Prokaryotic Cells




General organization of Prokaryotic Cells:
The prokaryotic cells are morphologically the most primitive cells.They seem to have appeared about 3.5 billion years ago. A prokaryotic cell is essentially a single __ membrane or one__ envelope system. The membrane surrounds the cell.There is no membrane enveloping the genetic material and the membranes pervading the  cytoplasm are scarce, if present they are usually associated with respiration or photosynthesis. The prokaryotic cells occur in bacteria,e.g., Pneumoccus, Vibrio, Clostridium, Yersinia, Escherichia;and in the group generally called blue _ green algae. The term algae was applied to the latter organisms because of their photosynthetic activities before their structural relationship to bacteria was discovered. They are more properly called blue_ green bacteria, or cyanobacteria. These include Nostoc and Oscillatoria among many others. The bacteria and blue _ green algae are together known as prokaryotes.Prokaryotes also include organisms known as mycoplasmas,e.g., Mycoplasma gallisepticum; rickettsias, e.g., Rickettsia rockettsii; spirochaetes, e.g; Treponema palladium; and Chalmydias, e.g., Chlamydia trachomatis.The prokaryotes are said to.form the super kingdom Prokaryotae.The prokaryotic cells have the following important features 

Size and Form

The prokaryotic cells generally have a size range of 1 to 10ųm. The smallest bacteria are just 0.5ųm.Bacteria may occur singly or form temporary or permanent colonies of a few to many independent cells.Cells very a great deal in form. The common shapes are spheres ( cocci), rods (bacilli), commas ( vibrios) and helices ( spirilla,spirochaetes) .Some bacteria assume different forms in their life cycle.They are said to be Pleomorphic, e.g., Bacillus radicicola.






Staining properties 

The bacteria are stained with Gram stain¹( weakly alkaline solution of crystal violet or gentian violet). The stained ( blue or purple) bacteria are then treated with 0.5% iodine ( KI) solution.They are then washed with water and finally with alcohol or acetone .The bacteria which retain the stain are termed as  Gram Positive bacteria whereas, the bacteria which lose the stain are termed as Gram negative  bacteria. Variation is due to difference in the structure of the cell wall. The thick cell wall of Gram Positive bacteria does not let the alcohol or acetone solvents to penetrate. 

Outer Coverings 

A bacterial cell is covered by an outer slime layer or capsule, a middle cell wall and an inner cell membrane. 

(i) Slime Layer and Capsule

Outside the cell wall, in the Gram negative bacteria, there may be a coating of macromolecules called glycocalyx.It may be a loose gelatinous sheath termed Slima layer  or a thick and tough covering known as capsule .The slime layer and capsule are formed of  polysaccharides but may  contain polypeptide also. They protect the cell against desiccation,antibiotics and virus attacks. They also enable bacteria to stick to surfaces_ soil particles or rocks in streams, or host cell.Gelatinous capsules glue together the cells of many bacteria that live in colonies.




(ii) Cell Wall

The cell wall  protects like cell, maintains its shape, provides support and strength to it, and keeps it from bursting in hypertonic media.Prokaryotic cell wall consists of peptidoglycan that contains polymers of modified sugars( N_ acetyl glucosamine and N_ acetyl muriatic acid) cross_ linked by short peptides.Peptidoglycan is also called murein or mucopeptid.It may also contain  teichoic acid in it. Some forms, such as mycoplasmas ( formerly called pleuro__ pneumonia__ like organisms or PPLOs), lack cell wall.







Cell wall varies considerable in composition in the Gram Positive and Gram negative bacteria. 
Many antibiotics, such as penicillin, cephalosporins, prevent the formation of peptidoglycan for bacterial cell wall, whereas the enzyme called   lysozyme present in saliva, nasal secretion and tears hydrolyzes peptidoglycan. Both the chemical defend against certain bacterial infections. 

(iii) Cell Membrane 

Cell membrane controls the movement of molecules into and out of the cell. It also bears respiratory molecules to detect chemicals in the surroundings. Cell membrane forms mesosomes by unfolding.The bacterial cell membrane lacks sterols, such as cholesterol. Certain bacteria contain penta cyclic sterol_ like molecules called hopanoids.The latter serve to stabilize the molecular structure of cell membrane. 


     Streptococcus mutants, the chief agent of tooth decay, adheres to the teeth.It uses sucrose ( but not other sugars) as a raw material. This is why candy and other sugary foods are harmful for our teeth.

Cytoplasm

(i)   Cytoplasm  lacks most organelles, namely,  endoplasmic  reticulum, mitochondria, Golgi apparatus, centuresome, vacuoles,  lysosomes, microfilaments¹, intermediate fibres and microtubules ².It contains a variety of inorganic and organic molecules including glycogen granules, lipid droplets, sulphur granules and poly phosphate granules that function as storage reservoirs.Polyphosphate granules ( also called  volutine granules) and sulphur granule take different stains with basic dyed, and are, therefore, called metachromatic granules .The cytoplasm also contains, biosynthetic enzymes, tRNAs and  mRNAs.






(ii)   Ribosomes the only cytoplasmic organelles found in a prokaryotic cells.They are smaller ( 70S) than ribosomes of  eukaryotic cells .They may occur singly or in rows called  poly ribosomes or polysomes. The ribosomes function as the site for proteins synthesis. 


( iii) Mesosomes: Some bacteria, such as Bacillus  subtitles, have Mesosomes, the unfolds of cell membrane, to bear respiratory enzymes. 

( iv) Thylakoids:Photosynthetic bacteria and blue_ green algae have membranous Thylakoids or photosynthetic lamellae to hold pigments.These are derived from the celle membrane. 

(v)  Vacuoles:There are no true, fluid_ filled Vacuoles.Some prokaryotic cells, usually the photosynthetic, free_ floating forms ,from  gas Vacuoles in the cytoplasm. The gas Vacuoles are surrounded by a very thin membrane, not a unit membrane, are permeable to atmospheric gases, and they keep the bacteria floating on or near the surface of water to receive light for photosynthesis or to avoid strong light.






Genetic Material 

(i)  Genetic material ( DNA) is not enclosed by an envelope and lies in direct contact with the cytoplasm, hence the name prokaryotic cell ( G.pro= before, karyon= nucleus). The naked genetic material may be 250 _ 700 times the length of the cell and lies compactly coiled up in a region called nucleoide ( nucleo+G.edios= resemblance).The nucleoid region stains less dense than the surrounding cytoplasm in the electron micrographs.


(ii)   DNA is double _ stranded, helical and circular. It occurs as a single chromosome having Relativity little  proteinassociated with it. Such a simple chromosome is often called a  prochromosome or a genophore to distinguish it from eukaryotic chromosomes. The prochromosome is attached to the cell membrane at least at one point.The cell membrane helps in separating the duplicated DNA molecules into daughter cells during cell division. 

( iii) Prokaryotic cells are haploid only one copy of a  chromosome. 


(iv)  Prokaryotic cells have much less DNA that codes for far fewer proteins than the eukaryotic cells.


(v) Many prokaryotes have small, circular DNA molecules, called plasmids, in addition to the prochromosome. The plasmids may encode proteins  that enable the organisms to resist antibiotics or other toxic materials. 


(vi) There is no nucleolus.


Appendages 

(i)  Bacterial cells may bear  flagella that may be scattered over the entire cell surface or confined to one or both ends of the cell. Flagellum is simple and formed of many intertwined chains of the subunits of a protein flagellin. Ultra _ structure reveals that it consists of two main parts:  basal  body and filament.The basal body further comprises two regions: shaft and hook.Flagella are inserted into the cell wall and cell membrane by a basal apparatus ( flagellar motor) , and not enclosed by cell surface membrane. They are only 20nm is diameter and 10 to 15 ųm long.The basal apparatus rotates the filament, which propels the cell by proton H+) gradient. Basal body is the most complex part of the flagellum. It has four rings connected to a central rod in Gram_ negative bacteria. The Gram_ Positive bacteria, however, have only two basal body rings; the inner ring is connected to the  plasma membrane and outer ring to peptidoglycan membrane. The basal body and hook permit the filament to rotate by 360° rather than undulating back and forth like a whip.The flagellum, therefore, spins the body in the opposite direction and pushes the organism in forward direction.There are no flagella in blue_ green algae. The latter may, however, slow, gliding movements or peculiar back_ and__ forth oscillatory movements. 







(ii) Many bacteria bear short, tubular, nonmotile processes termed pili, or fimbriae, on the surface. The police are formed of a protein pilin.They are used for attachment to surfaces, to food or to one another.  Neisseria gonorrhoeae attaches to the host cells by pili and causes gonorrhea.Some bacteria, such as Escherichia coli, have tubular   sex polishe   for use in the exchange of genetic material during conjunction. These have been reported only in Gram _ negative bacteria. 

    The fimbriae are small bristles _ like fibres sprouting out of the bacterial cell.These slender tubes are composed of helically arranged protein subunits, 3_ 10 nm in diameter. Fimbrae attach bacteria to solid surface e.g. rocks in streams as well as to the host tissues. Fimbrae are also responsible for mutual clinging of bacterial cells forming a film on the liquid or other thick aggregates. 


Biological Activities 

(i) Cytoplasm does not show streaming movements or cyclosis.

(ii) Phagocytosis, pinocytosis and exocytosis do not occur due to the presence of cell wall. Substances enter and leave the cell directly through the cell membrane. 


(iii) Mitotic apparatus is not formed during cell division. The cell membrane helps to segregate replicated products of the  chromosome into the daughter cells. 


(iv) Replication of DNA is normally continuous throughout the cell cycle. 

(v) Both transcription are nonsexual organisms and have asexual reproduction only.Therr is no process comparable to meiosis, or gamete formation,  or true fertilization. 

(vii) Some prokaryotes have the ability of nitrogen fixation. 

( viii) Respiration in prokaryotes may be aerobic. Respiratory  enzymes are associated with the cell membrane. 

( ix) Nutrition may be autotrophic ( photosynthetic or chemosynthetic) or heterotrophic( saprotrophic or parasitic).

Versatility:

The prokaryotic cells,  through simple in structure, have all the biochemical mechanisms required to synthesize organic materials precursors. In this respect, the  prokaryotes are more versatile in their synthetic activities than the eukaryotes.





Utility

 Prokaryotic cells are very suitable for the study of biochemistry and molecular biology on account of their simple structure, rapid growth and simple mode of reproduction. The  bacterium Escherichia coli is abundantly used for this purpose. 


































































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RNA                    Nucleic Acids









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