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Procaryotic cell

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Structure of Prokaryotes cell in detail

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Procaryotic cell

  1. 1. Prokaryotes cell Dr. Shalini Purwar Centre for food technology University of Allahabad
  2. 2. Shape, Arrangement and Size Prokaryotes are small, relatively simple organisms. Prokaryotes would be uniform in shape and size. Bacteria have two shapes. Cocci (s., coccus) are roughly spherical cells. They can exist as individual cells, but also are associated in characteristic arrangements that are frequently useful in bacterial identification. Diplococci (s., diplococcus) arise when cocci divide and remain together to form pairs. Rod, often called a bacillus (pl., bacilli).
  3. 3. Shapes of Bacterium Cocci – spherical bacteria Spirilla – spiral bacteria Bacilli – rod-shaped bacteria Vibrios – comma-shaped bacteria branching filamentous bacteria— lack cell wall -- flexible spiral forms Spirochetes Actinomycetes Mycoplasmas
  4. 4. Other shapes of bacteria Comma shaped Spirochetes Spirilla Most bacteria retain a particular shape; a few are pleiomorphic
  5. 5. Arrangement of bacteria: Cocci Cocci in pair – Diplococcus Sarcina – groups of eight Tetrad – groups of four Cocci in chain - Streptococci Cocci in cluster - Staphylococci Coccus 5
  6. 6. Size S. No. Bacteria Size 1 Escherichia coli 1.1 to 1.5 wide by 2.0to 6.0 µm 2 Mycoplasma 0.3µm in diameter 3 spirochaetes 500µm in length 4 Oscillatoria 7 µm in diameter Streptococcus 0.8 to 1.0µm 5 Acanthurus nigrofuscus (intestinal 600 by 80 µm 6 Epulopiscium fishelsoni 600 by 80 µm 7 Thiomargrita namibiensis Nanobacteria range from around 0.2 µm to less than 0.05 µm
  7. 7. Prokaryotic Cell Organization Plasma Membrane Selectively permeable barrier, mechanical, boundary of cell, nutrient and waste, transport, location of many metabolic, processes (respiration, photosynthesis), detection of environmental cues for chemotaxis, Gas Vacuole Buoyancy for floating in Aquatic environments. Ribosomes Protein synthesis Inclusion Bodies Storage of carbon, phosphate, and other substances Nucleiod Localization of genetic material (DNA) Cell wall Gives bacteria shape and protection from lysis in dilute solutions Periplasmic Space Contains hydrolytic enzymes and binding. Proteins for nutrient processing and uptake Capsule and slime layer Resistance to phagocytosis, adherence to surfaces Fimbriae and pili Attachment to surfaces, bacterial mating Flagella Movement Endospore Survival under harsh environmental conditions Bacterial and archaeal cell share common cell organization . Bacterial and archaeal Cells are surrounded by a cell envelope with complex cell walls; they lack many internal features common in eukaryotic cells
  8. 8. Procaryotic Cell Membranes • Membranes are an absolute requirement for all living organisms • Cell able to acquire nutrients and eliminate wastes but they also have to maintain their interior in a constant, highly organized state in the face of external changes. • The cell envelope includes the plasma membrane, cell wall, and other external layers of the cell .
  9. 9. •Bacterial Plasma Membranes The plasma membrane serves several functions: •It retains the cytoplasm and separates the cell from its environment. • It serves as a selectively permeable barrier. •It contains transport systems used for nutrient uptake, waste excretion, and pro secretion . •It is the location of a variety of crucial metabolic processes including respiratio photosynthesis, lipid synthesis, and cell wall synthesis. •It contains special receptor molecules that enable detection of and response to chemicals in the surroundings
  10. 10. The fluid mosaic model of membrane structure This model, proposed by Singer and Nicholson, states that membranes are lipid bilayers with floating proteins. Cell membranes are very thin (5–10 nm thick); the lipids are amphipathic, having hydrophilic (interact with water) head groups and long hydrophobic (insoluble in water) tails; the head groups face out of the membrane while the tails are buried in the membrane to form bilayers. Two types of proteins are associated with the lipid bilayer of the membrane: peripheral (loosely associated and easily removed) and integral (embedded within the membrane and not easily removed)
  11. 11. Bacterial Plasma Membranes
  12. 12. Bacterial lipids • The plasma membrane of bacteria consists of a phospholipid bilayer with hydrophilic surfaces and a hydrophobic interior; bacterial membranes lack sterols, but many contain sterol-like molecules called hopanoids that help stabilize the membrane • Bacteria do not have membranous organelles, but can have internal membrane systems with specialized functions such as photosynthesis or respiration
  13. 13. Bacterial Cell Walls • The cell wall is a rigid structure that lies outside the plasma membrane; it creates characteristic shapes for the bacteria and protects from osmotic lysis and toxins, often increasing pathogenicity. • Overview of bacterial cell wall structure – The cell walls of most bacteria contain peptidoglycan (murein). – The cell walls of gram-positive bacteria and gram- negative bacteria differ greatly, but both have the periplasmic space between the cell wall material and the plasma membrane
  14. 14. Bacterial Cell Walls
  15. 15. Peptidoglycan structure – Peptidoglycan is a polysaccharide polymer composed of two sugar derivatives with peptide linkers; the polysaccharide polymer is a linear chain of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid(NAM) moieties – Polysaccharide chains of peptidoglycan are cross-linked via a peptide interbridge attached to the sugar backbone via a short peptide chain; these peptides contain some amino acids not found in proteins; cross-linking adds strength to the peptidoglycan mesh – Variations in peptidoglycan structure are seen in certain bacterial groups and can be diagnostic – Archaea do not have peptidoglycan, they have pseudomurein
  16. 16.  They consist of a thick wall composed of many layers of peptidoglycan and large amounts of teichoic acids .  Techoic acids are polymers with a glycerol and phosphate backbone that span the cell wall and likely enhance its structural stability .  The periplasmic space of gram-positive cells is usually thin and contains only a few secreted proteins (exoenzymes).  Many gram-positive bacteria also have a layer of proteins (S-layer proteins) on the outer.  Surface of the peptidoglycan that have a role in wall synthesis and virulence.  Acid-fast bacteria include mycolic acids in their cell walls. Gram-positive cell walls
  17. 17. The Gram Positive Envelope Teichoic Acid Structure
  18. 18. Gram-negative cell walls  The gram-negative cell wall is more complex than the gram- positive cell wall; and has a thin layer of peptidoglycan surrounded by an outer membrane .  The periplasmic space is often wide and contains many different proteins; some are involved with energy conservation or nutrient acquisition .  The outer membrane is composed of lipids, lipoproteins, and lipopolysaccharides (LPS); lipoprotein attaches the outer membrane to the peptidoglycan .  LPS are large complex molecules composed of lipid A, core polysaccharides, and O antigen carbohydrate side.  The thinner, less cross-linked peptidoglycan layer of gram- negative bacteria does not retain the stain, and thus more readily decolorized when treated with alcohol
  19. 19. The Gram Negative Envelope
  20. 20. Cell walls and osmotic protection • The cell wall prevents swelling and lysis of bacteria in hypotonic solutions; in hypertonic habitats, the plasma membrane shrinks away from the cell wall in a process known as plasmolysis. • Bacteria without cell walls (by removal with lysozyme or through peptidoglycan synthesis inhibition by penicillin) called spheroplasts are osmotically sensitive. • Mycoplasmas lack a cell wall and tend to be pleomorphic.
  21. 21. Cell Envelope Layers Outside the Cell Wall
  22. 22. Cell Envelope Layers Outside the Cell Wall • Capsules and slime layers • Capsules and slime layers (also known as glycocalyx) are layers of polysaccharides lying outside the cell wall; they protect the bacteria from phagocytosis, desiccation, viral infection, and hydrophobic toxic materials such as detergents; they also aid bacterial attachment to surfaces and gliding motility • Capsules are well organized, whereas slime layers are diffuse and unorganized
  23. 23. S-layers • S-layers are regularly structured layers of protein or glycoprotein outside of the cell wall • S-layers protect against ion and pH fluctuations, osmotic stress, hydrolytic enzymes; can help maintain cell shape and envelope rigidity, promote cell adhesion, protect against host defenses • S-layers may be the only structural component to many of the archaea
  24. 24. Bacterial Cytoplasm A. The cytoplasmic matrix is the substance bounded by the plasma membrane; it is often packed with ribosomes and inclusion bodies B. The prokaryotic cytoskeleton has homologs of the elements seen in eukaryotes, filaments of actin, tubulin, and intermediate filament proteins C. Intracytoplasmic membranes are often involved in energy metabolism in nitrifying and photosynthetic bacteria
  25. 25. Inclusions • Many inclusions are granules of organic or inorganic material that are stockpiled by the cell for future use; some are not bounded by a membrane, but others are enclosed by a single-layered membrane . • Storage inclusions include glycogen (carbon storage), poly-- hydroxybutyrate (carbon storage), polyphosphate granules (energy and phosphorus storage), sulfur globules (wastes), and cyanophycin granules (nitrogen storage in cyanobacteria) • Carboxysomes are microcompartments that accumulate carbon dioxide and the enzyme ribulose-1,5-bisphosophate carboxylase • Gas vacuoles are composed of hollow protein sacs that are filled with gases and used for buoyancy control in aquatic environments; magnetosomes are magnetite granules that provide orientation in the Earth's magnetic field
  26. 26. Microcompartments • These are not used just for storing substances for later use by the cell. • Relatively large polyhedrom formed by one or more different proteins. • Best studied are the carboxysomes, found in many of the cyanobacteria and other CO2 fixing bacteria.
  27. 27. Bacterial Ribosomes • Ribosomes are the site of protein synthesis (translation) • They are complex structures consisting of protein and rRNA (ribosomal RNA) • Bacterial and archaeal ribosomes are 70S with 50S and 30S subunits; although differences are apparent in ribosomal protein and rRNA components, ribosomes are similar across cells from all three Domains (eukaryotic cells have 80S ribosomes)
  28. 28. • The procaryotic chromosome is located in an irregularly shaped region called the nucleoid. • Procaryotes contain a single circle of double- stranded deoxyribonucleic acid (DNA). • The nucleoid is visible in the light microscope and electron microscope after staining with the Feulgen stain, which specifically reacts with DNA. The Nucleoid
  29. 29. Plasmids • Many bacteria possess plasmids in addition to their chromo-some. • These are double-stranded DNA molecules, usually circular. • Plasmid can exist and replicate independently of the chromosome or may be integrated with it. • Plasmids are not re-quired for host growth and reproduction, although they may carry genes that give their bacterial host a selective advantage.
  30. 30. External Structures A. Pili and fimbriae are short, thin, hairlike appendages that mediate bacterial attachment to surfaces (fimbriae) or to other bacteria during sexual mating (sex pili); fimbriae tend are narrower in diameter and shorter than sex pili B. Flagella 1. Flagella are threadlike locomotor appendages extending outward from the plasma membrane and cell wall; they may be arranged in various patterns: a. Monotrichous—a single flagellum b. Amphitrichous—a single flagellum at each pole c. Lophotrichous—a cluster (tuft) of flagella at one or both ends d. Peritrichous—a relatively even distribution of flagella over the entire surface 2. The flagellum consists of a hollow filament composed of a single protein known as flagellin; the hook is a short, curved segment that links the filament to the basal body, a series of rings that drives flagellar rotation
  31. 31. Bacterial Motility and Chemotaxis • A. Motility 1. Motility in prokaryotes is not aimless; responses are made to temperature, light, oxygen, Osmotic pressure, and gravity; chemotaxis is directed movement of bacteria either toward a chemical attractant or away from a chemical repellent 2. Prokaryotic flagella rotate to create motion (like a propeller); the direction of flagellar rotation determines the nature of bacterial movement: counterclockwise rotation causes forward motion (called a run) and clockwise rotation disrupts forward motion (resulting in a tumble) 3. The basal body is the motor that drives the flagellum; it is powered by a proton motive force 4. Prokaryotes can move by other mechanisms: in spirochetes, axial fibrils cause movement by flexing and spinning; other prokaryotes exhibit twitching or gliding motility, a mechanism involving pili by which they move along solid surfaces with the help of the slime layer
  32. 32. Chemotaxis 1. The concentrations of attractants and repellents are detected by chemoreceptors in the periplasmic space or the plasma membrane 2. Directional travel toward a chemoattractant is caused by lowering the frequency of tumbles (twiddles), thereby lengthening the runs when traveling up the gradient, but allowing tumbling to occur at normal frequency when traveling down the gradient 3.Directional travel away from a chemorepellent involves similar but opposite responses
  33. 33. Bacterial Endospores A. The bacterial endospore is a special, resistant, dormant structure formed by some bacteria; it enables them to resist harsh environmental conditions B. Endospore structure is complex, consisting of an outer covering called the exosporium, a spore coat beneath the exosporium, the cortex beneath the spore coat, and the spore cell wall, which is inside the cortex and surrounds the core C. Endospore formation (sporulation) normally commences when growth ceases because of lack of nutrients; it is a complex, multistage process D. Transformation of dormant endospores into active vegetative cells is also a complex, multistage process that includes activation (preparation) of the endospore, germination (breaking of the endospore’s dormant state), and outgrowth (emergence of the new vegetative cell)

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