ScientificEnglishThestructureandfunctionoflargebiologicalmolecules(PartII)Concept3Lipidsareadiversegroupofhydrophobicmolecules•Thecompoundscalledlipidsaregroupedtogetherbecausetheyshareoneimportanttrait:Theymixpoorly,ifatall,withwater.•Thehydrophobicbehavioroflipidsisbasedontheirmolecularstructure.Althoughtheymayhavesomepolarbondsassociatedwithoxygen,lipidsconsistmostlyofhydrocarbonregions.•Lipidsarevariedinformandfunction.Theyincludewaxesandcertainpigments,butwewillfocusonthemostbiologicallyimportanttypesoflipids:fats,phospholipids,andsteroids.Fats•Afatisconstructedfromtwokindsofsmallermolecules:glycerolandfattyacids.•Glycerolisanalcohol;eachofitsthreecarbonsbearsahydroxylgroup.•Afattyacidhasalongcarbonskeleton,usually16or18carbonatomsinlength.Thecarbonatoneendoftheskeletonispartofacarboxylgroup,thefunctionalgroupthatgivesthesemoleculesthenamefattyacid.•Inmakingafat,threefattyacidmoleculesareeachjoinedtoglycerolbyanesterlinkage,abondbetweenahydroxylgroupandacarboxylgroup.•Theresultingfat,alsocalledatriacylglycerol,thusconsistsofthreefattyacidslinkedtooneglycerolmolecule.•Stillanothernameforafatistriglyceride,awordoftenfoundinthelistofingredientsonpackagedfoods.•Thetermssaturatedfatsandunsaturatedfatsarecommonlyusedinthecontextofnutrition.•Thesetermsrefertothestructureofthehydrocarbonchainsofthefattyacids.•Iftherearenodoublebondsbetweencarbonatomscomposingachain,thenasmanyhydrogenatomsaspossiblearebondedtothecarbonskeleton.Suchastructureissaidtobesaturatedwithhydrogen,andtheresultingfattyacidthereforecalledasaturatedfattyacid.•Anunsaturatedfattyacidhasoneormoredoublebonds,withonefewerhydrogenatomoneachdouble-bondedcarbon.•Nearlyalldoublebondsinnaturallyoccurringfattyacidsarecisdoublebonds,whichcauseakinkinthehydrocarbonchainwherevertheyoccur.•Afatmadefromsaturatedfattyacidsiscalledasaturatedfat.•Mostanimalfatsaresaturated:Thehydrocarbonchainsoftheirfattyacids--the“tails”ofthefatmolecules--lackdoublebonds,andtheirflexibilityallowsthefatmoleculest...
