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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