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.

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.

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.

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.