PROTOZOAN PROTISTS
Include unicellular protists with animal-like behaviour
They were first studied by Leeuwenhoek ( 1677). Protozoan
protists may be aquatic, terrestrial or parasites. They can
cause several diseases in humans and animals.
General characteristics of protozoans are described below
(i) They are microscopic, unicellular and colourless
organisms with different shapes.
(ii) Locomotion occurs with the help of finger-like
pseudopodia, flagella or hairy cilia.
(iii) All protozoans are heterotrophs and live as predators
or parasites.
(iv) Respiration occurs through the general surface of the
body.
(v) Asexual reproduction occurs by binary fission,
multiple fission or budding. Sexual reproduction
occurs by syngamy and conjugation.
There are four major groups of protozoans
1. Amoeboid Protozoans
These organisms live in freshwater, seawater or moist soil,
some of like Entamoeba are parasite.
Examples include Amoeba, Entamoeba, Radiolarians,
Pelomyxa, Foraminiferans and Heliozoans.
General features of this group are following
(i) They move and capture their prey by putting out
pseudopodia (false feet) as in Amoeba (as mouth is
absent).
(ii) The body is without periplast. It may be naked or
have a calcareous shell (marine).
(iii) Flagella are present in some developmental stages.
They also develop when food becomes scarce.
(iv) Nutrition is holozoic.
(v) Asexual reproduction occurs by binary fission,
multiple fission, spores and budding and sexual
reproduction occurs by syngamy.
2. Flagellated Protozoans
The members of this group are either free-living or
parasitic. Examples include Giardia, Trypanosoma,
Leishmania, Trichonympha and Trichomonas.
General features of this group are following
(i) They have flagella for locomotion as their name
suggests.
free-living, parasitic,
(ii) They may be aquatic,
commensals or symbiotic. They are generally
uninucleate (occasionally multinucleate).
(iii) The body is enclosed by a firm pellicle.
(iv) Nutrition is holozoic, saprobic and parasitic.
(v) Asexual reproduction is by binary fission and cyst
formation.
(vi) Sexual reproduction is observed in some forms only.
(vii) Various species of these protozoans causes diseases in
humans. For example
Trypanosoma (sleeping sickness)
• Leishmania (kala-azar, dum-dum fever)
• Giardia
(giardiasis)
• Trichomonas (leucorrhoea .

3. Ciliated Protozoans
These are aquatic, actively moving organisms because of the
presence of thousands of cilia.
Examples include Paramecium, Opalina, Vorticella,
Podophyra, Balantidium, etc.
General features of this group are following
(i) Many ciliates live as free-living individual in fresh
and marine water (Paramecium).
(ii) A large number of cilia present on whole body surface.
Cilia are used to capture food and for locomotion.
(iii) Ciliated protozoans have a cavity (gullet) that opens
to the outside of the cell surface. The coordinated
movement of rows of cilia causes the water laden
with food to be steered into the gullet. Nutrition is
holozoic except in some parasitic forms where it is
absorptive.
(iv) The body is covered with flexible pellicle.
(v) Ciliates have a larger macronucleus and smaller
micronucleus for sexual reproduction.
(vi) They have small ejectable trichocysts for defense.
(vii) Osmoregulation occurs by contractile vacuoles.
(ix) Asexual reproduction occurs by transverse binary
fission or budding. Cyst formation also occurs
during unfavourable condition.
(x) Sexual reproduction by means of conjugation.
4. Sporozoan Protozoans
This group includes organisms that have an infectious
spore-like stage in their life cycle.
Examples include Plasmodium (malarial parasite),
Monocystis, Eimeria.
General features of this group are following
(i) All sporozoans are endoparasites and pathogenic.
(ii) Locomotory organs and contractile vacuoles are
absent.
(iii) Nutrition is parasitic (absorptive).
(iv) Body is covered with an elastic pellicle or cuticle and
contractile vacuoles are absent.
(v) Asexual reproduction occurs through multiple
fission and sexual reproduction by syngamy.
(vi) Life cycle may include two different hosts
(digenetic), e.g., Plasmodium requires two hosts
female Anopheles mosquito and human beings.
Knowledge Plus
• The search for malarial vaccines has hamgprai by the
fact that Plastna:ium scpnds most of its tirne Inside hutTW1
cells, hidden from host’s immune system. It also continue
changes its surface proteins.
• By 2CD3, researchers had tracked the expression of most of
the parasitic genes at numerous points in its life cycle. Thß
research could help to identity the potential new targets Ü
vaccines.