Lecture biology (6e) chapter 7 campbell, reece

CHAPTER7ATOUROFTHECELL SectionA:HowWeStudyCells Microscopesprovidewindowstotheworldofthecell Cellbiologistscanisolateorganellestostudytheirfunction Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 1.Microscopesprovidewindowstothe worldofthecell ã Thediscoveryandearlystudyofcellsprogressed withtheinventionandimprovementofmicroscopes inthe17thcentury ã Inalightmicroscope(LM)visiblelightpasses throughthespecimenandthenthroughglasslenses ã Thelensesrefractlightsuchthattheimageismagnified intotheeyeorontoavideoscreen Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings • Microscopes vary in magnification and resolving  power • Magnification is the ratio of an object’s image to  its real size • Resolving power is a measure of image clarity • It is the minimum distance two points can be separated  byandstillbeviewedastwoseparatepoints ã Resolutionislimitedbytheshortestwavelengthofthe source,inthiscaselight Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Theminimumresolution ofalightmicroscopeis about2microns,thesize ofasmallbacterium • Light microscopes can  magnify effectively to  about 1,000 times the  size of the actual  specimen • At higher magnifications,  the image blurs Fig. 7.1 Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Techniquesdevelopedinthe20thcenturyhave enhancedcontrastandenabledparticularcell componentstobelabeledsothattheystandout Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Whilealightmicroscopecanresolveindividual cells,itcannotresolvemuchoftheinternal anatomy,especiallytheorganelles • To resolve smaller structures we use an electron  microscope (EM), which focuses a beam of  electrons through the specimen or onto its surface • Because resolution is inversely related to wavelength  used, electron microscopes with shorter wavelengths  than visible light have finer resolution • Theoretically, the resolution of a modern EM could  reach0.1nanometer(nm),butthepracticallimitis closertoabout2nm Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Transmissionelectronmicroscopes(TEMs)are usedmainlytostudytheinternalultrastructureof cells ã ATEMaimsanelectronbeamthroughathinsectionof thespecimen • The image is focused  and magnified by  electromagnets • To enhance contrast,  the thin sections are  stained with atoms  of heavy metals Fig. 7.2a Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Scanningelectronmicroscopes(SEMs)are usefulforstudyingsurfacestructures ã Thesamplesurfaceiscoveredwithathinfilmofgold ã Thebeamexciteselectronsonthesurface ã Thesesecondaryelectronsarecollectedandfocusedon ascreen ã TheSEMhasgreat depthoffield, resultinginan imagethatseems threeưdimensional Fig.7.2b Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Electron microscopes reveal organelles, but they  can only be used on dead cells and they may  introduce some artifacts • Light microscopes do not have as high a  resolution, but they can be used to study live cells • Microscopes are a major tool in cytology, the study  of cell structures • Cytologycoupledwithbiochemistry,thestudyof moleculesandchemicalprocessesinmetabolism, developedintomoderncellbiology Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 2.Cellbiologistscanisolateorganellesto studytheirfunctions ã Thegoalofcellfractionationistoseparatethe majororganellesofthecellssothattheirindividual functionscanbestudied Fig.7.3 Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Inmusclecells,thousandsofactinfilamentsare arrangedparalleltooneanother ã Thickerfilamentscomposedofamotorprotein, myosin,interdigitatewiththethinneractinfibers ã Myosinmoleculeswalkalongtheactinfilament,pulling stacksofactinfiberstogetherandshortening thecell Fig.7.21a Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Inothercells,theseactinưmyosinaggregatesareless organizedbutstillcauselocalizedcontraction ã Acontractingbeltofmicrofilamentsdividesthe cytoplasmofanimalcellsduringcelldivision • Localized contraction also drives amoeboid movement • Pseudopodia, cellular extensions, extend and contract  through the reversible assembly and contraction of  actin subunits into microfilaments Fig. 7.21b Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • In plant cells (and others), actin­myosin interactions  andsolưgeltransformationsdrivecytoplasmic streaming ã Thiscreatesacircularflowofcytoplasminthecell ã Thisspeedsthedistributionofmaterialswithinthecell Fig.7.21c Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Intermediatefilaments, intermediate in size at 8 ­ 12  nanometers, are specialized  for bearing tension • Intermediate filaments are  built from a diverse class of  subunits from a family of  proteins called keratins • Intermediate filaments are  morepermanentfixturesof thecytoskeletonthanarethe othertwoclasses ã Theyreinforcecellshape andfixorganellelocation Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings Fig.7.26 CHAPTER7ATOUROFTHECELL SectionG:CellSurfacesandJunctions Plantcellsareencasedbycellwalls 2.Theextracellularmatrix(ECM)ofanimalcellsfunctionsinsupport, adhesion,movement,andregulation 3.Intercellularjunctionshelpintegratecellsintohigherlevelsofstructure andfunction 4.Thecellisalivingunitgreaterthanthesumofitsparts Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 1.Plantcellsareencasedbycellwalls ã The cell wall, found in prokaryotes, fungi, and some  protists, has multiple functions • In plants, the cell wall protects the cell, maintains its  shape, and prevents excessive uptake of water • It also supports the plant against the force of gravity • The thickness and chemical composition of cell  walls differs from species to species and among cell  types Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Thebasicdesignconsistsofmicrofibrilsofcellulose embeddedinamatrixofproteinsandother polysaccharides ã Thisislikesteelưreinforcedconcreteorfiberglass ã Amaturecellwallconsistsofaprimarycellwall,a middlelamellawithstickypolysaccharidesthatholds celltogether,andlayersofsecondarycellwall Fig.7.28 Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 2.Theextracellularmatrix(ECM)of animalcellsfunctionsinsupport,adhesion, movement,andregulation ã Lackingcellwalls,animalscellsdohavean elaborateextracellularmatrix(ECM) • The primary constituents of the extracellular matrix  are glycoproteins, especially collagen fibers,  embedded in a network of proteoglycans • In many cells, fibronectins in the ECM connect to  integrins, intrinsic membrane proteins • The integrins connect the ECM to the cytoskeleton Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • The interconnections from the ECM to the  cytoskeleton via the fibronectin­integrin link  permit the interaction of changes inside and  outside the cell.  Fig. 7.29 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • The ECM can regulate cell behavior • Embryoniccellsmigratealongspecificpathwaysby matchingtheorientationoftheirmicrofilamentstothe grainoffibersintheextracellularmatrix ã Theextracellularmatrixcaninfluencetheactivityof genesinthenucleusviaacombinationofchemicaland mechanicalsignalingpathways ã Thismaycoordinateallthecellswithinatissue. Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 3. Intercellular junctions help integrate  cells into higher levels of structure and  function • Neighboring cells in tissues, organs, or organ  systems often adhere, interact, and communicate  through direct physical contact • Plant cells are perforated with plasmodesmata,  channels allowing cysotol to pass between cells Fig.7.28inset Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Animalhave3maintypesofintercellularlinks: tightjunctions,desmosomes,andgapjunctions ã Intightjunctions,membranesofadjacentcellsare fused,formingcontinuousbeltsaroundcells ã Thispreventsleakageofextracellularfluid. Fig.7.30 Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Desmosomes(oranchoringjunctions)fastencellstogetherintostrongsheets,muchlikerivets ã Intermediatefilamentsofkeratinreinforcedesmosomes. ã Gapjunctions(orcommunicatingjunctions)providecytoplasmicchannelsbetweenadjacent cells ã Specialmembraneproteinssurroundthesepores ã Saltions,sugar,aminoacids,andothersmallmoleculescanpass ã Inembryos,gapjunctionsfacilitatechemicalcommunicationduringdevelopment Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 4.Acellisalivingunitgreaterthanthe sumofitsparts ã Whilethecellhasmanystructuresthathavespecific functions,theymustworktogether ã Forexample,macrophagesuseactinfilamentstomove andextendpseudopodia,capturingtheirprey,bacteria ã Foodvacuolesaredigestedbylysosomes,aproductofthe endomembranesystemofERandGolgi Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Theenzymesofthelysosomesandproteinsofthe cytoskeleton are synthesized at the ribosomes • The information for these proteins comes from  genetic messages sent by DNA in the nucleus • All of these processes require energy in the form of  ATP, most of which is supplied by the  mitochondria • A cell is a living unit greater  than the sum of its parts.  Fig. 7.31 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings ... Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig.? ?7. 7 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig.? ?7. 8 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings CHAPTER? ?7? ?A TOUR OF THE CELL... inconnectingcellularstructureandfunction Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings CHAPTER7 ATOUROFTHECELL SectionB:APanoramicViewoftheCell Prokaryoticandeukaryoticcellsdifferinsizeandcomplexity... the polypeptides they are synthesizing Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings CHAPTER? ?7? ?A TOUR OF THE CELL Section D: The Endomembrane System Theendoplasmicreticulummanufacturesmembranesandperformsmany