华中农业大学微生物学第二章.ppt

Chapter 2 Cell Biology.2.1 Overview of the structure of microbial cells 2.2 Procaryotic cell wall 2.3 Cytoplasmic membrane 2.4 Cellular genetic information 2.5 Cytoplasmic matrix Ribosome and Inclusions 2.6 Components external to the cell wall 2.7 Bacterial endospores 2.8 Comparison of the prokaryotic and eukaryotic cellChapter Outline.ConceptsProkaryotes are small and simple in structure when compared with eukaryotes,yet they often have characteristic shape and size.Prokaryotic genetic material is located in an area called the nucleoid and is not enclosed by a membrane.The prokaryotic cell wall almost always has peptidoglycan and is chemically and morphologically complex.A procaryotic cellA eucaryotic cell 2.1 Overview of Cell Structure.3.Their cell wall almost always contain the complex polysaccharide peptidoglycanTheProkaryoticCell1.Their genetic material(DNA)is not enclosed within a membrane and they lack other membrane bounded organelles2.Their DNA is not associated with histidine4.They are very small!.Schematic diagram of a bacterial cell.Size:Mostbacteriafallwithinarangefrom0.2to2.0umindiameterandfrom2to8uminlengthA rod-shaped prokaryote is typically about 1-5 micrometers(m)long and about 1 m wideMicroorganisms in general are very small and are completely invisible to the naked eye.A cyanobacterium 8 x 50 um.size comparison of microorganismsVisibility scale Meters Relative size of MicrobesProkaryotesEukaryotesVirusesNaked eyeLight microscopeElectron microscope.a cell increases in size,its surface area to volume ratio decreasesSurface area and volume relationships in cells.spirallumShape:Bacteria have a few basic shapesspherical coccusRod-shaped bacillus.2.2 Prokaryotic Cell WallCell Wall.The cell wall of the bacterial cell is a complex,semi-rigid structure that is responsible for the characteristic shape of the cell.The cell wall surrounds the underlying,fragile plasma(cytoplasmic)membrane and protects it and internal parts of the cell from adverse changes in the surrounding environment.Almost all prokaryotes have cell walls.Gram+Gram-Schematic diagram of bacterial cell wallsBacteria can be divided into two major groups,called gram-positive and gram-negative.The original distinction between gram-positive and gram-negative was based on a special staining procedure,the Gram stain.The Gram-positive cell wall has a peptidoglycan layer that is relatively thick(ca.40 nm)and comprises approximately 90%of the cell wall.The cell walls of most Gram-positive eubacteria also have teichoic acids.Gram-positiveCellWall.StructureoftheRepeatingUnitinPeptidoglycanThese constituents are connected to form a repeating structure,the glycan tetrapeptide.Peptidoglycan is composed of two sugar derivatives,N-acetylglucosamine(NAG)and N-acetylmuramic acid(NAM),and a small group of amino acids consisting of L-alanine,D-alanine,D-glutamic acid,and either lysine or diaminopimelic acid(DAP).Peptide and glycan units are connected in formation of the peptidoglycan sheet.Gram-positive Bacteria frequently have acidic polysaccharides called teichoic acids attached to their cell wall.The term teichoic acids includes all wall,membrane,or capsular polymers containing glycerophosphate or ribitol phosphate residues.These polyalcohols are connected by phosphate esters and usually have other sugars and D-alanine attached.TeichoicAcids.Teichoic acids and lipoteichoic acids are arranged in the overall wall structure of gram-positive Bacteria.TeichoicacidLipoteichoicacid.The Gram-negative cell wall is a thin layer attached to an outer membrane via lipoproteins.The outer membrane contains phospholipid on its inner surface and lipopolysaccharide(LPS)on its outer surface.The space between the outer membrane and the cytoplasmic membrane is called the periplasmic space.Teichoic acids do not occur in Gram-negative bacterial cell walls.O side chainCore polysaccharideLipid AChemical Structure of Lipopolysaccharide.Molecular Model of E.coli Lipopolysaccharide.The bonds between the carbohydrates in pseudopeptidoglycan are 1-3 instead of1-4 as in peptidoglycan.Cell Walls of ArchaebacteriaThe archaebacteria do not contain peptidoglycan in their cell walls as occurs in eubacteria.N-acetylmuramic acid and D-amino acids are not found in the cell walls of archaebacteria.(Differences from eubacteria).Some archaebacteria have walls composed of pseudopeptidoglycan,which resembles the peptidoglycan of eubacteria but contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid and L.-amino acids instead of the D-amino acids in eubacterial cell walls.Protoplast Formation Peptidoglycancan be destroyed by certain agents for instancelysozyme,that breaks the 1,4-glycosidic bonds between N-acetylglucosamine and N-acetylmuramic acid in the molecule.The difference between gram-positive and gram-negative bacteria is due to the physical nature of their cell walls.If the cell wall is removed from gram-positive bacteria,they become gram negative.The peptidoglycan seems to act as a permeability barrier preventing loss of crystal violet.Gram-negative peptidoglycan is very thin,not as highly cross-linked,and has larger pores.Alcohol treatment also may extract enough lipid from the gram negative wall to further increase its porosity.For these reasons,alcohol more readily removes the purple crystal violet-iodine complex from gram-negative bacteria.The Mechanism of Gram Staining.Procedures of Gram Staining.Gram positive or Gram negative?.Structure of cytoplasmic membrane 2.3 Cytoplasmic Membrane Function of cytoplasmic membraneMembrane.B.Colorized electron micrograph of the cytoplasmic membrane(CM)of the bacterium Bacillus subtilis reveals the characteristic railroad track appearance of this lipid bilayer.Structure of Cytoplasmic MembraneIt is a typical UNIT MEMBRANE!A.The typical cytoplasmic membrane of prokaryotic and eukaryotic cells is a lipid bilayer,as illustrated here showing the orientations of the hydrophilic(tan spheres)and hydrophobic(black)ends of phospholipids that make up this structure.The cytoplasmic membrane,a highly selective barrier,is constructed principally of lipid,within which certain proteins are embedded.Membranes contain both lipids and proteins,although the exact proportions of lipid and protein vary widely.Diagramofthestructureofcytoplasmicmembrane.3.Energyconservation-siteofgenerationanduseoftheprotonmotiveforce.Function of Membrane1.Permeabilitybarrier-preventsleakageandfunctionasgatewayfortransportofnutrientsintoandoutofthecell.2.Proteinanchor-siteofmanyproteinsinvolvedintransport,bioenergetics,andchemotaxis.Rateofpermeability*100 0.1 0.001 0.001 0.000001 0.0000001 0.00000001 SubstanceWaterGlycerolTryptophanGlucoseChlorideion(Cl-)Patassiumion(K+)Sodiumion(Na+)*Relativerate:Permeabilitywithrespecttopermeabilityofwatergivenas100Comparative permeability of membrane molecules to various.Intracellular Membrane SystemBacteria cells dont contain membrane-enclosed organelles.However,bacteria may have specialized invaginations of the cytoplasmic membrane.Their function may be to provide a larger membrane surface for greater metabolic activity.Structure of MesosomeMesosome may be involved in wall formation during division or play a role in chromosome replication and distribution to daughter cells.It may also be involved in secretory processes.2.4 Cellular Genetic InformationNucleoid.1.Bacterial ChromosomeSupercoiling and chromosome structureChromosomal copy number2.Plasmids.Micrograph of a bacterium showing the nucleoid region(green)within the cytoplasm where the bacterial chromosome occurs.The bacterial chromosome is a circular DNA macromolecule except in Streptomyces where it is linear and Rhodobacter sphaffoides,which has two separate chromosomes.Bacterial ChromosomeThe bacterial chromosome is usually a single covalently closed circular molecule.The term nucleoid is used to describe aggregated DNA in the prokaryotic cell.Range of genome sizes in virous groups of organisms and the organellesof eukarya.Thebacterialchromosomeandsupercoiling:.Example of E.coli cellThere are over 50 supercoiled domain in the E.coli chromosome.The total amount of DNA is about 4600 kb.If the total DNA is opened and linearized,it would be 1 mm in length.The the cell is only about 2-3 um long.So to package this much DNA into the cell requires that the DNA be highly supercoiled.Electron micrograph of an isolated nucleoid released from E.coli.Chromosomecopynumber Bacteria that reproduce asexually are typically haploid in genetic complement.Rapidly growing cells contain more than 1 copy of the chromosome,and only when cell growth has ceased does the chromosome number approach one per cell.Reproductionofabacterialcellrequiresthereplicationofthebacterialchromosome.ThemicrographshowsthesequenceofsynthesisofnewcircularloopsofdoublehelicalDNA.Bacteria normally reproduce by binary fission.The inward growth of the septum divides the parent cell to produce two equal-sized progeny cells.Plasmids dont contain the genetic information for the essential metabolic activities of the cell,but they generally do contain genetic information for special features.PlasmidProkaryotic cells have small extra-chromosomal genetic elements called plasmids.Resistant plasmidsCol plasmidsConjugative plasmidsMetabolic plasmids Major Types of Plasmids.2.5 Cytoplasmic Matrix Ribosome and InclusionsRibosomeCarbon storage polymers PHB and glycogenPhosphate polymersSulfur Granules Gas Vacuoles.All eucaryotic and procaryotic cells contain ribosomes,which function as the sites of protein synthesis.Ribosomes are composed of two subunits Procaryotic ribosomes are called 70S ribosomes,and those of eucaryotic cells are known as 80S ribosomes RibosomesThe letter S refers to Svedberg units,which indicate the relative rate of sedimentation during ultra-high-speed centrifugation.Within the cytoplasm of procaryotic(and eucaryotic)cells are several kinds of reserve deposits,known as inclusions.Some inclusions are common to a wide variety of bacteria,whereas others are limited to a small number of species and therefore serve as a basis for identification.Among the more prominent bacterial inclusions are the following:Inclusion Carbon storage polymers PHB and glycogen Phosphate polymers Sulfur Granules Gas Vacuoles.Polyhydroxybutyricacid(PHB)PHB is a lipidlike compound-one of the most common inclusion bodies in prokaryotic organisms.PHB is commonly found as a storage material and unique to bacteria Glycogen is a starchlike polymer of glucose subunits.Glycogen granules are usually smaller than PHB granules.A Vibrio species.Many microorganisms accumulate granules of polyphosphate,which are large reserves of inorganic phosphates that can be used in the synthesis of ATPPolyphosphategranuleinabacterialcellA Pseudomonas species.ThesulfurglobulesinsidethecellsofpurplesulfurbacteriumChromatium buderiSome bacteria,including many photosynthetic bacteria,accumulate elemental sulfur granules as a result of their metabolism.Gasvacuoles(blue)andstoragegranules(red)inthecyanobacteriumMicrocystisThe formation of gas vacuoles by aquatic bacteria provides a mechanism for adjusting the buoyancy of the cell.Many aquatic cyanobacteria use their gas vacuoles to move up and down in the water column.2.6 Components external to the cell wall Flagella Fimbriae and pili Capsules and slime layers.Motility allows the cell to reach different regions of its environment.In the struggle for survival,movement to a new location may mean the difference between survival and death of the cell.But,as in any physical process,cell movement is closely tied to an energy expenditure,and the movement of flagella is no exception.Many prokaryotes are motile,and this ability to move independently is usually due to a special structure,the flagellum(plural,flagella).Four basic types of flagellar arrangementsa.monotrichousb.amphitrichousc.lophotrichousd.peritrichous.Flagella are arranged differently on different bacteria.In polar flagellation the flagella are attached at one or both ends of the cell.Occasionally a tuft(group)of flagella may arise at one end of the cell,an arrangement called lophotrichous.In peritrichous flagellation the flagella are inserted at many places around the cell surface(peri means around).The type of flagellation,polar or peritrichous,is often used as a characteristic in the classification of bacteria.TheflagellumofaGram-negativebacterium.The filament of bacterial flagella is composed of subunits of a protein called flagellin.The base of the flagellum is different in structure from that of the filament.There is a wider region at the base of the flagellum called the hook.The hook consists of a single type of protein and functions to connect the filament to the motor portion of the flagellum.The basal body is anchored in the cytoplasmic membrane and cell wall.The basal body consists of a small central rod that passes through a system of rings.In gram-positive Bacteria,which lack the outer lipopolysaccharide layer,only the inner pair of rings is present.Surrounding the inner ring and anchored in the cytoplasmic membrane are a pair of proteins called Mot.These proteins actually drive the flagellar motor causing a torque that rotates the filament.A final set of proteins,called the Fli proteins function as the motor switch,reversing rotation of the flagella in response to intracellular signals.In gram-negative Bacteria,an outer ring is anchored in the lipopolysaccharide layer and another in the peptidoglycan layer of the cell wall,and an inner ring is located within the cytoplasmic membrane.The movement of a procaryotic flagellum results from rotation of its basal body and is similar to the movement of the shaft of an electric motor.Bacterial cells can alter the speed and direction of rotation of flagella and thus are capable of various patterns of motility.Fimbriae can be evenly distributed over the entire surface of the cell.They can number anywhere from a few to several hundred per cell.Fimbriae allow a cell to adhere to surfaces including the surfaces of other cells.Piliare usually longer than fimbriae and number only one or two per cell.Pili function to join bacterial cells prior to the transfer of DNA from one cell to another.F pilus.vMany prokaryotes contain a cell surface layer composed of a two-dimensional array of protein.These layers are called S-layers.vS-layers have been detected in representatives of virtually every phylogenetic grouping of Bacteria and are nearly universal among Archaea.vIn some species of Archaea the S-layer is also the cell wall.Paracrystalline Surface Layers(S-Layers).vThe major function of S-layers is unknown.vAs the interface between the cell and its environment,it is likely that in cells that produce them the S-layer at least functions as an external permeability barrier,allowing the passage of low-molecular-weight substances while excluding large molecules.2.7 Bacterial EndosporesCertain species of bacteria produce special st