architechture of microbial cell
TRANSCRIPT
By :-Vikas C J
ARCHITECTURE OF MICROBIAL
CELL
Contents Introduction Classification of bacteria Structure of bacteriaConclusion Reference
Cells
Smallest living unit Most are microscopic
Cell size:
Dimensions of most bacterial cells: Diameter: 0.2 to 2.0 mm.
Human red blood cell is about 7.5-10 mm in diameter.
Length: 2 to 8 mm. Some cyanobacteria are up to 60 mm long.
Bacterial cells have large surface to volume ratios. Therefore all parts of the cell: Are close to the surface. Can be quickly reached by nutrients
Cell Types Prokaryotic
Eukaryotic
Distinguishing Features of Prokaryotic Cells:
1. DNA is: Not enclosed within a nuclear
membrane. A single circular chromosome. Not associated with histone proteins.
2. Lack membrane-enclosed organelles like mitochondria, chloroplasts, Golgi, etc.
3. Cell walls usually contain peptidoglycan, a complex polysaccharide.
4. Divide by binary fission
Distinguishing Features of Eukaryotic Cells:
1. DNA is: Enclosed within a nuclear membrane. Several linear chromosomes. Associated with histones and other
proteins.2. Have membrane-enclosed organelles
like mitochondria, chloroplasts, Golgi, endoplasmic reticulum, etc.
3. Divide by mitosis.
Differences between Prokaryotes and Eukaryotes
Prokaryotes EukaryotesCell size 0.2-2 um in diameter 10-100 um in diameterNucleus Absent PresentMembranousOrganelles Absent PresentCell Wall Chemically complex When present, simpleRibosomes Smaller (70S) Larger (80S) in cell
70S in organellesDNA Single circular Multiple linear
chromosome chromosomes (histones)Cell Division Binary fission MitosisCytoskeleton Absent Present
Structure of Prokaryotes
BACTERIAMicrscopic Unicellular , organisms belonging to Kingdom monera that possess a prokaryotic cell structure, which means their DNA (usually circular) can be found throughout the cytoplasm rather than within a membrane-bound nucleus. They reproduce by fission or by forming spores. They can practically live everywhere. They can inhabit all kinds of environment, such as in soil, acidic hot springs, radioactive waste, seawater, deep in the Earth's crust, in stratosphere,.
The electronic microscope studies have revealed that the ultra structure of bacterial cell possesses the following structure
Capsule or slim layer Cell wall Plasma membrane Flagella and fibriae and pilus Cytoplasm which includes ribosomes ,
mesosomes, fat vacuoles , inclusion bodies and nucleoides .
CAPSULE Certain bacteria like diplococcus and pneumococcal are
externally covered by a layer of that layer is called capsule. Glycocalyx - Polysaccharide on external surface Forms an envelope around the cell wall and can be observed under light
microscope. Chemically capsules are made up of proteins, polysaccharide and lipids The bacterial capsule is species specific and can be used for immunological
detections. amount these polymers vary with bacterial species. It is sticky in nature ,secreted by the cell which gets firmly attached to the
cell wall. If the substances are unorganized and loosely attached to cell wall, - slime
layer. Many gram positive and gram negative bacteria have a regular structured
layer called an S-layer on their surface. These are very common among Archea. where they may be the only wall structure outside the plasma membrane.
Functions of Capsule Prevent attachment of bacteiophages. Prevents bacterial cell against
desiccation. Survive in natural environment –sticky
property. Prevents from phagocytosis. It may protect the cell against ion and ph
fluctuation.
CELL WALL A typical bacterial cell possesses a rigid structure
called cell wall that lies outside the plasma membrane It is about 10 -25 nm thick and accounts for about 20 -
30% of the dry weight of the cell It is responsible for maintaining the shape of the
bacterium and prevents the cell from osmotic lysise The chemical subsatance that comfers rigidity to the
cell wall is peptidoglycon. it is a strong polymer found only in prokaryotes. peptidoglycon is a polymer of N-acetyl glucosomine
and N-acetyle muramic acid .
Bacteria are classified into gram positive and gram negative depending on the nature of the cell wall Gram’s positive bacteria will have thick
peptidoglycon layer where as gram negative bacteria are having thin peptidoglycon layer. Gram positive bacteria also contains teichoic acid which is absent in gram negative bacteria.
Gram negative bacteria contain an outer membrane that surrounds the peptidoglycon layer which is made up of lipopoly saccharides. The important function of this outer membrane is to serve as protective barrier and prevents or slows the entry of toxic substance that might kill or injure the bacteria. In addition this membrane is anchored to the peptidoglucon by Braun’s lipoprotein.
PLASMA MEMBRANE The contents of the bacterial cell with in the boundries of the cell
wall form the protoplast. It consists of outer plasma membrane, cytoplasm , and nucleoids.
The plasma membrane of bacterial cell is similar to that of eukaryotic cell. The most widely accepted current model for membrane structure is the fluid mosaic model of Singer and Nicolson.
According this membrane structurally is a lipid bilayer consisting of two types of proteins peripheral proteins are loosely connected to the membrane and can be easily removed and make 20-30% of the total membrane proteins. about 70-80% of the membrane protein are integral proteins, they are not easily extracted from membrane
Functions of plasma membrane Organic and inorganic nutrients are transported
through plasma membrane. Consists of enzymes of biosynthetic pathways. The inner membrane invaginates to form
mesosomes. It has selective permiability.
PILI or FIMBRIAE Many gram positive bacteria have short five hairy
appendages that are thinner than flagella and not involved in motility, they are usually called fimbriae
Although a cell may be covered by 1000’s of fimbriae, they are visible only under electron microscope due to their small size.
They seem to be slendour tubes composed of helically arranged proteins subunits and are about 3-10nm in diameter and upto several micrometer long. Atleast some types of fimbriae attach bacteria to solid surfaces such as rocks in streams and host tissue.
Flagella About half of all known bacteria are
motile, most use flagella. Long, thin, helical appendages. A bacterium may have one or several
flagella, which can be in the following arrangements: Monotrichous: Single polar flagellum at one
end. Amphitrichous: Two polar flagella, one at
each end. Lophotrichous: Two or more flagella at one or
both ends. Peritrichous: Many flagella over entire cell
surface.
Monotrichous; Lophotrichous
Amphitrichous Peritrichous
Flagella have three basic parts1. Filament: Outermost
region. Contains globular protein
flagellin. Not covered by a sheath like
eucaryotic filaments.2. Hook: Wider segment that
anchors filament to basal body.
3. Basal Body: Complex structure with a central rod surrounded by a set of rings.
Gram negative bacteria have 2 pairs of rings.
Gram positive bacteria only have one pair of rings.
Cytoskeleton These are Filaments & fibers
Made of 3 fiber types Microfilaments Microtubules Intermediate filaments
3 functions: mechanical support anchor organelles help move substances
VIRUSES WHAT ARE VIRUSES????
Viruses are considered to be the smallest ‘living units’. Viruses may be defined as acellular , Sub microscopic
entity consisting of a single nucleic acid surrounded by a protein coat and capable of replication only within the host cells using host metabolic machinery.
Exsists in both living and non-living form.
Morphology of viruses Shape Morphology of viruses Shape
Viruses has different shapes such as
Spheroid (adenovirus) Elongated (potato viruses) Coiled (beet yellow virus) Bullet shaped (rabies virus) Filamentous (bacteriophage)
Typical structure of virus
Capsid Protein coat surrounding nucleic acid Composed of capsomeres
The arrangement is characteristic for a particular virus Single protein type Several protein types
Envelope Covers capsid in some viruses Combination of
Lipids Proteins Carbohydrates
It is about 10-15µm thick Can be derived from host cells plasma membrane
• Spikes– Carbohydrate – protein complexes– Project from envelope– Attachment mechanism– Means of identification– Hemagglutination
• Clumping of RBC’s
Based on symmetry of capsule or shape Polyhedral viruses Helical viruses Complex virusesBased on presence or absence of envelop Naked capsid viruses Enveloped viruses
Polyhedral Capsids are many sided
They are of 3 typesTerahedral – 4
sidesOctahedral – 8
sidesIcosahedral – 20
triangular faces and 12 corners
In icosahedral capsids are made of many subunits called capsomeres.
Ex: adenovirus, herpes virus, etc.
Helical
Capsid and nucleic acid are helically coiled.
Ex: TMV, mumps viurs, influenza virus, Rabies, Ebola, etc
Complex Capsid is attached with
additional structuresEx: vaccinia virus, phages of T-even series
Bacteriophages - Capsid is polyhedral in shape where as sheath is helical
Conclusion
Reference Dr.R.C Dubey and Dr.D.K Maheshwari. A
text book of microbiology. https://scholar.google.co.in/scholar?
q=architecture+of+microbial+cell https://www.google.co.in/webhp?
sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=characteristics+of+anaerobes
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