Protozoa are unicellular, eukaryotic organisms belonging to the phylum Protozoa, characterized by their simple body organization and various modes of locomotion. They can reproduce asexually or sexually, exhibit diverse nutritional modes, and occupy various habitats including free-living and parasitic lifestyles. Additionally, certain protozoa play important roles in ecosystems, provide food for other organisms, and have both beneficial and harmful effects on human activities.

1. Protozoa
Pragya Atrey
Department of Microbiology

2. General Characters of Protozoa:
Term Protozoa (From Greek, protos meaning first, zoon-meaning animals) was given by Goldfass.
According to five-kingdom classification system, protozoans belong to the phylum Protozoa of kingdom Protista.
1. The protozoans are minute, generally microscopic and eukaryotic organisms. They are the simplest and primitive of
all the animals with very simple body organization, i.e. Protoplasmic grade of organization.
2. They are unicellular organisms without tissues and organs.
3. The protozoans are small, generally microscopic animalcules.
4. Simplest & primitive of all animals with very simple body organization, i.e. protoplasmic grade of organisation.
5. Acellular, animals, without tissues & organs.
6. Body naked or covered by pellicle but in some forms body is covered with shells and often provided with internal
skeleton.
7. Protozoans are solitary or colonial; in colonial forms the individuals are alike and independent.

3. 8. Body shape variable; it may be spherical; oval, elongated or flattened.
9. Body protoplasm is differentiated into an outer ectoplasm and inner endoplasm.
10. Protozoans may have one or more nuclei; nuclei may be monomorphic or dimorphic, vesicular or massive.
Vesicular nuclei are commonly spherical, oval or biconvex, consist of a central body, the endosome (nucleolus)
encircled by a zone of nuclear sap.
11. Locomotory organelles are pseudopodia, flagella, cilia or none.
12. Nutrition may be holozoic (animal-like), holophytic (plant-like), saprozoic or parasitic. Digestion intracellular, takes
place inside the food vacuoles.
13. Respiration occurs by diffusion through general body surface.
14. Excretion occurs through general body surface but in some forms through a temporary opening in the ectoplasm or
through a permanent pore, the Contractile vacuoles perform osmoregulation in fresh water & also help in removing
excretory products.

4. 15. Reproduction asexual or sexual; asexual reproduction occurs by binary fission, multiple fission, budding or
sporulation and sexual reproduction is performed by gamete formation or conjugation.
16. Life cycle often exhibits alternation of generation, i.e. it includes asexual and sexual phases.
17. Encystment usually occurs to tide over the unfavorable conditions and it also helps in dispersal.
18. The protozoans exhibit mainly two modes of life, free-living inhabiting fresh water, salt-water and damp places,
and parasitic living as ectoparasites & endoparasities. They are also commercial in habit.

5. Habit and habitat:
They may either be free-living (inhabiting fresh water, salt water or damp places) or parasitic (living as Ecto- or
endoparasities). Some are commensals in habit.
Body is either naked or covered by a pellicle (plasma lemma or theca or lorica).
Protozoans are either solitary or colonial; in colonial forms, the individuals are alike and independent.
They are motile and can move by:
• Cilia - tiny hair like structures that cover the outside of the microbe. They beat in a regular continuous pattern like
flexible oars.
• Flagella - long thread-like structures that extend from the cell surface. The flagella move in a whip-like motion that
produces waves that propel the microbe around.
• Amoeboid movement - the organism moves by sending out pseudopodia, temporary protrusions that fill with
cytoplasm that flows from the body of the cell.

6. Size
• The Protozoa are usually microscopic and not visible to the naked eyes. Their size varies from 2 microns to 250
microns (one micron is equal to 1/1000 mm or 0.001 mm).
• Leshmania and Plasmodium are the smallest protozoans known so far.
• Some of the larger protozoa, like Amoeba and Paramecium, can be seen with naked eyes
• Spirostomum, a ciliate, grows to 3 mm long.
• Pelomyxa, a giant amoeba, attains a diameter of about 1 to 5 mm.
• Porospora gigantea, a sporozoa (strictly parasitic non-motile protozoans) grows to about 16 mm long.
• Cycloclypeus foraminifera, (the largest members of the group of single-celled microbes known as Protists) exceeds a
diameter of about 50 mm and some shelled marine protozoans have diameters of about 63 mm.

7. Fig. : Various types of skeletons in protozoa. A. Plasma lemma of Amoeba; B. Pellicle of Paramecium;
C. Thecal plates of Gle1todillium; D. Lorica of Poteriode1ldro1l; E. Pseudochitinous shell of Arcella;
F. Sand grain shell of Diffugia; G. Calcareous shell ofDiscorbis; H. Siliceous shell of Euglypl,a; 1,1. Radiolarian
skeleton showing lattice network; K. Helmet-shaped radiolarian skeleton

8. Cytoplasm:
The cytoplasm of protozoa is generally colorless but certain colored species are also found; Blepharisma lateritia is
rose-red and Stentor coeruleus is blue.
The cytoplasm is commonly divided into peripheral clear ectoplasm and inner granular endoplasm. The cytoplasm
contains various organelles like mitochondria, Golgi bodies, endoplasmic reticulum, ribosomes, lysosomes, centrioles,
microtubules, plastids, etc.

9. Nucleus
The nuclei of Protozoa exhibit a greater variety of size, shape and structure than the nuclei of Metazoa (multicellular
and are divided into invertebrates and vertebrates).
The nucleus of Protozoa has a nuclear membrane, nucleoplasm, oxychromatin, basichromatin, and there may be a
nucleolus.
The nuclear membrane remains intact even in cell division.
There are various kinds of nuclei in protozoa
1. Vesicular nucleus has a large amount of nucleoplasm, the chromatin is small in quantity and it forms small granules,
the achromatin (oxychromatin) is more fluid and its network, if present, is coarse, there is a round endosome of
basichromatin or oxychromatin, or of both e.g., Euglena, Arcella, Entamoeba
2. Massive nucleus has a small amount of nucleoplasm, there is a large amount of chromatin forming evenly scattered
small granules, the achromatin is viscid forming a fine network, e.g., Amoeba.
In the majority Of protozoa, the nuclei show a structure intermediate between the vesicular and massive nuclei.
3-Polyenergid nucleus has several sets of chromosomes, instead of one set inside the nuclear membrane; this is due to
mitosis occurring repeatedly 'inside the nuclear membrane.

10. Cell structure:
• Body shape is variable; it may be spherical, oval, elongated or flattened.
• They are usually asymmetrical but Giardia is bilaterally symmetrical.
• The protoplasm is differentiated into outer ectoplasm and inner endoplasm. They may have one or more nuclei.
Nucleus may be monomorphic or dimorphic, vesicular (e.g. Ent amoeba) or massive (e.g. Amoeba).
• Vesicular nucleus is commonly spherical, oval or biconvex.
• Dimorphic nuclei are found in Ciliata, one larger macronucleus (with trophochromatin) and other small micronucleus
(with idiochromatin)
Locomotion
The Protozoa perform locomotion or movement by various organelles; pseudopodia characteristic of Sarcodina,
flagella characteristic of flagellate (Mastigophora), cilia characteristics of Ciliata and other contractile structures of
pellicle, myonemes, characteristic of Sporozoa and few others.

11. (A) Pseudopodia: Pseudopodia are generally temporary out growths of protoplasm from any part of the body, they are found
in those protozoa, which are 'naked' or have a very thin pellicle. Pseudopodia may be of ectoplasm or they may also have a
core of endoplasm. Following kinds of pseudopodia are found.
(i) Lobo podia are blunt, short or finger-like, they are made of ectoplasm with a core of fluid endoplasm, e.g. Arcella &
Amoeba.
(ii) Filo podia are fine, long threads, often with rounded ends, at times they may branch, they are made of only hyaline
ectoplasm.
(iii) Rhizopodia or reticulopodia are thin, long and branching, the branches of adjacent pseudopodia may anastomose to
form a network which also serves as a trap for capturing food, ego Elphidium
(iv) Axopodia are long, stiff threads made of ectoplasm, with a hard central axial filament of endoplasm, unlike others they
are semi-permanent, ego ActinophnJs, Axopodia are not organelles of locomotion but are only for capturing food.
Fig. : Types of pseudopodia. L. Lobopodia of Amoeba; B. Filopodia of Euglypha; C.
Reticulopodia of Globigeri1la D. Axopodia of Acti1lopltnjs

12. Life processes:
There is no physiological division of labour and all the vital activities of life are performed by a single cell. Nutrition
may be holozoic (animal like), holophytic (plant like) sporozoic or parasitic.
*In Euglena, the mode of nutrition is mixotrophic (both holozoic and halophytic).Digestion takes place inside the
food vacuoles, i.e. intracellular. Respiration occurs by diffusion through general body surface.
Excretion occurs through general body surface like respiration. They are ammonotelic (excrete nitrogenous waste
product in the form of ammonia).
In some forms, egestion occurs through a temporary opening in the ectoplasm or through permanent opening called
Cytopyge.
Contractile vacuoles perform osmoregulation in fresh water forms and also help in removing excretory products
*Contractile vacuole is absent in marine and parasitic forms.

13. Life cycle stages – Protozoa often live in habitats subject to change due to climate and seasons, while some live parts
of their lives inside different types of hosts. To survive variations in living conditions, protozoa can switch between two
different stages.
a) Trophozoites – Trophozoites (troph = activity), are active protozoa sometimes called vegetative cells. While in their
trophozoites form, protozoa are engaged in feeding, reproducing and moving about actively. Vernal pools, i.e., those
filled with water during the spring, contain many trophozoites. As the weather warms up and pools dry out in the
summer sun, the protozoa go into a resting state.
b) Cysts – Cysts are dormant structures produced by many types of protozoa under certain circumstances. They are
metabolically inactive and much more resistant to heat, drying, radiation and chemicals than are trophozoites (active
vegetative cells). Cysts allow protozoa to survive when their watery habitats dry out during summer months or freeze
solid during the winter. They also allow gastrointestinal parasites to survive passage through the stomach without being
damaged by stomach acids.

15. Reproduction:
• Reproduction is either sexual or asexual; asexual binary reproduction occurs by fission, multiple fission, budding or
sporulation and sexual reproduction occurs by gamete formation or conjugation.
• Binary fission may be simple, transverse, longitudinal, or oblique.
• Life cycle often exhibits alternation of generation, i.e. it includes asexual and sexual phases.
• Encystment usually occurs to protect the cell from the unfavorable conditions and it helps in dispersal.
1. Asexual reproduction – Asexual reproductive processes allow individuals to reproduce without interacting with other
cells. In eukaryotic organisms, asexual reproduction requires mitosis (the separation of the chromosome) and cytokinesis
(the separation of the cytoplasm forming new daughter cells).
Some specific examples include:
a) Binary fission – Binary fission is a process involving the separation of the cytoplasm across the long axis of the cell.
Most protozoa reproduce by binary fission.
b) Schizogony – Schizogony or multiple-fission is a process involving the splitting of one cell into many daughter cells.
Sporozoans in the genus Plasmodium reproduce by means of schizogony while inside human RBCs.
c) Budding – Budding involves an unequal division of the protoplasm and results in the formation of a bud at the
margin of a cell. If conditions are favorable, the bud will grow and eventually separate from the cell, but if conditions
are poor, the bud may die with little consequence to the cell.

17. 2. Sexual reproduction – In order to reproduce sexually, protozoa must interact with other, genetically different cells.
Sometimes this involves plasmogamy, Karyogamy and meiosis, but not always, and these terms are rarely used in zoology
texts. Two examples of sexual reproduction include:
a) Syngamy – Syngamy involves the fusion of two haploid cells to form a diploid zygote. Protozoa in the genus
Plasmodium undergo Syngamy while inside mosquitoes.
b) Conjugation – Conjugation requires that two cells with different genetic content meet and position themselves side-by-
side. Portions of the cell membranes fuse allowing the formation of a cytoplasmic bridge, and then segments of genetic
material (DNA) are exchanged between the two cells. Following conjugation, the cells separate again, but each one is now
carrying a new combination of genetic material.

18. Economic Importance:
Beneficial Protozoa:
Food: Protozoa provide food for insect larvae, crustaceans and worms, which are taken by large animals like fishes,
lobsters, clams, and crabs, which are eaten by man. Thus they form sources of food supply to man both directly and
indirectly.
Symbiotic Protozoa: Certain protozoa like Trichonympha and Colonymphya etc. live in the gut of termite, which help in
the digestion of cellulose. The host utilizes the digested cellulose.
Insect Control: Several protozoa control harmful insects by persisting their bodies.
Helpful in Sanitation: A large number of protozoa living in polluted water feed upon waste organic matters and thus
purify it. Many protozoa feed upon bacteria and play important role in the sanitary betterment and keeping water safe for
drinking.
Industry: The skeletal deposits of marine protozoa (Foraminifera and Radiolaria) from oceanic ooze (a type of
sedimentary deposit found on the ocean floor that is made up of soft mud and the remains of microscopic organisms) at the
sea-bottom. About 30% of oceanic bed is covered with the Globigerina ooze (Calcareous shell). These skeletal deposits are
put to many uses. Some are employed as filtering agents, others are made into chalk and still others are used for abrasive
(rough).
Building Material: The skeletal deposits in due course of time change into the limestone rock. The Globigerina ooze
provides limestone. Comerina (Formally known as Nummulites, are the largest protozoa, which form limestone. Thus,
limestone-beds are used as building material.
Oil Exploration: Petroleum is organic origin. The skeletal deposit of Foraminifera and Radiolaria are often found in
association with oil deposits. In this mean, they help in the exact location of oil.

19. Harmful protozoa:
Pollution of Water: Drinking water in natural condition is made unpalatable by the reproduction of some free-living
protozoa in it. For example, Uroglenopsis brings flashy odour like that of cod- liver oil. Peridinium emits smell
resembling that of clamshells
Destruction of Animals of Food Value: Dinoflagellates like Noctiluca and Gonyaulax, when become abundant, are
responsible for turning the ocean red. The water becomes foul and cause toxic reaction to molluscs like clam oysters and
mussels and they become unfit for eating by human beings.
Destruction of Wooden Articles: Some flagellates like Trichonympha and Colonymphya live in the gut of termites and
help in the process of cellulose digestion. In the absence of these flagellates, the termites will die or change their diet.
Thus, such protozoa indirectly help in distraction of wooden articles and books.
Reduction in Fertility of Soil: It has been observed that about 200- 300 species of protozoa are present in soil. These
protozoa feed on nitrogen-fixing bacteria thus reduce the fertility of soil.
Parasitic Protozoa: Parasitism is an association of two organisms in which one-organism lives at the expense of the
other giving nothing in return. The first animal is called parasite and other the host.
Parasitism is a specialized mode of life and various level of it are found in protozoa. There are two type of parasites
based on their survival Facultative parasites are those that can live for various lengths of time without the host. The
obligatory parasites are those, which cannot survive when separated from their host.

20. THANK YOU