eukaryotic cells
DESCRIPTION
Eukaryotes are organisms whose cells have nuclei and organelles that are enclosed in a membrane. The maintypes of eukaryotic cells include animal and plant cells and each have specific organelles (specialised structureswithin the cell) that have specific tasks.TRANSCRIPT
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EukaryoticCellsEukaryotesareorganismswhosecellshavenucleiandorganellesthatareenclosedinamembrane.Themaintypesofeukaryoticcellsincludeanimalandplantcellsandeachhavespecificorganelles(specialisedstructureswithinthecell)thathavespecifictasks.
AnimalCells
TheNucleusEukaryoticcellsgettheirnameduetothepresenceofmembraneenclosedorganellesprokaryoticcells,suchasbacteriaandarchaeacells,donothavemembraneenclosednuclei.Thenucleusisahighlyspecialisedorganellewhichisfoundinitsowndoublemembrane,calledthenuclearenvelope.ThenucleusstoresthechromosomesandthecellsDNAintheformofchromatins.TheDNAdeterminesthesequencesinwhichaminoacidsarelinkedtogetherinthecytoplasmtoformproteinmolecules.Thenucleusisthelargestorganelle,usuallyaround1020mindiameter.Communicationbetweenthenucleusandcytoplasm(andtheorganelleswithinit)isveryimportant.
TheNucleolusWithintheNucleusisanotherdensestructurewhichisnotsurroundedbyamembraneandisoftenreferredtoasasuborganelle:thenucleolus.Anucleuscanhavemorethanonenucleolus.ThemainfunctionofthenucleolusistomakeribosomalRNA(rRNA)whichitcombineswithproteinstomakeribosomes,whicharediffusedoutofthenuclearenvelopethroughnucleoporins.Manyribosomesarefoundattachedtothenuclearenvelopeitself.
TheNuclearEnvelopeThenuclearenvelope,apairofmembranes,completelyenclosesthenucleusandseparatesthegeneticmaterialandnucleoplasmfromthecellscytoplasm.Thenuclearenvelopehaslittleproteinpores,alsoknownasnucleoporins,whichallowonlycertain,small,watersolublemoleculestoenterandexitthenucleus.
ThePlasmaMembraneTheplasmamembraneenclosesalltheorganellesandithasselectivepermeability:itcontrolswhichsubstancescanenterandleavethecell.Mostoftheorganelleswithinthecellhavetheirownmembranewhichallowthemtocarryoutcertainchemicalreactionswithoutinterferingwithotherorganelles.
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TheEndoplasmicReticulumTheendoplasmicreticulum(ER)isanetworkoffoldedmembranesformingsheets,tubesandsacs.TheERcomplexoriginatesfromthenuclearenvelopeandisoftenstillattachedorveryclosetoit.TherearetwotypesofERroughandsmooth.
TheRoughEndoplasmicReticulum(RER)RERisanextensivenetworkofmembranesfoundinthecytoplasmandisfoundnearesttothenuclearenvelope.Themembranesfoldoneachothertoformsmallspacescalledcisternae.Theseswellupandarepinchedofftoformvesicles:usedforthetransportofsubstancesaroundthecell.RibosomesarefoundattachedtotheRER.
TheSmoothEndoplasmicReticulum(SER)SERissmallerthanitscounterparttheRERanddoesnothaveribosomesattachedtoit.Itisthesiteofsynthesisofsubstancesrequiredbycells.TheSERisalsothesiteofstorageofcalciumionswhichisvitalinthecontractionofmusclefibres.TheSERcontainsenzymeswhichhelpintheproductionoflipidsandsteroidhormonesandtheSERisalsoinvolvedinthebreakdownoftoxins.FurthermorethecisternaeoftheSERaremoreflatcomparedtothoseoftheRER.
TheRibosomeRibosomesaretinystructuresmadeofrRNAandproteinsandcanbefoundfreelyinthecytoplasmorattachedtothenuclearenvelopeandtheRERandareoftenaround25nmindiameter.Numeroustypesofcellscontainlargenumbersofthese.Ribosomesaremadeoftwosubunitsandtogethertheyassembleaminoacidsbylinkingthemtogethertoformpolypeptides.ThepolypeptidechainissenttotheRER.IftheproteinsproducedneedprocessingorexportationoutsidethecelltheymustbesenttotheGolgiapparatus.PolypeptidesmoveintothecisternaeoftheRERandthecisternaebreaksofftoformvesicles,smallfluidfilledsacs,wheretheytravelto
theGolgiapparatus.
TheGolgiApparatusSometimesreferredtoastheGolgibodyorGolgicomplex,theGolgiapparatusmodifiesproteinsintootherformsandalsoexportthemiftheproteinisneededbyanothercellinthebody.TheGolgiapparatuscanturntheproteinsintohormonesandtheycanalsocombinecarbohydrategroupswiththeproteinstoformother,importantmolecules.Theyarethentransportedinvesiclestotheplasmamembranewherethevesicleandmembranefuse.Here,theproductissecretedfromthecellbyaprocesscalledexocytosis.TheGolgiapparatusisalsoresponsibleforproducinglysosomes.
TheLysosomeLysosomesaresmallvesiclesthatcontainaconcentratedmixtureofhydrolytic(digestive)enzymesproducedintheGolgiapparatusorbytheRER.Lysosomesdigestbacteriaandothercellstakenintothecellbyphagocytosis.Theycanalsobeusedtobreakdownunwantedordamagedorganelles.
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TheMitochondrionAmitochondrionisasmooth,oblongorganellewhichisrelativelylargeataround310mlong.Theyarefoundinmostcellsinverylargenumberse.g.musclefibresandhormonesecretingcellsoftencontainthousandsofmitochondriaintheircytoplasm.Similarlytothenucleus,mitochondriahaveanenvelope,meaningitismadeoftwomembranes,surroundingit.Theinnermembraneisfoldedtoformwhatiscalledcristae.MitochondriaarethesitesofaerobicrespirationandtheyproduceAdenosinetriphosphate(ATP).Thefirststageofrespiration,theKrebscycle,takesplaceinthematrix:thisisanaqueoussolutionofmetabolitesandenzymes.Thefinalstage,oxidativephosphorylation,takesplaceonthemembranesofthecristae.
TheCytoplasmThecytoplasmisanythingthatisfoundoutsidethenucleusbutwithintheplasmamembraneboundaries.Thecytoplasmismadeupoftwothings.Thefirstarethemembraneboundorganelleswhicharefoundfloatinginthecytoplasmribosomescanalsobefoundfreelyinthecytoplasm.Thesecondisthecytosol,agraysolublefluidthatfillstheinteriorofaneukaryoticcellthecytosolismadeupmostlyofwaterwithsolutes,suchasaminoacids,dissolvedinit.Thenucleushasitsowntypeofcytoplasmwhichiscalledthenucleoplasm.
TheCytoskeletonThecytoskeletonisaproteinstrandstructurethatreinforcesthecellandalsohelpstomovetheorganellesaroundthecell.Microtubulesprovideascaffoldingforthecell.Microfilamentsarefoundalongtheperimeterofthecellandprovidesupportthereandallowthecelltoendureacertainamountofstrainorpressure.Themicrofilamentsaremuchthinnerthanmicrotubules.Somecells,suchasthespermcell(malegamete),usethemicrotubulestopowerorganellescalledciliaandflagellathiscreatesmovement.Furthermorethemicrotubulesformthecentrosometwocentriolesfoundatrightanglestooneanother.
TheCentrioleCentriolesarefoundonlyinanimalcells.Centriolesaremadeofmicrotubulesarrangedina(9+0)circularpatternfoundnearthenucleusofthecell.Microtubulesarehollowcylinders,approximately25nmlongandmakeupthecytoskeletonofeukaryoticcells,alongsidemicrofilaments.Centriolesarefoundinpairsperpendiculartoeachother.Fromthispositiontheyareabletomakethemicrotubulesformspindlesduringcellulardivisioninanimalcells.
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PlantCellsManyultrastructuresfoundinanimalscellscanalsobefoundinplantcells,theseincludethenucleus,nucleolus,nuclearenvelope,plasmamembrane,mitochondria,ribosomesandtheendoplasmicreticulum.Plantcellsdonothavecentriolesbuttheydohaveacytoskeletonwhichhelpsreinforcetheshapeoftheplantcell,whichisveryimportant.
TheCellWallNotfoundinanimalcells,thecellwallprovidesarigidperimetertothecell.Itismadeupofcellulose,anextremelystrongpolysaccharide,andprovidestheplantwithstabilityandprotection.Thecellwallisexternaltothecellandsurroundstheplasmamembrane.Thoughthecellwallisnotanorganelle,itisafundamentallyimportantlayerofthecellandisproducedbythereactionsoftheorganelleswithintheplant.
TheVacuoleAnimalplantsoftenhavesmallortemporaryvacuoleswhichareoftenformedbytheintuckingoftheplasmamembrane,howeverplantcellshavelarge,permanentvacuoles.Vacuolesarefilledwithfluidandarecavities,boundedbythetonoplast:asinglelayermembrane.Thetonoplastcontainsproteinsthatallowthetransportofsmallsolutesandsalts.Thevacuoleoccupiesupto80%ofthecellsvolumeandisfilledwithcellsap.Thecellsapisasolutionwithsolutes,suchasfoodmaterials,ionsandwasteproducts,dissolvedintoit.Thecellsapcansometimesalsocontainwatersolublepigments.Thevacuoleprovidesturgorpressurefrominsidethecellwhichreinforcestheshapeofthecellfurther.
TheChloroplastChloroplastsandamyloplastsarepartofalargergroupoforganellescalledplastids.Theyarenotfoundinanimalcellsbutareoftenfoundinmanyplantcells.Allplastidshaveadoublemembrane,oftencalledanenvelope.Chloroplastsarefairlylargeorganelles,410mlong,withabiconvexshapeandtheyaregreenincolour.Chloroplastsoftenoccurinthemesophyllcellsofleavesofgreenplantsandarethesiteofphotosynthesis.Morethan50chloroplastscanbepackedintoamesophyllcellandphotosynthesisoftenoccursinthosefoundintheleavesoftheplants.Theinsidelayerofthechloroplastsformlumenfilledthylakoidswithasystemofbranchingmembranes,oftenreferredtoaslamellae.Thylakoidsarearrangedinflat,circularpilescalledagranumandphotosyntheticpigments,calledchlorophylls,arelocatedinthegrana.Thethylakoidsarelooselyarrangedallowingspaceforanaqueousmatrixcalledthestroma.
TheAmyloplastAnothertypeofcolorlessplastidsaretheamyloplasts.Theseorganellessynthesizeandstorestarchgrainsapolysaccharidecomposedofamyloseandamylopectin.Theamyloplastscanalsoconvertthestarchbackintosugariftheplantneedsit.