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Date Created: 10/28/15
Membranes A review Phospholipids The major subclass of glycerolipids and The mosT imporTanT class in mammalian cells are The phospholipids or phosphoglycerides By definiTion phospholipids are lipids which conTain phosphaTe Phospholipids are The major componenT of The cell membranes of higher organisms Therefore Their major funcTion is sTrucTural The chemical sTrucTure of phospholipids is derived from diacylglycerol A phospholipid can be considered To be a diacylglycerol where one of The Terminal hydroxyl groups has a phosphaTe molecule bound To iT This resulTs in The simplesT phospholipid iTs name being phosphaTidic acid There are oTher Types of phospholipids which are more commonly found in cells The mosT common phospholipid is phosphaTidylcholine where a choline group is esTerified To The phosphaTe group in phosphaTidic acid OVERHEAD OTher groups which can be bound To phosphaTe To form differenT Types of phospholipids are shown on Today39s overhead These include eThanolamine To form phosphaTidyleThanolamine serine To form phosphaTidylserine glycerol To form phosphaTidylglycerol myoinosiTol To form phosphaTidylinosiTol and phosphaTidylglycerol To form cardiolipin NexT I39d like To inTroduce you To a liTTle vernacular regarding lipid represenTaTions Common Jargon used in Talking abouT phospholipids makes an analogy beTween The lipid sTrucTure and a person The phosphaTe group and whaTever group is bound To H is called The headgroup The glycerol molecule is ofTen referred To aT The backbone and The faTTy acid chains are called The Tails The faTTy acids bound To The Two posiTions on The glycerol backbone of phospholipids are mosT ofTen differenT in naTurally occurring phosphoglycerides This is The resulT of The sTrucTure of The glycerol backbone relaTive To The phosphoglyceride funcTion The hydroxyl oxygens on The 1 and 3 posiTions of glycerol exTend from glycerol along The plane of The Three carbon molecule The hydroxyl group on The cenTer carbon exTends perpendicular To The glycerol axis OpTimum funcTion of The phosphoglyceride requires ThaT These Two faTTy acid chains exTend side by side approximaTely The same disTance ThaT is The Two chains need To be abouT The same physical lengTh However The faTTy acid aT The cenTer glycerol has a head sTarT iTs hydroxyl group has direcTed The faTTy acid in The correcT fashion while The faTTy acid bound To The end of The glycerol has To curve abouT To line up wiTh The oTher faTTy acid There are Two ways To help To even The lengTh of These Two chains The firsT is simply To have more carbon aToms on The faTTy acid bound aT posiTion 1 on The glycerol AnoTher meThod which is more ofTen used in naTure is To increase The number of double bonds in The faTTy acid aTTached To The 2posiTion of glycerol MosT naTurally occuring faTTy acid double bonds are E so adding a double bond resulTs in a quotkinkquot in The faTTy acid chain This sends The chain off in a direcTion oTher Than normal decreasing The run per carbon aTom and allowing The sn1 chain To caTch up OVERHEAD CholesTerol A final Type of lipid which is imporTanT in many cellular and organismal funcTions is cholesTerol CholesTerol has This sTrucTure CholesTerol is an imporTanT componenT of cell membranes IT is also an imporTanT precursor To Two differenT Types of molecules LIPID PHASE BEHAVIOR As a resulT of Their amphipaThic naTure and The bulk of The hydrophobic porTion of lipids relaTive To Their hydrophilic porTion lipid molecules are noT parTicularly soluble in aqueous soluTion as isolaTed hydraTed molecules InsTead lipids in waTer aggregaTe and form a separaTe discernable phase from waTer similar in some respecTs To a simple hexanewaTer Two phase sysTem and differenT in many respecTs The driving force behind The formaTion of separaTe lipid phases in aqueous soluTion is The hydrophobic effecT We discussed The hydrophobic effecT iniTially when we discussed proTein folding To review The hydrophobic effecT resulTs from unfavorable inTeracTions beTween hydrophobic parTs of amphipaThic molecules and waTer An aggregaTion of amphipaThic molecules resulTing in The burying or hiding of hydrophobic groups from The aqueous medium wiTh corresponding exposure of hydrophilic parTs of amphipaThic molecules To The aqueous medium provides The mosT energeTically sTable sTrucTure for These amphipaThic molecules WiTh proTeins The amphipaThic molecule is The proTein iT has boTh hydrophobic and hydrophilic side chains By proper folding The proTein can hide iTs hydrophobic parTs and display iTs hydrophilic parTs Therefore The IIaggregaTeII I JusT spoke of can be a single proTein Lipids are much smaller molecules wiTh a small number of hydrophobic and hydrophilic regions per molecule WiTh lipids some aggregaTe sTrucTure is formed which gives The same resulT as proTein folding hydrophobic groups are hidden from waTer while hydrophilic groups are exposed To waTer Remember The clusTering of hydrophobic enTiTies away from waTer is driven enTropically To solvaTe a hydrophobic molecule or parT of molecule a waTer claThraTe sTrucTure would have To form around The hydrophobic parT of The molecule This would resulT in The ordering of many waTer molecules By hiding The hydrophobic parTs wiThin The inTerior of some shelTered sTruc Ture The waTer was ordered around The hydrophobic parT is released free inTo soluTion resulTing in a favorable increase in enTropy In addiTion wiTh lipids more Than proTein favorable Van der Waals inTeracTions exisT beTween The hydrophobic regions of lipids furTher sTabilizing The sTrucTures which are formed Our bodies don39T have micelles however They are useful in Terms of inTroducing lipid sTrucTure The major sTrucTural lipid in our bodies are phospholipids When solid phospholipids are dispersed in aqueous soluTion sTrucTures called bilayer vesicles are formed OVERHEAD I39m going To explain The concepT of lipid bilayers in The same way as a geography Teacher would use a map To explain a globe A bilayer is simply a planar array of phospholipids IT39s called a bilayer because iT39s made up of one layer of phospholipids on Top of anoTher layer of phospholipids The Two layers inTerconnecT aT The hydrophobic acyl chain Tails of The phospholipids This effecTively hides all of The hydrophobic acyl chains from waTer while exposing The hydrophilic phospholipid headgroups To The aqueous medium In soluTion This exTended plane of bilayer does noT form because The ends of The plane aren39T infinaTe and There would always exisT some hydrophobic surface represenTed by The crosssecTion of This bilayer which would inTeracT unfavorably wiTh waTer InsTead liTTle hollow globes of lipid form which are called bilayer vesicles Why do phospholipids form bilayer vesicles and faTTy acids form micelles in aqueous soluTion The major reason appears To be The Three dimensional shape of The differenT lipids A molecule wiTh a large headgroup To acyl chain diameTer like for example a faTTy acid or a lysophospholipid which is a phospholipid wiTh one faTTy acid group removed will Tend To form micelles A molecule wiTh a small headgroup To acyl chain diameTer like phosphaTidylcholine for example will Tend To form bilayers The differences in geomeTry beTween The Two Types of molecules deTermine how Tigthy The hydrophobic acyl regions of These lipids pack TogeTher There should be no room for waTer To fiT beTween The acyl chains Therefore The phase assumed by The lipids will minimize The space beTween The lipids or maximize The packing efficiency Phospholipid bilayers are The basic sTrucTure of membranes LeT me Take a minuTe To discuss The physical properTies of phospholipid bilayers 1 Phospholipids in bilayers are held TogeTher by noncovalenT inTeracTions Therefore individual phospholipids in The bilayer have exTensive freedom of movemenT Two Types of moTion exhibiTed by The phospholipids are roTaTion They can spin wiThin The plane of The bilayer and diffusion They can move in Two dimensions wiThin The bilayer A moTion which is noT easily allowed is movemenT from one half of The bilayer To The oTher This would force The charged phospholipid headgroup To pass Through a hydrophobic region which is energeTically unfavorable 2 Bilayers are impermeable To charged molecules As JusT menTioned iT is energeTically unfavorable for a charged molecule To pass Through a hydrophobic membrane inTerior Polar molecules pass Through The membrane aT very slow raTes The one excepTion is waTer which passes Through phospholipid membranes relaTively easily As a rule of Thumb The abiliTy for a small molecule To pass Through a membrane is correlaTed wiTh Their solubiliTy in a nonpolar solvenT relaTive To Their solubiliTy in waTer RelaTive permeabiliTies of some molecules Through membranes Na 103912 cms across a membrane K 5 x 103912 CI39 8 x 103911 urea 4 x 10396 H20 5 x 10393 Biological membranes The membranes in our bodies are comprised of boTh proTein and lipid Real biological membranes conTain many differenT proTeins and many differenT Types of lipids Membrane proTeins are divided inTo Two general classes SEE OVERHEAD InTeqral or inTrinsic membrane proTeins have exTended regions which are imbedded inTo The lipid bilayer ExTrinsic membrane proTeins are aTTached To The membrane surface and are noT imbedded inTo The bilayer inTerior How can you Tell The difference beTween an inTrinsic and exTrinsic membrane proTein An exTrinsic membrane proTein is removed from The membrane by relaTively mild means One way To remove exTrinsic proTeins is To TreaT membranes wiTh 1 M NaCl The salT disrust ionic conTacTs beTween The exTrinsic membrane proTein and The hydrophilic bilayer surface allowing The proTein To leave InTrinsic membrane proTeins are removed from bilayers only by more severe TreaTmenT This is defined as some TreaTmenT which solubilizes or parTially solubilizes The membrane specifically TreaTmenT wiTh a deTergenT or organic solvenT will disrupT The sTrucTure of The bilayer and remove inTrinsic proTeins