membrane | Definition, Structure, & Functions | Britannica

membrane, in biology, the thin layer that forms the outer boundary of a living cell or of an internal cell compartment. The outer boundary is the plasma ... membrane Sections&Media Article Introduction&QuickFacts FastFacts Facts&RelatedContent Media Videos Images AdditionalInfo MoreArticlesOnThisTopic Contributors ArticleHistory Home Health&Medicine Anatomy&Physiology membrane biology Alternatetitles:biologicalmembrane Print Cite verifiedCite Whileeveryefforthasbeenmadetofollowcitationstylerules,theremaybesomediscrepancies. Pleaserefertotheappropriatestylemanualorothersourcesifyouhaveanyquestions. SelectCitationStyle MLA APA ChicagoManualofStyle CopyCitation Share Share Sharetosocialmedia Facebook Twitter URL https://www.britannica.com/science/membrane-biology More GiveFeedback ExternalWebsites Feedback Corrections?Updates?Omissions?Letusknowifyouhavesuggestionstoimprovethisarticle(requireslogin). 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ExternalWebsites NationalCenterforBiotechnologyInformation-PubMedCentral-Biologicalmembranes BritannicaWebsites ArticlesfromBritannicaEncyclopediasforelementaryandhighschoolstudents. membrane-StudentEncyclopedia(Ages11andup) By TheEditorsofEncyclopaediaBritannica | ViewEditHistory FastFacts Facts&RelatedContent molecularviewofthecellmembrane Seeallmedia KeyPeople: ErwinNeher BertSakmann Charles-FrançoisBrisseaudeMirbel ...(Showmore) RelatedTopics: meninges endoplasmicreticulum villus tympanicmembrane nictitatingmembrane ...(Showmore) Seeallfactsanddata→ membrane,inbiology,thethinlayerthatformstheouterboundaryofalivingcellorofaninternalcellcompartment.Theouterboundaryistheplasmamembrane,andthecompartmentsenclosedbyinternalmembranesarecalledorganelles.Biologicalmembraneshavethreeprimaryfunctions:(1)theykeeptoxicsubstancesoutofthecell;(2)theycontainreceptorsandchannelsthatallowspecificmolecules,suchasions,nutrients,wastes,andmetabolicproducts,thatmediatecellularandextracellularactivitiestopassbetweenorganellesandbetweenthecellandtheoutsideenvironment;and(3)theyseparatevitalbutincompatiblemetabolicprocessesconductedwithinorganelles.Membranesconsistlargelyofalipidbilayer,whichisadoublelayerofphospholipid,cholesterol,andglycolipidmoleculesthatcontainschainsoffattyacidsanddetermineswhetheramembraneisformedintolongflatsheetsorroundvesicles.Lipidsgivecellmembranesafluidcharacter,withaconsistencyapproachingthatofalightoil.Thefatty-acidchainsallowmanysmall,fat-solublemolecules,suchasoxygen,topermeatethemembrane,buttheyrepellarge,water-solublemolecules,suchassugar,andelectricallychargedions,suchascalcium. BritannicaQuiz PartsofaCellQuiz Whatisthethinlayerthatformstheouterboundaryofacell?Whereisthesiteofphotosynthesisinaplantcell?Testyourknowledge.Takethisquiz. Embeddedinthelipidbilayerarelargeproteins,manyofwhichtransportionsandwater-solublemoleculesacrossthemembrane.Someproteinsintheplasmamembraneformopenpores,calledmembranechannels,whichallowthefreediffusionofionsintoandoutofthecell.Othersbindtospecificmoleculesononesideofamembraneandtransportthemoleculestotheotherside.Sometimesoneproteinsimultaneouslytransportstwotypesofmoleculesinoppositedirections.Mostplasmamembranesareabout50percentproteinbyweight,whilethemembranesofsomemetabolicallyactiveorganellesare75percentprotein.Attachedtoproteinsontheoutsideoftheplasmamembranearelongcarbohydratemolecules.differenttypesofmembranetransportThecellmembranecontainsproteinsthattransportionsandwater-solublemoleculesintooroutofthecell.Somemoleculesareabletofreelydiffuseacrossthemembraneinaprocessknownassimplediffusion.EncyclopædiaBritannica,Inc.Manycellularfunctions,includingtheuptakeandconversionofnutrients,synthesisofnewmolecules,productionofenergy,andregulationofmetabolicsequences,takeplaceinthemembranousorganelles.Thenucleus,containingthegeneticmaterialofthecell,issurroundedbyadoublemembranewithlargeporesthatpermittheexchangeofmaterialsbetweenthenucleusandcytoplasm.Theouternuclearmembraneisanextensionofthemembraneoftheendoplasmicreticulum,whichsynthesizesthelipidsforallcellmembranes.Proteinsaresynthesizedbyribosomesthatareeitherattachedtotheendoplasmicreticulumorsuspendedfreelyinthecellcontents.Themitochondria,theoxidizingandenergy-storingunitsofthecell,haveanoutermembranereadilypermeabletomanysubstances,andaless-permeableinnermembranestuddedwithtransportproteinsandenergy-producingenzymes. GetaBritannicaPremiumsubscriptionandgainaccesstoexclusivecontent. SubscribeNow ThisarticlewasmostrecentlyrevisedandupdatedbyAdamAugustyn. LearnMoreintheserelatedBritannicaarticles: cell:Thecellmembrane Athinmembrane,typicallybetween4and10nanometers(nm;1nm=10−9metre)inthickness,surroundseverylivingcell,delimitingthecellfromtheenvironmentaroundit.Enclosedbythiscellmembrane(alsoknownastheplasmamembrane)arethecell’sconstituents,… nervoussystem:Theneuronalmembrane Theprinciplesoutlinedabovecanbeappliedtotheneuronanditsioniccontents.… prenataldevelopment:Extra-embryonicmembranes Thewayinwhichtheencapsulatingmembraneoftheblastocystbecomesthechorion,andthemostdeeplyembeddedpartofitbecomesthefetalplacenta,hasalreadybeendescribed.Therearestillotherimportantmembranesthatdevelopfromthoseportionsoftheinnercell… Historyatyourfingertips SignupheretoseewhathappenedOnThisDay,everydayinyourinbox! Emailaddress Bysigningup,youagreetoourPrivacyNotice. Thankyouforsubscribing! 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