Anatomy, Classification, Life Cycle and Microscopy
Essentially, protozoa are single-celled eukaryotes.
This means that they are single celled organisms that have a nuclei as well as
a number of other important organelles within the cytoplasm and enclosed by a
membrane. They exist as free-living organisms or as parasites. This
makes protozoa a diverse group of unicellular organisms, varying in shape and
Anatomy (Bodily Structure)
Given that they are eukaryotes, protozoa are
larger cells of between 10 and 100 micrometer in diameter (compared to
prokaryotes) with a more complex structure. This means that they have a cell
membrane which bounds the organelles, a DNA that is also bound by a membrane, nucleoli,
ribosome, Golgi apparatus and multiple linear chromosomes with histones among
It is worth noting that organelles present in these cells
will vary from one type to another. There are also a number of organelles
that are exclusive to protozoa, these include:
- The Trichocysts of
- Certain skeletal structures
Compared to other ciliates, the nucleus of
protozoa is vesicular. As such, the chromatic is scattered resulting in a
nucleus that is diffuse in appearance. However, this also varies from one to another. For instance, in the Phylum Apicomplexa, the vesicular nucleus had
one or more nucleoli with DNA while the endosone of trypanosomes are lacking DNA.
Protozoa also have in place locomotory structures such as
pseudopodia, flagella and cilia which are used for movement. These structures
are also surrounded by the plasma membrane. On the other hand, the pellicle
(outer surface of some like the Giardia) is rigid enough to
support and maintain a distinctive shape while at the same time allowing for
twisting and bending when moving.
Because of their diversity, protozoa present
several problems when it comes to classification. They are considered to be under the sub-kingdom protista with more than 50,000 species being described as
free-living (these are the type that do not directly depend on
others for survival).
Free-living protozoa can be found in virtually every
possible habitat. Based on both light and electron microscopy morphology, they have been classified into six major phyla with a majority of disease
causing protozoa falling under the phyla Sacromastigophora and Apicomplexa.
The following are some of the sub-phyla and
classes within these sub-phyla based on locomotive structures:
Plasmodroma - The locomotive structures of this sub-phylum
may be flagella, pseudopodia or none at all. Classes that fall under this
sub-phyla include; Mastigophora (use one or more flagella for locomotion), Sarcodina (used pseudopodia for locomotion and for capturing food) and Sporozoa
which lack locomotive structures.
Ciliophora - These are in the Sub-phyla
Ciliophora use cilia or sucking tentacles in some stages or throughout their
life span. Ciliata (which use cilia throughout) and Suctoria (which use cilia
when young and tentacles as adults) are some of the class that fall under this
Sarcomastigopohora - The locomotive structures
used in this sub-phylum include pseudopodia or flagella. Here, the nuclei is
also of one kind (monomorphic). Super class Mastigophora, which falls under
this sub-phyla are flagellates and
thus use flagella for locomotion.
The Phytomastogophoerea also falls under this
sub-phyla and use flagella in some cases. Under the Class Phytomastogophoerea
is Order Chrysomonadida, which includes such organisms like Chrys amoeba,
synura and ochromonas among others.
** These are just a few in the classification. It
is extensive and contains many more organisms.
Classification Based on Mode of Existence
Of the existing protozoa, there are about 21,000
species that occur as free-living in a variety of habitats while another 11,000 species occur as parasitic
microbes in both vertebrate and invertebrates hosts.
The free-living species can be found
in various habitats and particularly in soil and water. These types of protozoa
have little impact on human health given that they do not directly depend other
organisms for their survival. However, some of the free-living can cause
pathology when introduced into a human host.
Others will also affect human
health by producing toxins. The following are some of the free-living amoebae
that can also cause human disease:
- Naegleria fowleri - This
species is mostly found in moist soil and can be located all across the world. It
causes acute primary amebic meningoencephalitis.
Acanthamoeba - Found in
soil and water, acanthamoeba can cause chronic granulomatous amebic
encephalitis, amebic keratitis, granulomatous skin as well as lung lesions.
- Balamuthia mandrillaris -
Causes sub-acute to chronic granulomatous amebic encephalitis as well as
granulomatous skin and lung lesions.
Parasitic protozoa are the type that
depend on the host for survival. As such, they live inside the host and even
cause health problems.
The following are some of the parasitic:
Sarcodina ( e.g. Entamoeba) - Entamoeba histolytica
is a type of amoeba that lives in the human alimentary canal. For most part,
they are harmless and feed on various bacteria and particles that may be
present in the intestine. Although they are mostly harmless, this parasite may
invade the intestinal wall or the rectum where they cause ulcerations and even
bleeding along with pain, vomiting, and diarrhea among other symptoms.
Trypanosomes - This is a flagellate that lives in the blood
stream. Various species of this parasite cause such diseases as:
- sleeping sickness,
- Chaga's disease
Mastigophora (e.g. Giardia) - This is a flagellate that is mostly found in the small
intestine of the host. The giardia typically attach themselves on to
the intestinal lining causing inflammation, diarrhea as well as abdominal pain
among other types of symptoms.
Sporozoa (e.g. Plasmodium) - The plasmodium species
is a parasite that lives in the blood stream of human beings, Once in the red
blood cells, the parasite feeds on their cytoplasm. As they continue
multiplying within the cells, this causes the cells to burst which in turn
results in many more parasites being released into the circulatory system.
For the parasitic forms, the life cycle stages
may occur intercellular, intracellular or in the lumen of given organs. Because
of the diversity, it is not possible to describe a single or one
common life cycle sequence. Here, therefore, we shall look at three of the most
common broad patterns exhibited by this group of protozoa.
This pattern is common in the phylum Apicomplexa
and involves an alteration between asexual and sexual reproductive stages.
The process starts with the cycles of asexual reproduction where the cycles of
schizogony (involving mitosis and cytokinesis) in the tissues of the host
results in increase population. Following this stage, some in the population
start undergoing gametogony (a sexual process) to produce gametes. These
gametes then unite and divide asexually to produce sporozoites through a
process known as sporogeny.
It is these sporozoites that are then capable of
infecting a new host and the process continues. Here, it is worth noting that transition
in to a new host is through cysts, which are tough under stressful conditions.
The cysts can survive external conditions (outside the body) and contain the
Once in a new host, the sporozoites start the reproduction cycle
again. Some of the species in this phylum (Apicomplexa) require two hosts to
complete their life cycle. This includes a vertebrate host where the parasite
goes through schizogony and gametogony and an invertebrate where the gametes
unite and sporiogony occurs in the tissues.
The second pattern is common among most
flagellates and involves asexual reproduction. For these, a number of
morphological transformations occur during the cycle. However, they all
reproduce through binary fission.
Some of the species in this group will
complete this cycle in a vertebrate host as they transmit from one host to
another through cysts, which can survive tough conditions better. Therefore, as
in the case with the Apicomplexa phylum, some species in this group will also
require two hosts to complete their life cycle.
This is particularly common among amoebas and
involves asexual reproduction. Unlike the others, a single host is required to
complete the reproduction cycle. Here, trophozoites in the host live in the
lumen of the gut and continue to
multiply through binary fission. Here, under certain conditions, the
trophozoites may be stimulated to encyst as they undergo nuclear division
within the cyst. Once the cyst is ingested by another host, the cycle
Life Cycle of Free Living Protozoa
For this group, the life cycle
largely involves the growth and increase in size of the organism which is then
followed by binary fission (or other forms of asexual reproduction). For the
free-living, sexual reproduction only occurs under unfavorable
conditions (unfavorable temperature, or reduced food supplies etc). However,
these factors often vary from one species to another.
During the growth and division cycle of the
free-living protozoa, there is a phase of DNA synthesis, chromosome replication
as well as the growth of the cells.
The phases of the cycle include:
- First division phase
- End of division phase and
beginning of DNA synthesis
- DNA synthesis
- End of DNA synthesis and
beginning of next division
Classification based on Nutrition (How they Obtain Energy)
There are three main categories based on nutrition.
Autotrophs like some of the flagellates
synthesize carbohydrates from carbon dioxide and water using chlorophyll. Here,
radiant energy from the sun is used.
Most of the photoautotrophic flagellates
including members of Euglenida, Cryptomonadida as well as Volvocida also tend
to combine autotrophy with heterotrophy. For this reason, they are often
described as acetate flagellates.
Some of their source of carbon include
acetates, simple fatty acids as well as alcohols. While they are autotrophs in
the light, these flagellates switch to heterotrophs in the dark.
A majority of the free-living protozoa fall
under this category. As such, they depend on a wide range of diet. Whereas some
feed on bacteria (microbivores) others feed on algae and are described as
herbivores. The carnivorous feed on both of the two trophics (herbivores
The free living are also divided into two groups (morphological). These include those with a mouth/cytostome and
those that lack a mouth or a definite point of entry for food. For instance,
whereas some flagellates and many ciliates (apart from some apostomatida) have
a cytostome the Sarcodina lack a mouth.
Chemoheterotrophic - This group includes
those that require energy and organic carbon sources.
As previously mentioned, protozoa are very
diverse. As such, they are distinguished from one another based on their
different structural features, means of locomotion as well as the formation of
Using a light microscope, it is possible to view different types of
Protozoa can be obtained from almost any given
habitat. Whereas the free-living species can be found in water as well as
various moist habitats, the parasitic can be found in most metazoan
For students, it would prove easier to use the free-living
protozoa, which can be obtained from such habitats as mud, ponds and transient
bodies. Here, it is worth noting these are very fragile. For this
reason, they should be handled with care.
It is also important to be careful given that even free-living protozoa can become parasitic.
Protozoa can also be cultured in order to increase their numbers for
observation. Some of the medium used include split pea (for Eglena) distilled
water with wheat grains (for chilomonas) as well as hay (for peranema) among
Some of the requirements for microscopy include:
- A microscope
- Microscope slides
- Microscope clips
- Distilled water (or tap
Wet Mounting Technique
Wet mounting technique is the technique that
simply introducing the sample/specimen on to a drop of water and viewing it
under the microscope. If the sample was obtained from a pond, then the
following process can be used:
- Gently shake the container
(to distribute the protozoa in the water)
- Using a dropper, obtain a
sample of the pond water from the container
- Place a drop of the sample
onto the center of a microscope slide and cover with a cover slip (always ensure that the microscope slide and
slip are clean to avoid introducing other microorganisms)
- Place the slide onto the
microscope stage for viewing
In some cases, staining may be used to increase
contrast and get a clearer view. Some of the stains used here
- Bismarck Brown
- Brilliant Cresyl Blue
- Bromothymol Blue
- Carmine Powder
- Methylene Blue
More on Cells:
Eukaryotes - Cell Structure and Differences
Prokaryotes - Cell Structure and Differences
Diatoms - Classification and Characteristics
Protists - Discovering the Kingdon Protista in Microscopy
Fungi - Mold Under the Microscope
Specifically learning about Vorticella
Take a more in-depth look at Parasitology as well
See Ciliates Microscopy
See Amoeba under the Microscope specifically Acanthamoeba
More information about Unicellular Organisms - Discussing Bacteria, Protozoa, Fungi, Algae and Archaea Here
Read about Parasites under the Microscope here
Also check out Microrganisms, especially in Pond Water.
Take a look at Microscope Slide Preparation.
And if in need of a microscope then be sure to read our Darkfield Microscope Buyer's Guide and Phase Contrast Microscope Buyer's Guide.
Return to Cell Biology - Components, Cycles, Processes and Microscopy Techniques
Return from Protozoa to Best Microscope Information and Research
Ward's Science (2005) Working with protozoa.
Johanna Laybourn-Parry (1984) A functional
biology of free-living protozoa.
Gary N. Calkins (1906) The Protozoan Life Cycle.
J. P. Kreier and J. R. Baker (1987) Anatomy and
physiology of the protozoa.
R. W. Hegner (1926) Homologies and Analogies
Between Free-Living and Parasitic Protozoa.
Martinez AJ, Visvesvara GS (1997) Free-living,
amphizoic and opportunisitic amebas. Brain Path. 7:583-598.
Mackean & Ian Mackean (2017)Parasitic
Protozoa, an Introduction.