Kingdom Protoctista: Protozoa
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
¾ Kingdom Protoctista versus Kingdom Protista
General classification has seen marked changes since the mid-70s to the present. The term
protista is still used in many schemes for classification. Protozoa was once considered a single
phylum in the Kingdom Protista. Now considered to consist of at least 29 to 45 phyla within
what used to be 3 different kingdoms (now the Kingdom Protoctista by some authors). Many
taxonomists consider it best to group the different groups as different phyla.
Margulis and Schwartz promote the term protoctista because protista denotes only organisms
which are unicellular. The protoctista scheme contains many multi-cellular organisms with
tissue specialization. The protista system places several of the protoctistan phyla into the
plant and fungi kingdoms.
According to the system used for classification by Lynn Margulis and Karlene Schwartz,
members of the protoctista are identified primarily by exclusion from all the other kingdoms.
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Animals develop from a blastula.
Plants develop from an embryo.
Fungi develop from spores, and lack flagella and cilia
Monerans (prokaryotes) lack a membrane bound nucleus.
All remaining organisms are placed into the kingdom protoctista. They are aquatic, living in
saltwater, freshwater, and of the watery tissues of other organisms. There are many protozoan
parasites of both invertebrates and vertebrates.
Depending on the source the Kingdom Protoctista is now considered to consist of at least 29 to
45 phyla within what used to be 3 different kingdoms.
¾ General Characteristics of Protoctistans
1. unicellular eukaryotes (some multinucleate, a few loosely multicellular), not all have
mitochondria (microspores, many flagellates).
2. up to about 400 micrometer in size (some larger)
3. all have at least one nucleus
4. most are free living, but many parasitic forms including entire phyla
5. motile by a variety of mechanisms but also several non-motile taxa
6. Many have cyst stages secreted by trophic or spore stages
Cysts/spores have four basic functions:
• protect against unfavorable conditions
• serve as sites for multiplication
• assist in attachment to surfaces such as hosts
• transmission stage from host to host
7. all types of nutrition are exhibited by the Kingdom.
• autotrophs: photosynthesis
• heterotrophs (holozoic vs. saprozoic)
• phagocytosis: ingestion of solid particles (e.g., bacteria)
• pinocytosis: same as phagocytosis but intake of liquid
• saprozoic or saprotrophy: diffusion or active transport across membrane
Kingdom Protoctista: Protozoa
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
¾ Characteristics of Protozoa
The protozoa are a diverse assemblage of unicellular eukaryotic organisms in the Kingdom
Protoctista having at least two animal-like properties: (1) absence of a cell wall and (2) presence
of at least one motile stage in the life cycle. All functions of life are performed within the limits
of a one cell membrane. Although protozoans have no organs or tissues, there is division of
labor within the cytoplasm, where various complex organelles are specialized to carry out
specific tasks. These organelles tend to be more specialized than those of an average cell of a
multicellular organism, functioning as skeletons, locomotory systems, sensory systems,
conduction mechanisms, defense mechanisms, and contractile systems.
Protozoa are often called "simple" organisms. However, many are quite complex. Ciliates, for
example, are not only the most complex of protozoans but also the most elaborately organized
of all known cells. Protozoans are widespread ecologically, being found in fresh, marine, and
brackish water and in moist soils. Some are free-living; others live as parasites or in some
other symbiotic relationship.
The protozoa are an artificial assemblage of organisms placed together for convenience.
Traditionally, four main groups of protozoa have been recognized: flagellates, amebas, spore
formers, and ciliates. These were assembled in a single phylum, Protozoa, within the kingdom
Animalia. A revision adopted by the Society of Protozoologists in 1980 recognized seven
separate phyla. However, more recent DNA sequence analyses of genes have shown that the
protozoa represent numerous clades of varying evolutionary relationships. For example,
ameboid organisms (formerly Sarcodina) fall into numerous lineages with undetermined
associations. Nevertheless, the amebas comprise recognizable morphological groups, which we
collect for convenience into the informal heading "Ameobas." (or “Amebas”).
¾ Protozoa Taxonomy & Classification
The following is a condensed Linnaean classification of the protozoan groups as presently
recognized.
Amebas = about 12,000 species; locomotion by pseudopodia; body naked or with external or
internal test or skeleton; asexual reproduction by fission; sexuality, if present, associated with
flagellated (rarely ameboid) gametes; most free-living, some parasitic. Several groups of
uncertain affinities.
Rhizopodans Locomotion by lobopodia, filopodia (thin pseudopodia that may branch but
do not rejoin), or by cytoplasmic flow without forming discrete pseudopodia. (The
rhizopodans are divided among several clades.) Examples: Amoeba, Endamoeba, Dif./lugia,
Arcella, Chlamydophrys.
Granuloreticulosans Locomotion by reticulopodia (thin pseudopodia that branch and often
rejoin). Includes foraminiferans. Examples: Globigerina, Vertebralima.
Actinopodans Locomotion by axopodia (long, slender pseudopodia). Includes radiolarians.
(The actinopodans are divided among several clades.) Examples: Actinophrys, Clathrulina.
Phylum Euglenozoa (yu-glen-a-zo'a) (Gr. eu-, good, true, + glime, cavity, socket, + zoon,
animal). Movement by flagella; cortical microtubules. Examples: Euglena,
Trypanosoma. About 7500 different species.
Kingdom Protoctista: Protozoa
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Phylum Chlorophyta (klor-of'i-ta) (Gr. chloros, green, + phyton; plant). Unicellular and
multicellular algae; photosynthetic chlorophyll pigments, flagella of equal length and smooth,
mostly free-living photo autotrophs. Example: Volvox.
Phylum Dinoflagellata (dy'no-fla-jel-at'a) (Gr. dinos, whirling, + flagellum, little whip). Typically
with two flagella, one transverse, one trailing; body usually grooved transversely and
longitudinally, each groove containing a flagellum; chromoplasts bearing chlorophyll; freeliving, planktonic, parasitic, or mutualistic. Examples: Noctiluca, Ceratium, Gonyaulax.
Phylum Apicomplexa (a'pi-com-plex'a) (1. apex, tip, + complex, twisted around, + a, suffix).
Characteristic set of organelles (apical complex) at anterior end in some stages; cilia and
flagella usually absent; all species parasitic; about 5500 species.
Class Gregarinea (gre-ga -ryn' e-a) (1. gregarius, belong to a herd or flock). Mature
gamete-producing individuals large, extracellular; gametes usually alike in shape and
size; parasites of digestive tract or body cavity of invertebrates; life cycle with one host.
Examples: Monocystis, Gregarina.
Class Coccidea (kok-sid'e-a) (Gr. kokkos, kernel, grain). Mature gamete-producing
individuals small, typically intracellular; parasites mostly of vertebrates.
Examples: Plasmodium, Toxoplasma, Eimeria.
Phylum Ciliophora (sil-i-of' o-ra) (1. cilium, eyelash, + Gr. phora, bearing). Cilia or ciliary
organelles present in at least one stage of life cycle; usually two types of nuclei; binary fission
across rows of cilia; budding and multiple fission also occur; sexuality involving conjugation,
autogamy, and cytogamy; heterotrophic nutrition; mostly free-living; contractile vacuole typically present. (This is a very large group, now divided into three classes and numerous orders.)
Examples: Paramecium, Colpoda, Tetrahymena, Stentor, Blepharisma, Epidinium, Vorticella,
Euplotes, Didinium; about 9000 different species.
¾ Biological Contributions of Protozoa
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Intracellular specialization
Division of labor between cells in colonial protozoa
Asexual reproduction by mitosis appears in unicellular eukaryotes
True sexual reproduction forming a zygote (occasionally found)
Simplest “reflexes”/responses
Shelled protozoa = first exoskeletons
All types of nutrition developed
Means of locomotion in aqueous media developed
¾ Types of Nutrition/Feeding Strategies Seen in Protozoans
Protozoa may use one or more of the following:
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Autotroph
Heterotroph
Phagocytosis = phagotroph or holozoic feeder
Ingest visible food particles
Osmosis = osmotrophs or saprozoic feeders
Ingest soluble food
Kingdom Protoctista: Protozoa
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
¾ Types of Protozoan Locomotion
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Flagella = flagellates
Cilia = ciliates
Pseudopodia = amebas
Lobopodia = blunt tipped
• Filipodia = thin and sharply pointed
• Rhizopodia = branched filaments
• Reticulopodia = branched filaments forming a
• net-like structure
• Axopodia = thin, pointed containing a central axial filament of microtubules
¾ FLAGELLATES (eg. Euglena)
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Move by means of a flagellum
Formed of an axoneme = 9 + 2 arrangement of microtubules with dynein arms that
use ATP to “walk” along one filament in an adjacent pair. This causes the tubules to
slide past one another and bend due to the shear resistance provided by fibril
spokes running to the central pair of microtubules
Kinetosome (basal body) = third microtubule joins each of the 9 pairs to form 9
triplets at the flagellum base near cell membrane
Kinetosomes have same structure as centrioles
¾ AMEBAS (eg. Ameba proteus)
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Cytoplasm is not homogeneous
Ectoplasm = more transparent and more rigid and gel-like
(“outer” plasma)
Endoplasm = more granular and contains cell organelles
(“inner” plasma)
¾ Pseudopodium Formation in Amebas
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Hyaline cap appears as an extension of the ectoplasm
Endoplasm begins to flow toward and into the hyaline cap
Actin subunits in the endoplasm attach to regulatory actin-binding proteins that
prevent actin from polymerizing
Endoplasm “fountains out” to the periphery when it nears hyaline cap
Interactions with phospholipids the cell membrane release the actin subunits from
the regulatory proteins so that the actin can now repolymerize into microfilaments
These microfilaments bind to actin binding protein to form a gel-like tube through
which the endoplasm flows as the pseudopodium extends
Near the trailing edge of the gel, calcium ions activate a protein that severs
releasing microfilaments and allowing myosin to bind the microfilaments and pull
on them
This contractile force at the trailing edge forces the fluid endoplasm (and actin
subunits) back toward the hyaline cap
Kingdom Protoctista: Protozoa
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
¾ CILIATES
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Have same basic structures as a flagellum, but they are shorter
All typical cilia and flagella have a kinetosome at their base
Ciliary movement is vital to many species for handling food, reproduction, excretion
and osmoregulation
¾ Protozoan Encystment
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Some protozoa can form dormant cysts to survive harsh environmental conditions
Form resistant external coverings
Metabolism shuts down
Can occur in some parasitic forms
Can be reactivated after decades or longer?
Appears to be cyclic and tied to a stage in the life cycle
Cilia and flagella are resorbed
Golgi apparatus secretes the cyst wall material
¾ Contractile Vacuoles
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For osmoregulation
Empty more often in freshwater protozoa than marine forms
Smaller species with greater surface-to-volume ratios have more rapid filling and
expulsion rates
Excretion of waste (ammonia for nitrogen metabolism) is by diffusion
Contractile Vacuoles
Remove excess water by osmosis
Proton pumps on vacuole surface actively transport Hydrogen ions (H+) and
bicarbonate ions (HCO3-).
These ions accumulate within a vacuole causing water to flow into the vacuole
As vacuole fuses to cell membrane water is expelled to the outside of the cell
These ions can be readily replaced by the action of the enzymes - carbonic
anydrase – using its substrates of CO2 and H2O
¾ Protozoan Reproduction
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Binary Fission
Most common type of reproduction in protozoa
When parent is larger than progeny with progeny growing to adult size = budding
Multiple fission = several nuclear division occur before cytokinesis so that a
number of individuals are produced simultaneously
Schizony (multiple fission) is common among apicomplexans and amebas
Somewhat like mitosis, but nuclear membrane often persists through mitosis
Sexual Reproduction
If all gametes looks alike = isogametes
If gametes are of two different types = anisogametes
Syngamy = fertilization of an individual gamete by another
Autogamy = fertilization in which gametic nucle form by meiosis and fuse to form a
zygote in the same organism
Conjugation = exchange of gametic nuclei between paired organisms
Kingdom Protoctista: Protozoa
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
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Reproduction in Paramecium
Binary fission = micronucleus divides mitotically into two daughter micronuclei
and move to opposite ends of the cell. The macronucleus elongates and splits, but
not by mitosis.
Autogamy = “self-fertilization”; similar to conjugation with no exchange of nuclei
with another Paramecium
Conjugation in Paramecium
Temporary union of two individuals to exchange chromosomes
Macronucleus degenerates and micronucleus undergoes meiosis to make four
haploid pronuclei. Three of these pronuclei degenerate
One pronucleus is exchanged with the other conjugant
Two pronuclei (in each Paramecium) fuse to restore diploid state
Allows for genetic recombination
Ciliate Reproduction Comparison
Conjugation is similar to zygote formation in other animals
Permits gene recombinations/variation
Ciliates in clone cultures can reproduce asexually for long periods, but often usually
lose vigor
Conjugation restores viability
Conjugation often seen with seasonal changes or a deterioration of the environment