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1、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 prokaryot
2、ic and eukaryotic cellChapter outlineA procaryotic cellA eucaryotic cell Overview of cell structure3.Their cell wall almost always contain the complex polysaccharide peptidoglycanThe prokaryotic cell1.Their genetic material(DNA)is not enclosed within a membrane and they lack other membrane bounded o
3、rganelles2.Their DNA is not associated with histidine4.They are very small!Size:Most bacteria fall within a range from 0.2 to 2.0 um in diameter and from 2 to 8 um in lengthA rod-shaped prokaryote is typically about 1-5 micrometers(m)long and about 1 m wideMicroorganisms in general are very small an
4、d are completely invisible to the naked eye.A cyanobacterium 8 x 50 umsize comparison of microorganismsVisibility scale Meters Relative size of MicrobesProkaryotesEukaryotesVirusesNaked eyeLight microscopeElectron microscopea cell increases in size,its surface area to volume ratio decreasesSurface a
5、rea and volume relationships in cellsspirallumShape:Bacteria have a few basic shapesspherical coccusRod-shaped bacillusThe 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 pla
6、sma(cytoplasmic)membrane and protects it and internal parts of the cell from adverse changes in the surrounding environment.Almost all prokaryotes have cell walls.Prokaryotic cell wallGram+Gram-Schematic diagram of bacterial cell wallsBacteria can be divided into two major groups,called gram-positiv
7、e 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
8、 Gram-positive eubacteria also have teichoic acids.Gram-positive cell wallStructure of the Repeating Unit in PeptidoglycanThese constituents are connected to form a repeating structure,the glycan tetrapeptide.Peptidoglycan is composed of two sugar derivatives,N-acetylglucosamine(NAG)and N-acetylmura
9、mic 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 sheetGram-positive Bacteria frequently have acidic polysaccharides called teichoic a
10、cids 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.Teichoic acidsTeichoic acids an
11、d lipoteichoic acids are arranged in the overall wall structure of gram-positive Bacteria.Teichoic acidLipoteichoic acidThe 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)
12、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 LipopolysaccharideMolecular model of E.coli lipopolysac
13、charide The bonds between the carbohydrates in pseudopeptidoglycan are 1-3 instead of1-4 as in peptidoglycan.CELL WALLS OF ARCHAEBACTERIA The 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
14、 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
15、 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
16、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 als
17、o 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 StainingGram positive or Gram negative?Morphol
18、ogy of a gram-positive bacterial cell Structure of cytoplasmic membrane Function of cytoplasmic membrane2.3 Cytoplasmic membraneA.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 hy
19、drophobic(black)ends of phospholipids that make up this structure.B.Colorized electron micrograph ofthe 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
20、!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.1.Permeability barrier-prevents leakage and function as gate
21、way for transport of nutrients into and out of the cell.2.Protein anchor-site of many proteins involved in transport,bioenergetics,and chemotaxis.3.Energy conservation-site of generation and use of the proton motive force.Function of membraneIntracellular membrane systemBacteria cells dont contain m
22、embrane-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 chro
23、mosome replication and distribution to daughter cells.It may also be involved in secretory processes2.4 Cellular genetic informationBacterial Chromosome1.Supercoiling and chromosome structure2.Chromosomal copy numberPlasmidsMicrograph of a bacterium showing the nucleoid region(green)within the cytop
24、lasm where the bacterial chromosome occursThe 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 mole
25、cule.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 eukaryaThe bacterial chromosome and supercoiling:Example of E.coli cellThere are over 50 supercoiled domain in the E.coli chromosome.The total am
26、ount 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.Chromosome c
27、opy number 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.Reproduction of a bacterial cell requires the replication of
28、 the bacterial chromosome.The micrograph shows the sequence of synthesis of new circular loops of double helical DNA.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 inform
29、ation 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 p
30、lasmids 2.5 Cytoplasmic matrix Ribosome and InclusionsAll 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
31、 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
32、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:Carbon storage polymers PHB and glycogenPhosphate polymersSulfur Granules Gas VacuolesINCLUSIONSPolyhydroxybutyric acid(P
33、HB)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 speciesMany microor
34、ganisms accumulate granules of polyphosphate,which are large reserves of inorganic phosphates that can be used in the synthesis of ATPPolyphosphate granule in a bacterial cellA Pseudomonas speciesThe sulfur globules inside the cells of purple sulfur bacteriumChromatium buderiSome bacteria,including
35、many photosynthetic bacteria,accumulate elemental sulfur granules as a result of their metabolism.Gas vacuoles(blue)and storage granules(red)in the cyanobacterium MicrocystisThe formation of gas vacuoles by aquatic bacteria provides a mechanism for adjusting the buoyancy of the cell.Many aquatic cya
36、nobacteria 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 layersMotility allows the cell to reach different regions of its environment.In the struggle for survival,movement to a new location may me
37、an 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 fla
38、gellum(plural,flagella).Four basic types of flagellar arrangementsa,monotrichousb,amphitrichousc,lophotrichousd,peritrichousFlagella 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 ma
39、y 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.The f
40、lagellum of a Gram-negative bacteriumThe 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 sing
41、le 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-negative Bacteria,an outer ring is anchored
42、 in the lipopolysaccharide layer and another in the peptidoglycan layer of the cell wall,and an inner ring is located within the cytoplasmic membrane.In gram-positive Bacteria,which lack the outer lipopolysaccharide layer,only the inner pair of rings is present.Surrounding the inner ring and anchore
43、d 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 si
44、gnals.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
45、 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.Pili are usually longer than fimbriae and number only one or two per cell.Pili function to join bacterial
46、 cells prior to the transfer of DNA from one cell to another.F pilusMany prokaryotic organisms secrete on their surfaces slimy or gummy materials.A variety of these structures consist of polysaccharide,and a few consist of protein.The terms capsule and slime layer are frequently used to describe pol
47、ysaccharide layers.Demonstration of the presence of a capsule is usually by means of negative staining Capsules and Slime LayersMany prokaryotes contain a cell surface layer composed of a two-dimensional array of protein.These layers are called S-layers.S-layers have been detected in representatives
48、 of virtually every phylogenetic grouping of Bacteria and are nearly universal among Archaea.In some species of Archaea the S-layer is also the cell wall.The major function of S-layers is unknown.However,as the interface between the cell and its environment it is likely that in cells that produce th
49、em the S-layer at least functions as an external permeability barrier,allowing the passage of low-molecular-weight substances while excluding large molecules.Paracrystalline Surface Layers(S-Layers)2.7 Bacterial endosporesCertain species of bacteria produce special structure called endospores.They a
50、re very resistant to heat and can not be destroyed easily,even by harsh chemicals.Endospores are also resistant to other harmful agents such as drying,radiation,acids and chemical disinfectants.Sporulating cellCentral coreCortexSpore coat/membraneexosporiumMicrograph of a endosporeVegetative cell Co