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17.3 Lipid Composition of Cell Membranes 529 (J tl CHr-O-C- t- (CHJ?CH :CH- (CH2)7CH3 lq ttl cH-o-c-(cH2)16cH3 o CHt -o -t - rcHz)zcH:cH- (cH2)7cH3 17.5Lipid compositionof cell membrones AIM: Torecognizethe generol structuresof the following three phosphoglycerides, typesof lipid molecules: sphingomyelins, ond glycolipids. Cellular membranes are made primarily of complex lipids. It is possible to break cells,empty them of their contents, and isolate the cell membranes.The cellmembrane is the "snck"that holds the contentsof cells and actsas a selectiuebarrier for the passageofcertain substancesin and out of the cell. The interior of cells also contains membrane structures, as described in A Closer Look Cells. Chemical analysis of the isolated membranes shows that lipids are the major components. These lipids are not triglycerides, but another group of compounds called complex lipids. Complex lipids contain parts madefrom substancesbesidesfatty acids and glycerol; some contain no glycerol.The complex lipids fall into two categories: phospholipids and glycolipids. PhosphoHpids are lipids that are estersof phosphoric acid. There are two main types of phospholipid molecules in cell membranes: phospho- y glycerides and sphingomyelins. Phosphoglycerrde moleculesare builtfrom long-chainfatty acids (14 to 24 carbons), glycerol, and phosphoric acid. Two fatty acids are covalently bonded to adjacent hydroxyl groups ofglycerol by ester linkages.The phosphoric acid is bonded through phosphate ester linkages to the remaining hydroxyl function of glycerol. The resulting molecule is called a phosphatidic acid, which is a phosphoglyceride. o cH2-o-cR CH2OH o CH-OH + 2RC-OH o + HO-P-OH I OH o + cH_o_cR o C H-, -tO - P - O H cH2oH OH Glycerol Fatty acids Phosphoric acid Aphosphatidic acid + 3H2O 550 CHAPTERl7 Lipids Cells The two major cell designs are prokaryotic and eukaryotic. The former is the more ancient of the two. Microscopic examination of fossilized.remains shows that prokaryotes were present on Earth at least 3 billion years ago, whereas eukaryotes did not appear until 2 billion yearslater. In the modern world, the prokaryotic cell design is limited to bacteria and blue-greenalgae.The cells of other cellular organisms, including green plants and people, are eukaryotic. Endoplasmic reticulum Mito chondrion_",.---- Golgibody "'=-.-.-.-/./ I Nucleus ', ' 1",' ', Both types of cells are essentially packagesof chemicalsnecessaryforlife encasedin a cell membrane. Eukaryotic cells are considerably larger and somewhat more complicated than prokaryotes, but the chemical processescarried out by both typesof cellsare very similar,and both are exceedingly efflcient chemicalfactories.The major feature that distinguishesprokaryotes from eukaryotes is the latter's organelles("little organs")-small membrane-enclosedbodies suspended in the interior cellular fluid or cltoplasm {seefigure). The organelles are the sites of many specialized functions in eukaryotes.The most prominent membrane-encasedorganellesand their functions are asfollows: Organelle Function Nucleus Mitochondrion Cellreproduction Production of most cellular energy in cells using oxygen for respiration Processing of proteins into glycoproteins Golgi body Lysosome Lysosome Cell membrane Theeukaryotic cell Digestion of cellular wastes and substancestaken into cells Yet another membrane structure in eukaryotes is the highly folded, netlike endoplasmic reticulum (ER).Among its variousfunctions,the ER servesas an attachment site for ribosomes-small organellesthat are not membrane-encased but are the sites where proteins are made. Endoplasmic reticulawith and without attachedribosomes are called, respectively,rough ER and smooth ER becauseof their appearanceunder a microscope. Living cells contain little or no free phosphatidic acid. Usually, the phosphorus of the phosphatidic acid is linked to the hydroxyl group of a second alcohol. Choline, a common amino alcohol constituent of cell membrane phospholipids, is an example.In Figure 17.2, the hydroxyl group of choline is attached to the phosphorus of the phosphatidic acid through a phosphateesterbond. This phospholipidis a phosphoglyceride 17.3 Lipid Compositionof Cell Membranes ffi ,!."ror." -7% Choline 551 zc*2crzc*zcHzcHzcHzcHzcH Fattyacid Glycerol o ll aCnrcslrcu"cH2cH2cH2cHzc]l'cHzcHzcHacH2cH3 II ccn crqcrlc:rrcn cfi?€.H-2€nt cfi,e.s!$ I Aphosphatidyl choline (or lecithin) # Charged head // \ \ Hydrophobic tail / Figure17.2 A phosphatidyl (phosphoglyceride) choline or lecithinmoleculeis constructed from the aminoalcoholcholine, phosphoric acid,glycerol, andtwo fattyacidmolecules. A simplifiedrepresentation of the molecule(lowerleft) hasbeenadoptedfor all membranelipids.Thehydrophilic (charged) headis shownasa purplesphere, andthe hydrophobic tailsasredwavylines.A space-filling modeloi a phosphatidyl choline moleculeis shownat the lowerrightof the figure. \ becauseit contains phosphorus and has a backbone of glycerol. It also m1y be called a phosphatidyl choline becauseit is an ester,of phosphatidic acid and choline . An older name for phosphatidyl choline;J leciitrin. It is important to recognize that the names phosphoglyceride,phosphatidic acid, and phosphatidyl choline are only general names for classesof compounds. The lengths of the hydrocarbon chains of the fatty acids may vary and these chains may be saturated or contain one or more double bonds. 5t2 CHAPTER l7 Lipids -,:-: PRACTICE EXERCISE I7.5 '.4-; := When the phosphorus of a phosphatidic acid is linked to the hydroxyl group of the amino alcohol ethanolamine, NH2CH2CH2OH,the com"== pound formed is phosphatidyl ethanolamine. Phosphatidyl ethanolamine, also called cephalin, is found in brain tissue and is important in = = blood clotting. Draw the general structure for cephalin. The second type of phospholipid moleculei encountered in cell rnetnbranes ls sphingomyelins. Sphingomyelins do not contain glycerol. Instead, they contain sphingosine, a long-chain unsaturated amino alcohol. Only one fatty acid is attached to sphingosine, as shovrrnin Figure 17.3, through an amide linkage. The structure of the nonpolar end of II ,[nnrcurcuzc:HzcTzcHzcHzcHzchrzc]HzcHzcH2cH3 HO Fatty acid *- Ho- cH, - H'- :; - * "" L.- : cHcH2cH2cH zcH2cHzcH2clr2cH2 cH2cH zcH2cHzcH s Phosphoric acid Sphingosine (an amino alcohol) ?" CHz-CH-CH-CH:CHCH2CH2CH,CH2CH.CH.CH2CH.CH.CH.CH?CH2CH3 Asphingomyelin p-cCharged head / \ Hydrophobictail Figure17.5 A sphingomyelin moleculeconsists of one moleculeeachof choline,phosphoric acid,sphingosine, anda fattyacid.A space-filling modelof a sphingomyelin moleculeis shownat the bottomof the figure. 17.3 Lipid Compositionof Cell Membranes The abnormal metabolism and accumulation of certain types of lipid molecules occur in a number of genetic diseases.For example, a glycolipid accumulates and damagesthe brain in Tay-Sachs disease. 555 sphingomyelins may differ somewhat, depending on the length and t degree of saturation of the fatty acid attached to the sphingosine amino\ group. Sphingomyelins are the only phospholipids that are not built on ' \ glycerol. Large amounts of sphingomyelins are found in brain and nervous tissue and in the myelin sheath,the protective coat of nerves. Glycolipids are lipid molecules that contain carbohydrates, usually simple sugars such as glucose or galactose.Figure 17.4 shows a glycolipid that consists of sphingosine, a fatty acid, and a sugar. These are called cerebrosidesbecause of their abundancein the brain. The cerebrosidesare not phospholipids, becausethey do not contain phosphorus. The classificationof lipids is summarizedin Figure 17.5. =ffi NH" OH tt Ho - cH, - cH - cF{-cH=cHCH2cH2cH zcKzcH2cHzcH2cHzcH2cH2cHzcH2cH3 Sphingosine (anaminoalcohol) Glucose cHz-cH- iOri-: l- CH- CH:CHCHzCH2CH2CH2CH2CH.CH.CH2CH2CH'CH2CH,CH3 A cerebroside # // \ Polar head \ Hydrophobic tail Figure17.4 (glycolipid) A cerebroside consists of a sugar,a sphingosine, andonefatty acidmolecule. Thesugarunitshownis glucose, but it maybe galactose' A space-filling modelof a cerebroside is shownat the bottomof the figure. CHAPTER 17 Lipids Sphingosine Triglyceride Phosphoglyceride Sphingomyelin Glycolipid Figure17.5 Lipidclassification diagram. 17,4Structareof liposomesond cellmembrones AIMS: To sketchsecfionsof the liposomolbiloyerin woter,lobeling Toexplointhe relotionthe polar end of the lipid molecules. ship betweenthe degreqof unsoturotionin phospholipid molecules ond membraleflexibility.To usethe fluid mosoic in modelto describethe movementof lipid molecules membrones. Complex Hpids form the lipid bilayers of liposomes and cell membranes. Phosphatidyl choline is a typical membrane phospholipid. It contains a charged head consisting of negatively charged phosphate and positively charged choline attached through glycerol to two hydrophobic fatty acid tails. If we vigorously shake a mixture of phosphatidyl choline and water, the lipid molecules form microscopic spheresrather than dispersing evenly in water. These lipid spheres,or liposomes, are packages of water surrounded by alipid,bilayet-a two-layer-thick wall of phosphatidyl choline. Figure 17.6showsa crosssection of a liposome.The lipid moleculesof the liposomal bilayer are more ordered than the sulfonic acid molecules in detergentmicelles (Sec.14.3). All the hydrophobic hydrocarbon tails of the lipids are protected from water, and all the hydrophilic phospholipid heads interact with water. Lipo-