Prokaryotic Cell Structure PDF

Course: General Microbiology (BIOE 230) Prokaryotic cell structure Dr. Sunil Mundra Assistant Professor [email protected]...

Course: General Microbiology (BIOE 230) Prokaryotic cell structure Dr. Sunil Mundra Assistant Professor [email protected] Department of Biology msunilmundra UAE University, Al-ain Cell theory Cells were discovered in 1665 Cell Theory by Robert Hooke. 1. All organisms are composed of cells. 2. Cells are the smallest living things. Early studies of cells were 3. Cells arise only from pre-existing cells. conducted by All cells today represent a continuous line of - Mathias Schleiden (1838) descent from the first living cells. - Theodor Schwann (1839) Schleiden and Schwann proposed the Cell Theory. Cell theory Microscopes are required to Cell Theory visualize cells. All cells have certain structures in common. Light microscopes can resolve 1. genetic material – in a nucleoid or nucleus structures that are 200nm apart. 2. cytoplasm – a semifluid matrix 3. plasma membrane – a phospholipid bilayer Electron microscopes can resolve structures that are 0.2nm apart What is cell? The cell is the fundamental unit of any living organism because it exhibits the basic characteristics of life. There are two categories of cells: 1. Prokaryotic Bacteria and archaea Originally defined in 1962 by the characteristics they lack compared to eukaryotes. 2. Eukaryotic algae, protozoa, fungi Some are not composed of cells viruses, prions, viroids What is cell Prokaryotic Cells Eukaryotic Cells Prokaryotic cells do not possess Contain a “true” nucleus, a complex system of A true nucleus consists of nucleoplasm, membranes and membrane- chromosomes, and a nuclear membrane. bound organelles. Possess a complex system of membranes Lack membrane-bound and membrane-bound organelles nucleus, a cytoskeleton, membrane-bound organelles, and internal membranous structures. Since the 1960s, biochemical, genetic, and genomic analyses have shown that Bacteria and Archaea are distinct from each other. Cocci shape and arrangement The microbial world has a variety of morphologies. Shape Cocci—spherical cells Rods—oblong cells Arrangement Determined by plane of division. Determined by separation or not. Size—varies (a) ©Science Source; (b, c) Source: CDC/Janice Haney Carr Cocci shape and arrangement Cocci (s., coccus)—spheres that can be single or can be associated in arrangements that is useful for identification. Diplococci (s., diplococcus)—divide and remain in pairs. Streptococci—divide on 1 plane to form chains. Staphylococci—divide in random planes making grape-like clusters. Tetrads—divide in 2 planes forming a square of 4 cocci. Sarcina—divide in 3 planes making cubic packet of 8 cocci. (a) ©Science Source; (b, c) Source: CDC/Janice Haney Carr Other shapes and arrangement Bacilli (s., bacillus)—rods. Coccobacilli—very short rods. Vibrios—resemble rods, comma shaped. Spirilla (s., spirillum)—rigid helices. Spirochetes—flexible helices. Mycelium—network of long, multinucleate filaments. Pleomorphic—organisms that are variable in shape. (a) ©Science Source; (b, c) Source: CDC/Janice Haney Carr Cell size Smallest—0.3 μm (Mycoplasma). Average rod—1.1 to 1.5 by 2 to 6 μm (E. coli). Very large—600 by 80 μm (Epulopiscium fishelsoni). The range of sizes shown by prokaryotes, relative to those of other organisms and biomolecules 1 m = 100 cm = 1,000mm = 1,000,000 µm = 1,000,000,000nm 1mm = 1000 µm = 1000000nm 1 µm = 1000nm Prokaryotic cells First cell type on earth Prokaryotic cells possess Cell envelope—3 layers Cytoplasm Internal structures -genetic material in the nucleoid -ribosomes -no membrane-bound organelles External structures -Pilli -Flagella Bacterial cell envelop 1. Plasma membrane. 2. Cell wall. 3. Layers outside the cell wall. Capsule. Slime layer. 1. Glycocalys/capsule Slimy layer of polysaccharides outside cell wall Capsules adhere to solid surfaces and to nutrients in the environment. Adhesive power of capsules is a major factor in the initiation of some bacterial diseases. Protective advantages. o Resistant to phagocytosis. o Protect from desiccation. o Exclude viruses and detergents. 2. Cell wall Cell wall functions. Maintains shape of the bacterium. Helps protect cell from osmotic lysis and toxic materials. May contribute to pathogenicity. Peptidoglycan (murein). Rigid structure lying just outside the cell plasma membrane. Two types of bacteria based on Gram staining of peptidoglycan. Gram-positive and Gram-negative: Mycoplasma spp. do not have a cell wall; they are pleomorphic Archaean cell walls lack peptidoglycan. Prokaryotic cell wall Peptidoglycan is a huge polymer of interlocking chains of identical peptidoglycan monomers. Provides rigid support while freely permeable to solutes. Backbone composed of two derivatives of glucose: - N-acetylglucosamine (NAG) - N-acetlymuramic acid (NAM) NAG / NAM strands are connected by inter-peptide bridges. Strands are crosslinked Peptidoglycan strands have a helical shape. Peptidoglycan chains are crosslinked by peptides for strength. Interbridges may form. Peptidoglycan sacs—interconnected networks. Various structures occur.  Prokaryotic cell wall Gram +ve. Gram -ve Thick peptidoglycan Thin peptidoglycan Outer membrane Teichoic acids Periplasmic space  Mechanism of Gram stain reaction Gram stain reaction due to nature of cell wall. Shrinkage of the pores of peptidoglycan layer of Gram-positive cells. Constriction prevents loss of crystal violet during decolorization step. Thinner peptidoglycan layer and larger pores of Gram-negative bacteria do not prevent loss of crystal violet. Alcohol may also remove/extract some lipids from outer layer of Gram- negative cell wall, making crystal violet dye removal easier. Two types of bacteria based on Gram stain. Gram-positive: stain purple; thick peptidoglycan Monoderm—single membrane Gram-negative: stain pink or red; thin peptidoglycan and outer membrane. Diderm—plasma membrane and an outer membrane Gram-Positive Cell Walls Composed primarily of peptidoglycan. May also contain teichoic acids (negatively charged). Polymers of glycerol. Help maintain cell envelope. Protect from environmental substances. May bind to host cells to initiate infection. Egbert Hoiczyk 18 Gram-Negative Cell Wall Basic Structure More complex than Gram-positive. Consist of a thin layer of peptidoglycan surrounded by an outer membrane. Outer membrane composed of lipids, lipoproteins, and lipopolysaccharides. No teichoic acids. Egbert Hoiczyk 19 Cell Walls and Osmotic Protection Hypotonic environments Solute concentration outside cell less than inside cell. Water moves into cell and cell swells. Cell wall protects from lysis. Hypertonic environments Solute concentration outside cell is greater than inside. Water leaves cell and cytoplasm shrivels up. Plasmolysis Cells That Lose a Cell Wall May Survive in Isotonic Environments Protoplast—Gram-positive cells that lose cell wall in isotonic environments. Spheroplast—Gram-positive cells that lose cell wall (outer membrane remains) in isotonic environments. 20 Cells That Lose a Cell Wall May Survive in Isotonic Environments Protoplast—Gram-positive cells that lose cell wall in isotonic environments. Spheroplast—Gram-positive cells that lose cell wall (outer membrane remains) in isotonic environments. Mycoplasma Lack a cell wall, but plasma membrane more resistant to osmotic pressure. 21 3. Cell membrane Most important membrane that is required for all living organisms. Innermost membrane that encompasses the cytoplasm. Selectively permeable barrier that acquires nutrients and eliminates waste. Many enzymes are attached to the cell membrane Encompasses the cytoplasm; absolute requirement for all living organisms. Interacts with external environment. o Receptors for detection of and response to chemicals in surroundings. o Transport systems. o Metabolic processes.  Membrane are lipid bilayer with floating proteins Membrane proteins Protein embedded in two layers of lipids (lipid bilayer). Peripheral protein —Loosely connected to membrane and can not be easily removed; 20 to 30% of the total membrane proteins. Integral protein —Amphipathic (embedded within membrane and not easily removed); carry out important functions. Amphipathic lipids Polar ends (hydrophilic—interact with water and present of surface). Non-polar tails (hydrophobic—insoluble in water and buried inside menbrane).  Structure of a Phospholipid and other bacterial lipids The plasma membrane is mainly composed of phospholipids. Other smaller lipids: Hopanoids—Hydrophobic molecule similar to cholesterol. Distort the bilayer, which impacts the fluidity and shape in membrane region. Form functional membrane microdomains that are platforms for protein complex assembly. 24 Nutrients Macronutrients—required in large amounts. Found in organic molecules (that is, proteins, lipids, nucleic acids, and carbohydrates). Cations contribute to activity and stability of molecules and cell structures. Important in cellular processes (that is, protein synthesis). Micronutrients—required in small amounts. Ubiquitous in nature and usually present in adequate amounts to support microbial growth. Work to assist enzyme catalysis and maintain protein structure. 25 Growth factors Organic compounds required for survival. Essential cell components (or their precursors) that the cell cannot synthesize and must be supplied by environment. Macronutrients Cationic Macronutrients Micronutrients Growth Factors Carbon Potassium Manganese Amino acids Oxygen Calcium Zinc Purines Hydrogen Magnesium Cobalt Pyrimidines Nitrogen Iron Molybdenum Vitamins Sulfur Nickel Phosphorus Copper 26 Methods for Uptake of Nutrients Microbes can only take in dissolved particles across a selectively permeable membrane. Microorganisms use transport mechanisms. Passive diffusion Facilitated diffusion Primary and secondary active transport Group translocation 27 Passive Diffusion Molecules move from a region of higher concentration to one of lower concentration. Requires a large concentration gradient for adequate nutrient uptake. The rate of diffusion decreases as more nutrients accumulate in the cell. H2O, O2, and CO2 easily cross the plasma membrane via passive diffusion. 28 Facilitated Diffusion Movement across the plasma membrane with the help of transport proteins. Channels—proteins that form pores for substances to pass through. Carriers—proteins that have high substrate specificity in transport. Truly diffusion because it is not energy dependent. Direction of movement is from high to low concentration. Size of concentration gradient impacts rate of uptake. Rate increases with the concentration gradient. If the gradient is lost, transport stops. 29 Facilitated versus Passive Diffusion Access the text alternative for slide images. 30 Active Transport Transport of molecules against the concentration gradient. Energy-dependent process. ATP or proton motive force used. Involves carrier proteins that control the rate of transport. When the solute concentration is high, carrier saturation effect is observed. 31 Primary Active Transport Use energy from A T P hydrolysis to move substances against concentration gradient without modifying them. Uniporters—single molecule transported across membrane. A T P-binding cassette (A B C) transporters Consist of: 2 hydrophobic membrane spanning domains. 2 cytoplasmic associated A T P- binding domains. Access the text alternative for slide images. 32 Secondary Active Transport Use potential energy of ion gradients to cotransport substances without modifying them. Move both the ion and the substance across the membrane. Symport—2 substances both move in the same direction. Antiport—2 substances move in opposite directions. Access the text alternative for slide images. 33 Group Translocation Energy dependent transport that chemically modifies the molecule as it is brought into cell. Best known translocation system is phosphoenolpyruvate: sugar phosphotransferase system (P T S). Imports sugars while phosphorylating them. 34 Iron Uptake Microorganisms require iron for building molecules important in energy-conserving processes. Ferric iron is very insoluble so uptake is difficult. Siderophores—secreted by bacteria and complex with ferric ion for transport into cell. Access the text alternative for slide images. 35 Bacterial cytoplasmic structure Cytoskeleton. Inclusions and gas vacuole Ribosomes. Nucleoid. Plasmids. Protoplast—plasma membrane and everything within. Cytoplasm—material bounded by the plasma membrane. Protoplasm and cytoplasm Protoplast Plasma membrane and everything within. Cytoplasm Semi-liquid that consists of water, enzymes, waste products, nutrients, proteins, carbohydrates and lipids – materials required for metabolic functions Bacterial cytoskeleton Homologs of all 3 eukaryotic cytoskeletal elements have been identified in bacteria. Actin filaments - responsible for cellular contractions, crawling, “pinching” Microtubules – provide organization to the cell and move materials within the cell Intermediate filaments – provide structural stability Functions are similar as in eukaryotes Participate in cell division Localize proteins Determine cell shape and provide mechanical support to cell Anchor organelles Help move substances Gas vacuoles Found in aquatic, photosynthetic bacteria and archaea. Provide buoyancy in gas vesicles. Involved in bacterial movement. Made of aggregates of hollow, cylindrical gas vesicles. ©Daniel Branton/Harvard University Ribosomes – Protein factory Found within the cytosol of the cytoplasm and attached to internal membranes Complex protein/RNA structures. Sites of protein synthesis. Bacterial and archaea ribosome = 70S. Bacterial ribosomal RNA. 16S small subunit. 23S and 5S in large subunit. The nucleoid Region of DNA concentration and not membrane bound (few exceptions). Location of chromosome and associated proteins. Prokaryotic chromosome usually consists of a single, long, supercoiled, circular DNA molecule – serves as the control center of the cell (3000 genes) Supercoiling and nucleoid proteins (different from histones) aid in folding. Plasmid Extrachromosomal DNA. - Usually small, closed circular DNA molecules (5 - 100 genes) Exist and replicate independently of chromosome. Episomes - may integrate into chromosome. Inherited during cell division. Not critical to everyday functions. Can provide genetic information to promote: - Antibiotic resistance - Promote conjugation (transfer of genetic material between bacteria through cell-to- cell contact) Cell conjugation Transfer of genetic material between bacteria through cell-to-cell contact) External structure Extend beyond the cell envelope in bacteria. Function in protection, attachment to surfaces, horizontal gene transfer, cell movement. Pili and fimbriae. Flagella. Cell appendages Fimbriae (s., fimbria); pili (s., pilus). Short, thin, hairlike, protein appendages (up to 1,000/cell). Can mediate attachment to surfaces, motility, DNA uptake. Sex pili (s., pilus). Longer, thicker, less numerous (1 to 10/cell). Genes for formation on plasmids. Required for conjugation. Flagella Threadlike, locomotor appendages extending outward from plasma membrane and cell wall. Functions. Motility and swarming behavior. Attachment to surfaces. May be virulence factors. Patterns of flagellation. Monotrichous—one flagellum. Polar flagellum—flagellum at end of cell. Amphitrichous—one flagellum at each end of cell. Lophotrichous—cluster of flagella at one or both ends. Peritrichous—spread over entire surface of cell. Flagella Thin, rigid protein structures that cannot be observed with bright-field microscope unless specially stained. Ultrastructure composed of 3 parts: Filament—extends from cell surface to the tip. Basal body—embedded in cell envelope. Hook—short curved segment. Motility Flagellar movement. Swarming Spirochete motility. Twitching and gliding motility. Chemotaxis. Move toward chemical attractants such as nutrients, away from harmful substances. Move in response to temperature, light, oxygen, osmotic pressure, and gravity. Swimming Flagellum rotates like a propeller. Very rapid rotation up to 1100 revolutions/sec. In general, counterclockwise (C C W) rotation causes forward motion (run). In general, clockwise rotation (C W) disrupts run causing cell to stop and tumble. 49 Swarming Occurs on when cells move in unison across a moist surfaces. Most swarmers have peritrichous flagella. Commonly, the cell produces a molecule that lowers surface tension. Dr. Daniel Kearn 50 Spirochete Motility Undulation of the entire cell. Multiple flagella form axial fibril which winds around the cell. Flagella remain in inside the cell wall. Corkscrew shape exhibits flexing and spinning movements. Jacques Izard 51 Twitching and Gliding Motility Occurs on solid surface. Does not involve flagella. May involve Type IV pili and slime. Twitching motility Pili at ends of cell. Short, intermittent, jerky motions. Cells are in contact with each other and surface. Gliding Smooth movements that do not require appendages. 52  Endospore structure Spore surrounded by thin covering called exosporium. Thick layers of protein form the spore coat. Cortex, beneath the coat, thick peptidoglycan. An endospore stained bacterial smear of Bacillus subtilis showing endospores as green Core has nucleoid and ribosomes. and vegetative cells as red. Endospore Formation Access the text alternative for slide images. 54 Formation of Vegetative Cell Three Stages: Activation Prepares endospores for germination. Germination Starts when germinant receptors detect small molecules (that is, sugars and amino acids). Outgrowth Emergence of vegetative cell. 55 Cell reproduction Binary Fission Prokaryotic Cell Reproduction Prokaryotic cells reproduce by a process known as binary fission – one cell splits in half to become two daughter cells. Before a prokaryotic cell divides in half, the chromosome must be duplicated. The time it takes for binary fission to occur is called the generation time. Generation time varies from one species to another and depends on growth conditions (under ideal conditions, E. coli has a generation time of about 20 minutes). Revision: cell structure 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 An inclusion that provides buoyancy for floating in aquatic environments Ribosomes Protein synthesis Inclusions Storage of carbon, phosphate, and other substances; site of chemical reactions (microcompartments); movement Nucleoid Localization of genetic material (DNA) Periplasmic space In typical Gram-negative bacteria, contains hydrolytic enzymes and binding proteins for nutrient processing and uptake; in typical Gram-positive bacteria, may be smaller or absent Cell wall Protection from osmotic stress, helps maintain cell shape Capsules and slime layers Resistance to phagocytosis, adherence to surfaces Fimbriae and pili Attachment to surfaces, bacterial conjugation and transformation, twitching Flagella Swimming and swarming motility Endospore Survival under harsh environmental conditions Take Home Message Bacteria can have many different structures, but there are some fundamental items that are present in virtually all of them. As you continue on to examine the other two domains of life, be thinking about what is similar and what is different between them. Structure always determines function—so be thinking about how different cellular structures (bacterial in this case) give the cells particular functions for their particular environments or needs. 58