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CBSEClass–12SubjectChemistry

NCERTSolutions

Chapter–14

Biomolecules

In-textQuestion

1. Glucose or sucrose are soluble in water but cyclohexane or benzene (simple six

memberedringcompounds)areinsolubleinwater.Explain.

Ans.Aglucosemoleculecontainsfive-OHgroupswhileasucrosemoleculecontainseight-

OHgroups.Thus,glucoseandsucroseundergoextensiveH-bondingwithwater.

Hence,thesearesolubleinwater.

But cyclohexane and benzene do not contain -OH groups.Hence, they cannot undergoH-

bondingwithwaterandasaresult,areinsolubleinwater.

2.Whataretheexpectedproductsofhydrolysisoflactose?

Ans.Lactoseiscomposedof -D-galactoseand -D-glucose.Thus,onhydrolysis,itgives -

D-galactoseand -D-glucose.

3.HowdoyouexplaintheabsenceofaldehydegroupinthepentaacetateofD-glucose?

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Ans. D-glucose reacts with hydroxylamine (NH2OH) to form an oxime because of the

presenceofaldehydic(-CHO)grouporcarbonylcarbon.Thishappensasthecyclicstructure

of glucose formsanopen chain structure in anaqueousmedium,which then reactswith

togiveanoxime.

ButpentaacetateofD-glucosedoesnotreactwithNH2OH.Thisisbecausepentaacetatedoes

notformanopenchainstructure.

4.Themeltingpointsandsolubilityinwaterofaminoacidsaregenerallyhigherthan

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thatofthecorrespondinghaloacids.Explain.

Ans.Bothacidic(carboxyl)aswellasbasic(amino)groupsarepresentinthesamemolecule

ofaminoacids. Inaqueous solutions, thecarboxylgroupcan loseaprotonand theamino

groupcanacceptaproton,thusgivingrisetoadipolarionknownasazwitterion.

Due to thisdipolarbehaviour, theyhave strong electrostatic interactionswithin themand

withwater.Buthalo-acidsdonotexhibitsuchdipolarbehaviour.

Forthisreason,themeltingpointsandthesolubilityofaminoacidsinwaterishigherthan

thoseofthecorrespondinghalo-acids.

5.Wheredoesthewaterpresentintheegggoafterboilingtheegg?

Ans. When an egg is boiled, the proteins present inside the egg get denatured and

coagulated. After boiling the egg, the water present in it is absorbed by the coagulated

proteinthroughH-bonding.

6.WhycannotvitaminCbestoredinourbody?

Ans.Vitamin C cannot be stored in our body because it iswater soluble. As a result, it is

readilyexcretedintheurine.

7.WhatproductswouldbeformedwhenanucleotidefromDNAcontainingthymineis

hydrolysed?

Ans.Whenanucleotide from theDNA containing thymine is hydrolyzed, thymine -D-2-

deoxyriboseandphosphoricacidareobtainedasproducts.

8.WhenRNAishydrolysed,thereisnorelationshipamongthequantitiesofdifferent

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basesobtained.WhatdoesthisfactsuggestaboutthestructureofRNA?

Ans.A DNA molecule is double-stranded in which the pairing of bases occurs. Adenine

always pairs with thymine, while cytosine always pairs with guanine. Therefore, on

hydrolysis of DNA, the quantity of adenine produced is equal to that of thymine and

similarly,thequantityofcytosineisequaltothatofguanine.

ButwhenRNAishydrolyzed,thereisnorelationshipamongthequantitiesofthedifferent

basesobtained.Hence,RNAissingle-stranded.

ChapterEndQuestion

1.Whataremonosaccharides?

Ans.Monosaccharidesarecarbohydratesthatcannotbehydrolysedfurthertogivesimpler

unitsofpolyhydroxyaldehydeorketone.

Monosaccharidesareclassifiedonthebasesofnumberofcarbonatomsandthefunctional

group present in them. Monosaccharides containing an aldehyde group are known as

aldoses and those containing a keto group are known as ketoses. Monosaccharides are

further classified as trioses, tetroses, pentoses, hexoses, and heptoses according to the

numberof carbonatomsthey contain.For example, aketose containing3 carbonatoms is

calledketotrioseandanaldosecontaining3carbonatomsiscalledaldotriose.

2.Whatarereducingsugars?

Ans.ReducingsugarsarecarbohydratesthatreduceFehling'ssolutionandTollen'sreagent.

Allmonosaccharidesanddisaccharides,excludingsucrose,arereducingsugars.

3.Writetwomainfunctionsofcarbohydratesinplants.

Ans.Twomainfunctionsofcarbohydratesinplantsare:

(i)Polysaccharidessuchasstarchserveasstoragemolecules.

(ii)Cellulose,apolysaccharide,isusedtobuildthecellwall.

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4.Classifythefollowingintomonosaccharidesanddisaccharides.

Ribose,2-deoxyribose,maltose,galactose,fructoseandlactose

Ans.Monosaccharides:Ribose,2-deoxyribose,galactose,fructose

Disaccharides:Maltose,lactose

5.Whatdoyouunderstandbythetermglycosidiclinkage?

Ans.Glycosidic linkage refers to the linkage formed between two monosaccharide units

throughanoxygenatombythelossofawatermolecule.

Forexample,inasucrosemolecule,twomonosaccharideunits, -glucoseand -fructose,

arejoinedtogetherbyaglycosidiclinkage.

6.Whatisglycogen?Howisitdifferentfromstarch?

Ans.Glycogen is a carbohydrate (polysaccharide). In animals, carbohydrates are storedas

glycogen.

Starchisacarbohydrateconsistingoftwocomponents-amylose(15-20%)andamylopectin

(80-85%).

However, glycogen consists of only one component whose structure is similar to

amylopectin.Also,glycogenismorebranchedthanamylopectin.

7.Whatarethehydrolysisproductsof(i)sucroseand(ii)lactose?

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Ans.(i)Onhydrolysis,sucrosegivesonemoleculeof -Dglucoseandonemoleculeof -

fructose.

(ii)Thehydrolysisoflactosegives -galactoseand -glucose.

8.Whatisthebasicstructuraldifferencebetweenstarchandcellulose?

Ans.Starchconsistsoftwocomponents-amyloseandamylopectin.Amyloseisalonglinear

chainof -D-(+)-glucoseunitsjoinedbyC1-C4glycosidiclinkage( -link).

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Amylopectin is a branched-chain polymer of -D-glucose units, in which the chain is

formed by glycosidic linkage and the branching occurs by glycosidic

linkage.

Ontheotherhand,celluloseisastraight-chainpolysaccharideof -D-glucoseunitsjoinedby

glycosidiclinkage( -link).

9.WhathappenswhenD-glucoseistreatedwiththefollowingreagents?

(i)HI

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(ii)Brominewater

(iii)

Ans.(i)WhenD-glucoseisheatedwithHIforalongtime,n-hexaneisformed.

(ii)WhenD-glucoseistreatedwith water,D-gluconicacidisproduced.

(iii)Onbeingtreatedwith ,D-glucosegetoxidisedtogivesaccharicacid.

10.EnumeratethereactionsofD-glucosewhichcannotbeexplainedbyitsopenchain

structure.

Ans. (1) Aldehydes give 2, 4-DNP test, Schiff's test, and react with to form the

hydrogensulphiteadditionproduct.However,glucosedoesnotundergothesereactions.

(2)Thepentaacetateofglucosedoesnotreactwithhydroxylamine.Thisindicatesthatafree

-CHOgroupisabsentfromglucose.

(3)Glucoseexistsintwocrystallineforms- and .The form(m.p.=419K)crystallises

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fromaconcentratedsolutionofglucoseat303Kandthe form (m.p=423K) crystallises

fromahotandsaturatedaqueoussolutionat371K.Thisbehaviorcannotbeexplainedby

theopenchainstructureofglucose.

11.Whatareessentialandnon-essentialaminoacids?Givetwoexamplesofeachtype.

Ans.Essentialaminoacidsarerequiredbythehumanbody,buttheycannotbesynthesised

inthebody.Theymustbetakenthroughfood.Forexample:valineandleucine

Non-essentialaminoacidsarealsorequiredbythehumanbody,buttheycanbesynthesised

inthebody.Forexample:glycine,andalanine

12.Definethefollowingasrelatedtoproteins

(i)Peptidelinkage

(ii)Primarystructure

(iii)Denaturation.

Ans.(i)Peptidelinkage:

The amide formed between -COOH group of onemolecule of an amino acid and

groupofanothermoleculeoftheaminoacidbytheeliminationofawatermoleculeiscalled

apeptidelinkage.

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(ii)Primarystructure:

The primary structure of protein refers to the specific sequence in which various amino

acidsarepresent in it, i.e., thesequenceof linkagesbetweenaminoacids inapolypeptide

chain. The sequence in which amino acids are arranged is different in each protein. A

changeinthesequencecreatesadifferentprotein.

(iii)Denaturation:

In a biological system, a protein is found to have a unique 3-dimensional structure and a

uniquebiologicalactivity.Insuchasituation,theproteiniscallednativeprotein.However,

whenthenativeproteinissubjectedtophysicalchangessuchaschangeintemperatureor

chemicalchangessuchaschangeinpH,itsH-bondsaredisturbed.Thisdisturbanceunfolds

theglobulesanduncoils thehelix.Asa result, theprotein loses itsbiologicalactivity.This

loss of biological activity by the protein is called denaturation. During denaturation, the

secondaryandthetertiarystructuresoftheproteingetdestroyed,buttheprimarystructure

remainsunaltered.

Oneoftheexamplesofdenaturationofproteinsisthecoagulationofeggwhitewhenanegg

isboiled.

ChapterEndQuestion

13.Whatarethecommontypesofsecondarystructureofproteins?

Ans.Therearetwocommontypesofsecondarystructureofproteins:

(i) -helixstructure

(ii) pleatedsheetstructure

-Helixstructure:

In this structure, the -NHgroupof an aminoacid residue formsH-bondwith the

groupoftheadjacentturnoftheright-handedscrew( -helix).

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pleatedsheetstructure:

Thisstructureiscalledsobecauseitlookslikethepleatedfoldsofdrapery.Inthisstructure,

allthepeptidechainsarestretchedouttonearlythemaximumextensionandthenlaidside

byside.Thesepeptidechainsareheldtogetherbyintermolecularhydrogenbonds.

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14.Whattypeofbondinghelpsinstabilisingthe -helixstructureofproteins?

Ans. The H-bonds formed between the -NH group of each amino acid residue and the

groupoftheadjacentturnsofthe -helixhelpinstabilisingthehelix.

15.Differentiatebetweenglobularandfibrousproteins.

Ans.

16.Howdoyouexplaintheamphotericbehaviorofaminoacids?

Ans.In aqueous solution, the carboxyl group of an amino acid can lose a proton and the

aminogroupcanacceptaprotontogiveadipolarionknownaszwitterion.

Therefore,inzwitterionicform,theaminoacidcanactbothasanacidandasabase.

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Thus,aminoacidsshowamphotericbehaviour.

17.Whatareenzymes?

Ans.Enzymesareproteinsthatcatalysebiologicalreactions.Theyareveryspecificinnature

andcatalyseonlyaparticularreactionforaparticularsubstrate.Enzymesareusuallynamed

after the particular substrate or class of substrate and sometimes after the particular

reaction.

Forexample,theenzymeusedtocatalysethehydrolysisofmaltoseintoglucoseisnamedas

maltase.

Again, the enzymes used to catalyse the oxidation of one substratewith the simultaneous

reductionofanothersubstratearenamedasoxidoreductaseenzymes.

Thenameofanenzymeendswith .

18.Whatistheeffectofdenaturationonthestructureofproteins?

Ans.Asaresultofdenaturation,globulesgetunfoldedandhelixesgetuncoiled.Secondary

and tertiary structures of protein are destroyed, but the primary structures remain

unaltered. It can be said that during denaturation, secondary and tertiary-structured

proteinsgetconvertedintoprimary-structuredproteins.Also,asthesecondaryandtertiary

structuresofaproteinaredestroyed,theenzymelosesitsactivity.

19.Howarevitaminsclassified?Namethevitaminresponsible for thecoagulationof

blood.

Ans.Onthebasisoftheirsolubilityinwaterorfat,vitaminsareclassifiedintotwogroups.

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(i)Fat-solublevitamins:Vitaminsthataresolubleinfatandoils,butnotinwater,belongto

thisgroup.Forexample:VitaminsA,D,E,andK

(ii)Water-solublevitamins:Vitamins that are soluble inwater belong to this group. For

example:Bgroupvitamins( ,etc.)andvitaminC

However,biotinorvitaminHisneithersolubleinwaternorinfat.

VitaminKisresponsibleforthecoagulationofblood.

20.WhyarevitaminAandvitaminCessentialtous?Givetheirimportantsources.

Ans.ThedeficiencyofvitaminAleadstoxerophthalmia(hardeningofthecorneaoftheeye)

andnightblindness.ThedeficiencyofvitaminCleadstoscurvy(bleedinggums).

ThesourcesofvitaminAarefishliveroil,carrots,butter,andmilk.ThesourcesofvitaminC

arecitrusfruits,amla,andgreenleafyvegetables.

21.Whatarenucleicacids?Mentiontheirtwoimportantfunctions.

Ans.Nucleic acids are biomolecules found in the nuclei of all living cells, as one of the

constituentsofchromosomes.Therearemainlytwotypesofnucleicacids-deoxyribonucleic

acid (DNA)andribonucleicacid (RNA).Nucleicacidsarealsoknownaspolynucleotidesas

theyarelong-chainpolymersofnucleotides.

Twomainfunctionsofnucleicacidsare:

(i)DNAisresponsibleforthetransmissionofinherentcharactersfromonegenerationtothe

next.Thisprocessoftransmissioniscalledheredity.

(ii)Nucleicacids (bothDNAandRNA)areresponsible forproteinsynthesis inacell.Even

though the proteins are actually synthesised by the various RNAmolecules in a cell, the

messageforthesynthesisofaparticularproteinispresentinDNA.

22.Whatisthedifferencebetweenanucleosideandanucleotide?

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Ans.Anucleosideisformedbytheattachmentofabaseto positionofsugar.

Nucleoside=Sugar+Base

On the other hand, all the three basic components of nucleic acids (i.e., pentose sugar,

phosphoricacid,andbase)arepresentinanucleotide.

Nucleotide=Sugar+Base+Phosphoricacid

23.ThetwostrandsinDNAarenotidenticalbutarecomplementary.Explain.

Ans.In thehelicalstructureofDNA, the twostrandsareheld togetherbyhydrogenbonds

betweenspecificpairsofbases.Cytosineformshydrogenbondwithguanine,whileadenine

formshydrogenbondwiththymine.Asaresult,thetwostrandsarecomplementarytoeach

other.

24.WritetheimportantstructuralandfunctionaldifferencesbetweenDNAandRNA.

Ans.ThestructuraldifferencesbetweenDNAandRNAareasfollows:

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ThefunctionaldifferencesbetweenDNAandRNAareasfollows:

25.WhatarethedifferenttypesofRNAfoundinthecell?

Ans.(i)MessengerRNA(m-RNA)

(ii)RibosomalRNA(r-RNA)

(iii)TransferRNA(t-RNA)

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