CBSE NCERT Solutions for Class 11 biology Chapter 8 Exercises Q.1. Which of the following is not correct? Robert Brown discovered the cell Solution: Robert Brown discovered the nucleus of the cell in 1831. The cell was discovered by Robert Hooke in 1665. Robert Hooke discovered cell while observing the dead cork under a microscope. The first living cells were observed by Anton Van Leeuwenhoek in 1672. The cell was discovered by Robert Hooke in 1665. Schleiden and Schwann formulated the cell theoryVirchow explained that cells are formed from pre-existing cellsA unicellular organism carries out its life activities within a single cell Q.2. Cell is the basic unit of life. Discuss in brief. Solution: Cells are the structural and functional units of life, which are responsible for performing all the biochemical processes necessary for all the organisms to live.The cell contains the genetic material that can be inherited. The necessary biochemical processes happening in all the living organisms are digestion of food, respiration for obtaining energy, excretion, etc. Cells are capable of performing all these metabolic functions of the body. Hence, cells are known as the basic units of life. Q.3. What are nuclear pores? State their function. Solution: The nucleus is covered by a double membrane called nuclear envelope which separates the nucleus from the cytoplasm. The nuclear membrane is interrupted by minute pores at some intervals, formed by the fusion of its two membranes. These pores are called nuclear pores. The nuclear pores on the membrane selectively allow the transport of RNA and ribosomal proteins from the nucleus to the cytoplasm and proteins (such as DNA polymerase and lamins), carbohydrates, signalling molecules and lipids from the cytoplasm to the nucleus. Q.4. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment. Solution: Lysosomes and vacuoles have very different functions in the cell. Functions of the vacuoles are: To store water and solutes to maintain turgidity in a cell. Contractile vacuoles in Amoeba help in excretion. Food engulfed by the process of phagocytosis is stored in food vacuoles before being digested in phagolysosomes. Whereas, lysosomes are digestive in nature. They have hydrolytic enzymes enclosed within which act at acidic pH. These enzymes help in the intracellular digestion of engulfed food and also help to digest cellular organelles in starved and stressed cells. Thus, they are known as the 'suicide bags' of the cell. Q.5. Describe the structure of the following with the help of labelled diagram. Nucleus NCERT Biology Grade 11 Chapter 8 Cell: the unit of life Practice more on Cell: the unit of life Page 1 www.embibe.com
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CBSENCERTSolutionsforClass11biologyChapter8
Exercises
Q.1. Whichofthefollowingisnotcorrect?RobertBrowndiscoveredthecell
Solution: RobertBrowndiscoveredthenucleusofthecellin1831.ThecellwasdiscoveredbyRobertHookein1665.RobertHookediscoveredcellwhileobservingthedeadcorkunderamicroscope.ThefirstlivingcellswereobservedbyAntonVanLeeuwenhoekin1672.
ThecellwasdiscoveredbyRobertHookein1665.
SchleidenandSchwannformulatedthecelltheoryVirchowexplainedthatcellsareformedfrompre-existingcellsAunicellularorganismcarriesoutitslifeactivitieswithinasinglecell
Q.2. Cellisthebasicunitoflife.Discussinbrief.
Solution: Cellsarethestructuralandfunctionalunitsoflife,whichareresponsibleforperformingallthebiochemicalprocessesnecessaryforalltheorganismstolive.Thecellcontainsthegeneticmaterialthatcanbeinherited.Thenecessarybiochemicalprocesseshappeninginallthelivingorganismsaredigestionoffood,respirationforobtainingenergy,excretion,etc.Cellsarecapableofperformingallthesemetabolicfunctionsofthebody.Hence,cellsareknownasthebasicunitsoflife.
Q.3. Whatarenuclearpores?Statetheirfunction.
Solution: Thenucleusiscoveredbyadoublemembranecallednuclearenvelopewhichseparatesthenucleusfromthecytoplasm.Thenuclearmembraneisinterruptedbyminuteporesatsomeintervals,formedbythefusionofitstwomembranes.Theseporesarecallednuclearpores.
ThenuclearporesonthemembraneselectivelyallowthetransportofRNAandribosomalproteinsfromthenucleustothecytoplasmandproteins(suchasDNApolymeraseandlamins),carbohydrates,signallingmoleculesandlipidsfromthecytoplasmtothenucleus.
Q.4. Bothlysosomesandvacuolesareendomembranestructures,yettheydifferintermsoftheirfunctions.Comment.
Solution: Lysosomesandvacuoleshaveverydifferentfunctionsinthecell.
Functionsofthevacuolesare:
Tostorewaterandsolutestomaintainturgidityinacell.ContractilevacuolesinAmoebahelpinexcretion.Foodengulfedbytheprocessofphagocytosisisstoredinfoodvacuolesbeforebeingdigestedinphagolysosomes.
Whereas,lysosomesaredigestiveinnature.TheyhavehydrolyticenzymesenclosedwithinwhichactatacidicpH.Theseenzymeshelpintheintracellulardigestionofengulfedfoodandalsohelptodigestcellularorganellesinstarvedandstressedcells.Thus,theyareknownasthe'suicidebags'ofthecell.
Q.5. Describethestructureofthefollowingwiththehelpoflabelleddiagram.
Nucleus
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Solution: Thenucleusconsistoffourparts:nucleolus,nucleoplasm,nuclearmembrane,nuclearpores.Thenuclearmembraneconsistsoftwoparallelmembranesandthespacebetweenthemiscalledperinuclearspace.Thenuclearmembraneseparatescytoplasmfromtheinsideofthenucleus.Theoutermembranecontinueswiththeendoplasmicreticulumandalsohasribosomesonit.
Thenuclearenvelopeisinterruptedbyminuteporesatsomeintervalswhichareformedbythefusionofitstwomembranes.Theseporesareknownasnuclearpores.ThenuclearporesonthemembraneselectivelyallowthetransportofRNAandribosomalproteinsfromthenucleustothecytoplasmandproteins,carbohydrates,signallingmoleculesandlipidsfromthecytoplasmtothenucleus.Thenucleoplasmornuclearmatrixcontainsnucleolusandchromatin.
Thenucleolusisasmallsphericaldensestructurefoundinthenucleusofacellduringinterphaseofcellcycle.
Q.6. Describethestructureofthefollowingwiththehelpoflabelleddiagrams.
Centrosome
Solution: Acentrosomeisanorganellefoundonlyinanimalcells.Thecentrosomeisthemainplacewherecellmicrotubulesareorganized.Thecentrosomehasonepairofcylindricalstructurescalledcentrioles.Thecentrioleslieperpendicularwitheachotherandaremadeupofnineevenlyspacedperipheralfibrilsoftubulinprotein.Eachfibrilisatriplet,andadjacentfibriltripletsarejoined.Thecentralpartofthefibriltripletisproteinaceousandcalledthehub,isconnectedtotheadjacentperipheralfibriltripletsbyradialspokesmadeofprotein.
Thechiefpurposeofacentrosomeistoorganisemicrotubulestoformspindlefibreswhichassistinpullingthesisterchromatidsapartduringanaphaseofcelldivision.
Q.7. Whatisacentromere?Howdoesthepositionofcentromereformthebasisofclassificationofchromosomes.Supportyouranswerwithadiagramshowingthepositionofcentromereondifferenttypesofchromosomes.
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Solution: Thecentromereisapointofnarrowconstrictioninthechromosomeswherethetwosisterchromatidsareheldtogether.
Chromosomescanbedividedintofourtypesbasedonthepositionofthecentromere:
(i)Metacentricchromosome:Thecentromereispresentinthecentreanddividesthechromosomeintotwoequalarms.TheyappearV-shapedduringanaphaseofcelldivision.
(ii)Sub-metacentricchromosome:Thecentromereisslightlyawayfromthecentre.Hence,onearmisslightlylongerthantheother.Duringanaphase,theyappearL-shaped.
(iii)Acrocentricchromosome:Thecentromereislocatedclosetooneoftheterminalends.Hence,onearmisextremelylong,andtheotherisextremelyshort.Duringanaphase,theyappearJ-shaped.
(iv)Telocentricchromosome:Thecentromereislocatedatoneoftheterminalends.Duringanaphase,theyappeari-shaped.
Q.8. NewcellsgeneratefromBacterialfermentationRegenerationofoldcellsPre-existingcells
Solution: Thenewcells(daughtercells)aregeneratedfrompre-existingcells(parentcells)bycelldivision.RudolfVirchowexplainedthatcellsareformedfrompre-existingcells.Hegavethephrase'Omniscellulaecellula'.Thisstatementwasaddedtothecelltheoryandthenthecelltheorywasrenamedasthemoderncelltheory.
Abioticmaterial
Q.9. Matchthefollowing
ColumnI ColumnII(a) Cristae (i) Flatmembranoussacsinstroma(b) Cisternae (ii) Infoldingsinmitochondria(c) Thylakoids (iii) Disc-shapedsacsinGolgiapparatus
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Solution: (a)-(ii)ToincreasethecapacityofthemitochondriontosynthesizeATP,theinnermembraneisfoldedtoformcristae.ThesefoldsallowamuchgreateramountofelectrontransportchainenzymesandATPsynthasetobepackedintothemitochondrion.Thecristaeincreasethesurfacearea.
(b)-(iii)TheGolgiapparatusconsistsofsetsofmembrane-delimitedsmooth-surfacedcisternae.Anumberofproteinssynthesisedbyribosomesontheendoplasmicreticulumaremodifiedinthecisternaeofthegolgiapparatusbeforetheyarereleasedfromitstransface.
(c)-(i)Anumberoforganisedflattenedmembranoussacscalledthethylakoids,arepresentinthestroma.Theyprovidetheplatformforthelightreactionsofphotosynthesis.Thylakoidsarearrangedinstackslikethepilesofcoinscalledgranaortheinter-granalthylakoids.Inaddition,thereareflatmembranoustubulescalledthestromalamellaeconnectingthethylakoidsofthedifferentgrana.Themembraneofthethylakoidsencloseaspacecalledalumen.
Q.10. Whichofthefollowingiscorrect?CellsofalllivingorganismshaveanucleusBothanimalandplantcellshaveawelldefinedcellwallInprokaryotes,therearenomembraneboundorganelles
Solution: Prokaryoticcellsdonothaveawell-definednucleus.TheyhaveDNAastheirgeneticmaterial,lyingnakedinthecytoplasm,knownasanucleoid.Noneoftheprokaryoticcellshavemembrane-boundorganelles.Theonlycellorganelleprokaryoteshaveisribosome.
Cellsareformeddenovofromabioticmaterials
Q.11. Whatisamesosomeinaprokaryoticcell?Mentionthefunctionsthatitperforms.
Solution: Mesosomesarethestructuresformedinprokaryoticcellsbytheinvaginationoftheplasmamembranetowardsthecytoplasm.
Mesosomesarerichinenzymesandhencehelptoperformmanyfunctionssuchascellularrespiration,DNAreplication,celldivision,secretionoftheglycocalyxandsynthesisofcell-wall.Theyalsohelpinequaldistributionofchromosomesintodaughtercells.
Q.12. Howdoneutralsolutesmoveacrosstheplasmamembrane?Canthepolarmoleculesalsomovecrossitinthesameway?Ifnot,thenhowarethesetransportedacrossthemembrane?
Solution: Diffusionisthemovementofmoleculesfromaregionofhigherconcentrationtoaregionoflowerconcentration.Thisdoesnotrequireenergyandhenceitisapassivetransportmechanism.Theneutralmoleculescanmovethroughthelipidbilayeroftheplasmamembranebytheprocessofdiffusion.
Polarmembranescannotbetransportedbysimplediffusion.Themovementofpolarmoleculesacrossthenon-polarlipidbilayeroccurswiththehelpofcarrierproteins.Thecarrierproteinsareintegralmembraneproteinshavingacertainaffinityforspecificsolutes.Theyfacilitatethetransportofmoleculesacrossthemembrane.
Q.13. Nametwocell-organellesthataredoublemembrane-bound.Whatarethecharacteristicsofthesetwoorganelles?Statetheirfunctionsanddrawlabelleddiagramsofboth.
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Solution: MitochondriaandChloroplastaredoublemembrane-boundorganelles.Mitochondriahelpinrespirationandarefoundinalleukaryoticcells.Chloroplasthelpsintheprocessofphotosynthesisandisfoundonlyinplantcells.
Mitochondria-Themitochondrionisadoublemembrane-boundcellorganellepresentineukaryoticcells.Itconsistsofanoutermembrane,aninnermembraneandagel-likematerialcalledthematrixintheinnercompartment.Thespacebetweentheoutermembraneandtheinnermembraneiscalledtheintermembranespace.Boththemembraneshavetheirownspecificenzymesfordifferentmitochondrialfunctions.
Theinnermembraneofmitochondriaconsistofseveralfoldsthatformalayeredstructurecalledcristae.Itincreasestheinnersurfaceareaoftheorganelle.ThecristaeandtheproteinsoftheinnermembranehaveacrucialroletoplayintheATPsynthesisthroughoxidativephosphorylation.MitochondrialmatrixalsocontainsDNA,RNA,70Sribosomeandallthecomponentsrequiredforproteinsynthesis.Theycandividebytheprocessoffission.
Thefunctionofmitochondria-Themostcrucialfunctionofthemitochondriaistoproduceenergythroughoxidativephosphorylation.Hence,itiscalledthepowerhouseofthecell.Theyarethesitesofaerobicrespiration.
Chloroplast-Thechloroplastisadoublemembraneorganellefoundonlyinplantcells.Ithascoinlikestructurescalledthylakoids.Thethylakoidwallshavemoleculesofthechlorophyllpigment.Thesethylakoidsarearrangedlikeastackofcoins.Thisstackiscalledagranum.Granumisjoinedtogetherbystromalamellae.Matrixofthechloroplastiscalledthestroma.Ithasalltheenzymesneededforcarbohydratesynthesis.Thechloroplastisasemi-autonomousorganelleasithasDNA.Ithas70sribosomethathelpsinthesynthesisofitsownproteins.
Functionofchloroplast-Thylakoidsofchloroplasthelpinthefixationofsunlightintochemicalenergy.Italsohelpsinthefixationofatmosphericcarbondioxideintocarbohydrateandreleasesmolecularoxygentotheatmosphere.
Q.14. Whatarethecharacteristicsofprokaryoticcells?
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Solution: Theexamplesofprokaryoticcellsarebacteria(1-2µm),blue-greenalgae(10-20µm),mycoplasma(10-20µm),PPLO(PleuroPneumoniaLikeOrganisms–about0.1µm).Prokaryoteshavefourmainstructures:plasmamembrane,cytoplasm,ribosomes,DNAandRNA.Allprokaryoteshaveacellwallsurroundingacellexceptinmycoplasma.Manyprokaryoticcellsarecoveredbythreelayers:outermostglycocalyx,middlecellwallandinnermostplasmamembrane.Thecell-matrixisfilledwithajelly-likefluidcalledcytoplasm.Thenucleusinprokaryotesisnotwell-definedandhence,itiscalledthenucleoidandtheDNAiscallednaked-DNA.Theprokaryoticcellshaveasinglecircularchromosome.TheymayalsohaveextrachromosomalpiecesofcircularDNAcalledplasmids.
Theprokaryoticcellshaveinvaginationsoftheplasmamembrane,whicharecalledmesosomes.Theyarerichinenzymesandhencehelptoperformmanytaskslikecellularrespiration,DNAreplication,celldivision,secretionoftheglycocalyxandsynthesisofcellwall.Theyalsohelpinequaldistributionofchromosomesintodaughtercells.Themostcrucialfunctionofmesosomesisthatitincreasesthesurfaceareaofthecellmembrane.Prokaryoticcellsdonothaveanymembrane-boundcellorganelles.Theycontainribosomes.Allprokaryoteshavethe70Sribosomeswhileeukaryotescontainlarger80Sribosomesintheircytosol.
Q.15. Multicellularorganismshavedivisionoflabour.Explain
Solution: Multicellularorganismsaremadeupofmillionsofcells.Allthesecellsarespecialisedinperformingaspecificfunctionbytheprocessofcelldifferentiation.Agroupofdifferentiatedcellswhichperformsimilarfunctionsaregroupedastissues,andtheyperformspecialisedfunctionsinthebody.Hence,eachtissuehasaparticularfunctiontocarryoutatadefiniteplaceinthebody.So,differentfunctionsarecarriedoutbydifferentgroupsofcellsinanorganism.Hence,itiscalledthe'divisionoflabour'inthemulticellularorganisms.
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