
In prokaryotes the chlorophyll, whes present, is associated with free lamellae and the lamellae are not enclosed by a membrane. In eukaryotes the pigmented lamellae are found in distinct chloroplast.
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VIRUSES:
Virus is a Latin word which means poison. Adolf Mayer described for the first time a disease of tobacco plant.
Dimitri Ivanowski, a Russian botanist in 1892 demonstrated experimentally that sap of mosaic tobacco plant was capable of inducing the mosaic disease in healthy tobacco plants after it had been passed through bacteria proof filter.
This indicated that the infective agent was smaller than any known bacteria and so he was the first to give clear cut evidence of virus.
Beizerinck, a Dutch microbiologist in the year 1898 confirmed the observation of Ivanowski and named the infectious fluid obtained from the diseased tobacco plant as 'contagium vivum fluidium' and referred it as virus.
Stanley (1935) isolated a crystaline protein from the diseased tobacco leaves.
Bawden and Pirie (1937) established that viruses are nucleoproteins. Marcham (1949) isolated tobacco mosaic virus (TMV).
Viruses comprise a unique group of infectious agents which are characterised by their small size, simple composition and parasitic mode of life.
Sizes and shapes of Viruses:
Viruses are much smaller than bacteria and their size is variable.
The larger viruses may be about 300A in diameter. i.e., they may be as large as some of the small bacteria.
The majority of the viruses are about 200Å in diameter.
Viruses occur mainly in the following three shapes:
(1) Spherical or Polyhedral, as for example, Polio virus, Adeno viruses and Herpes viruses.
(2) Helical or Cylindrical, as for example, Tobacco mosaic virus. Influenza virus, etc.
(3) Complex symmetry, as for example, Vaccinia viruses and some bacteriophages.
Viruses cannot grow and multiply outside the living cell (total parasite). They do not have independent metabolic system of their own and are inactive when they are outside the host cell.
Structure:
Viruses have simple morphology.
They consist of two distinct parts:
1. A core of nucleic acid
2.The protein coat
The protein coat is known as capsid. The protein coat or capsid is composed of several closely packed morphological units called capsomeres.
In this respect viruses differ from typical bacterial cells which are made up of proteins, carbohydrates, lipids, nucleic acids etc.
Some viruses, e.g. Myxoviruses have additional membranous envelope containing proteins, lipids and carbohydrates outside the usual protein coat. They do not have plasma membrane.
Viruses lack cytoplasm and thus cell organelles such as mitochondria,golgi bodies, ribosomes and lysosomes as well as enzyme systems are absent.
Viruses usually have either DNA or RNA where as the typical cell contains both DNA and RNA. Certain animal viruses, e.g.
Rous sarcoma virus (RSV) have both DNA and RNA.
Thus with respect to nucleic acid, the viruses are of three types:
(1) RNA virus
(2) DNA virus
(3) DNA-RNA virus
Plant viruses contain only RNA. Most bacteriophages (vi- ruses infecting bacteria) contain only DNA (some bacteriophages like MS- 2, F, coliphages, R-17 are the excep tions which contain RNA).
Animal viruses contain RNA or DNA or rarely both DNA-RNA.
The nucleic acid component of viruses may be composed of either single stranded or double stranded RNA or double stranded DNA or single stranded DNA.
Bacteriophages are the viruses which multiply within bacteria.
Twort (1915) and d'Herelle (1917) discovered them and named them.
Brenner and his associates studied the ultra-structure of bacteriophages.
Most of the bacteriophages have complex symmetry.
Bacteriophages of T- even series (TTT) which grow in the colon bacterium Escherichia coli are tadpole shaped consisting of a head and a tail.
The head is pyramidal prolate icosahedran made up of about 2,000 protein sub-units or capsomeres and is packed with DNA.
The tail is cylindrical which varies in diameter and structure.
The tail is composed of a helical core tube, 80 Å in diameter surrounded by a protein sheath.
Core tube is connected with the head by a disc called called collar at the upper end and with a hexagonal plate at the lower end (end plate or tail plate).
The base plate has six corners, each with a spike. Each of the six corners of base plate is pro- vided with a long tail fibre which is about 1300 Å in length. These are meant for attach- ment of bacterioph- age on surface of -v2 bacterial cell.
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MYCOPLASMA
Mycoplasmas are tiny, virus-like nonmotile prokaryotic organisms.
They are the simplest and smallest known organisms capable of growing on cell free laboratory culture media containing sterols.
Mycoplasmas lack definite cell wall and are characteristically delimited by a distinct, soft, flexible trilaminar unit membrane of lipoprotein.
Because of the soft and plastic membrane they can assume a variety of shapes and sizes (Pleomor- phic).
The electron micrographs have revealed that mycoplasmas are small spherical bodies or branched worm-like structures which are smaller than bacteria but are within the size range of large viruses.
They can pass easily through bacterial filter and can be squeezed through filter pores smaller than their actual diameter (Morowitz 1972). They are highly resistant to many antibiotics.
The unit membrane of mycoplasma surrounds the cytoplasm which is packed with ribosomes, fibrilar DNA, one or more electron dense areas, and some empty vacuoles.They are capable of performing all life activities.
The genetic material of mycoplasma consists of a naked circular chromosome of fibrilar DNA which is about 3nm in thickness and 1000x 10' daltons in weight (1 dalton = 1/16 the atomic weight of oxygen 1.650 x 10 g).
This replicates in the same way as does the bacterial chromosome. The ribosomes are generally 72 S and are about 14 nm in diameter.
Reproduction in mycoplasma has not yet been conclusively proved.
They can reproduce by binary fission, by formation of spores (elementary bodies), by filamentous growth and by budding.
Structure of Pleuropneumonia like organism(PPLO):
The Pleuropneumonia like organisms (PPLO) are bacteria like organisms and show structural organization some what intermediate between those of viruses and bacteria.
They are more complex than the primitive self- duplicating system from which life presumably arose, Nevertheless, they may be more close to the minimum size and complexity required for independent cellular life and reproduction under present day condition (Morowitz, 1966).
They are filterable like viruses but unlike viruses they can grow in cell free non-living medium.
Mycoplasma, group of widely studied PPLO, measure nearly 0.1m to 0.25m in diameter.
They differ from bacteria in lacking cell wall and mes- osomes and exhibiting very simple organization.
The PPLOS possess a thick plasma membrane of about 75 Å thickness and a hyaloplasm.
In hyaloplasm there occurs a single free DNA molecule, numerous ribosomes, some granular structures and empty spaces of unknown significance.
The vacuole like structures presumably arise due to infoldings of the plasma membrane.
The PPLOS are free living and have all materials and enzymes required in the completion of some biological processes as DNA replication, protein synthesis, anaerobic break-down of sugar, ATP synthesis and so on.
These primitive microorganisms unlike viruses do not require host cells for their multiplication.
They multiply by binary fission, budding, fragmenta tion etc.
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BACTERIAL CELL
The bacteria are unicellular, achlorophyllous prokaryotic microorganisms leading either saprophytic or parasitic mode of life.
An average bacterial cell may measure 1.25m in diameter.
The smallest among the known bacteria is Dialister pneumosintes (0.15 to 0.3m in length) and the largest bacterium is Spirillum volutans (13 to 15 m in length).
The bacterial cell may be,
(i) Spherical (coccus)
(ii) Rod shaped or cylindrical (bacillus)
(iii) Spiral (spirillum) or spirochetes
The spherical bacteria (cocci) may occur either singly (micro or monococci) or in pairs (diplococci) or in a group of 4(tetracocci) or in chains (streptococci) or in irregular bunches (staphylococci).
Rod shaped bacteria occur usually singly but may occasionally be found in pairs (diplobe- cilli) or in chains (streptobacilli).
Structure:
The bacterial cells show a typical prokaryotic organization consisting of the following parts :
(i) Outer covering
(ii) Cytoplasm
(iii) Nucleoid
(1) Outer covering of the cell:
The outer covering in most of the bacterial cells consists of the following layers:
(a) Capsule
(b) Cell wall
(c) Plasma membrane
(a) Capsule:
In some bacteria there may be a slimy layer outside the cell wall which is called capsule.
The slimy capsule is secreted by protoplasm and is made up of di or polysaccharides. The polysaccharides of most bacterial capsules contain more than one type of sugar residues and so they are heteropolysaccharides such as D-glucose, D-galactose, D-Mannose, D-glucouronic and L-rhamnose residues etc.
In some bacteria the polysaccharides do contain single sugar residue and so they are homopolysaccharides such as polyfructose.
Capsules may be divided into two categories:
(i) Macrocapsules
(ii) Microcapsules
Macrocapsules are about 0.2m thick and can be seen under light micro- scope after special staining.
Microcapsules are extremely thin and cannot be seen under light microscope.
They can however be demonstrated immuno- logically.
The capsule layer serves as a protective covering for bacterial protoplasm.
In pathogenic bacteria capsule provides enough resistance against drugs and phagocytes.
Appreciable number of bacterial species are motile and they are provided with flagella.
Bacterial flagella are about 100 to 200 Å thick and show varied length.
They develop from basal granules lying below the cell wall.
The bacterial flagella, unlike multistranded flagella of eukaryotes, are made up of special type of proteins.
In addition to flagella, there occur some hair-like or peg-like outgrowths on the surface of some bacterial cells.
These are called pili or fimbriae. They are composed of helically arranged units of special protein called pilin.
Pilin help in the attachment of bacterial cells to some other objects, and in some they act as conjugation channels through which DNA of donor cells moves into the female cell.
(b) Cell wall:
Cell wall is rigid structure, 10 nm thick which is present just below the capsule and outside the plasma membrane.
The cell wall maintains the shape of the cell and protects the cell from high osmotic pressures gradient.
The chemical composition of bacterial cell wall is different from that of higher plant cell. It does not contain cellulose.
The main structural components of almost all bacterial cell walls are peptidoglycans or mucopeptides consisting of acetylglucosamine and acetyl muramic acid molecules linked alternately in chain.
In certain bacteria, Teichoic acid molecules are covalently linked to peptidoglucan.
Other compounds found in the cell wall are proteins, polysaccharides, lipids, certain inorganic salts, phosphorous and aminoacids, diaminopimallic acid (a substance found only in bacteria and blue-green algae).
The polysaccharides of bacterial cell wall consist of differ ent sugars such as glucose, galactose and mannose or corresponding aminosugars depending upon bacteria.
The electron microscopy of bacterial cell wall reveals that it is composed of granular units of 50 to 140 mm diameter arranged in regular hexagonal or rectangular pattern.
(c) Plasma membrane:
Beneath the cell wall there lies a thin living membrane, the plasma membrane which forms the outer boundary of cytoplasm.
The plasma membrane is 75 Å thick unit membrane and is composed chiefly of proteins and phospholipids.
Some amount of carbohydrate, DNA and RNA have also been reported from plasma membrane but it still needs substantial proof.
The lipids in the prokaryotic plasma membrane are polar lipids which may be glycophosphates or glycolipids.
Small amount of qui- none, Co-enzyme-Q, vitamin K and carotenoids may also be found in the bacterial plasma membrane.
This membrane is a selective barrier to the surrounding medium.
The plasma membranes of bacteria contain enzymes involved in oxidation of metabolites or respiratory chain and many multienzyme complexes.
It performs the functions including oxidative phosphorylation which are usually done by mitochondria in eukaryotic cells.
Bacterial plasma membrane contains many specific transport systems for compounds of the sugars, amino acids, mineral ions etc.
The plasma membrane is capable of not only transporting materials by simple diffusion but is also involved in active transport against concentration gradient.
At certain places membrane may be infolded to form whorls of convoluted membranes called mesosomes.
The mesosomes perform several important metabolic activities such as respiration, secretion, etc.
They are thought to increase the surface aea for transporting systems of the cells. They are also sites of DNA replication enzymes and nucleoid separation.
Cytoplasm:
The cytoplasm of bacterial cell is dense and colloidal substance containing a variety of organic compounds such as proteins, glyco- gens and lipids.
Besides, granulose (a polymer of glucose), volutis (polymetaphosphate), polybeta hydroxibutyric acid and elemental sulphur may also be found in the form of granules.
The majority of cytoplasmic organelles such as endoplasmic reticulum, golgi complex. plastids, mitochondria, lysosomes and centrioles are lacking in the bacterial cytoplasm.
Photosynthetic bacteria contain a pigment called bacteriochlorophyll which is some what different from the chlorophylls found in eukaryotes.
The photosynthetic pigments and enzymes are generally found associated in the groundplasm in the lamellae, tubules or vesicles called chromatophores.
The bacterial cells contain ribosome particles of about 25nm diameter which exist in the cytoplasm in free state or in the form of polysomes but are not attached to the membrane,
Ribosome particles constitute upto 30% of the bacterial weight and they are the sites for protein synthesis.
Nucleoid:
Nuclear region or nucleoid contains genophore which is single circular double stranded molecule of deoxyribonucleic acid (DNA).
DNA molecule of bacterium is about 1mm long (10 nm) and contains all the genetic informations.
One molecule is sufficient to code for about 2000 to 3000 different proteins. DNA molecule is folded and packed within the nuclear region.
It lies free in the cytoplasm and is not bounded by nuclear membrane. DNA molecule of bacterial cell appears to be free and not complexed with histone but contains polyamines (non-histone type) which may be linked to some of its phosphate groups.
Under certain conditions bacterial cell may contain two or more DNA molecules, because of replication of original DNA.
Evidence from studies of Bacillus subtilis indicates that the circular DNA may be attached to the plasma membrane via mesosome.
So it is possible that mesosome plays an important role in DNA replication by providing a mechanism for unwinding of DNA double helix as well as energy.
In general, during division of the bacterial cell DNA molecule replicates and the two molecules go to different daughter cells.
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BLUE GREEN ALGAE
Blue-green algae are the most primitive organisms in the plant kingdom and show typical prokaryotic organization.
A typical cell of blue- green alga is composed of the following components.
1. Outer cellular covering.
2. Cytoplasm.
3. Nucleic material.
1. Outer cellular covering:
The outer covering of cell includes:
(a) Mucilaginous layer
(b) Cell wall
(c) Innermost plasma
(a) Mucilaginous layer:
Mucilaginous sheath is the outermost layer covering the cell wall.
In some cases the mucilaginous layer is very conspicuous and forms mucilaginous sheath but in others it may be inconspicuous.
It protects the cell from the injurious factors of the environment.
(b) Cell wall:
Just below the mucilaginous layer is present cell wall. Electron microscopy has revealed that the cell wall is relatively complex structure.
The cell wall is 2 or 3-layered and the inner layer lies in between outer wall layer and plasma membrane.
The cell wall is formed of polysaccha rides and mucopeptides.
(c) Plasma membrane:
The plasma membrane is selectively permeable living membrane enclosing the cytoplasm and is lipoproteinic in nature.
2. Cytoplasm:
Below the plasma membrane is seen the groundplasm which contains structures of different shapes and functions.
In the peripheral of cytoplasm are located lamellae which contain pigmemts.
Fine structure study has made it clear that the pigmented lamellae are not organised into plastid.
Lamellae or membranes are derived from plasma membrane.
The pigments in lamellae include chlorophylls, carotenes, xanthophylls, c-phycoerythrin and c-phycocyanin, the last two are characteristically found in blue-green algae only.
In addition to lamellae, several mem- brane bound vesicles may also be seen in the cytoplasm and they may some times be stacked in layers.
Besides, ribosomes may be found scattered in the groundplasm.
3. Nuclear material:
The nucleoplasm or DNA containing region is centrally located in the cell and shows a fibrillar structure.
Nucleoplasm is Feulgen-positive but is not organised into a nucleus, i.e., there is no nuclear boundary and no nucleolus.
During division the nucleoplasmic material dispersed throughout the cell divides into two and no spindle apparatus participates in this process.
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