NCERT Solutions | Class IX | Science (Biology) | Chapter 5 | The Fundamental Unit of Life

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1. NCERT
SOLUTIONS
BIOLOGY
CLASS –IX

2. The Fundamental
Unit of Life

3. In-text Exercise 1
Whodiscoveredcells,andhow?
Whyisthecellcalledthestructuralandfunctionalunitoflife?
Cellswerediscoveredin1665byanEnglishBotanist,RobertHooke.He
usedaprimitivemicroscopetoobservecellsinacorkslice.
In-text Exercise 2
CO2isproducedduringcellularrespiration.Therefore,itispresentin
highconcentrationsinsidethecell.ThisCO2mustbeexcretedoutofthe
cell.Inthecell’sexternalenvironment,theconcentrationofCO2islow
ascomparedtothatinsidethecell.Therefore,accordingtotheprinciple
ofdiffusion,CO2movesfromaregionofhigherconcentration(inside
thecell)towardsaregionoflowerconcentration(outsidethecell).
Similarly,O2entersthecellbytheprocessofdiffusionwhenthe
concentrationofO2insidethecellislowascomparedtoits
surroundings.
BiswarupMajumder| Bio-World
HowdosubstanceslikeCO2andwatermoveinandoutofthe
cell?Discuss.
1.
2.
1.
Cellsarestructuralunitastheyformthestructureoftheorganism.Cells
combinetoformtissues,whichfurthercombinetoformorgans,organs
combinetoformorgansystems,whichfurthercombinetoform
organism.So,cellisthebasicstructuralunitforallunicellularand
multicellularorganisms.
Someofthebasiclifeprocessesorfunctionstakesplaceineverycellto
keeptheorganismsalivesuchasdigestion,respiration,excretion,etc.,
whetheritisasingle-celledorganismormulticellularorganism.So,the
cellsarealsocalledthefunctionalunitoflifeowingtotheseprocesses.

4. Thecellmembraneortheplasmamembraneisknownasaselectively
permeablemembranebecauseitregulatesthemovementofsubstances
inandoutofthecell.Thismeansthattheplasmamembraneallowsthe
entryofonlysomesubstancesandpreventsthemovementofsome
othermaterials.
BiswarupMajumder| Bio-World
Movementofwater:
Watermovesfromaregionofhighconcentrationtoaregionoflow
concentrationthroughtheplasmamembrane.Theplasmamembrane
actsasasemi-permeablemembrane,andthismovementofwateris
knownasosmosis.However,themovementofwateracrosstheplasma
membraneofthecellisaffectedbytheamountofsubstancedissolved
inwater.
Whyistheplasmamembranecalledaselectivelypermeable
membrane?
2.
In-text Exercise 3
Fillinthegapsinthefollowingtableillustratingdifferences
betweenprokaryoticandeukaryoticcells.
1.
Prokaryotic cell Eukaryotic cell
Size:generallysmall(1-10µm)
1µm=10m
Size:generallylarge(5-100
µm)
Nuclearregion:
_________________________
andknownas______________
Nuclearregion:welldefined
andsurroundedbyanuclear
membrane
Chromosome : single More than one chromosome
Membrane-boundcell
organellesabsent
_______________________
_______________________
-6

5. BiswarupMajumder| Bio-World
Prokaryotic cell Eukaryotic cell
Size:generallysmall(1-10µm)
1µm=10m
Size:generallylarge(5-100
µm)
Nuclearregion:poorlydefined
becauseoftheabsenceofa
nuclearmembrane,andknown
asnucleoid.
Nuclearregion:well-defined
andsurroundedbyanuclear
membrane
Chromosome : single More than one chromosome
Membrane-bound cell
organelles absent
Membrane-bound cell
organellessuch as
mitochondria,plastids,etc.,
arepresent
In-text Exercise 4
Canyounamethetwoorganelleswehavestudiedthatcontain
theirowngeneticmaterial?
1.
Mitochondriaandplastidsarethetwoorganellesthatcontaintheirown
geneticmaterial.BoththeseorganelleshavetheirownDNAand
ribosomes.
Iftheorganizationofacellisdestroyedduetosomephysical
orchemicalinfluence,whatwillhappen?
2.
Cellisthesmallestunitoflife,whichiscapableofalllivingfunctions.If
theorganizationofacellisdestroyedduetosomephysicalorchemical
influence,thentheabilityofthecelltoperformalllivingfunctionssuch
asrespiration,nutrition,excretion,etc.wouldbeaffected.
-6

6. BiswarupMajumder| Bio-World
Why are lysosomesknown as suicide bags? 3.
Lysosomesaremembrane-boundvesicularstructuresthatcontain
powerfuldigestiveenzymes.Theseenzymesarecapableofbreaking
downanyforeignfoodparticleormicrobesenteringthecell.
Sometimes,lysosomescancauseself-destructionofacellbyreleasing
thesedigestiveenzymeswithinthecells.Hence,theyarealsoknownas
‘suicidebags’ofcell.
Where are proteins synthesized inside the cell? 4.
Ribosomesarethesiteforproteinsynthesis.Ribosomesareverysmall
structuresfoundeitherinafreestate,suspendedinthecytoplasm,or
attachedtothesurfaceoftheendoplasmicreticulum.Theyare
composedofribonucleicacidsandproteins.
NCERT Exercise
Makeacomparisonandwritedownwaysinwhichplantcells
aredifferentfromanimalcells.
1.

7. BiswarupMajumder| Bio-World
Howisaprokaryoticcelldifferentfromaeukaryoticcell?2.
Whatwouldhappeniftheplasmamembranerupturesor
breaksdown?
3.
Iftheplasmamembraneofacellisruptured,thenthecellwilldie.The
plasmamembraneregulatesthemovementofsubstancesinandoutof
thecellbydiffusionorosmosis.Thus,iftheplasmamembraneis
ruptured,thenthecellmightleakoutitscontents.
WhatwouldhappentothelifeofacelliftherewasnoGolgi
apparatus?
4.
IftherewasnoGolgiapparatusinthecell,thenmostactivities
performedbytheGolgiapparatuswillnottakeplace.
MembranesoftheGolgiapparatusareoftenconnectedtoER
membranes.Itcollectssimplermoleculesandcombinesthemtomake
morecomplexmolecules.Thesearethenpackagedinsmallvesiclesand
areeitherstoredinthecellorsentoutaspertherequirement.Thus,if
theGolgiapparatusisabsentinthecell,thentheaboveprocessof
storage,modification,andpackagingofproductswillnotbepossible.
(i)

8. Theformationofcomplexsugarsfromsimplesugarswillnotbepossibleas
thistakesplacewiththehelpofenzymespresentinGolgibodies.
TheGolgiapparatusisinvolvedintheformationoflysosomesor
peroxisomes.Thus,iftheGolgibodyisabsentinacell,thesynthesisof
lysosomesorperoxisomeswillnotbepossibleinthecell.
(ii)
(iii)
Mitochondriaareknownasthepowerhouseofcells.
Mitochondriacreateenergyforthecell,andthisprocessofcreating
energyforthecellisknownascellularrespiration.Mostchemical
reactionsinvolvedincellularrespirationoccurinthemitochondria.
Theenergyrequiredforvariouschemicalactivitiesneededforlifeis
releasedbythemitochondriaintheformofATP(Adenosine
triphosphate)molecules.Forthisreason,mitochondriaareknownas
thepowerhouseofcells.
Whichorganelleisknownasthepowerhouseofthecell?Why?5.
BiswarupMajumder| Bio-World
Wheredothelipidsandproteinsconstitutingthecell
membranegetsynthesized?
6.
Lipidsandproteinsconstitutingthecellmembrane
aresynthesizedintheendoplasmicreticulum.
HowdoesanAmoebaobtainitsfood?7.
Amoebaobtainsitsfoodthroughtheprocessof
endocytosis.Theflexibilityofthecellmembrane
enablesthecelltoengulfthesolidparticlesoffood
andothermaterialsfromitsexternalenvironment.
SER(Smoothendoplasmicreticulum)helpsinthe
manufacturingoflipids.
RER(Roughendoplasmicreticulum)hasparticles
attachedtoitssurface,calledribosomes.These
ribosomesarethesiteforproteinsynthesis.
See Figure

9. BiswarupMajumder| Bio-World
What is osmosis? 8.
Osmosisisthediffusionofwatermoleculesacrossasemi-permeable
membranefromamoredilutesolution(withalowersolute
concentration)toalessdilutesolution(withahighersolute
concentration).
Carryoutthefollowingosmosisexperiment:
Takefourpeeledpotatohalvesandscoopeachoneouttomake
potatocups.Oneofthesepotatocupsshouldbemadefroma
boiledpotato.Puteachpotatocupinatroughcontainingwater.
Now,
KeepcupAempty
PutoneteaspoonsugarincupB
PutoneteaspoonsaltincupC
PutoneteaspoonsugarintheboiledpotatocupD.
Keepthesefortwohours.Thenobservethefourpotatocups
andSolutionthefollowing:
9.
See Figure
Forexample,ifthe
medium
surroundingthecell
hasahigherwater
concentrationthan
thecelli.e.,ifthe
solutionisadilute
solution,thenthe
cellwillgainwater
byosmosis.
(a)
(b)
(c)
(d)

10. ExplainwhywatergathersinthehollowedportionofBand
C.
WhyispotatoAnecessaryforthisexperiment?
Explainwhywaterdoesnotgatherinthehollowedout
portionsofAandD.
Experimental set up
BiswarupMajumder| Bio-World
(i)
(ii)
(iii)
Watergathersinthehollowedportionsofset-upBandCbecause
waterentersthepotatoasaresultofosmosis.Sincethemedium
surroundingthecellhasahigherwaterconcentrationthanthecell,
thewatermovesinsidebyosmosis.Hence,watergathersinthe
hollowedportionsofthepotatocup.
PotatoAintheexperimentactsasacontrolset-up.Nowatergathers
inthehollowedportionsofpotatoA.
WaterdoesnotgatherinthehollowedportionsofpotatoAbecause
potatocupAisempty.Itisacontrolset-upintheexperiment.
WaterisnotabletoenterpotatoDbecausethepotatousedhereis
boiled.Boilingdenaturestheproteinspresentinthecellmembrane
andthus,disruptsthecellmembrane.Forosmosis,asemi-
permeablemembraneisrequired,whichisdisruptedinthiscase.
Therefore,osmosiswillnotoccur.Hence,waterdoesnotenterthe
boiledpotatocup.
(i)
(ii)
(iii)

11. Whichtypeofcelldivisionisrequiredforgrowthandrepairof
bodyandwhichtypeisinvolvedinformationofgametes?
10.
Mitosisisrequiredforgrowthandrepairofbody.Meiosisisrequired
fordiploidorganismsandmitosisisrequiredforhaploidorganisms.
BiswarupMajumder| Bio-World
Biology is the most powerful technology ever created. DNA
is software, protein are hardware, cells are factories.