introduction

1. 1
Parasitic Protozoa:Form,Function,and Classification
FORMAND FUNCTION
Protozoaconsistof a single cell,althoughmanyspeciescontainmore thanone nucleusduringall or
portionsof theirlife cycles.Bymid–19thcentury,manyprotozoangenerahadbeendescribed,andtheir
enormousstructural diversity,complexity,andevenbeautywerewidelyrecognized.20Earlyelectron
microscopists,ashadthe lightmicroscopists,foundunicellulareukaryotesfascinatingsubjects,andsoon
afterWorldWar II,researchersrecognizedthatthe groupwasa heterogeneousassemblagewhose
membersdidnotall conformto a single bodyplan. In1980 a committee of the Societyof
Protozoologistsrevisedthe classification,recognizingsevenphyla,andfurtherrevisionswere
recommendedbyasimilargroupof expertsin1985.23 More recentclassifications,incorporating
moleculardata,propose groupingsthatseemquite contrarytothose of oldersystems.Anexample of
the latteris the superphylumAlveolata,whichincludesdinoflagellates,phylumApicomplexa,phylum
Ciliophora, andHaplosporidia.
Ultra structural studieshave shown,however,thatregardlessof how elaborate orelaboratelyarranged
theyare,most componentsof protozoanorganellesdonotdifferinanybasicwayfromthose of
metazoancells.14,19 Indeed,Pitelka28concluded“thatthe fine structure of protozoaisdirectlyand
inescapablycomparablewiththatof cellsof multicellularorganisms,” andthe “morphologisthastostart
out byadmittingthatprotozoaare, at the least,cells.”Muchof the apparentupheaval ineukaryotic
systematicisa resultof suchadmission,withthe distinctionsbetweenunicellularandmulticellular
organismsbecomingquite blurredatthe ultrastructural and molecularlevels.4,17 On the otherhand,
to a studentwhofirstencountersthem,protozoanstructurescanseembizarre,oftenmultiple versions
and arrangementsof the familiarmitochondria,microtubules,flagella,andmembranesstudiedin
introductorybiology.If itseemslike the wordsmay,usually,typically,andoftenoccurmore frequently
inthischapter thanin others,thensuchuse isa reflectionof the greatdiversity,atthe subcellularlevel,
foundin single-celledeukaryotes.
NucleusandCytoplasm
 Plasma membrane like all cells,the bodiesof protozoaiscoveredby a plasma membrane,
whichisthe lipidbilayer,fluidmosaic,describedinintroductorybiologytexts.Manyprotozoa
have more than one such membrane aspart of theirpellicle.
 Additional membranes may be presentasalveoli,orsacs,whichinsome ciliatesare enlarged,
producingridgesandcraterson the cell surface.Protozoamayalsopossessa thickglycocalyx,or
glycoproteinsurface coat,which,inthe case of parasiticforms,hasimmunological importance.
Othermembrane proteinsmayserve asbinding sitesthatfunctionduringuptake of intracellular
parasitesbytheirhostcell.
 Pellicularmicrotubules maycourse justbeneaththe plasma membrane,the numberand
arrangementof suchtubulesbeingtypical of agroup.The pellicle maybe thrownintomore or
lesspermanentfolds,supportedbymicrotubules,asingregarine parasitesof insects.Orsuch

2. 2
microtubulesmayunderlie aflexiblemembrane,asinkinetoplastidflagellates.The structural
elaborationof membranes,throughfoldingandadditionof electron-densematerials,also
occurs intissue-dwellingcysts.Adjoiningmembranesmayhave anelectrondenseorfibrous
connectionbetweenthem,suchasthatbetweenthe bodyandundulatingmembrane of
trypanosomesandtrichomonads.
 OrganellesProtozoapossessagreatdiversityof membranousorganellesintheircytoplasm.
 Mitochondria, the organellesthatbearenzymesof oxidative phosphorylationandthe
tricarboxylicacidcycle,oftenhave tubularratherthanlamellarcristae.Inaddition,some
amebashave branchedtubularcristae,butinotherprotozoangroupsthe cristae maybe absent
altogether.Mitochondriamaybe presentasa single,large body,asinsome flagellates,or
arrangedas elongated,sausage-shapedstructures,asoccur inpellicularridgesof some ciliates.
 The Golgi apparatus (dictyosome) isquite elaborate insome flagellates,occurringaslarge
and/ormultiple parabasal bodies inassociationwithkinetosomes, the “basal bodies”of flagella,
and ispresent,althoughnotalwaysasprominent,inamebasandciliates. Dictyosome canplay
diverse rolesinthe livesof protozoa— forexample,theycanbe the source of skeletal platesin
some amebasandpolar filamentsinmicrosporidianparasites
 Microbodiesare usually,butnotalways,spherical membrane-boundstructures withadense,
granularmatrix. In mostanimal andmany plantcells,Microbodiescontainoxidasesand
catalase.The oxidasesreduce oxygentohydrogenperoxide,andcatalase decomposeshydrogen
peroxide towaterandoxygen.Thus,Microbodiesinthesecellsare calledperoxisomesbecause
of theirbiochemical activity.Peroxisomesare foundin many aerobic protozoa in which oxygen
is a terminal electronacceptor in metabolism.Inat leastsome anaerobes,suchasparasitic
Trichomonadsspp.,Microbodiesproduce molecularhydrogenandare called hydrogenosomes.
Microbodiesmayalsocontainenzymesof the glyoxylate cycle,a seriesof reactionsthatfunction
inthe synthesisof carbohydrate fromfat.Microbodiesof Kinetoplastidaare calledglycosomes
and containmostof the glycolyticenzymes(whichinothereukaryoticcellsare foundinthe
cytosol)
 Othermore unusual membrane-boundorganellesincludeaboutadozenkindsof extrusomes,
whichgenerallyoriginate inthe dictyosome andcome tolie beneaththe cell membrane.Upon
properstimulusextrusomesfuse withthe cell membrane,releasingtheircontentstothe
exterior.Extrusomesas toxosomesmayrelease toxicsubstances,evidentlyasadefensive
mechanismorfunctionaskinetocystsinfoodcapture,ashaptocyststo paralyze prey,oras
trichocystsinmechanical resistance topredators.The dark(electron-dense),elongated bodies
perpendiculartothe cell membrane are mucocystsof a parasiticciliate,Ichthyophthirius
multifiliis.Mucocystsare thoughttoprovide acoating that protects the cell againstosmotic
shock. Not all extrusomes,however,have obviousfunctions.
 The cytoplasmicmatrix consistsof verysmall granulesandfilamentssuspendedinalow-density
mediumwiththe physical propertiesof acolloid;thatis,withthe capabilityof existingina
relativelyfluid(sol state) orrelativelysolid(gel state)condition.Centralandperipheral zonesof
cytoplasmcan oftenbe distinguishedasendoplasmandectoplasm. Endoplasmisinthe sol
state,and itbearsthe nucleus,mitochondria,Golgi bodies,andsoon.Ectoplasm isofteninthe

3. 3
gel state;underthe lightmicroscope itappearsmore transparentthansol,and inthisphysical
state cytoplasmfunctionstomaintaincell shape.The basesof flagellaorciliaandtheir
associatedfibrillarstructures,whichmaybe verycomplex,are embeddedinthe ectoplasm
 Protozoa,like fungi,plants,andanimals,are eukaryotes;thatis,theirgeneticmaterial—
deoxyribonucleicacid(DNA)—iscarriedonwell-definedchromosomescombinedwithbasic
proteinscalledhistones,andthe chromosomesare containedwithinamembrane-bound
nucleus.Atthe lightmicroscope level,protozoannucleiare typicallyoval,discoid,orround,and
theyare usuallyvesicular,withanirregulardistributionof chromatinmaterialand“clear”areas
inthe nuclearsap.But inciliates,whichcontainatleastone micronucleusandone
macronucleus,the lattermaybe dense,elongated,chainlike,orbranched.Micronuclei are
reproductive nuclei,undergoingmeiosispriortosexual reproduction(conjugation).Macronuclei
are considered“somatic”;theyfunctionincell metabolismandgrowthbutdo notundergo
meiosis.
 In electronmicrographsnucleoplasmappearsfinelygranular,withaggregationsof dense
chromatin.Chromosomesmayremainasrecognizablebodiesthroughoutthe cell cycle.Nucleoli
are usuallypresent,but theytypicallydisappearduringnucleardivision.Endosomes,
conspicuousinternal bodies,are nucleoli,althoughtheydonotdisappearduringmitosis.
Parasiticamebasandtrypanosomeshave Endosomes.The termEndosomesmayalsobe usedin
reference tovesiclesarisingbyendocytosis.17The nuclearenvelope issimilartothatof most
eukaryoticcells,consistingof twomembranesthatfuse inthe regionof pores,butthe envelope
may be thickenedbyafibrouslayerorhave strange honey comb like tubesonthe outerorinner
face.The nuclearenvelope mayormaynot persistduringmitosis,againdependingonthe
species,andmitoticspindlescanbe intra- orextranuclear.
Locomotory Organelles
 Protozoamove bythree basictypesof organelles:pseudopodia, flagella,andcilia;flagella
and ciliaare alsocalledundulipodia.
 Some amebaspossessbothflagellaandpseudopodia,althoughtransformationfrom
flagellatedto amoeboid celloccursinresponse toenvironmentalconditionsandisa
recognizedlifecycle event.
 Flagellamayalso occur inlarge numbersandin rows,thussuperficiallyresemblingcilia.In
Ciliophorathe ciliabasesare connectedbyacomplex fibrousnetwork,or infraciliature
Flagella
Flagella(undulipodia) are slender,whiplike structures,eachcomposedof acentral axoneme andan
outersheaththat isa continuation of the cell membrane.Anaxoneme consistsof nine peripheral and
one central pair of microtubules(the nine-plus-twoarrangementfoundinciliaandflagellathroughout
the animal kingdomwithafewexceptions).Central microtubulesare singlets,butperipheralonesare
oftendoubletsorevendoublets“witharms.”The central twomicrotubulesare bilateral,andthe
peripheral onescanthusbe numberedwithreferencetoa plane perpendiculartothe line betweenthe

4. 4
central pair.The axoneme arisesfromakinetosomes(basalbody),whichisultrastructurally
indistinguishable fromcentriole of othereukaryoticcells,beingmade upof the nine peripheral
elements,typicallymicrotubule tripletsarrangedinacartwheel manner.Kinetosomesmaylie atthe
bottomof flagellarpocketsorreservoirsof differingdepths,dependingonthe species.Whenaflagellate
has at leasttwoflagellawithdifferingstructures,the conditionistermed heterokon
 The entire unit—flagellum,kinetosomes,andassociatedorganelles—iscalledamastigont or a
mastigont system.
 Kinetosomesare more or lessfixedinpositionrelative tootherorganelles;thus,flagellamaybe
directedanteriorly,laterally,orposteriorly,independentof theirmovements.Mostflagellates
have more than one flagellum,andthese maybe insertedintothe cell atdifferentangles.The
flagellummayalsobe bentbackalongand looselyattachedtothe lateral cell surface,forminga
finlike undulatingmembrane,whichmaybe an adaptationtolife inrelativelyviscous
environments.
 Flagellarmovements are generallyhelical wavesthatbeginateitherthe base or tip,pushing
fluidsalongthe flagellaraxis.The resultingbodymovementmay be fastor slow,forward,
backward,lateral,orspiral.Insome cases,such as withtrichomonadsparasites,movementis
highlycharacteristicandrecognizedinstantlybymostParasitologistswhohave previously
studiedthese flagellatesinfreshintestinal contents.
 A mastigont systemmay alsoinclude aprominent,striatedrod,orCosta,that coursesfromone
of the kinetosomesalongthe marginof the organismjustbeneaththe recurrentflagellumand
undulatingmembrane.A tube like axostyle,formedbyasheetof microtubules,mayrun from
the area of the kinetosomestothe posteriorend,whereitmayprotrude.InphylumParabasalia,
kinetosomesof the three anteriorlydirectedflagellaare numbered1,2, and3, and have lamina
(sheets) of microtubulesthatincross sectionsappeareitherashooks(kinetosomes1and 3) or
as sigmoidprofiles(kinetosomes2).A Golgi body(dictyosome) maybe present;if aperiodic
fibril,orparabasal filament,runsfromthe Golgi bodyto contact a kinetosomes,the Golgi body
isreferredtoas a parabasal body.A fibril runningfromkinetosomestoa pointnearthe surface
of the nuclearmembrane iscalledarhizoplast,andthe entire complex of organellesandan
associatednucleusisthusreferredtoasa karyomastigont.
 In class Kinetoplastida, whichincludesthe trypanosomes,adark-stainingbodyorkinetoplastis
foundnearthe kinetosomes. The kinetoplastisactuallyadisc made of DNA circles,calledkDNA,
locatedwithinasingle large mitochondrion. KDNA hasdifferentgeneticpropertiesfromnuclear
DNA.Kinetoplastidalsohave aparaxial (crystallinerod) thatliesalongside the axoneme,within
the flagellum.And,finally,manyfree-livingflagellatespossessfinefringesor hairlike
mastigonemesontheirflagella,makingthemlooklikemotile test-tubebrushesinthe electron
microscope.Tubularflagellarhairswiththree finefilamentsatthe tipare a structural character
that unitesthe so-called“stramenopiles”.

5. 5
Cilia
Cilia(alsoundulipodia) are structurallysimilartoflagella,withkinetosomesandanaxoneme composed
of twocentral and nine peripheral microtubules.Ciliatypicallyappeartobeatregularly,withaback-and-
forthstroke ina two-dimensional plane, whereasflagellaoftenappeartobeatirregularly,turningand
coilingina three-dimensional space.However,ciliamaybeatina helical movement,some flagellabeat
ina plane,andbothtypesof undulipodiabeatinmetachronal waves,reminiscentof afieldof waving
grain,whentheyoccur inlarge numbers.
Body cilia (somatic ciliature) are arranged in rows, known as kinetics, which in turn are composed of
kinetids,the basicunitsof ciliate pellicularorganization.Monokinetids contain single kinetosomes and
associatedfibers;dikinetidscontainapairof kinetosomes;andsoon.The pellicle of Dexiotricha media,
a ciliate found in an Illinois pig wallow is simple enough to serve as an introduction to ciliate
organization. A kinetids consists of the kinetosomes; a small membranous pocket, the parasomal sac;
and a number of fibers or sheets, made from microtubules that extend in various directions from the
kinetosomes.
A tapering banded fiber, the kinetodesma (plural kinetodesmata), arises from the clockwise side of
each kinetosomes (whenviewedfromthe anteriorendof the cell),coursesanteriorly,andjoinsasimilar
fiberfromthe adjoiningciliuminthe same row.The resultingcompoundfiberof kinetodesmataiscalled
a kinetodesmose. Flat sheets of microtubules, the postciliary microtubules, run posteriorly from each
kinetosomes, andsimilarlyconstructedbands,the transversemicrotubules,lieperpendicularto kinetics.
Kinetosomes and associated fibrils constitute the infraciliature. Ciliates differ significantly in the
structure of theirinfraciliature,andsuchdifferencesare of majortaxonomicimportance.Obviously, the
great diversityinstructure isassumed to reflect an equal diversity in functional details, but we still do
not have much knowledge about those details.
Oral ciliature can be amazingly complex and is an outstanding example of the elaboration of familiar
organelles. Oral membranes are actually polykinetids; that is, fields or rows of cilia and their
kinetosomeslinkedbyelectrondensefibrousnetworks.The adoral zone of membranelles is a series of
such oral membranes located to the “left” of or counterclockwise from the side of the oral area of the
more complex ciliates.Polykinetidsmayalsobe found on the body as cirri (singular cirrus), tufts of cilia
that functiontogether,usuallyinlocomotion along a substrate. Much of the wonder that ciliates seem
to produce instudentscomesfromthe actionof polykinetids;forexample,the “walking” motion of cirri
tendsto make the organismlookas if itis behavinginarather purposeful way. A group of kinetosomes
forming a tuft of ciliary organelles in the aboral region of peritrich ciliates is called the scopula. It is
involved in stalk formation.
Ciliabeatwitha powerful backstroke,pushingthe surroundingfluid posteriorly, in metachronal waves.
Membranelleshave theirown beat cycles that are usually independent of the somatic ciliature. When
ciliates divide, the ciliature is usually reorganized according to a precise sequence of events.
Reorganizationof oral polykinetids is a complex process. This “embryological development” has been
the basis for much of the class level taxonomy in the Ciliophora, but current classifications also rely
heavily on ultrastructural details of body ciliature

6. 6
The mechanismby which flagellaand ciliamove requires ATP and involves the interaction of the arms
of each microtubule pair with the neighboring pair of microtubules. These interactions cause one
memberof a pair to slide lengthwise relative tothe other microtubule in the pair (sliding microtubule
model).
Pseudopodia
Pseudopodia are temporary extensions of the cell membrane and are found in amebas as well as in a
variety of cell types in other organisms. Pseudopodia function in locomotion and feeding. In some
amebas,movementisbyflow of the entire body, with no definite extensions. Such amebas are called
Limax forms, after the slug genus Limax.
 Four general types of pseudopodia occur in amebas; three of these types are illustrated
 Lobopodia are finger-shaped, round-tipped pseudopodia that usually contain both ectoplasm
and endoplasm. Most free-living soil and freshwater amebas and all parasitic and commensal
amebas of humans have this kind of pseudopodium.
 Filopodia are slender, sharp-pointed organelles, composed only of ectoplasm. They are not
branchedlike rhizopodia,whichbranchextensivelyandmayfuse togethertoformnetlike mesh.
 Axopodia are like Filopodia, but each contains a slender axial filament composed of
microtubulesthatextendsintothe interiorof the cell.Bothpseudopodshape and the shapes of
uroids (membranous extensions at the posterior end of the cell) are taxonomic characters in
amebas.Uroidsmay be bulbous,spiny,morulate (like a grape cluster), or papillate. Movement
by meansof pseudopodiais a complex form of protoplasmic streaming involving protrusion of
the cell, adhesion to substrate, and subsequent contraction.
 Candelas give anexcellentreview of the signalingsystems and protein interactions involved in
cell crawling. Evidence suggests that the mechanism requires coordinated structural
modification, polymerization, and crosslinking of actin filaments, myosin-mediated filament
sliding, adhesion, and deadhesion. Although protoplasmic streaming is well studied, the
mechanisms that determine pseudopod shape are not known. Amebas obviously have some
characteristics that function to produce extensions of plasma membrane that are indeed
temporarybutare alsoconsistentenoughinstructure sothattheymay be usedinidentification
and classification. Pseudopodformationiscertainlynolesswondrousthanpolykinetidsfunction
 In many apicomplexans (gregarines, coccidia, and malaria parasites, , the merozoites,
ookinetes,andsporozoitesappeartoglide throughfluidswithno subcellularmotion whatever.
Gregarines, for example, exhibit a variety of slow, sometimes almost snakelike movements,
depending on the species and the kind of fresh tissue preparation that is examined. Electron
microscope studies reveal longitudinal pellicular ridges (epicytic folds) on these cells, which
often appear to have been fixed in the process of forming an undulatory wave. Subpellicular
microtubulesare foundinthe folds, and it has been proposed that these fibers function in the
gliding locomotion. Experimental work, however, reveals that contact with a substrate is
essential to gregarine movement and suggests that mucous secretion may also play a role in
locomotion.

7. 7
Reproductionand Life Cycles
Protozoanreproductionmaybe either asexualorsexual,althoughmanyspeciesalternatethe twotypes
intheirlife cyclesorperformone or the otherreproductive functionsinresponse toenvironmental
conditions.Mostoftenasexual reproductionisby binary fission,inwhichone individualdividesinto
two.The plane of fissionisrandominamebas,longitudinal inflagellates(betweenkinetosomesor
flagellarrows;thatis,symmetrogenic),andtransverse inciliates(acrosskinetics,orhomothetogenic).
The sequence of divisionis
1. Kinetosomes
2. kinetoplast(ifpresent),
3. nucleus,and
4. Cytokinesis.
A. asexual reproduction
Nucleardivisionduring asexual reproductionisbymitosis,exceptinmacronuclei of ciliates,whichare
highlypolyploidanddivide amitotically.However,patternsof mitosisare muchmore diverse among
unicellulareukaryotesthanamongmetazoa.Aninventoryof thesepatternsisbeyondthe scope of this
book,butexamplesinclude nuclearmembranesthatpersistthroughmitosis,spindlefibersthatform
withinthe nuclearmembrane,missingcentriole,andchromosomesthatmaynotgo througha well-
definedcycle of condensationanddecondensation.Nevertheless,the essentialfeaturesof mitosis—
replicationof chromosomesandregulardistributionof daughterchromosomestodaughternuclei—are
alwayspresent.
Multiple fission (merogony,schizogony) occursinsome amebasandinApicomplexa.Inthistype of
divisionthe nucleusandotheressential organellesdividerepeatedlybefore cytokinesis.Thus,alarge
numberof daughtercellsare producedalmostsimultaneouslyandare,theoretically,inthe same or
similarphysiological condition.Cellsundergoingschizogonyare calledschizonts,meronts,or
segmenters.Dependingonthe species,the schizontsdaughternucleimayarrange themselves
peripherally,withmembranesof daughter cellsformingbeneaththe cell surface of the mothercell. The
daughtercellsare merozoites,andtheyeventuallybreakawayfroma small residual massof protoplasm
remainingfromthe mothercell toinitiate anotherphase of merogony(schizogonyproducing more
asexuallyreproducingmerozoites) ortobegingametogony(gametocyte formation).
Sporogony
Anothertype of multiplefissionoftenrecognizedis sporogony,whichismeiosisimmediatelyafterthe
unionof gametes,typicallyfollowedbymitosis.The productsof merogonyare additional parasitesof
the same life-cyclestage,suchasthose that invade redbloodcellsduringamalarial infection.The
productsof sporogony,however,are usuallyof acompletelydifferentlife-cycle stage,suchas
sporozoites inresistantoocysts(“spores”) of gregarines
Several formsof buddingcanbe distinguished. Plasmotomy,sometimesregardedasbudding,isa
phenomenoninwhichamultinucleateindividualdividesintotwoormore smallerbutstill multinucleate

8. 8
daughtercells.Plasmotomyitself isnotaccompaniedbymitosis.Externalbuddingisfoundamongsome
ciliates,suchassuctorians.Here nucleardivisionisfollowedbyunequal cytokinesis,resultingina
smallerdaughtercell,whichthenswimsawayfromthe sessile parentandsubsequentlysettles,
metamorphoses,andgrowsto itsadultsize.Internal budding,orendopolyogeny,differsfrom
schizogonyonlyinthe locationwhere daughtercellsare formed.Inthisprocessdaughtercellsbegin
formingwithintheirowncell membranes,distributedthroughoutthe mothercell’scytoplasmrather
than at the periphery.The processoccursinschizontsof some coccidians.Endodyogenyis
endopolyogenyinwhichonlytwodaughtercellsare formed.Protozoan’s,itseems,are asvaried and
elaborate intheirasexual reproductionastheyare in theirstructure.
B. Sexual reproduction
Sexual reproductioninvolves reductiondivision inmeiosis,resultinginachange fromdiploidyto
haploidy,withasubsequentunionof twocellstorestore diploidy.Cellsthatjointorestore diploidyare
gametes,andthe processof producinggametesisgametogony.Cellsresponsible forgamete production
are gamonts.Reproductionmaybe amphimictic,involvingthe unionof gametesfromtwoparents,or
automictic,inwhichone parentgivesrise tobothgametes.Unitinggametesmaybe entire cellsoronly
nuclei. Whengametesare whole cells,the union iscalled syngamy.
Syngamy
In syngamy,gametesmaybe outwardlysimilar(isogametes)ordissimilar(anisogametes).Although
isogameteslooksimilar,theywillfuse onlywithisogametesof another“matingtype.”Anisogametes
oftendifferincytoplasmiccontents,insize (sometimesmarkedly),andinsurface proteinsthat
determine matingtype.The larger,more quiescentof the pairisa macrogamete;the smaller,more
active partnerisa microgamete.Itistemptingtocall these formsfemaleandmale,respectively,butitis
debatable whethergender,inthe commonlyusedsense,canorevenshouldbe distinguishedin
protozoa.Fusionof a microgamete andmacrogamete producesazygote,whichmaybe a restingstage
that overwintersorformssporesthatenable survival betweenhosts.
Conjugation
Conjugation,inwhichonlynuclei unite,isfoundonlyamongciliates,whereas syngamyoccursinall
othergroupsin whichsexual reproductionisfound.Twoindividualsreadyforconjugationunite,and
theirpelliclesfuseatthe pointof contact. The macronucleusineachdisintegrates,andtheirmicronuclei
undergomeioticdivisions intofourhaploidpronuclei (of whichtwodegenerate).A migratorypronucleus
fromeach conjugantpassesintothe otherto fuse withastationarypronucleus,restoringthe diploid
condition.The cellsseparate andsubsequentnucleardivisionsproduce one ormore macronuclei.The
exconjugants,whichare nowgeneticrecombinants,thenactivelyreproduce byfission.
The detailsof conjugation,includingexconjugants’relationships,extentof cytoplasmicsharing,andfate
of exconjugants,varywidelyamongciliates.Undernatural conditions conjugatingpairsare seen
occasionally,especiallywhenenvironmental conditionsdeteriorate.Clone cultures,descendedfrom
single individuals,canbe preparedinthe laband stressedtoproduce cellsthatare readyto conjugate

9. 9
and will dosoen masse whenmixedwithotherclones(the matingtype reaction).Thistechnique has
beenuseful inthe studyof matingspecificity,genetics,andsurface proteinfunctioninciliates.
Variationsof conjugationare cytogamy,inwhich twoindividualsfuse butdonotexchange pronuclei,
withtwopronuclei ineachcell rejoiningtorestore diploidy,andautogamy,inwhichhaploidpronuclei
fromthe same cell fuse butthere isnocytoplasmicfusionwithanotherindividual.InApicomplexa,
meiosisoccursinthe firstdivisionof the zygote (zygoticmeiosis),andall otherstagesare haploid.
Intermediarymeiosis,whichoccursonlyinthe Foraminiferaamongprotozoabutwhichiswidespreadin
plants,exhibitsaregularalternationof haploid anddiploidgenerations.
Encystment
 Many protozoacan secrete aresistantcoveringandenterarestingstage,or cyst.Cyst formation
isparticularlycommonamongparasiticprotozoaas well asamongfree-livingprotozoafoundin
temporarybodiesof waterthatare subjecttodryingor otherharsh conditions.
 In additiontoprovidingprotectionagainstunfavorable conditions,cystsmayserve assitesfor
reorganizationandnucleardivision,followedbymultiplicationafterexcystation.
 In a fewforms,such as Ichthyophthiriusmultifiliis,aciliate parasite of fish,cystsfall fromthe
hostto the substrate andstickthere until excystationoccurs.Cellulosehasbeenfoundinthe
cyst wallsof some amebas,andotherscontainchitin.Cystscanbe highlycomplexandlayered
structures,as seenwithanelectronmicroscope inthe filamentouscystsof Giardiaspecies.
 The outerlayersmayalsoreact withimmunodiagnosticreagents,althoughnotalwaysina
highlyspecificmanner.ConditionsfavoringEncystmentare notfullyunderstood,buttheyare
thoughtinmost casesto involve some adverse environmental eventssuchasfooddeficiency,
desiccation,increasedtonicity,decreasedoxygenconcentration,orpH or temperature change.
 It isvitallyimportantforParasitologiststounderstandthe elusive factorsthatinduce cyst
formationwithinthe host,the role thatcystsplayincompletionof aparasite’slife cycle,and
factors thatwork to disseminate cysts.
 For example,humanamebiasis,causedbyEntamoebahistolytica,isspreadbypersonswho
oftenhave noclinical symptomsbutwhopasscystsin theirfeces.DuringEncystmentacystwall
issecreted,andsome foodreserves,suchasstarch or glycogen,are stored.Projectingportions
of locomotorsorganellesare partiallyorwhollyresorbed,andcertainotherstructures,suchas
contractile vacuoles,maybe dedifferentiated.Duringthe processorfollowingsoonthereafter,
one or more nucleardivisionscangive acyst more nuclei thana trophozoite
 . In flagellatesandamebas, cytokinesisoccursina characteristicdivisionpatternafter
excystation.
 In coccidians the cystic formis oocysts,whichare formedaftergamete unionandinwhich
multiple fission(sporogony)occurstoproduce sporozoites.
 In eimeriancoccidians, oocystscontainingsporozoitesserve asresistantstagesfortransmission
to newhosts,whereasinhaemosporidians(includingthe causativeagentsof malaria,
Plasmodiumspp.) oocystsserve asdevelopmentalcapsulesforsporozoiteswithintheir insect
host

10. 10
 In speciesinwhich the cyst is a resistant stage,a returnof favorable conditionsstimulates
excystation.Inparasiticformssome degreeof specificityinthe requisite stimuliprovidesthat
excystationwill nottake place exceptinthe presenceof conditionsfoundinahost’sgut.
Mechanismsforexcystationmayinclude absorptionof waterwithconsequentswellingof the
cyst,secretionof lyticenzymesbythe protozoan,andactionof hostdigestive enzymesonthe
cyst wall.Excystationmust includereactivationof enzymepathwaysthatwere “turnedoff”
duringthe restingstage,internal reorganization,andredifferentiationof cytoplasmicand
Locomotory organelles
Feeding and Metabolism
Some protozoaare photosyntheticandsynthesize carbohydratesinchloroplasts,the organellesof
“typical”plants.Suchorganismsare oftenconsideredalgae andclaimedbythe phycologists,butafew
participate insymbioticrelationshipsof interesttoParasitologists.Zooxanthellae(dinoflagellates) are
very importantmutual’slivingincellsof reef-formingcoralsandotherinvertebrates(includingsome
otherprotozoa),contributing significantamountsof carbohydratestotheirhosts.
 Protozoalackingchloroplastsare all heterotrophic,requiringtheirenergyinthe formof complex
carbon moleculesandtheirnitrogeninthe formof a mixture of preformedaminoacids.Protozoa
are typically particle feeders—thatis,grazersandpredators—andmanysymbioticspeciesfeedon
hostcells.Theirmouthopeningsmay be temporary,asinamebas,orpermanentcytostomes,asin
ciliates.A submicroscopicmicropore ispresentinEimeriaandPlasmodiumspeciesand,incertain
stages,isinvolvedin takinginnutrients.
 Particulate foodpassesintoafoodvacuole,whichisa digestive organelle thatformsaroundany
foodthusingested.Indigestible material isvoidedeitherthroughatemporaryopeningorthrougha
permanentcytopyge,whichisfoundinmanyciliates.Pinocytosisisanimportantactivityinmany
protozoa,as is phagocytosis.Both Pinocytosisandphagocytosisare examplesof endocytosis,
differingonlyinthat Pinocytosisdealswithdropletsof fluid,whereasphagocytosisis the process
of internalizingparticulate matte
 Like mostothereukaryoticcells,protozoa generallycarryout the manyreactionsof glycolysis,Krebs
(citricacid) cycle,pentose-phosphate shunt,electrontransport,transaminations,lipidoxidations
and syntheses,nucleicacidmetabolism, andthe multitude of othermetaboliceventsthatmake
biochemical pathwayslooklikeprintedcircuitsof high-techelectronicequipment.ATPisthe most
commonform of immediatelyusable energy,althoughafew parasitesuse inorganicpyrophosphate
ina similarrole. Polysaccharides, especiallyglycogenorrelatedmolecules,functionasdeepenergy
storage.Genesare transcribed in the nucleus,and polypeptidesare synthesizedonribosomes,as
in other cells.
 Comparative biochemical studiesreveal thatdetailsof protozoanmetabolismare asvariedas the
detailsof protozoansex.Some importantbiological factorstoconsiderare thatmany parasites
occupy environmentsinwhichthe oxygensupplyisquite limited.Otherslive intissues,suchas
blood,where neitheroxygennorglucose islimited.Inthe lattercase,there isnoenergyadvantage
incompletelyoxidizingglucose.Organismsthatare adaptedtosuch environments,includingmany

11. 11
protozoanparasites,oftenderive all theirenergyfromglycolysisandexcrete the partiallyoxidized
productsas waste.The complete Krebscycle andCytochrome systemthenbecomeexcess
metabolicmachinery,atleastintermsof energyproduction.However,the problemof reoxidation
of reducedNADremains,becauseoxidizedcompoundsmustbe availableforcontinuousfunctioning
of glycolysis.Insome parasitesthe electronsare transferredtopyruvate,andthe resultingethanol
or lactate is excreted,althoughmanyorganismsexcrete suchcompoundsassuccinate,acetate,and
short-chainfattyacidsas endproductsof glycolysis.
 Metabolicflexibilityisafeature of obligate heterotrophprotozoa.Forexample,the Krebscycle
requiresacontinuoussupplyof the 4-carbonmolecule oxaloacetate,one of the cycle’s ownend
products,as an acceptorof 2-carbon unitsduringformationof citricacid.Krebscycle intermediates
are routinelytakenoutof circulationandusedinsyntheticreactionssuchastransaminations.Thus,
an alternate source of oxaloacetate isrequired,whichinmanyprotozoansisthe glyoxylate cycle,a
metabolicpathway especiallyimportantinthose speciesthatrelyheavilyonethanol,fattyacids,
and acetate fortheirenergyandcarbon skeletons.The glyoxylate cycle usestwoacetyl-CoA
moleculestomake asingle oxaloacetate molecule;the enzymesforthiscycle are foundinthe
glyoxysomes(peroxisomes).
Protozoaalsomay utilize avarietyof hydrogenacceptorsinthe final oxidationscoupledwithATP
production.
 In aerobic metabolismofmost animals, thisfinal acceptorismolecularoxygen.
 Under anaerobic conditionsprotozoamayproduce lactic acidor ethanol byusingpyruvate asa
hydrogenacceptor.Ciliatesof genusLoxodesevidentlyuse NO3–as a terminal hydrogen
acceptor inthe mitochondriaandcontainenzymesmore typical of bacteriathaneukaryotesto
carry out thisfeat.
 In parasitic protozoa without mitochondria—Trichomonasvaginalis,T. foetus,Giardia
duodenalis,andEntamoeba histolytica—the final acceptorcanbe pyruvate,a keymolecule in
carbohydrate metabolism,inwhichcase the endproductislactate or ethanol.These protozoa
take up molecularoxygen,butavailabilityof oxygenmakeslittle ornodifference intheirenergy
metabolism.Absenceof mitochondriahasbeenvariouslyinterpretedaseitheraprimitive
character, reflectinganancientevolutionaryorigin,oraderivedcharacter resultingfrom
secondaryloss. Oddandparasite-specificmetabolicpathwaysare,of course,invitingtargetsfor
chemotherapy.
 Some of the more effectiveantimalarial drugsinterfere withthe parasites’abilitytometabolize
1-carbon unitsduringnucleicacidsynthesis.Intracellularstagesof the flagellategenus
Leishmaniadonotbuildtheirnucleicacidprecursorsbutinstead salvage themfromtheirhost
cells.
 Allopuranol,apurine analog,cannotbe metabolizedbythe parasitesbutcanbe takenup from
the host cell andusedto buildnucleicacidsthatdonot functionproperlyinthe parasite.
 Needlesstosay, Parasitologists leavefew metabolicpathwaysunexploredintheireffortstofind
waysof treatingdiseases.Manyparasiticprotozoaare intracellular.Insome,entryintoahost

12. 12
cell isby hostphagocytosisof the parasite.Anexampleis Leishmaniadonovani,whichiseaten
by free roamingmacrophagesandreticulo endothelialcells.
 (Formingvacuole)The hostcell formsa membrane-boundparasitophorousvacuolearoundthe
parasite,butinsteadof killingthe parasite withdigestive enzymes,asmightbe expectedfroma
macrophage,the hostcell providesitwithnutrients.
 Membersof the important apicomplexans generaBabesia,Eimeria,Plasmodium, and
Toxoplasmaare all intracellularatleastat some stagesintheirlives,anduptake isbyactive
invasionof hostcellsbymotile infective stages,probablyaidedbydigestivesecretions.
 Microsporidian employ adifferentmode of entryintohostcells.These parasites’cyststages
containa coiled,hollow filamentthatevidentlyisundergreatpressure.Wheneaten byahost,
whichisusuallyanarthropod,thistubule isforciblyextrudedfromthe cystandpenetratesan
adjoininghostcell.The organismwithinthe spore (sporoplasm) thencrawlsthroughthe tube
and entersitshost.Inthiscase the membrane of the parasite isindirectcontact withthe
cytoplasm
Excretionand Osmoregulation
 Most protozoaappear to be ammonotelic;thatis,theyexcrete mostof theirnitrogenas
ammonia,mostof whichreadilydiffusesdirectlythroughthe cell membrane intothe
surroundingmedium.
 Othersometimesunidentifiedwaste productsare alsoproduced,atleastbyintracellular
parasites.Afterthese substancesare secretedtheyare accumulatedwithintheirhostcell
and,on the deathof the infectedcell,have toxiceffectson the host.Carbondioxide,lactate,
pyruvate,andshort-chainfattyacidsare alsocommon waste products.
 Contractile vacuolesare probablymore involvedwithOsmoregulationthanwithexcretion
perse. Because free-living,freshwaterprotozoaare hypertonictotheirenvironment,they
imbibe watercontinuouslybyosmosis.Contractile vacuoleseffectivelypumpoutthe water.
 Marine speciesandmostparasitesdonot formthese vacuoles,probablybecause theyare
more isotonictotheirenvironment.However,Balantidiumspecies have contractile
vacuoles