Prokaryotic and eukaryotic cell

Prokaryotic and eukaryotic cell

1. Amandeep Singh
Assistant Professor
Department of Biotechnology
GSSDGS KhalsaCollege Patiala

2. Comparison Prokaryotic and Eukaryotic
cells
Characteristics of prokaryotes
1.DNA is not enclosed within a membrane and is usually a singular
circularly arranged chromosome.
2.DNA is not associated with histones (special chromosomal proteins
found in eukaryotes).
3.They lack membrane-enclosed organelles.
4.Their cell walls almost always contain the complex polysaccharide
peptidoglycan.
5.They usually divide by binary fission.
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3. 1. DNA is not enclosed within a membrane and is
usually a singular circularly arranged chromosome.
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4. 2. DNA is not associated with histones
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5. 3. They lack membrane-enclosed organelles
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6. 4. Their cell walls almost always contain the
complex polysaccharide peptidoglycan
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7. 5.Theyusuallydividebybinaryfission
Duringthisprocess,theDNAiscopied,andthecellsplitsintotwocells.Binary
fissioninvolvesfewerstructuresandprocessesthaneukaryoticCelldivision
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8. Comparison Prokaryotic and Eukaryotic
cells
Characteristics of Eukaryotes
1.Their DNA is found in the cell’s nucleus.
2.Their DNA is consistently associated with chromosomal proteins
called Histonesand with Non-histones.
3.They have a number of membrane-enclosed organelles, including
mitochondria, endoplasmic reticulum, Golgi complex, lysosomes,
and sometimes chloroplasts.
4.Their cell walls, when present, are chemically simple.
5.Cell division usually involves mitosis.
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9. 1. Their DNA is found in the cell’s nucleus.
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10. 2. Their DNA is consistently associated with
chromosomal proteins called Histonesand with
Non-histones.
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11. 3. They have a number of membrane-
enclosed organelles.
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12. 4. Their cell walls, when present, are
chemically simple
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13. 5. Cell division usually involves mitosis
In which chromosomes replicate and an identical set is distributed into each of two nuclei. This process is guided by the
mitotic spindle, a football-shaped assembly of microtubules. Division of the cytoplasm and other organelles follows so
that the two cells produced are identical to each other
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14. The Prokaryotic Cell
•Themembersoftheprokaryoticworldmakeupavastheterogeneous
groupofverysmallunicellularorganisms.
•Prokaryotesincludebacteriaandarchaea.
•Themajorityofprokaryotes,includingthephotosynthesizing
cyanobacteria,arebacteria.
•Bacteriaaredifferentiatedbymanyfactors,includingmorphology
(shape),chemicalcomposition(oftendetectedbystainingreactions),
nutritionalrequirements,biochemicalactivities,andsourcesof
energy(sunlightorchemicals).
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15. The size, shape, and arrangement of
Bacterial Cells
•Bacteriacomeinagreatmanysizesandseveral
shapes.Mostbacteriarangefrom0.2to2.0µmin
diameterandfrom2to8µminlength.Theyhavea
fewbasicshapes:sphericalcoccus(plural:cocci,
meaningberries),rod-shapedbacillus(plural:bacilli,
meaninglittlestaffs),andspiral.
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16. •Cocciareusuallyroundbutcanbeoval,
elongated,orflattenedononeside.
•Whencoccidividetoreproduce,thecellscan
remainattachedtooneanother.Coccithatremain
inpairsafterdividingarecalleddiplococci;those
thatdivideandremainattachedinchainlike
patternsarecalledstreptococci.
•Thosethatdivideintwoplanesandremainin
groupsoffourareknownastetrads.
•Thosethatdivideinthreeplanesandremain
attachedincubelikegroupsofeightarecalled
sarcinae.
•Thosethatdivideinmultipleplanesandform
grapelikeclustersorbroadsheetsarecalled
staphylococci.
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17. •Bacillidivideonlyacrosstheirshortaxis,
sotherearefewergroupingsofbacilli
thanofcocci.
•Mostbacilliappearassinglerods,called
singlebacilli.
•Diplobacilliappearinpairsafter
division,andstreptobacillioccurin
chains.
•Somebacillilooklikestraws.Others
havetaperedends,likecigars.Stillothers
areovalandlooksomuchlikecoccithat
theyarecalledcoccobacillii.
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18. “Bacillus”hastwomeaningsinmicrobiology
1
st
:referstoabacterialshape
2
nd
:Whencapitalizedanditalicized,itreferstoa
specificgenus.
•Forexample,thebacteriumBacillusanthracisisthe
causativeagentofanthrax.
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19. •Spiralbacteriahaveoneor
moretwists;theyarenever
straight.Bacteriathatlook
likecurvedrodsarecalled
vibrios.
•Others,calledspirilla,have
ahelicalshape,likea
corkscrew,andfairlyrigid
bodies.
•Yetanothergroupofspirals
arehelicalandflexible;they
arecalledspirochetes.
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20. •Theshapeofabacteriumisdeterminedbyheredity.
Genetically,mostbacteriaaremonomorphic;thatis,they
maintainasingleshape.
•However,anumberofenvironmentalconditionscanalterthat
shape.Iftheshapeisaltered,identificationbecomesdifficult.
Moreover,somebacteria,suchasRhizobiumand
Corynebacteriumaregeneticallypleomorphic,whichmeans
theycanhavemanyshapes,notjustone.
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21. Structure of a typical prokaryotic cell
Components according to the following organization:
(1) Structures external to the cell wall
(2) The cell wall itself
(3) Structures internal to the cell wall.
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22. Structures External to the Cell Wall
•Amongthepossiblestructuresexternaltothe
prokaryoticcellwallaretheglycocalyx,flagella,
axialfilaments,fimbriae,andpili.
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23. Glycocalyx
•Manyprokaryotessecreteontheirsurfaceasubstancecalled
glycocalyx.
•Glycocalyx(meaningsugarcoat)isthegeneraltermusedfor
substancesthatsurroundcells.
•Itisaviscous(sticky),gelatinouspolymerthatisexternaltothecell
wallandcomposedofpolysaccharide,polypeptide,orboth.
•Ifthesubstanceisorganizedandisfirmlyattachedtothecellwall,
theglycocalyxisdescribedasacapsule.
•Ifthesubstanceisunorganizedandonlylooselyattachedtothecell
wall,theglycocalyxisdescribedasaslimelayer.
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24. Function of Glycocalyx
•InformationofBiofilms.Aglycocalyxthathelpscellsinabiofilm
attachtotheirtargetenvironmentandtoeachotheriscalledan
extracellularpolymericsubstance(EPS).
•TheEPSprotectsthecellswithinit,facilitatescommunicationamong
them,andenablesthecellstosurvivebyattachingtovarioussurfaces
intheirnaturalenvironment.
•Throughattachment,bacteriacangrowondiversesurfacessuchas
rocksinfast-movingstreams,plantroots,humanteeth,medical
implants,waterpipes,andevenotherbacteria.
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25. •Streptococcusmutans,animportant
causeofdentalcaries,attachesitself
tothesurfaceofteethbya
glycocalyx.
•Vibriocholerathecauseofcholera,
producesaglycocalyxthathelpsit
attachtothecellsofthesmall
intestine.
•Glycocalyxalsocanprotectacell
againstdehydration.
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26. Flagella
•Someprokaryoticcellshaveflagella(singular:flagellum),whichare
longfilamentousappendagesthatpropelbacteria.
•Bacteriathatlackflagellaarereferredtoasatrichous(without
projections).
•Flagellamaybeperitrichous(distributedovertheentirecell;orpolar
(atoneorbothpolesorendsofthecell).
•Ifpolar,flagellamaybemonotrichous(asingleflagellumatone
pole;lophotrichous(atuftofflagellacomingfromonepole);or
amphitrichous(flagellaatbothpolesofthecell).
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27. Department of Biotechnology, GSSDGS Khalsa College Patiala 27

28. FunctionofFlagella
•FunctionofFlagellaistoprovide
motilitytobacteria.
•Oneadvantageofmotilityisthatit
enablesabacteriumtomovetoward
afavorableenvironmentoraway
fromanadverseone.
•Themovementofabacteriumtoward
orawayfromaparticularstimulusis
calledtaxis.Suchstimuliinclude
chemicals(chemotaxis)andlight
(phototaxis).
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29. Axial Filaments
•Spirochetesareagroupofbacteria
thathaveuniquestructurefor
motility.
•Oneofthebest-knownspirochetes
isTreponemapallidum,the
causativeagentofsyphilis.
•Spirochetesmovebymeansof
axialfilaments,orendoflagella,
bundlesoffibrilsthatariseatthe
endsofthecellbeneathanouter
sheathandspiralaroundthecell.
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30. Fimbriae and Pili
•Manygram-negativebacteriacontainhairlikeappendagesthatare
shorter,straighter,andthinnerthanflagellaandareusedfor
attachmentandtransferofDNAratherthanformotility.
•Thesestructures,whichconsistofaproteincalledpilinarranged
helicallyaroundacentralcore,aredividedintotwotypes,fimbriae
andpili,havingverydifferentfunctions.
•Fimbriae(singular:fimbria)canoccuratthepolesofthebacterial
cellorcanbeevenlydistributedovertheentiresurfaceofthecell.
•Fimbriaehaveatendencytoadheretoeachotherandtosurfaces.Asa
result,theyareinvolvedinformingbiofilmsandotheraggregationson
thesurfacesofliquids,glass,androcks.
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31. •Pili(singular:pilus)areusuallylongerthanfimbriaeandnumberonly
oneortwopercell.
•PiliareinvolvedinmotilityandDNAtransfer.
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32. The Cell Wall
•Itisacomplex,semi-rigidstructureresponsiblefortheshapeofthe
cell.
•Thecellwallsurroundstheunderlying,fragileplasma(cytoplasmic)
membraneandprotectsitandtheinteriorofthecellfromadverse
changesintheoutsideenvironment.
•Preventcellsfromrupturingwhenwaterpressureinsideincreasesthan
outside.
•Alsoserveasanchorageforflagella.
•Siteofactionofantibiotics.
•Itschemicalcompositionisresponsiblefordifferentiation.
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33. Composition and Characteristics of Cell Wall
•ComposedofPeptidoglycan.
•Peptidoglycanconsistofrepeatingdisaccharide
attachedbypolypeptide.
•Thedisaccharideportionismadeupof
monosaccharidescalledN-acetylglucosamine(NAG)
andN-acetylmuramicacid(NAM).
•RepeatingNAG&NAMlinkedbyPolypeptide.
NAG
NAM
NAG
NAM
NAG
Polypeptide
Carbohydrate
Backbone
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34. Gram Positive Cell Wall
•Inmostgram-positivebacteria,thecellwallconsistsofmanylayers
ofpeptidoglycan,formingathick,rigidstructure.
•Bycontrast,gram-negativecellwallscontainonlyathinlayerof
peptidoglycan.
•ContainTeichoicacids:consistprimarilyofanalcohol(suchas
glycerolorribitol)andphosphate.
•Twoclassesofteichoicacids:Lipoteichoicacid,whichspansthe
peptidoglycanlayerandislinkedtotheplasmamembrane,andWall
teichoicacid,whichislinkedtothepeptidoglycanlayer.
•Teichoicacids(negativecharge)controlmovementofpositiveions
intoandoutofcell.Italsoprovideantigenicitytocell.
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35. Gram Negative Cell Wall
•Consistofoneoraveryfewlayersof
peptidoglycanandanoutermembrane.
•Thepeptidoglycanisbondedtolipoproteinsinthe
outermembraneandisintheperiplasm,agel-
likefluidbetweentheoutermembraneandthe
plasmamembrane.
•Theoutermembraneofthegram-negativecell
consistsoflipopolysaccharides(LPS),
lipoproteins,andphospholipids.
lipoproteins
lipopolysaccharides
phospholipids
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36. •Lipopolysaccharide(LPS)consistsof3
components:
1.LipidA
2.Corepolysaccharide
3.Opolysaccharide
•LipidAisthelipidportionoftheLPSandis
embeddedinthetoplayeroftheouter
membrane.
•Whengram-negativebacteriadie,theyrelease
lipidA,whichfunctionsasanendotoxin.
•Responsibleforsymptomslikefever,dilation
ofbloodvessels,shock,andbloodclotting.
•Corepolysaccharideroleisstructural—to
providestability,
•Opolysaccharidefunctionsasanantigen.
Lipid A
Core polysaccharide
O polysaccharide
Plasma membrane
Outer membrane
Peptidoglycan
CELL WALL
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37. Atypical Cell Walls
•Amongprokaryotes,certaintypesofcellshavenowallsorhavevery
littlewallmaterial.TheseincludemembersofthegenusMycoplasma
andrelatedorganisms.
•Mycoplasmasarethesmallestknownbacteriathatcangrowand
reproduceoutsidelivinghostcells.
•Becauseoftheirsizeandbecausetheyhavenocellwalls,theypass
throughmostbacterialfiltersandwerefirstmistakenforviruses.
•Theirplasmamembranesareuniqueamongbacteriainhavinglipids
calledsterols,whicharethoughttohelpprotectthemfromlysis
(rupture).
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38. Acid-Fast Cell Walls
•Acid-faststainisusedtoidentifyallbacteriaofthegenusMycobacterium
andpathogenicspeciesofNocardia.
•Thesebacteriacontainhighconcentrations(60%)ofahydrophobicwaxy
lipid(mycolicacid)intheircellwallthatpreventstheuptakeofdyes.
•Acid-fastbacteriacanbestainedwithcarbolfuchsin;heatingenhances
penetrationofthestain.
•Thecarbolfuchsinpenetratesthecellwall,bindstocytoplasm,andresists
removalbywashingwithacid-alcohol.
•Acid-fastbacteriaretaintheredcolorofcarbolfuchsinbecauseitismore
solubleinthecellwallmycolicacidthanintheacid-alcohol.
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39. ComparisonofCellwallsofGramPositive,
GramNegativeandAcidFastBacteria
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40. NAG
NAM
NA
G NAM
NA
G
Polypeptide
Damage to the Cell Wall
•Onewaythecellwallcanbedamagedisbyexposuretothedigestiveenzyme
lysozyme.
•Thisenzymeoccursnaturallyinsomeeukaryoticcellsandisaconstituentof
perspiration,tears,mucus,andsaliva.
•Lysozymecatalyzeshydrolysisofthebondsbetweenthesugarsintherepeating
disaccharide“backbone”ofpeptidoglycan.
Carbohydrate
Backbone
Peptidoglycan in Cell wall
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41. Effect of Lysozyme on Gram Positive and
Gram Negative Cell wall
•Grampositivecellwall:Completelydestroyedbylysozyme.
•Thecellularcontentsthatremainsurroundedbytheplasmamembrane
mayremainintactiflysisdoesnotoccur;thiswall-lesscellistermeda
protoplast.
•Gramnegativecellwall:Notcompletelydestroyedbylysozyme.
•Inthiscase,thecellularcontents,plasmamembrane,andremaining
outerwalllayerarecalledaspheroplast,alsoasphericalstructure.
•Forlysozymetoexertitseffectongram-negativecells,thecellsarefirsttreatedwithEDTA
(ethylenediaminetetra-aceticacid).EDTAweakensionicbondsintheoutermembraneand
therebydamagesit,givingthelysozymeaccesstothepeptidoglycanlayer.
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42. EffectofAntibioticsonGrampositiveand
GramNegativebacteria
•Certainantibiotics,suchaspenicillin,destroybacteriabyinterfering
withtheformationofthepeptidecross-bridgesofpeptidoglycan,
thuspreventingtheformationofafunctionalcellwall.
•Grampositivehavemorepeptidoglycancontent,thereforesusceptible
toantibioticsuchaspenicillin.
•Gramnegativehavelesspeptidoglycanandtherefore,notsusceptible
topenicillin.
•However,gram-negativebacteriaarequitesusceptibletosome
ß-lactamantibioticsthatpenetratetheoutermembranebetterthan
penicillin.
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43. Structures Internal to the Cell Wall
StructuresinternaltotheCellWallconsistsofThe
Plasma(Cytoplasmic)Membrane,Cytoplasm,the
nucleoid,ribosomes,inclusions,Endospores.
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44. ThePlasma(Cytoplasmic)Membrane
•Theplasma(cytoplasmic)membrane(orinnermembrane)isathin
structurelyinginsidethecellwallandenclosingthecytoplasmofthe
cell.
•Theplasmamembraneofprokaryotesconsistsprimarilyof
phospholipids,whicharethemostabundantchemicalsinthe
membrane,andproteins.
•Eukaryoticplasmamembranesalsocontaincarbohydratesand
sterols,suchascholesterol.
•Becausetheylacksterols,prokaryoticplasmamembranesareless
rigidthaneukaryoticmembranes.Oneexceptionisthewall-less
prokaryoteMycoplasma,whichcontainsmembranesterols.
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45. Structure
•Theplasmamembraneofprokaryotesconsistsprimarilyofphospholipids.
•Thephospholipidmoleculesarearrangedintwoparallelrows,calledalipid
bilayer.
•Eachphospholipidmoleculecontainsapolarhead,composedofa
phosphategroupandglycerolthatishydrophilic(water-loving)andsoluble
inwater,andnonpolartails,composedoffattyacidsthatarehydrophobic
(water-fearing)andinsolubleinwater.
•Proteins:(i)PeripheralProteins(functionasenzymesthatcatalyzechemical
reactions)
•(ii)IntegralProteins/Transmembraneproteins(arechannelsthathavea
pore,orhole,throughwhichsubstancesenterandexitthecell).
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47. Cytoplasm
•Itreferstothesubstanceofthecellinsidetheplasmamembrane.
•Cytoplasmisabout80%waterandcontainsprimarilyproteins
(enzymes),carbohydrates,lipids,inorganicions,andmanylow
molecular-weightcompounds.
•Themajorstructuresinthecytoplasmofprokaryotesareanucleoid
(containingDNA),particlescalledribosomes,andreservedeposits
calledinclusions.
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48. TheNucleoid
•Thenucleoidofabacterialcellusuallycontainsasinglelong,
continuous,andfrequentlycircularlyarrangedthreadofdouble-
strandedDNAcalledthebacterialchromosome.
•Unlikethechromosomesofeukaryoticcells,bacterialchromosomes
arenotsurroundedbyanuclearenvelope(membrane)anddonot
includehistones.
•Inadditiontothebacterialchromosome,bacteriaoftencontainsmall
usuallycircular,double-strandedDNAmoleculescalledplasmid
(extrachromosomalgeneticelements,replicateindependently,carry
genesforantibioticresistance,tolerancetotoxicmetals,the
productionoftoxins,andthesynthesisofenzymes).
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49. Ribosomes
•Functionasthesitesofproteinsynthesis.
•Ribosomesarecomposedoftwosubunits,eachofwhichconsistsofprotein
andatypeofRNAcalledribosomalRNA(rRNA).
•Prokaryoticribosomesaresomewhatsmallerandlessdensethanribosomes
ofeukaryoticcells.
•prokaryoticribosomesarecalled70Sribosomesandthoseofeukaryotic
cellsareknownas80Sribosomes.
•Severalantibioticsworkbyinhibitingproteinsynthesisonprokaryotic
ribosomes.Antibioticssuchasstreptomycinandgentamicinattachtothe
30Ssubunitandinterferewithproteinsynthesis.Otherantibiotics,suchas
erythromycinandchloramphenicol,interferewithproteinsynthesisby
attachingtothe50Ssubunit.
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50. Inclusions
•Withinthecytoplasmofprokaryoticcellsareseveralkindsofreserve
deposits,knownasinclusions.
•Cellsmayaccumulatecertainnutrientswhentheyareplentifulanduse
themwhentheenvironmentisdeficient.
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51. Endospores
•Whenessentialnutrientsaredepleted,certaingram-positivebacteria,
suchasthoseofthegeneraClostridiumandBacillus,formspecialized
“resting”cellscalledendospores.
•Uniquetobacteria,endosporesarehighlydurabledehydratedcells
withthickwallsandadditionallayers.
•Whenreleasedintotheenvironment,theycansurviveextremeheat,
lackofwater,andexposuretomanytoxicchemicalsandradiation.
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52. The Eukaryotic Cell

53. Introduction
•Eukaryoticorganismsincludealgae,protozoa,fungi,plants,and
animals.
•Structureincludes:
•FlagellaandCilia,theCellWallandglycocalyx,ThePlasma
(Cytoplasmic)Membrane,Cytoplasm,Ribosomes,Organelles
(Nucleus,EndoplasmicReticulum,GolgiComplex,Vacuoles,
Mitochondria,Chloroplasts,Peroxisomes,Centrosome).
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54. Flagella and Cilia
•Manytypesofeukaryoticcellshaveprojectionsthatareusedfor
cellularlocomotionorformovingsubstancesalongthesurfaceofthe
cell.
•Theseprojectionscontaincytoplasmandareenclosedbytheplasma
membrane.
•Iftheprojectionsarefewandarelonginrelationtothesizeofthecell,
theyarecalledflagella.
•Iftheprojectionsarenumerousandshort,theyarecalledcilia.
AlgaeofthegenusEuglenauseaflagellumfor
locomotion,whereasprotozoa,suchas
Tetrahymenauseciliaforlocomotion.
Euglena Tetrahymena
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55. The Cell Wall and Glycocalyx
•Mosteukaryoticcellshavecellwalls,althoughtheyaregenerally
muchsimplerthanthoseofprokaryoticcells.
•Principlecomponentofcellwall:
•Cellulose(Inalgaeandmostplants)
•Chitin(InFungi)
•GlucanandMannan(Inyeast)
•Inothereukaryoticcells,includinganimalcells,theplasmamembrane
iscoveredbyaglycocalyx,alayerofmaterialcontainingsubstantial
amountsofstickycarbohydrates.
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56. The Plasma (Cytoplasmic) Membrane
•Theplasma(cytoplasmic)membraneofeukaryoticand
prokaryoticcellsisverysimilarinfunctionandbasicstructure.
•Thereare,however,differencesinthetypesofproteinsfoundinthe
membranes.
•Eukaryoticmembranesalsocontaincarbohydrates,whichserveas
attachmentsitesforbacteriaandasreceptorsitesthatassumearolein
suchfunctionsascell–cellrecognition.
•Eukaryoticplasmamembranesalsocontainsterols,complexlipidsnot
foundinprokaryoticplasmamembranes(withtheexceptionof
Mycoplasmacells).
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57. •Substancescancrosseukaryoticmembraneandprokaryoticplasma
membranesbysimplediffusion,facilitateddiffusion,osmosis,or
activetransport.
•Grouptranslocationdoesnotoccurineukaryoticcells.However,
eukaryoticcellscanuseamechanismcalledendocytosis.
•Thisoccurswhenasegmentoftheplasmamembranesurroundsa
particleorlargemolecule,enclosesit,andbringsitintothecell.
•Thethreetypesofendocytosisarephagocytosis,pinocytosis,and
receptor-mediatedendocytosis.
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58. Department of Biotechnology, GSSDGS Khalsa College Patiala 58

59. Cytoplasm
•Thecytoplasmofeukaryoticcellsencompassesthesubstanceinside
theplasmamembraneandoutsidethenucleus.
•Thetermcytosolreferstothefluidportionofcytoplasm.
•Amajordifferencebetweeneukaryoticand
prokaryoticcytoplasmisthateukaryotic
cytoplasmhasacomplexinternalstructure,
consistingofexceedinglysmallrods
(microfilamentsandintermediatefilaments)
andcylinders(microtubules).
•Together,theyformthecytoskeleton.
Thecytoskeletonprovidessupportandshapeandassistsintransportingsubstancesthroughthecell(andevenin
movingtheentirecell,asinphagocytosis).
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60. •Themovementofeukaryoticcytoplasmfromonepartofthecellto
another,whichhelpsdistributenutrientsandmovethecellovera
surface,iscalledcytoplasmicstreaming.
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61. Ribosomes
•Attachedtotheouter
surface of rough
endoplasmicreticulumare
ribosomes,whicharealso
foundfreeinthecytoplasm.
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62. •Asinprokaryotes,ribosomesarethesitesofproteinsynthesisinthecell.
•Theseeukaryoticribosomesare80Sribosomes,eachofwhichconsistsofa
large60SsubunitcontainingthreemoleculesofrRNAandasmaller40S
subunitwithonemoleculeofrRNA.
•Someribosomes,calledfreeribosomes,areunattachedtoanystructurein
thecytoplasm.TheySynthesizeproteinsusedinsidethecell.
•Otherribosomesaremembrane-boundribosomes,attachtothenuclear
membraneandtheendoplasmicreticulum.Theseribosomessynthesize
proteinsdestinedforinsertionintheplasmamembraneorforexportfrom
thecell.
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63. Organelles
•Organellesarestructureswithspecificshapesandspecialized
functionsandarecharacteristicofeukaryoticcells.
•Theyincludethenucleus,endoplasmicreticulum,Golgicomplex,
lysosomes,vacuoles,mitochondria,chloroplasts,peroxisomes,and
centrosomes.
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64. The Nucleus
•Thenucleusisusuallysphericaloroval,is
frequentlythelargeststructureinthecell,and
containalmostallofthecell’shereditary
information(DNA).
•Thenucleusissurroundedbyadoublemembrane
calledthenuclearenvelope.
•Tinychannelsinthemembranecallednuclear
poresallowthenucleustocommunicatewiththe
cytoplasm.
•Withinthenuclearenvelopeareoneormore
sphericalbodiescallednucleoli(singular:
nucleolus).
•Nucleoliareactuallycondensedregionsof
chromosomeswhereribosomalRNAisbeing
synthesized.
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65. •Thenucleusalsocontainsmostofthecell’sDNA,whichis
combinedwithseveralproteins,includingsomebasicproteinscalled
histonesandnonhistones.
•Whenthecellisnotreproducing,theDNAanditsassociated
proteinsappearasathreadlikemasscalledchromatin.
•Duringnucleardivision,thechromatincoilsintoshorterandthicker
rodlikebodiescalledchromosomes.
•Prokaryoticchromosomesdonotundergothisprocess,donothave
histones,andarenotenclosedinanuclearenvelope.
•Eukaryoticcellsrequiretwoelaboratemechanisms:mitosisand
meiosistosegregatechromosomespriortocelldivision.Neither
processoccursinprokaryoticcells.
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66. Endoplasmic reticulum
•Withinthecytoplasmofeukaryotic
cellsistheendoplasmicreticulum,or
ER,anextensivenetworkofflattened
membranoussacsortubulescalled
cisternae.
•TwoformsofER:
•Roughendoplasmicreticulum(RER)
•Smoothendoplasmicreticulum(SER)
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67. Rough endoplasmic reticulum (RER)
•It is continuous with the nuclear membrane and usually unfolds into a
series of flattened sacs.
•The outer surface of rough ER is studded with ribosomes, the sites of
protein synthesis.
•Enzymes within the cisternae attach the proteinsto carbohydratesto
form glycoproteins, also attach proteins to phospholipids.
•These molecules may be incorporated into organelle membranes or the
plasma membrane.
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68. Smooth endoplasmic reticulum (SER)
•SmoothERextendsfromtheroughERtoformanetworkof
membranetubules.
•UnlikeroughER,smoothERdoesnothaveribosomesontheouter
surfaceofitsmembrane.
•SmoothERdoessynthesizephospholipids,asdoesroughER.
•SmoothERalsosynthesizesfatsandsteroids,suchasestrogensand
testosterone.
•Inlivercells,enzymesofthesmoothERhelpreleaseglucoseintothe
bloodstreamandinactivateordetoxifydrugsandotherpotentially
harmfulsubstances(forexample,alcohol).
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69. Golgi Complex
•Mostoftheproteinssynthesizedby
ribosomesattachedtoroughERare
ultimatelytransportedtootherregions
ofthecell.
•Thefirststepinthetransportpathway
isthroughanorganellecalledthe
Golgicomplex.
•Itconsistsof3to20cisternaethat
resembleastackofpitabread.
•Thecisternaeareoftencurved,giving
theGolgicomplexacuplikeshape.
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70. •ProteinssynthesizedbyribosomesontheroughERare
surroundedbyaportionoftheERmembrane,which
eventuallybudsfromthemembranesurfacetoforma
transportvesicle.
•ThetransportvesiclefuseswithacisternoftheGolgi
complex,releasingproteinsintothecistern.
•Theproteinsaremodifiedandmovefromonecisternto
anotherviatransfervesiclesthatbudfromtheedgesof
thecisternae.
•Enzymesinthecisternaemodifytheproteinstoform
glycoproteins,glycolipids,andlipoproteins.
•Someoftheprocessedproteinsleavethecisternaein
secretoryvesicles,whichdetachfromthecisternand
delivertheproteinstotheplasmamembrane,wherethey
aredischargedbyexocytosis.
•Someprocessedproteinsleavethecisternaeinvesicles
thatarecalledstoragevesicles.Themajorstoragevesicle
isalysosome.
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71. Lysosomes
•LysosomesareformedfromGolgi
complexesandlooklikemembrane-
enclosedspheres.
•Lysosomeshaveonlyasingle
membraneandlackinternalstructure.
•Theycontainasmanyas40different
kindsofpowerfuldigestiveenzymes
capableofbreakingdownvarious
molecules(forexamplebacteria).
•Humanwhitebloodcells,whichuse
phagocytosistoingestbacteria,
containlargenumbersoflysosomes.
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72. Vacuoles
•Avacuoleisaspaceorcavityinthecytoplasmofacellthatisenclosedby
amembranecalledatonoplast.
•Inplantcells,vacuolesmayoccupy5–90%ofthecellvolume,depending
onthetypeofcell.
•VacuolesarederivedfromtheGolgicomplexandhaveseveraldiverse
functions.
•Somevacuolesserveastemporarystorageorganellesforsubstancessuch
asproteins,sugars,organicacids,andinorganicions.
•Othervacuolesformduringendocytosistohelpbringfoodintothecell.
•Manyplantcellsalsostoremetabolicwastesandpoisonsthatwould
otherwisebeinjuriousiftheyaccumulatedinthecytoplasm.
•Finally,vacuolesmaytakeupwater,enablingplantcellstoincreaseinsize
andalsoprovidingrigiditytoleavesandstems.
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73. Mitochondria
•Sphericalorrod-shapedorganellescalledmitochondriaappear
throughoutthecytoplasmofmosteukaryoticcells.
•Amitochondrionconsistsofadoublemembranesimilarinstructureto
theplasmamembrane.
•Theoutermitochondrialmembraneissmooth,buttheinner
mitochondrialmembraneisarrangedinaseriesoffoldscalledcristae
(singular:crista).
•Thecenterofthemitochondrionisasemifluid
substancecalledthematrix.
•Mitochondriaareoftencalledthe
“powerhousesofthecell”becauseoftheir
centralroleinATPproduction.
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74. Chloroplasts
•Algaeandgreenplantscontainauniqueorganellecalleda
chloroplast,amembrane-enclosedstructurethatcontainsboththe
pigmentchlorophyllandtheenzymesrequiredforthelight-gathering
phasesofphotosynthesis.
•Thechlorophylliscontainedinflattenedmembranesacscalled
thylakoids;stacksofthylakoidsarecalledgrana(singular:granum).
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75. Peroxisomes
•Organellessimilarinstructuretolysosomes,butsmaller,arecalled
peroxisomes.
•Peroxisomescontainoneormoreenzymesthatcanoxidizevariousorganic
substances.
•Forexample,substancessuchasaminoacidsandfattyacidsareoxidizedin
peroxisomesaspartofnormalmetabolism.
•Inaddition,enzymesinperoxisomesoxidizetoxicsubstances,suchas
alcohol.
•Aby-productoftheoxidationreactionsishydrogenperoxide(H
2O
2),a
potentiallytoxiccompound.
•However,peroxisomesalsocontaintheenzymecatalase,which
decomposesH
2O
2.
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76. Centrosome
•Acentrosomeisacellular
structureinvolvedintheprocessof
celldivision.Beforecelldivision,the
centrosomeduplicatesandthen,as
divisionbegins,thetwocentrosomes
movetooppositeendsofthecell.
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