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Osazone formation Phenylhydrazinein acetic acid, when boiled with reducing sugars, forms osazones in a reactionsummarizedin Fig,2, As is evident from the reaction, the first two carbons Cr and C2 are involved in osazone formation.

The sugars that differ in their configuration on these two carbons give the same type of osazones,since the differenceis maskedby bindingwith phenylhydrazine. Thus glucose,fructoseand mannosegive the same type needle-shaped osazones. Reducingdisaccharidesalso give osazones- maltose sunflower-shaped,and lactose powder- puff shaped.

RrcprcsentsCrto Crofglucose. The nomenclatureof glycosidic bonds is based on the Iinkagesbetweenthe carbon atomsand the status of the anomeric carbon o or p. For instance,lactose-which is formed by a bond between C1 of p-galactoseand Ca of glucose- is namedas 0. The other glycosidicbonds are describedin the structure of di- and polysaccharides.

Physiologieally important glycosides 1. Glucovanillin vanillin-D-glucoside is a naturalsubstancethat impartsvanilla flavour. Cardiac glycosides steroidalglycosides: Digoxin and digitoxin contain the aglycone steroidand they stimulatemusclecontraction. Streptomycin, an antibiotic used in the treatmentof tuberculosisis a glycoside. Sugar acids: Oxidation of aldehyde or primaryalcoholgroupin monosaccharideresults in sugaracids.

Cluconic acid is producedfrom glucose by oxidation of aldehyde C1 group whereasglucuronicacid is formedwhen primary alcoholgroup C6 is oxidized. Sugar alcohols polyols: They are producedby reductionof aldosesor ketoses. For instance,sorbitol is formed from glucose and mannitol from mannose. The monosaccharides, on reduction,yield polyhydroxyalcohols,known as alditols.

Ribitol is a constituent of flavin coenzymes; glycerol and myo-inositol are componentsof lipids. Xylitol is a sweetenerused in sugarlessgumsand candies.

Amino sugars: When one or more hydroxyl groups of the monosaccharidesare replaced by amino groups, the products formed are amino sugarse. D-glucosamine, D-galactosamine.

They are present as consti- tuentsof heteropolysaccharides. The amino groups of amino sugars are sometimes acetylated e. N-acetyl D-gluco- samrne. It is an important constituentof glycoproteins and glycolipids. The term sialicacid is usedto include NANA and its other derivatives.

Certain antibiotics contain amino sugars which may be involvedin the antibioticactivity e. These are the sugarsthat contain one oxygen lessthan that presentin the parent molecule. L-Ascorbic acid vitamin C: This is a water-solublevitamin, the structureof which closelyresemblesthat of a monosaccharide. The structuresof selected monosaccharide derivativesare depictedin Fig.

Among the oligosaccharides,disaccharides are the most common Fig. As is evident from the name, a disaccharideconsistsof two monosaccharideunits similaror dissimilar held together by a glycosidic hond. They are crystalline,water-solubleand sweetto taste. The disaccharidesare of two types '1. Reducingdisaccharideswith free aldehyde or keto group e. Non-reducingdisaccharideswith no free aldehyde or keto group e. Thefreealdehydegrouppresenton C1of second glucoseanswersthe reducingreactions,besides Structuresol monosaccharidederivatives selectedexamples.

Maltosecan be hydrolysedby dilute acid or the enzyme maltaseto liberate two moleculesof cr-D-glucose. Cellobioseis another disaccharide,identical in structurewith maltose,exceptthat the former has p 1 -r 4 glycosidiclinkage. Cellobioseis formedduringthe hydrolysisof cellulose. Suoroee Sucrose canesugar isthe sugarof commerce, mostlyproducedby sugarcane and sugarbeets. Sucrose is made up of a-D-glucose and p- D-fructose.

The reducing groups of glucose and fructose are involvedin glycosidicbond, hencesucroseis a non-reducing sugar,and it cannot form osazones. Sucroseis the major carbohydrateproduced in photosynthesis.

Sucroseis the mostabundantamong the naturallyoccurring sugars. This is due to the fact that in sucrose,both the functional groups aldehyde and keto are held togetherand protectedfrom oxidativeattacks. Sucrose is an important source of dietary carbohydrate. Sucroseis employed as a sweeteningagentin food industry.

The intestinal enzyme-sucrase-hydrolysessucroseto glucose and fructosewhich are absorbed. F-aetsse Lactoseis more commonlv known as milk sugarsinceit is the disaccharidefound in milk. Lactoseis composed ol p-D-galactoseand B-D- glucoseheld togetherby 0 1 -r a glycosidic bond. The anomericcarbonof C1glucoseis free, hence lactoseexhibits reducing propertiesand formsosazones powder-puffor hedgehogshape.

Lactose of milk is the most important carbohydratein the nutritionof young mammals. It is hydrolysedby the intestinalenzyme lactase to glucoseand galactose. But, r,r,hen hydrolysed, sucrose becomes levorotatory The Structures of disaccharides -maltose, sucrose and lactose.

The processof inversionis explainedbelow. Hydrolysisof sucroseby the enzyme sucrase invertasdor dilute acid liberatesone molecure each of glucoseand fructose. However, p-D- fructofuranoseis lessstableand immediatelygets converted to p-D-fructopyranose which is stronglylevorotatory ". Polysaccharides or simply glycans consistof repeat units of monosaccharides or their derivatives,held togetherby glycosidicbonds.

Theyareprimarilyconcernedwith two important functions-structural,and storageof energy. Polysaccharides are linear as well as branched polymers. This is in contrast to structureof proteinsand nucleicacidswhich are only linear polymers. The occurrence of branchesin polysaccharidesis due to the fact that glycosidic linkagescan be formed at any one of the hydroxylBroupsof a monosaccharide. Polysaccharidesare of two types 1. Homopolysaccharideswhich on hydrolysis yield only a singletype of monosaccharide.

They are named based on the nature of the monosaccharideunit. Thus,glucans arepolymers of glucose whereas fructosans are polymers of fructose. Heteropofysaccharideson hydrolysisyield a mixture of a few monosaccharidesor their derivatives.

High contentof starchis found in cereals,roots,tubers,vegetables etc. Starch is a homopolymer composed of D-glucoseunits held by a-glycosidicbonds.

Amylopectin,on the other hand, is a branchedchain with a 1 --r 6t glycosidicbondsat the branchingpointsand c 1 -; 4 linkages everywhere else Fig. Amylopectin molecule containing a few ChapteF 2: Starches are hydrolysed by amylase pancreaticor salivary to liberatedextrins,and finally maltoseand glucoseunits. Dextrins Dextrins are the breakdown products of starchby the enzyme amylaseor dilute acids. Starch is sequentially hydrolysed through different dextrins and, finally, to maltose and glucose.

The variousintermediates identifiedby iodine colouration are soluble starch blue , amylodextrin violet , erythrodextrin red and achrodextrin no colour. Inulin fnulin is a polymerof fructosei. It occursin dahlia bulbs,garlic,onion etc. Inulin is not utilized by the body. Glycogen Clycogen is the carbohydrate reserve in animals,henceoften referredro asanimal starch.

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It is present in high concentration in liver, followed by muscle,brainetc. Clycogenis also found in plantsthat do not possesschlorophyll e. The structureof glycogenis similarto that of amylopectin with more number of branches. The molecularweight up to 1 x and the numberof glucoseunits up to 25, vary in glycogendependingon the sourcefrom which glycogenis obtained. Structureofglycogen A Generalstructure B Enlargedat a branchpoint.

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Cellulose Celluloseoccursexclusivelyin plantsand it is the most abundant organic substancein plant kingdom. Celluloseis totally absent in animal body. Cellulosecannot be digestedby mammals- includingman-due to lack of the enzymethat cleavesB-glycosidicbonds a amylasebreakscr bondsonly. Certainruminantsand herbivorous animalscontainmicroorganismsin thegutwhich produce enzymesthat can cleave p-glycosidic bonds.

Hydrolysis of cellulose yields a disaccharide cellobiose, followed by P-D- glucose. Cellulose, though not digested, has great importancein human nutrition. The functions of dietary fiber include decreasing the absorption of glucose and cholesterolfrom the intestine,besidesincreasing the bulk of feces.

When the polysaccharidesare composedof differenttypesof sugarsor their derivatives,they are referred to as heteropolvsaccharidesor heteroglycans. Theseare more commonly known as glycosaminoglycans GAG. Acetylatedamino groups,besidessulfate and carboxyl groups are generally present in CAC structure. The presenceof sulfate and carboxyl groups contributesto acidity of the molecules, making them acid mucopoly,- saccharides. Someof the mucopolysaccharidesare found in combination with proteins to forrn mucoproteins or mucoids or proteoglycans Fig.

Structureof cellulose The repeat: Diagrammaticrepresentationof a prateoglycan complex. Mucopolysaccharidesareessentialcomponents of tissue structure. The extracellularspacesof tissue particularlyconnective tissue-cartilage, skin, blood vessels,tendons consistof collagen and elastinfibersembeddedin a matrixor ground substance.

Thegroundsubstanceis predominantly composedof CAC. The importantmucopolysaccharidesincluoe hyaluronicacid, chondroitin4-sulfate,heparin, dermatansulfateand keratansulfate Fig. Hyaluronicacid servesas a lubricantand shock absorbantin joints.

The bulk of dietary carbohydrote starch is dlgestedond finally obsorbedas glucose into the body. Ea Dextrose glucosein solution in dextrorotatory form is frequently used in medical Rq'- CF practice. Fructoseis obundantly found in the semen which is utilized by the spermsfor energy. Seueral diseoses are associated with carbohydrate. These include decreasing the intestinal absorption ol glucose and cholesterol, qnd increasingbulk of feces to ouoid eonstipation.

The enzgmehgaluronidaseof semendegradesthe gel contains hyaluronic acid around the ouum. This qllows eft'ectiuepenetration of sperm into the ouum. The mucopolysaccharideheparin is an onticoagulant preuentsblood clotting. The suruiual of Antarctic lish below -2"C is attributed to the antit'reeze glycoproteins. This enzyme is present in high concentrationin testes,seminalfluid, and in certainsnakeand insectvenoms. Hyaluronidase of semen is assignedan important role in fertilization as this enzyme clears the gel hyaluronicacid around the ovum allowing a better penetration of sperm into the ovum.

Hyaluronidaseof bacteriahelps their invasion into the animaltissues. Ghondroitin sulfates Chondroitin 4-sulfate Greek: Structurally,it is comparablewith hyaluronicacid. Chondroitin 4-sulfateconsistsof repeatingdisaccharideunits composedof D-glucuronicacid and N-acetyl D-galactosamine4-sulfate Fig.

Chondroitin5-sulfateis alsopresentin many tissues. As evident from the name, the sulfate group is found on C6 insteadof Ca. Heparin Heparin is an anticoagulant preventsblood clotting thatoccursin blood,lung,liver,kidney, spleenetc.

Heparin helps in the releaseof the enzyme lipoprotein lipase which helps in clearingthe turbidityof lipemic plasma. Heparin is composedof alternatingunits of N-sulfoD-glucosamine6-sulfateand glucuronate 2-sulfate Fi9. Dermatan sulfate The name dermatansulfateis derived from the fact that this compoundmostlyoccursin the skin. Structuresof commonglycosaminogi',-;-: Promotes woundhealing Chondroitinsulfate D-Glucuronicacid, N-acetylgalactosamine 4-sulfate Cartilage,bone,skin,bloodvessel walls Helpstomaintainthestructure andshapesoftissues Heparin D-Glucuronate2-sulfate,Blood,lung,liver,kidney,spleen N-sulfoglucosamine 6-sulfate Actsasananticoagulant Dermatansulfate L-lduronicacid,N-acetyl- galactosamine4-sulfate Bloodvesselvalves,heartvalves, Maintainstheshapesoftissues skin Keratansulfate D-Galactose,N-acetyl- glucosamine6-sulfate Cartilage,cornea,connective tissues Keepscorneatransparent 4-sulfate.

The only differenceis that there is an inversion in the configuration around C5 of D-glucuronic acid to form L-iduronic acid Fi9. Keratan sulfate It is a heterogeneousCAG with a variable sulfate content, besides small amounts of mannose, fructose, sialic acid etc. Keratan sulfateessentiallyconsistsof alternatingunitsof D-galactosamine and N-acetylglucosamine 6-sulfate.

A summaryof the glycosaminoglycanswith regardto composition,distributionand functions is given in Table 2. Several proteins are covalently bound to carbohydrateswhich are referredto as glyco- proteins.

Clycoproteins are very widely distributedin the cells and perform variety of functions. Theseincludetheir role as enzymes, hormones,transportproteins,structuralproteins and receptors.

A selectedlist of glycoproteins and their major functionsis given in Table2. The carbohydratesfound in glycoproteins include mannose, galactose, N-acetyl- glucosamine, N-acetylgalactosamine,xylose, L-fucoseand N-acetylneuraminicacid NANA.

Antifreeze glycoproteins: The Antarctic fish live below -2oC, a temperatureat which the Glycoprotein s Major function s Collagen Hydrolases,proteases, glycosidases Ceruloplasmin lmmunoglobulins Synovialglycoproteins Thyrotropin,erylhropoietin Bloodgroupsubstances Fibronectin,laminin Intrinsicfactor Fibrinogen Structure Enzymes Transport Defenseagainstinfection Lubrication Hormones Antigens Cell-cellrecognitionand adhesion Absorptionofvitamin8,, Bloodclotting Antifreezegiycoproteins consistof 50 repeatingunits of the tripeptide, alanine-alawine-threonine.

The carbohydrate contentalso playsa determinantrole in blood Eroup! Carbohydrs,tesare the polyhydroxyaldehydesor ketones,or campounds whichproduce them on hydrolysis. The term sugor is applied to carbohydratessoluble in water and stDeetto taste. Carbahgdratesqre the major dietary energy sources,besidestheir inualuementin cell structure and uariousother t'unctions.

It' two rnonosaccharidesdiffer in their structurearound o singlecarbonatom, they ore known as eplmers. Glucoseand galactoseare C4-epimers. D'Glucose is the most predominant naturally occurring aldosdmonosaccharide.

Giucoseexisfscs a and p anemerswith dit'Jerentopticalrotations. The interconuersion of a and B anomericforms with changein theopticalrotatianis knoun asmutsratation.

Manosaccharidespariicipate in seuercl recctions"Theseinclude oxidation, reduction. Formatian ol esters and glycosides by manosacchqridesis af specialsignificanceln biochemical reactions. Among the oligosacchqrides,disoccharidesare the most common. Theseinclude the reducing disaccharidesnamely lactose rnilk sugar and maltase malt sugar and the non-reducingsucrose canesugar.

Palysacclwridesare the poiymersot' monosaccharidesor their deriuatiues,held together by glycosidic bonds. Homopalysaccharidessre compasedot' a single manosaccharicle e. Heteropolysaccharidescontain a mixture af Jew monasaceharidesor thetr derluatiues e. Slorch and glgcogensre the carbohydratereseruesot' plants and animalsrespectiuelg.

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Cellulose,exclusiuelyt'ound in plants, is the structural constituent. Inulin is utilized to ossesskidney tunction bg measuringglomerular t'iltration rate GFR. Theyprouide the mstrix or grownd substanceof extracellular tissuespacesin whtchcollagenand elastinfibers are embedded. Hyaluranic ocid,chondroitin 4'sult'ote, heporin, are amang the important glycosaminaglgcdns. Glycoproteins are a group of biochernically important compaunds with a uariable composition of carbohyd.

Seueralenzyrnes, hormanes,structura! Essayquestions 1. Define and classifycarbohydrateswith suitableexamples. Add a note on the functionsof carbohydrates. Describethe structureand functionsof mucopolysaccharides. Cive an accountof the structuralconfigurationof monosaccharides,with specialreferenceto glucose.

Discussthe structureand functionsof 3 biochemicallyimportantdisaccharides. Definepolysaccharidesand describethe structureof 3 homopolysaccharides. Fill in the blanks 1. Namea non-reducingdisaccharide 2. The carbohydratethat is taken as a referencefor writing the configurationof others 3. The s and B cyclicformsof D-glucoseare referredto as The non-carbohydratemoietyfound in glycosidesis known as Cive an exampleof a glycosideantibiotic Theglycosidicbondsat the branchingpointsin the structureof starchare The polysaccharideemployedfor the assessmentof kidneyfunction The glycosaminoglycanthat servesas a lubricantand shockabsorbantof joints Namethe sialicacid,mostlyfound in the structureof glycoproteinsand glycolipids IV.

Multiple choice questions Riboseand deoxyribosedifferin structurearounda singlecarbon,namely a Cr b Cz c C: One of the followingis not an aldose a Clucose b Calactose c Mannose d Fructose. The glycosaminoglycanthat servesas an anticoagulant a Heparin b Hyaluronicacid c Chondroitinsulfate d Dermatansulfate. The followingpolysaccharideis composedof B-glycosidicbonds a Starch b Clycogen c Dextrin d Cellulose.

The carbonatomsinvolvedin the osazoneformation a 'l and 2 b 2 and 3 c 3 and 4 d 5 and 6. Lirpirdls fl? As such. Lipidsmay be regarded as organic substances relatively insoluble in water, soluble in organic solvents alcohol, ether etc. Unlike the polysaccharides,proteins and nucleic acids,lipids are not polymers. Further, lipidsare mostlysmall molecules. Lipids are broadlyclassified modifiedfrom Bloor into simple, complex, derived and miscellaneouslipids,whicharefurthersubdivided into differentgroups 1.

Simple lipids: Estersof fatty acids with alcohols. Theseare mainly of two types a Fatsand oils triacylglycerols: Theseare estersof fatty acids with glycerol. The difference between fat and oil is only physical. Thus,oil is a liquid while fat is a solid at room temperature.

Estersof fattyacids usuallylong chain with alcoholsotherthan glycerol. These alcohols may be aliphatic or alicyclic. Cetylalcoholis mostcommonly found in waxes. Complex or compound lipids: Theseare estersof fatty acids with alcohols containing additional groups such as phosphate, nitrogenousbase, carbohydrate,protein etc They are furtherdividedas follows a Phospholipids: They containphosphor,c acid and frequentlya nitrogenousbase This is in addition to alcohol and fai:.

Chapter 3: Thesephospho- lipids containglycerolas the alcohol e. Sphingosineis the alcohol in this group of phospho- lipidse. Theselipids contain a fatty acid, carbohydrateand nitrogenousbase. The alcohol is sphingosine,hence they are also called as glycosphingolipids.

Clycerol and phosphateare absente. Macromolecularcomplexes of lipids with proteins. Sulfolipids,amino- lipidsand lipopolysaccharidesareamong the othercomplex lipids.

Derived lipids: Theseare the derivatives obtainedon the hydrolysisof group 1 and group 2lipids which possessthe characteristicsof lipids. Theseincludeglycerolandotheralcohols, fatty acids,mono- and diacylglycerols,lipid fat soluble vitamins, steroid hormones, hydro- carbonsand ketonebodies.

The lipids which are unchargedare referredto asneutrallipids. These are mono-, di-, and triacylglycerols,cholesterol and cholesterylesters. Functions of lipids Lipids performseveralimportantfunctions 1. They are the concentratedfuel reserveof the body triacylglycerols. Lipids are the constituentsof membrane structure and regulate the membrane permeability phospholipidsand cholesterol. They serve as a source of fat soluble vitamins 4, D, E and K.

Lipidsare importantas cellularmetabolic regulators steroidhormonesand prostaglandins. Lipidsprotectthe internalorgans,serveas insulatingmaterialsand give shapeand smooth appearanceto the body.

Fatty acids are carboxylic acids with hydrocarbonside chain. They are the simplest form of lipids. Occurrence Fattyacidsmainly occur in the esterifiedform as major constituentsof variouslipids.

They are also present as free unesterified fatty acids. Fattyacidsof animalorgin are much simplerin structure in contrast to those of plant origin which oftencontaingroupssuch asepoxy,keto, hydroxy and cyclopentanerings.

Even and odd carbon fatty acids Most of the fatty acids that occur in natural lipids are of even carbons usually 14C-2OC. This is due to the fact that biosynthesisof fatty acidsmainly occurswith the sequentialaddition of 2 carbon units. Among the odd chain fatty acids, propionic acid 3C and valericacid 5C are well known. Saturated and unsaturated fatty acids Saturatedfatty acids do not contain double bonds,while unsaturatedfattyacidscontainone or more double bonds.

Both saturated and unsaturatedfatty acids almost equally occur in the natural lipids. Fatty acids with one double bond are monounsaturated,and thosewith 2 or more double bonds are collectivelv known as polyunsaturated fafty acids PIJFA.

Nomenclature of fatty acids The namingof a fatty acid systematicname is basedon the hydrocarbonfrom which it is derived. The saturatedfatty acids end with a suffix -anoic e. Numbering of carbon atoms: The carbonsadjacentto this carboxylC are2, 3, 4 and so on or alternatelya, F, T and so on. The terminalcarbon containingmethyl group is known omega or carbon. Starting from the methylend, the carbonatomsin a fattyacid are numberedas omega 1, 2, 3 etc.

Shorthand representation of latty aclds lnstead of writing the full structures, biochemists employ shorthand notations by numbers to representfatty acids. The general rule is that the total numberof carbonatomsare written first,followed by the nunrberof double bonds and finally the firstcarbon position of Common Name Systematicname Abbreviationx Structure l.

Ghapten 3: Thus,saturatedfattyacid, palmiticacid iswritten as. There are other conventionsof representing the double bonds. Ae indicatesthat the double bond is between9 and 10 of the fatty acid. Naturallyoccurringunsaturated fatty acids belongto ro 9, ol 6 and o 3 series. The fatty acidsthat cannotbe synthesizedby the body and, therefore, should be supplied in the diet are known asessentialfattyacids EFA.

Chemically, they are polyunsaturated fatty acids, namely linoleic acid Arachidonic acid The structuresof EFA are given in the Table3. Biochemical basis for essentiality: Linoleic acid and linolenic acid are essentialsince humans lack the enzymesthat can introduce double bonds beyond carbons9 to Functionsof EFA: Essentialfatty acids are required for the membrane structure and function, transportof cholesterol,formation of lipoproteins,preventionof fatty liver etc.

They are also needed for the synthesisof another important group of compounds, namely eicosanoids Chapter Deficiency of EFA: The deficiency of EFA results in phrynoderma or toad skin, characterizedby the presenceof hornyeruptions H.. Cis-trans isomerism in unsaturated fattv acids.

On the other hand, if the groups occur on the opposite side, it is a trans configuration. Thus oleic acid is a cis isomer while elaidic acid is a transisomer,as depicted in Fig.

Cis isomersare lessstablethan frans isomers. Most of the naturally occurring unsaturatedfatty acids exist as crs isomers. In the cis isomericform, there is a molecular binding at the double bond. Thus, oleic acid exists in an L-shapewhile elaidic acid is a straightchain. Increasein the numberof double bonds will cause more bends kinks and arachidonicacid with 4 doublebondswill have a U-shape.

Hydroxy fatty acids: Someof the fatty acids are hydroxylated. Cerebronic acid and recinoleic acid are long chain hydroxy fatty acids. Cyclic fatty acids: Fatty acids with cyclic structuresare ratherraree. Elaldicacid fransform Thesecompoundsare relatedro eicosapolyenoicfatty acids and include prosta- glandins,prostacyclins,leukotrienesand throm- boxanes. Theyarediscussedtogether Chapter Triacylglycerols formerly triglycerides are the estersof glycerol with fatty acids.

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The fats and oils thatarewidely distributedin both plants and animals are chemically triacylglycerols. They are insolublein water and non-polarin characterand commonly known as neutralfats. Fatsas stored fuel: Triacylglycerolsare the most abundantgroup of lipids that primarily function as fuel reservesof animals. Fats primarily occur in adipose tissue: Adipocytes of adipose tissue-predominantly found in the subcutaneouslayer and in the abdominalcavity-are specializedfor storageof triacylglycerols.

The fat is storedin the form of globulesdispersedin the entirecytoplasm. And surprisingly,triacylglycerolsarenot the structural componentsof biological membranes. Structures of acylglycerols: Monoacyl- glycerols, diacylglycerolsand triacylglycerols, respectivelyconsistingof one, two and three moleculesof fatty acidsesterifiedto a molecule of glycerol,are known Fi5.

Among these, triacylglycerols are the most important biochemically. Simpletriacylglycerolscontainthe sametype of fattyacid residueat all the threecarbonse. Mixed triacylglycerols are more common. They contain2 or 3 different typesof fattyacid residues.

In general,fatty acid attachedto C1 is saturated,that attached to C2 is unsaturated while that on C3 can be either. Triacylglycerols are named according to placement of acyl radicalon glycerole.

Triacylglycerols of plants, in general, have higher content of unsaturated fatty acids compared to that of animals. Thisis not true in a 3-dimensionalstructure. In orderto represent the carbonatomsof glycerolin an unambiguous manner, biochemists adopt a stereospecific numbering sn and prefixglycerolwith sn. Cells possess enzymes that can distinguish these two carbons. Thus glycerokinasephosphorylatessn-3 andnot sn-l glycerolto give sn-glycerol3-phosphate. Triacylglycerols undergo stepwiseenzymatichydrolysisto finally liberate free fatty acids and glycerol.

The processof hydrolysis,catalysedby lipasesis importantfor digestionof fat in the gastrointestinaltract and fat mobilizationfrom the adiposetissues. The hydrolysisof triacyl- glycerolsby alkalito produceglyceroland soaps is known as saoonification. Rancidityis the term used to represent the deterioration of fats and oils resultingin an unpleasanttaste.

Fatscontaining unsaturatedfatty acids are more susceptibleto ranciditv. Rancidity occurs when fats and oils are exposed to air, moisture, light, bacteria etc.

Hydrolytic rancidity occurs due to partial hydrolysis of triacylglycerols by bacterial enzymes. Oxidativerancidityis due to oxidation of unsaturatedfatty acids.

This results in the formation of unpleasant products such as dicarboxylic acids, aldehydes, ketones etc. Rancid fats and oils are unsuitablefor human consumotion.

The substanceswhich can preventthe occurrenceof oxidativerancidityare known as antioxidants. Trace amounts of antioxidantssuch as tocopherols vitamin E , hydroquinone,gallic acid and c,-naphtholare addedto the commercialpreparationsof fatsand oilsto preventrancidity. Propylgallate,butylated hydroxyanisole BHA and butylated hydroxy- toluene BHT are the antioxidantsused in food preservation.

In the living cells, lipids undergo oxidation to produce peroxidesand free radicalswhich can damage the tissue. Tests to check purity of fats and oils Adulterationof fatsand oils is increasingday by day.

Several tests are employed in the laboratoryto check the purity of fats and oils. Some of them are discussedhereunder lodine number: Thus lower is the iodine number,lessis the degreeof unsaturation.

FaUoil lodine number Coconutoil Butter Palmoil Oliveoil Groundnutoil Cottonseedoil Sunfloweroil Linseedoil 7- 10 28 4C- 55 85 Determinationof iodinenumberwill help to know the degreeof adulterationof a given oil. Thevalueis higher for fats containing short chain fatty acids.

The saponificationnumbersof a few fatsand oils are given below Humanfat: RM number is useful in testingthe purity of buttersince it containsa goodconcentrationof volatilefattyacids butyric acid, caproicacid and caprylicacid.

This is in contrastto other fats and oils which have a negligibleamount of volatile fatty acids. Butter hasa RM numberin the range,while it is lessthan I for mostotheredibleoils. Thusany adulteration of hutter can be easily tested by this sensitiveRM number. Acid number: In normalcircumstances,refinedoils shouldbe free from any free fatty acids. Oils, on decomoosition-due to chemical or bacterial contamination-yield freefatty acids.

Therefore, oils with increasedacid number are unsafefor humanconsumption. These are complex or compound lipids containingphosphoricacid,in additionto fatty acids,nitrogenousbaseand alcohol Fig. There are two classesof phospholipids 1. Clycerophospholipids or phosphoglyce- rides that contain glycerolas the alcohol. Sphingophospholipids or sphingomyelins that containsphingosineas the alcohol. Theyconsist of glycerol 3-phosphateesterifiedat its C1 and C2 with fatty acids.

Usually, C1 contains a saturated fatty acid while C2 contains an unsaturatedfatty acid. This is the simplest phospholipid. Basically, phosphatidicacid is an intermediatein the synthesisof triacylglycerolsand phospholipids. The other glycerophospholipidscontaining differentnitrogenousbasesor other groupsmay be regardedas the derivativesof phosphatidic acid.

Lecithins phosphatidylcholine zTheseare the mostabundantgroupof phospholipidsin the cell membranes. Chemically,lecithin Creek: Phosphatidylcholines represent the storage form of hody's choline.

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Thts can be preuented by the addition ol certain antioxidants uitamin E, hgdroquinone, gallic acid. Sttucturesof phospholipids. Respiratory distresssyndromein infantsis a disorder characterizedby the absenceof dipalmitoyl lecithin.

Cephafins phosphatidylethanolamine: Ethanolamineis the nitrogenousbasepresentin cephalins,Thus,lecithinandcephalindifferwith regardto the base.

The steroisomer myo-inositolis attachedto phosphatidicacid to givephosphatidylinositol Pl. Thisisan important comDonentof cell membranes. The action of certain hormones e.

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The amino acid serineis presentin this group of glycerophos- pholipids. Phosphatidylthreonineis alsofound in certaintissues. When a fatty acid is attachedby an etherlinkageat C1 of glycerolin the glycerophospholipids, the resultant compound is plasmalogen.

Phosphatidal- ethanolamineis the most imoortantwhich is similarin structureto phosphatidylethanolamine but for the ether linkage in place of ester.

An unsaturatedfatty acid occurs at C1. Choline, inositoland serinemay substituteethanolamine to give other plasmalogens.

Structurally, a cardiolipin consists of two molecules of phosphatidicacid held by an additionalglycerol through phosphategroups. Cardiolipin is the only phosphoglyceridethat possessesantigenic properties. Sphingomyelins Sphingosineis an amino alcohol presentin sphingomyelins sphingophospholipids.

They do notcontainglycerolat all. Sphingosineisattached by an amide linkageto a fatty acid to produce ceramide. The alcohol group of sphingosineis bound to phosphorylcholinein sphingomyelin structure Fig. Sphingomyelinsare important constituentsof myelin and are found in good quantityin brain and nervoustissues.

Action of phospholipases Phospholipasesare a group of enzymesthat hydrolysephospholipids. Thereare four distinct phospholipases Ar, 42, C and D , eachone of them specificallyactson a particularbond. For details,refer lipid metabolism Chapter l4. Functions of phospholipids Phospholipidsconstitutean importantgroup of compound lipids that performa wide variety of functions 1. In associationwith proteins,phospholipids form the structural components of membranes and regulatemembranepermeability.

Phospholipids lecithin, cephalin and cardiolipin in the mitochondriaare responsible for maintaining the conformation of electron transportchain components,and thus cellular respiration. Phospholipidsparticipatein the absorption of fat from the intestine. Check Here. Board Secretly Enforcing 3 Attempts Only? Thursday, April 4, Forgot your password? Get help. Create an account. Click the download link on page which will direct you to Google Drive for Satyanarayana book. It is a very good book and have very simple wording which we can better understand.

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