KnowYourInner:AllaboutLipids
Haveyoueverthoughtunderstandinglipidswasasimpletask…untilyoutriedtotackletheexamquestions?Thistutorialwillmainlyserveasaguidedoutlinetocomprehendingandcompletingthetoughestandmostintensivequestionsaboutlipids.
PartA:WhatisaLipid,Anyway?Bydefinition,alipidis“an[organic]moleculeofbiologicaloriginthatissolubleinsolventsoflowpolarityandinsolubleinsolventsofhighpolarity.”1
Atfirstglance,thereseemtobemanycomplexpartstothisdefinition,solet’sbreakitdown:
First,alipidisanorganicmolecule,meaningthemoleculemustcontaincarbon.Furthermore,lipidsarebiomolecules,meaningthattheyderivefrombiologicalorigins(ie.aresynthesizedbyalivingorganism).Assuch,atitsmostbaselevel,themolecularformulaforlipidswillcontaincarbons,hydrogens,andoxygens.
Secondly,lipidsaresolubleinsolventsoflowpolarity,whichsimplymeansthatlipidsaresolubleinnonpolarsolvents.Thissolubilitypropertyisaresultofthelargenonpolarregionsthatareindicativeofthelipidmolecule.
HOWCANYOUDISTINGUISHALARGENONPOLARREGION?
Longhydrocarbonchainsinlipidmoleculesdevelopthelargelynonpolarregionalpropertyoflipids.Duetocarbonandhydrogenformingnonpolarbondsduetotheirlowelectronegativedifference(asopposedtoOxygen‐Hydrogen,whichhaveahighelectronegativedifferenceandarethuspolar),alargehydrocarbonchainorringcreatesanonpolarregion.
Forexample:PHOSPHOLIPID
NONPOLARREGION
Anotherexample:STEROIDS
FATTY
Thelargehydrocarbonrings,methylgroups,andhydrocarbonchainsallconsistofnonpolarC‐Hbondsduetoitslowelectronegativedifference,andassuchcontributedtoitsnon‐polarity.
Whenfinalsdayapproaches,itwillbeimportanttounderstandwhatareasofthelipidmoleculecontributetoitsnon‐polarity,sotakeheed!
What’sthesignificanceofthenon‐polarity/lowpolarity?
Themostsimplistic(andnotentirelychemicallytrue)rule‐of‐thumbisthat“likedissolveslike”,whichindicatesthatasolutewilldissolveinasolventofidenticalpolarproperty,buttwosubstancesofopposingpolarpropertieswillnotinteract.Forexample,anonpolarmolecule(ie.afattyacid)willnotdissolveinapolarsubstance(ie.water),andviceversa.
Assuch,thirdly,lipidsareinsolubleinwater(apolarmoleculeduetothehighelectronegativeO‐Hbonddifference),fornon‐polarmoleculesareinsolubleinpolarmolecules.Thus,thepresenceofanonpolarmolecule(orregionofmolecule)meansthattheregionormoleculeishydrophobic(Greek,“waterfearing”),andproducesahydrophobiceffect,“whichcausesthepolarendstobeorientedoutwardstowardsthesolventandthenonpolarregionsorientedinwardsawayfromthepolarsolvent.”2,whenincontactwithapolarsolvent(ie.solvent),makingthenonpolarmoleculeisinsolubleinwater.Thistermisinterchangeablewithlipophilic(Greek,“fatloving”),indicatedthatnonpolarlipidsaresolubleinothernonpolarsolvents.
Thus,polarmoleculesareconsideredtobehydrophilic(“waterloving)andlipophobic(“waterfearing”),duetotheabilityofhighlypolarbondsbeingsolubleinpolarsolvents,andinsolubleinnonpolarsolvents(ie.fats,lipids).
However,therearecategoriesoflipidsthatarebothhydrophobicandhydrophilic.Theselipidsarecalledamphiphiles,moleculesthatcontainbothlargeareasofpolarityandnonpolarity,andarethusbothhydrophilicandlipophilic.Phospholipidshavebothalonghydrophobichydrocarbontail,andahydrophilichead,givingphospholipidsintegralbiologicalproperties,whichwillbelaterdiscussed.
SUMMARY,PLEASE!:Lipidshavelargenonpolarregionsderivingfromnonpolarhydrocarbonbonds(whichhavelowelectronegativedifference).Thus,lipidsarelargelyinsolubleinpolarsolvents(water),andaresolubleinnonpolarmolecules.Assuch,thenonpolarregionsarehydrophobic/lipophilic,andthepolarregionsarehydrophilic/lipophobic.
Part2:WhataretheDifferentkindsofLipids?Thereare8majorclassificationsoflipids(with7ofthembeingimportantforthepurposeofChem14C)
1. FattyAcids
2. Waxes
3. Triacylglycerides
4. Phospholipids
5. Prostaglandins
6. Steroids
7. LipophilicVitamins
8. Terpenes(plant‐basedlipids)
Part3:HowintheworlddoItelltheselipidsapart?Thissectionwillbethemostinvaluabletoyoufortheexam.Mostexamquestionsfrompreviousyearscenteronhowtodistinguishandclassifytypesoflipids,andtheimportantdefinitionsbehindsaidmolecules.
FattyAcids:
Fattyacidsarelipidsthatconsistof(1)Carboxylicacid(‐COOH)witha(2)long,un‐branchedhydrocarbonchain.
Example:Arachidicacid(C20H40O2)
CARBOXYLICACID(‐COOH)LONGUNBRANCHEDHYDROCARBONCHAIN(C‐H)
Fattyacidscommonlyhavethefollowingcharacteristics(besuretolookforthesepropertiesontheexam)
‐ Evennumberofcarbons(ex.arachidicacidhas20carbons).Themostcommonfattyacidsconsistof12‐20carbons,whilethemostbiologicallyimportantfattyacidshave18carbons(forexample,stearic,oleic,andlinoleicacids)
‐ Actasabasic“buildingblock”formorecomplexlipidmolecules(actasaprecursortootherlipids).Forexample,thelonghydrocarbonchainprovidesanessentialnonpolarregionforthehydrophobictailofphospholipids;waxes,triacylglycerides,andphospholipidshaveatleastonefattyacidasanattachment.
‐ LowpolarityduetothelowelectronegativedifferenceintheC‐Hbondsofthelonghydrocarbonchain,producingahydrophobiceffect.
Furthermore,fattyacidscanbedividedintotwosubcategories:saturatedandunsaturated.ThisismaincategorizedbythepresenceofC=Cbondsinthehydrocarbonchain.SaturatedcarbonshavefullC‐Hbondsinthehydrocarbonchain,andassuchhavenonC=Cbonds.Unsaturatedcarbons,however,haveatleastoneC=Cbond(monounsaturated,morethanoneispolyunsaturated)inthechaintoreplacetwohydrogenC‐Hbonds.
Withinunsaturatedfattyacids,cis‐C=Cbonds(Carbonbondsareonthesameside)aremuchmoreprevalentthantrans‐C=Cbonds(carbonbondsonoppositesides)innature.“Trans‐fats”arenotaseasilymetabolizedbycatalyticenzymesascis‐fats,andareleftuncatalyzedandcandamagehealth(plaquebuildup).
Waxes:
Waxesare(1)estersthathaveattached(2)afattyacidand(3)along‐chainalcohol.Thesynthesisofwaxesusuallyderivesfromthedehydrationsynthesisofafattyacidandalong‐chainalcoholgroup.
Duetothelowpolaritycausedbythetwolonghydrocarbonchainsoneithersideoftheesterlinkage,waxesarehighlyinsolubleinpolarsolventsandarehydrophobic;extremelyrepellenttowardwater.Asaresult,themajorbiologicalfunctionwaxesaretoactasawaterbarrier(ie.birdfeathersarecoatedinwaxlipids,whichminimizeswettingofthefeathers,leavesarecoatedinwaxtopreventtheevaporationofwater,damagingplantnourishment.
Myricyl cerotate Present in beeswax, carnauba
wax
ESTER
Long‐chainalcohol(‐OH)
Fattyacid
Triacylglycerols:
Triacyglycerols,ortriacylglycerides,arelipidssimilartothatofwaxes(inthattheycontainesterandanalcohol‐likefunctiongroup),butconsistof(1)atriester,(2)afattyacid,and(3)glycerol,orglycerin,athreecarbonalcoholstructure:
Glycerolandthefattyacidundergodehydrationsynthesis,whichinturncreatesthetriacylglycerolmolecule.
Where“R”indicatesthelong,un‐branchedhydrophobictailofhydrocarbonsthatarerepresentativeofthefattyacid.
Triacylglycerolshavethefollowingimportantcharacteristics
‐ Varyingmeltingpoints.Ifsolidatroomtemperature,thenthelipidisconsideredafat,ifliquidatroomtemperature,oil.
‐ Themostabundantnaturallysynthesizedlipid.‐ Itsmainbiologicalfunctionisforenergystorage(ie.consumptionoffatsprovidesaslowburninglong‐
termstorageofenergythatcanbemetabolized).‐ Withtheadditionofwaterinahydrolysisreaction,the“breaking”offatsderivedfromanimalsyields
soaps,whicharehydrophilicCO2‐groupsattachedtoahydrophobichydrocarbonchain.
Glycerol Fatty acids
Triacylglycerol
HOWDOSOAPSWORK?
1. Thepolar,hydrophilicheadofsoap(CO2‐)isattractedtothepositivelychargedendsofwater,whilethe
hydrophobichydrocarbontailcanavoidthewater.2. Thishydrophobic,lipophilictailthencanattachtodirts,fattyacids,andotherlipids,duetotheirlow
polarity.3. Thesetailssurroundthe“dirts”inspherescalledmicelles,whichisolatethedirtandremovethem
frominteractionwiththewatermolecules.4. Finally,whenthewaterisremoved,themicellesencapsulatingthedirtarecarriedaway,leavingthe
areafreeofthedirts.
Phospholipids:
Aphospholipidsimilarlyconsistsof(1)glycerol,three‐carbonalcoholstructureswhichthenformsan(2)triesterwith(3)twofattyacidsand(4)onephosphategroup
Genericexample:
Where“R”representsthelongun‐branchedhydrocarbonchainrepresentativeofafattyacid.
Phospholipidshavethefollowingkeyproperties:
‐ Secondmostabundantnaturallysynthesizedlipid‐ Itsmajorbiologicalfunctionistoformaintegralphospholipidbilayerinthecellmembraneofliving
organisms,acriticalcomponentthatallowsfortheselectivediffusionofionsandmoleculesthroughthebarrier
HOWDOESTHEPHOSPHOLIPIDBILAYERWORK?
Twophospholipidsfaceapartfromeachother,thepolar,hydrophilicheadofthephosphategroupfacingoutwards,andthehydrophobic,hydrocarbonchainonthefattyacidsfacinginwards.Thiscreatesthehydrophobiceffectofthetails,avoidingwater.Thisbilayerofthecellmembraneallowsforthecelltofreelymovethroughwater,yetmaintaintheinternalstructureofthecell
Phosphategroupester
2Fattyacidesters
Prostoglandins:
Prostoglandinsarelipidmoleculesthatcontaina(1)prostanoicacidskeleton,whichconsistofa(2)cyclopentaneringattachedtoaupperchainofa(3)fattyacidwithsevencarbons,andalower(4)long,unbranchedchainofhydrocarbons,with8carbons.
Ex.ProstaglandinF2alpha
Thenomenclatureofprostaglandinsisdependentonthestereochemistryofthelipid,basedonthenumberofOH,C+C,andC=Ogroupsinthemolecule.
Majorcharacteristicsofprostaglandinsinclude:
‐ Majorbiologicalfunctiontoactasaregulatorandsignalmolecule(regulationofhormones,inflammation,calciummovement;controlofcellulargrowth;constrictionofsmoothmusclecells;regulationofplateletgrowth;spinalneuronsensitization)
‐ Prostoglandinsoftensynthesizeatwoundsitestoregulateandsignalaninflammatoryresponse,leadingtoinflammation.
‐ Shorthalf‐lifefromorigination,about5minutesorless.‐ Whileeachstructurehasthesameprostanoicacidskeleton,thedifferingstereochemistryofeach
prostaglandinderivesvastlydifferingbiologicalfunctionsforeachlipid.
Prostaglandin F2α (PGF2α
)
Upperchainoffattyacids
Lowerchainofhydrocarbons
Cyclopentane
ring
Steroids:
Steroidmoleculesconsistofabasic(1)4‐carbonringskeletonformingaflatcore.
Examplesofsteroidsinclude:cholesterol,testosterone,estrone,cortisone,aldosterone,corticoidhormones,bileacids,andcholicacids.
Whileallsteroidshaveaverysimilarcarbonringskeletalstructure,thevariousfunctiongroupattachmentsgivesteroidsaverywiderangeoffunctions.
Abbreviatedlistofbiologicalfunctions:
1. Hormones:chemicallyregulatethebodya. Testosteroneandestrone,sexhormones,regulatethesexualdevelopmentoforganismsb. Corticoidsteroids/hormones,regulatethemetabolicprocessesofthebodyc. Cortisone:regulatesthebodyinflammatoryresponse,glucosemetabolismd. Digestive:emulsifiesfatsinintestinestoaidinthedigestiveprocess,integralcomponentofbile
acid.
Biosynthesisofsteroidsisalongandcomplexprocess,startingfromAcetylCoenzymeA;variousenzymaticstepsformintermediatesteroidmolecules,formingcholesterolattheend.
Cholesterolistheprecursortoallvertebratesteroids,themostbasicsteroidbiologicallyproducedbylivingorganisms.
LipophilicVitamins:
First,vitaminsareorganiccompounds(containingcarbon,hydrogen),thatarenotclassifiedasfats,proteins,orcarbohydrates,thatarerequiredforthegrowth,regulation,andmaintenanceofanimals.Thisbroaddefinitionofvitaminsthusencompassescompoundswithawidevarietyofstructuresandfunctions.
Therearetwomajorlipophilic(andthushydrophobic)vitamins:VitaminEandVitaminA(retinol)
VitaminEisamixtureofstereoisomers,mainlyconsistingofamoleculecalledalpha‐tocopherol
Withahydrophobictailcreatedbythelonghydrocarbontail.Itsmainfunctionistoprotectcellsagainstfreeradicals,unstableionswithanopenvalencethatcancauseoxidativedamagetothecells.
VitaminA(retinol)containsalonghydrophobicnonpolarhydrocarbonchain,withitsmainfunctionbeingusedinphoton‐harvestingproteins,essentialtothedevelopmentofvision.
Theeasiestmethodofdistinguishinglipophilicvitaminsfromotherlipidcategoriesisthattheyhaveastructureunlikethatofotherlipids,andcanwidelyvary.Whenindoubt,Useamethodofeliminationtorejectthepossibilityofthemoleculebeinganyotherkindoflipidfirst.
α-Tocopherol Hydrophobic antioxidant
vitamin
Part4:Finally,whatshouldIpracticethemostformytestonlipids?Thefollowingisanabbreviatedlistoftopicsyououghttorememberforthefinalexam.Thebestrecommendationistodoalloftheproblemsfrompreviousfinalexams,astheywillbemostindicativeoftherigorandextentinwhichitdelvesintolipids.
‐Moleculeswiththelargesthydrocarbonchainshavelowerpolarity,andarethusmostsolublewithfats,andinsolublewithwater.
‐Whenaskedwhichmoleculewitheasilypassthroughahydrophobicmembrane,lookforthemoleculewhichexhibitsthelowestpolarity(lowestelectronegativedifferenceinbonds,diploemoments,etc.);rememberthesimple“likedissolvelike”rule.
‐Knowinsideandoutthekeyfundamentalcharacteristicsofeachcategoryoflipid(asoutlineineachbox),andthecommonnamesofafewmoleculesofeachcategory.
‐Ifdescribingthestructurecangetconfusingforyouattimes,learninghowtodrawthebasicstructureofeachlipidwouldbeinvaluableinyouranalysisofthelipidanditsfunctionalproperties.
‐Knowthemostbasispropertiesoflipids(insolubleinwater,solubleinnonpolarsolvents,organicbiologicalmolecule)
‐Finally,whenindoubtaboutdrawingthestructureoflipidcategories,drawthemostgenericexample.
Suggestedpracticereview:Problem16,FinalExamPartA,Fall2012;Problem13,FinalExamPartA,Winter2012.Allproblemscanbefoundonhttp://www.chem.ucla.edu/harding,underPastExams.
Goodluck,anddigdeep,theanswerismostdefinitelyinyourgut.
Works Cited
Brandt, Mark. "Introduction to Lipids." Rose-Hulman.edu. Rose-Hulman Institute of Technology, 2011. Web. 9 June 2012. <http://www.rose-hulman.edu/~brandt/Chem330/Lipid_properties.pdf>. Web.
“Fatty Baby”. Picture from http://musicallmorning2.files.wordpress.com/2009/03/fattybaby.jpg. Google Images, 9 June 2012. Web.
Hardinger, Steven. "Biomolecules Survey 2: Lipids." Chemistry 14C, Lecture 1. UCLA, Young CS50, Los Angeles. 4 June 2012. Lecture. (Majority of information from this tutorial derives from this lecture)
Hardinger, Steven. Chemistry 14C: Organic Molecular Structures and Interactions : PowerPoint Lectures for Chemistry 14C with PowerPoint CD. Plymouth, MI: Hayden-McNeil, 2012. Print. (All chemical diagrams, pictures, and figures are taken from Prof. Hardinger’s Powerpoint Slides provided with the Course Supplement.)
1,2 Hardinger, Steven. “Illustrated Glossary of Organic Chemistry: A product of the Institute for Reduction of Cognitive Entropy in Organic Chemistry”. Organic Chemistry at UCLA. http://www.chem.ucla.edu/harding/ 2012. Web.
