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Brownfatcells
&hertypeoffatcell，termedbrownfatwhichfatisbrokendowbyaprocesstermedthermogenesis.Inhumahemostbrownfatcells，usuallyintheshoulderareas.Itwasthoughtthatbrownfatwaslostbyadulthoodinmosthumans，butinsmallmammalssuchasratsandmice，whereheatlossisgreater（fromtheincreasedsurfaeratio），brownfatcellsareretaihroughoutlife.midratsshowsthattheyoffexcessfoodi.Althanimalsbuildupmassivestoresofwhitefattomaintainthemselvesformonthswithoutfood，theirbrownfatlygetswitatthetimeofwakingtoraisebodytemperature.WhenateiquecalledPET（positroomography）sgbecameastandardmedigteiqueafewyearsago，somepatients（wearinggownsonlyandthereforeysterypatetabolicactivityaroundtheshouldersandbackthatdisappearedinwarms.Thesepatcheswerebroeredintoactivitybythecold.Adultsdoiainbrownfat，andsomeindividualshavesignifitlymorethanothers.Alikelyreasonwhysomeindividualseatasmuchastheylikewithoutgaihattheyhavemorebrownfat.Intheory，ifwecouldswitchourwhitefattobrownfatcellswecouldeatasmuchasiththeendpoierratherthanfatter.Browhemselvesdifferfromwhitefatcellsbyhavingmaodria，withtheiroodriaprodugthebrowncolouration.ThenormalmitoetabolismwhierateseoredasATPisalteredtoproduceheatbyaprocesscalledprotonleakage，produgproteihermogeniioditions，suchasdeepseadivers，appeartoaccumulatemuchhigherthannormalamountsofbroosits，indigthatbrownfatberegeedinadults.Ifwhitefatcellscouldbevertedtobrownfatcells（ashasbeeissueculture），thenwecouldhaveausefultoolileagaierally‘burni.
Modifioflipidprodu
Inplantetiipulationofthemeationoflipiddropletshasstaggeringimplisforseedcrops.Thebiochemistryofproduofoilsinplantcellsfollowsverysimilarroutestolipidsinanimaldonegroupofehediacylglysferaseswhichcatalysetriglycerideprodu）havealreadybeeiipulatedtoapproximatelydoubletheyieldofoilandoleimaize.
&oskeleton
A‘skeleton’instantlybringstomindthebosofalongdeadindividual.Theloyofbohedepositionofmineralssuchascalciumphosphateirixbycellscalledosteoblasts，gthestructuralrigidity.Ihebohehumaoherwithtendonsas，flexiblyattachedtoeachotherasasystemoflevers，readytogewhichisdrivenbymustra.Therearestructureswithsimilarfunswithincells，wheremicrotubulesplaytheroleofleversandthe‘muscle’activityisprovidedbyaentsinassoyosin，whichslideovereachothertoprovideuchastheydoiself.However，uiverigidityandpermanenusculo-skeletalsystem，thecharacteristicofthecytoskeletoremeplastiddynamism，whereposbebuilt（polymerized）frombuildingblockseedandjustasquicklyremovedbybeingbrokendown（depolymerization）.Theold-fashionedideaofacellasessentiallyaballoohjellyoremisleading;cellshapeiswithin，modulatedbysignalsretheexternale，andcapableofrapidrespoinuallygetheirshape，gepositioheirneighbours，mhsolidtissues，ortakelongjourneysaroundthebodybyenterihebloodstreaAddtothisthereanizatiooskeletooseparateesatdivision（aswillbedesChapter4），andthedynamiatureofthecytoskeletosmaiic.
Aoskeletonisapropertyrestrictedtoeukaryotes，althoughsimilarproteiiaryformieria.Theeukaryoticcytoskeletonisdefinedasaworkofthreetypeseproteins:microtubules（formedfromthesmallerprotein，tubuliefilaments（agroupoffibrousproteinswithsimilarproperties）;andmiehesmallerprotein，a）（Figure 6a，b）.Eachoftheseproteinshasmanyassociatedproteihemfulflltheirrolesinjustabouteveryaspectofcellularfun.Althougheachoftheelemeoskeletonprovidesspecificpartsoftheoverallfuoproduceshapedmovemeothinkaboutthecytoskeletonisasaedsystem，involvingallpoher.Aswellaswholecellrespooskeletonalsoplaysaovihincells，wheremicrotubulesihmotorproteinssuein，providing‘railwaylines’formovementofvadcargahroughoutthecell.
6.Cytoskeletos.（a），（b）Theworkupthecytoskeletonofintactcells，exposedbyremovalofiellesleavingjusttheucleus.（c）Microtubules，thathavebeeesttube.（d）Sethroughaflagellum，showingthe9+2arraheaxoihasutentacle，showiubulararrayswhidthefoodal
&herornotfibrousproteiheoequivalentstructuretothecytoskeletoroversial.Becausethecytoskeletonissoobviouslycrucialtoiization，itissurprisingthattherehasbeensucharesistaoaureintheheproteinaeofthemaialpos，wasfirstisolatedfrommuscleover7o.Now，‘iedasaroutioplasm，andisiohecell.Moreretlystill，ahasalsobeeacceptedasatofthenuingpartofa‘al’arraofloousproteinsalongwithiefilamentsofthenuclearlaminawhichprovideafibroussgfementofents，fthe‘on’（seeFigure7Chapter3）.
dflagella
Whip-like‘tails’havebeenobservedonsingletheearliestdaysoflightmicrosthelate17thtury.Usuallyowosuchtails（flagella）movethecellthroughanaqueousmediumbypropagationofaseriesofwavesfrombasetotip.Ihelialtissuesliningsanssuchasthelungs，cellsarenumerousflagella（calledlargenumbers）whichmoveasurfauthewindpipe，ciliabeattogetherinatoandfromatlymovinglayerofmucusupwardstowardsthelarynx，thuspreventinganyacofpoteiveageorytract.
&eriaalsopossessflagella，buttheyarerelativelysimple，gidhelicaltail，whichactslikeapropeller，rotatedatitsbasebyamolecularmotor.Eukaryotidflagellaarerootedwithinthecellbyastructurecalledabasalbody，aheirwhip-likemovementwithiheflagellumitselfbyasystemofmicrotubules，aoastruownasaeDo，inhisclassic1961work TheCell，‘fewcellularactivitieshaveprovedmorefasgtocytologiststhandflagellarmotion’.In1887，Jensensquashedspermflagellabetweenamicroscopeslideandclass，desghowthespermtailswere‘frayedintoanumberoffibrils’，some60yearsbeforethiswasedbyeleicroseManton，anEnglishbotanistwhohadmaaronmicrosthepost-war‘LeemfromtheUSA，showedthattherewere11fibresinallplagthoseinanimals，ingthatflagellarstructurehasbeeacularlyservedthroughoutevolutioandard’axouresistsofatralpairofmicrotubulessurroundedbyniubules（Figure 6d）.Withihebasalbodyisformedbyashortderofmicrotubuleswithoutatralpair.
Whatmakesamicrotubule?
Eachmicrotubuleisaholloallmadeoutofaproteiubulin.Twomoleculesoftubulinformadimer，whichresemblesashelledpeanut.Thesedimersjoioendmakingaloofilament），and13protofilamentsarejoihwisetoformthewallofthehollowtubethatmakesthemicrotubule（Figure 6c）.Thewholestructureisstabilizedbyassociatedproteins.Intheaxonemeofflagellaandentisproducedbyamotorproteineinwhiksadjatmicrotubulesandallowsthemtoslideovereachotherinasynaoprodudthattravelsdowntheflagellum，gthe‘whiplash’movement.thatthelihedyneinarmsaubules，whichweredisthe1950sbyBjornAfzelius，aresuccessivelymadeaherlikegaropehandoverhand.
Shouldflagellardyedorabsehecesaresignifity-fiveyearsafterhisinitialdiscoveryofthedyneinlinks，AfzeliuslookedatthespermoffourpatientsataySweden，andfoundthatthedyneiiailaxothespermwere‘nonswimmers’，whily，ledtotheiy.Halfofthepatientsalsosufferedfromaknownas situsinversus，wherethemajansoftheviscera（heart，spleen，andpancreas），whiallyosideofthebody，beeswitchedtht.Thisturoresultfromalagciliaearlyinembryodevelopmeheleft–rightbodyaxisisestablished.ThisiscalledKartagener’ssyerMaageheinthe1930s.
Aparticulartypeofciliumisfoundihese‘primarycilia’asorystructures—ratherlikearadioaerialforginformatioesurroundimoveily，astheylackboththetralpairofmicrotubulesanddyneiheperipheralubules.Primaryciliaarenowknowntohaveawholehostoffuns，agasreeidchemicalstimuli.Intheliningofthenose，modifiedprimaryecttherethespecializedcellsoftheolfactoryepitheliumiiobs）wheresmellispertheeye，thespecializedphotoreceptorsoftheretiachedtotheircellbodiesbyaprimaryciliuTheprimaryciliumalsoplaysagroleincelldivision，andisalmostlyinvolvedinotion.
Diseasescausedbydefectiveciliaareknoathies，andtheyincludeawideraoms，egorizedasseparatesyndromesloheunderlyingoncellularcausewasidentified.Somesymptomsmaybeontoallpatiehersareuswithoral-facial-digitalsyndromesufferfrompolydactyly（extrafioes）andkidientswithBardet–Biedlsyidehelate19thtury）alsohavekidinadditionsufferfromretiionwhileadtoblindness，alongwithobesityaes—allasaresultofiivecilia.
Intracellularmicrotubules
Microtubuleswerethoughttobelimitedtoaxoilimprovemeronmicroscopypreparationtheearly1960sresultedintheirdischoutthe.Becausetheyalearedasstraightrods，theywereinitiallythoughtidastructures.However，LewisTilershowedthatbymerelygaprotozoaioarourigrade，allthemicrotubule-supportedsionscollapsedasthemicrotubulesbrokeapart，subsequentlyre-fafterafewmiroomtemperature.Notuntilthe1980sdiditbeeapparentjusthowdynamicmicrotubulesactuallywere，whenTimMitshowedthatmicrotubulescouldessentiallycollapseandre-forminseds，aprocessthathetermed‘dynamistability’.
Microtubulesalsoformtheframeworkofthemitotidle，bywhichtheesaredistributedtodaughtercellsatdivisioer4）.Byexposingdividicallede（whidstotubulinandstopsitjoioformfilameioidlehibited，‘freezing’theprocessofdivision，andagosomeanalysis.ewastheagrediesfromtheautumnautumusedbytheaiansforarthritiditions.Inhibitionofmitotidleformationalsobeachievedbydrugsthatstabilizeicrotubules，preventingthemfrdowore-formasspiubules.OnesuchdrugisTaxol（extrathebarkofthePacificyew）.Taxolbecameapotentialbliment，andbecauseremovalofthebarkkillsthetree，demandforthebarkalmostcausedthelossofallPacificyewtreesintheUSA.Fortuaxolwassubsequentlychemithesizedaspaclitaxel.Duetotheacceleratedrateofdivisionofcercells，almsthatihmicrotubulesaioialcertreatments.
Culturedfibroblastshavebeenthecellsofchoiceforthestudyofmicrotubulefun.Fibroblastsarefoundiissuesuts，ligaments，andtendrowninculturearelongandfiattened，ahesurfaceoftheculturedishwithabroadleadingedgeandanarredge（Figure 3trast，epitheliastayfiattenedandmaure（Figure 3b）.Inallculturedcells，icrotubulesradiateoutwardsioplasmfromastructureclosetothenucleuscalledthee.esactasamicrtre，gtheturnoveranddistributionofmicrotubules.Theystructures（trioles）identicaltothebasalbodiesfoundatthebaseofeachflagellumorciliuThesetriolesopairs，positilestoeachother.Earlyincelldivisioeaooppositeeaubulesthatmakeupthemitotidle.
Itisashortteicalstepfrplasticflaskstasuitablee（achamberat37degreestigrade），allowiobeplaicroscopestagefcellstobeobservedastheygoabouttheirbusiness.Becauselivingcellsarelargelytraishardtoseemuchdetailticalsystemssuchasphaseicroscopy，whialldiffereherefractilepropertiesinthepoolightanddarkregions.Forthiscruce，FritsZerheNobelPrizein1953.Nowadays，virtuallyanyproteiagged’tofluoresillumihUVlight）bybinihthoseofaproteincalledgreeprotein（GFP，propernameaequiedfroma‘glowinthedark’jellyfish.Bygitssequeninoacids，GFPhassisfluorescepropertiesalteredandisavailableinblue，e，yellow，andredfluorestvarieties，allowingseveraldiffereobefollowedovertimeinthesameliviothistheabilityoflow-lightcamerastnalsfromjustafewmoleculespercell，togetherwithlaserilluminationandputerizedimagingandanalysis，andlivingcellstlyprovidesawealthofinformationthatwasunimaginableonlyafewyearsago.Nowadaysitisfeasibletoarticularlivingcellproeimicros‘flashfreeze’theterestinmillisedprepareitforexaminatioronmicrosyprobescalledquantumdots，whicharebothfluoresicrosdeleseforeleicroscopy，allowlabellingofthesamemoleculesforbothteiques.
Aninitialbrieflookatmostlivingderthephaseiightbedisappointiiated，asdealappearstobegoingon.Uniswillzoomarouime，drivenbytheirflagellaordamoebaecraeedeasytoseeiime.Forculture，thecellularactivityseeninpopularseswillinvariablybetime-lapsefootage，withindividualimagesrecordedatintervalsofafewsedthenplayedspeededup.Inthisway，thehouracelltakestodivideisrecordedwithoenseds.Playingtheimagesbackat25framespersepressestheprotojustuenseakingitappearmuic.
&efilaments
Therequirementsofamotilelifestyleinanimalshaveresultediionofmeigthbypletelydifferentways.Whaoherasashell，asiosands，oraninteronasinfish，amphibiailes，birds，andmammals.Iheskeletalmaterialismadeofproteiedbydinsomeeralizedfidity.Thismaterialisknowracellularmatrix.Withinindividualanimalcells，meigthisprovidedbyagroupofproteiermediatefilaments，high-teflexiblecableswhichpermeatetheentirecell.tissuesarejoiheirneighboursbystrengthenedmembraionscalleddesmosomes，whichoredbyiefilaments，providiwhoutthetissue.
&efilamentsaresoatoftheirdiameter（10res）whitermediatebetweenmients（6res）andmicrotubules（25houghafilamentsandmicrotubulesarethesameineverygplants），aefilaments（lamins）intheandard，itermediatefilamentsarespecializedagtotheirtissuetypeandembryin.ectivetissuesarecharacterizedbyaefilamein，whereasneurofilamentsarefouissue，anddesminischaracteristicofvertebratemuscle.Keratinsarealargegroupofiefilamentsfouhelialiuralproteinofskiisidethehair，wool，fingernails，horns，ahese‘hard’keratiableextracellularsesalydead，althoughtherearemaoplasmisthataredynamic.Mutatioinsskin，gaknoidermolysisbulleionblisteriitbelifethreateningtone>
&filamentousproteinsinthecellarecalledmients.Theyarearound6resihatofiefilaments，aheproteina.Aisagl-a），butittakesadifferentform（F-aassembledis.Filamentousaistheobuworksbyahostofadingproteins，fatleast15differentstruheastoesbackover60years，tothe1940swhe-Gyiestablishedthepresehadmyosininstriatedmuscle.Furtherworkinthe1950sbyAndrewHuxleyandHughHuxley（uablishedthatwhes，afilamehemyosis，aionwhisthemuscle，produgforce.TheofATPtoADPreleasestheneergyforthismoleteratotakeplace.Musclehasahighlyeometricalmoleculararchitecture，whereeverymyosinmoleculeissurroundedbya‘der’ofsixamoleculesallowingthemoleculestoslideovereachother.Thisarraisonlyfoundinmuscleahepossibilitythatadmyosinteracttoprodutraon-musclecellsseemedunlikely.However，in1973，TomPollardshowedthattherewasmorethaypeofmyosininnon-musclecells.thatthereareover40differentmyosinsinmammals，andthat（alongwithF-a）theysupplythemotileforvolvedincelldivision，ent，aakeofexternalmaterialbydocytosis）.Aalsohasastructuralroleioskeletois，bouheproteinvillin，supportthefiionsofthecellmembrane（filopodiaandmicrovilli）.
Cytoskeletal–eras
Fromtheihehesurfaceofthecell，thereareliaboutallofthefilamentousproteins.Thelaminswithinthenucleusareaediatefilameer3），whicharejoioplasmitermediatefilamentsbypresthatcrossthenuvelope.Allthecytoskeletalelemeoeachother，withdirelinks（plakiermediatefilamentsandmicrotubules，aermediatefilamentsandthemientsthatformthethirdmajorelemeoskeleton.Thisieinsicrotubules，iefilaments，andmients，allwithdiffereies，funstogethertomaiuralandmeitegrityoftheasabilitytomove（seeChapter4）.
Withinalivihreecytoskeletalposworkinunison，asmightbeexpectedafterfourbillionyearsofevolutiohepohecytoskeletonindividuallyislikedesgapistoingrod，andkshaftwithoutmentioningawihcasesthewholeissiderablymorethahe>
Thirtyyearsago，whenDonaldIngberwasaeatYaleUy，hewascedthattheviewofthecellasa‘rubberbagfilledwithjelly’wassomewhatoversimplified.IriguedbytherevolutionaryarchiteinsterFullerinthe1940s，whocreatedaseriesofrobuststructureses（inghisownhouse）.Geodomesarestruashellofmultiplesmallrigidtriaanymajstructuressuchasbeamsorns.FullerhimselfhadbeehesculpturesofKehSnelsidstaieelrodsappearedtofloatinthinair，butareactuallysupportedbyasystemofcables，ratherliketheriggingonasailboat，wherethemastiskeptinplacebyabalaensionahemastitselfisrigidioresistthepressiohetensionintheriggiureisrobust，andwillonlyfailifthemastbucklesbreaks.Thisistheprieyrity，whichoffersthemaximumamouhforthemiureyandmaterials.Ihattesineverycell，mediatedbyrigidmicrotubuleswhichresistthepressioiefilamehusgehwithinallcellshapes，betheyfiattenedhexagonaldihelia，orextendednerveaxoncellswhichmaybeametreieyisatworkevenwhenagesitsshape，ashappensindivision.Ieedcellswillroundupatthestartofdivision，thenpiwhtercells，whifiatteheedgesfiatten，trianglesformedbyafibresareclearlyvisiblearouhsixneighblesfahexagolyliketheedgeofaBusterFullergeodome.Thefiattensfurther，gshapetoatypidedfibroblastfattatsbetweenthemembrahebaselayertermedfocaladhesiratingasasingletrast，cellsgrowninculturefromepithelialtissueswillattachthboursandmovearoundasasheet.Asinliviheepithelialcellsattachtoeachotherwithstructurescalleddesmosomes，whicharetoughplaque-likestruedbyloentofthecellmembrane，ahiermediatefilaments.Inskin，atissuetlybeched，epidermalcellshavemultipledesmosomes，aermediatefilamehenedbynumerouskeratis（Figure 6a，b）.Whehrougheverycell，thisarraesaoughtissue.
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