Classification, Structure, Function and Characteristics
Paramecium is a unicellular organism with a shape resembling
the sole of a shoe. It ranges from 50 to 300um in size which varies from species to species. It is mostly found in a freshwater
It is a single-celled
eukaryote belonging to kingdom Protista and is a well-known genus of ciliate
As well, it belongs to the phylum Ciliophora. Its whole body is covered with small hair-like filaments called the cilia which helps in locomotion. There is also a deep oral groove containing not so clear oral cilia. The
main function of this cilia is to help both in locomotion as well as dragging
the food to its oral cavity.
Classification of Paramecium
Paramecium can be classified
into the following phylum and sub-phylum based on
their certain characteristics.
well-known ciliate protozoan, paramecium exhibits a high-level cellular differentiation containing several complex
organelles performing a specific function to make its survival possible.
Besides a highly specialized structure, it also has a complex
reproductive activity. Out of the 10 total species of Paramecium, the most common two are P.aurelia and P.caudatum.
Structure and Function
1. Shape and Size
cadatum is a
microscopic, unicellular protozoan. Its size ranges from 170 to 290um or up to
300 to 350um. Surprisingly, paramecium is visible to the naked eye and has an
elongated slipper like shape, that’s the reason it’s also referred to as a
The posterior end of the body is pointed, thick and cone-like while the anterior part is broad and
blunt. The widest part of the body is below the middle. The body of a paramecium is asymmetrical. It has a
well-defined ventral or oral surface and has a convex aboral or dorsal body
Its whole body is covered with a flexible, thin and firm membrane called pellicles. These pellicles are elastic in nature which supports the cell membrane. It is made up of a gelatinous substance.
Cilia refers to the multiple,
small hair-like projections that cover the whole body. It is arranged in longitudinal rows with a uniform length
throughout the body of the animal. This condition is called holotrichous. There are also a few longer cilia
present at the posterior end of the body forming a caudal tuft of cilia, thus
The structure of cilia
is the same as flagella, a sheath made of protoplast or plasma membrane with
longitudinal nine fibrils in the form of a ring. The outer fibrils are much
thicker than the inner ones with each cilium arising from a basal granule.
Cilia have a diameter of 0.2um and helps
in its locomotion.
It contains the following parts:
- Oral groove: There
is a large oblique shallow depression on the ventrio-lateral
side of the body called peristome or an oral grove. This oral groove gives an
asymmetrical appearance to the animal. It further extends into a depression
called a vestibule through a short conical funnel. This vestibule further
extends into the cytostome through an oval-shaped
opening, through a long
opening called a cytopharynx and then the esophagus
leads to the food vacuole.
- Cytopyge: Lying
on the ventral surface, just behind the cytostome is the cytopyge also called a cytoproct. All the
undigested food gets eliminated through the cytopyge.
Cytoplasm is a jelly-like substance
further differentiated into the ectoplasm. The ectoplasm is a narrow peripheral layer.
It is a dense and clear layer with an inner mass of endoplasm or semifluid plasmasol that is granular in shape.
Ectoplasm forms a thin, dense and clear
outer layer containing cilia, trichocysts, and
fibrillar structures. This ectoplasm is further bound to pellicle externally
through a covering.
Endoplasm is one of the most detailed parts
of the cytoplasm. It contains several different granules. It contains different
inclusions and structures like vacuoles, mitochondria, nuclei, food vacuole,
contractile vacuole etc.
Embedded in the cytoplasm are small spindle-like
bodies called trichocysts. Trichocysts are filled with a dense refractive
fluid containing swelled substances. There is a conical head on the spike at
the outer end. Trichocysts are perpendicular to the ectoplasm.
The nucleus further consists of a macronucleus
and a micronucleus.
- Macro Nucleus: Macronucleus is kidney like or ellipsoidal in
shape. It's densely packed within the DNA (chromatin granules). The macronucleus controls all the vegetative
functions of paramecium hence called the vegetative nucleus.
- Micro Nucleus: The micronucleus is found close to the macronucleus. It is a small and compact structure, spherical in
shape. The fine chromatin threads and granules are uniformly distributed
throughout the cell and control
reproduction of the cell. The number
in a cell varies from species to species. There is no nucleolus present in caudatum.
Paramecium consists of two
types of vacuoles: contractile vacuole and
- Contractile vacuole: There
are two contractile vacuoles present close to the dorsal side, one on each end
of the body. They are filled with fluids and are present at fixed positions
between the endoplasm and ectoplasm. They disappear periodically and hence
are called temporary organs. Each contractile vacuole is connected to at
least five to twelve radical canals. These radical canals consist of a long ampulla, a terminal part and an injector canal which is short in
size and opens directly into the contractile vacuole. These canals pour all the liquid collected from the whole
body of paramecium into the contractile vacuole which makes the vacuole increase in size. This liquid is discharged to the outside through a permanent
pore. The contraction of
both the contractile vacuoles is irregular. The posterior contractile vacuole
is close to the cytopharynx and hence contract more quickly because of more
water passing through. Some of the main functions of contractile vacuoles include osmoregulation, excretion, and respiration.
- Food vacuole: Food
vacuole is non-contractile and is roughly spherical in shape. In
the endoplasm, the size of food vacuole varies and digest food
particles, enzymes alongside a small
amount of fluid and bacteria. These food vacuoles are associated with the
digestive granules that aid in food digestion.
1. Habit and Habitat
Paramecium has a worldwide distribution and is a free-living organism. It usually lives in the
stagnant water of pools, lakes, ditches, ponds, freshwater and slow flowing
water that is rich in decaying organic matter.
2. Movement and Feeding
Its outer body is covered by the tiny hair-like
structures called cilia. These cilia are in constant motion and help it move with a speed that is
four times its body’s length per second. Just as the organism moves forward,
rotating around its own axis, this further helps it to push the food into the
gullet. By reversing the motion of cilia, paramecium can move in the reverse
direction as well.
Through a process known as phagocytosis, the food is
pushed into the gullet through cilia which further goes into the food vacuoles.
The food is digested with the help of certain enzymes and hydrochloric acid.
Once the digestion is completed the rest of the food content is quickly emptied
into cytoproct also known as the pellicles.
The water absorbed from the
surroundings through osmosis is continuously expelled from the body with the
help of the contractile vacuoles present on either end of the cell. P. bursaria
is one of the species which forms a symbiotic relationship with
In this case, the
paramecium provides a safe habitat for the algae to grow and live in its own
cytoplasm, however, in return the
paramecium might use this algae as a
source of nutrition in case there is a scarcity
of food in the surroundings.
Paramecium also feeds on other microorganisms
like yeasts and bacteria. To
gather the food it makes use of its cilia, making quick movements
with cilia to draw the water along with its prey organisms inside the mouth
opening through its oral groove.
The food further passes into the gullet
through the mouth. Once there is enough food accumulated a vacuole is formed
inside the cytoplasm, circulating through the cell
with enzymes entering the vacuole through the cytoplasm to digest the food
Once the digestion is completed the vacuole starts to shrink and the
digested nutrients enter into the cytoplasm. Once the vacuole reaches the anal
pore with all of its digested nutrients it ruptures and expels all of its
waste material into the environment.
Symbiosis refers to the
mutual relationship between two organisms to benefit from each other. Some
species of paramecium including P. bursaria
and P. chlorelligerum form a symbiotic
relationship with green algae from which they not only take food and nutrients
when needed but also some protection from certain predators like Didinium nasutum.
There has been a lot of endosymbioses
reported between the green algae and paramecium with an example being that of
the bacteria named Kappa particles giving paramecium the power to kill other
paramecium strains which lack this bacteria.
Just like all the other
ciliates, paramecium also consists of one or more diploid micronuclei and a
polypoid macronucleus hence containing a dual nuclear apparatus.
of the micronucleus is to maintain the
genetic stability and making sure that the desirable genes are passed to the
next generation. It is also called the germline or generative nucleus.
The macronucleus plays a role in non-reproductive
cell functions including the expression of genes needed for the everyday
functioning of the cell.
asexually through binary fusion. The micronuclei
during reproduction undergo mitosis while
the macronuclei divide through amitosis. Each new cell, in the end, contains a
copy of macronuclei and micronuclei after the cell undergoes a transverse
division. Reproduction through binary fission may occur spontaneously.
It may also undergo autogamy (self-fertilization) under certain
conditions. It may also follow a sexual reproduction process in which there is an exchange of genetic material because of mating
between two paramecia who are compatible for mating through a temporary
There is a meiotic division of the micronuclei
during the conjugation which results in haploid gametes and is further passed on from cell to cell. The old
macronuclei are destroyed and formation
of a diploid micronuclei takes place
when gametes of two organisms fuse together.
Paramecium reproduces through
conjugation and autogamy when conditions are not favorable and there is a scarcity
There is a gradual loss of
energy as a result of clonal aging during the mitotic cell division in the asexual
fission phase of growth of paramecium.
P. tetraurelia is a well-studied species and it has been known that
the cell expires right after 200 fissions if the cell relies only on the asexual
line of cloning instead of conjugation and autogamy.
There is an increase in
the DNA damage during clonal aging specifically the DNA damage in the macronucleus
hence causing aging in P. tetraurelia.
As per the DNA damage theory of aging the whole process of aging in single-celled protists is the same as that of
the multicellular eukaryotes.
Strong evidence for the
three whole-genome duplications has been
provided after the genome of species P. tetraurelia has been
sequenced. In some of the ciliates including Stylonychia and Paramecium UAA and
UAG are designated as sense codons while UGA as a stop codon.
There have been some ambiguous results yielded, based on
different experiments regarding whether
or not paramecium exhibits the learning behavior.
There was a study published in 2006 which showed that P. causatum can be
trained to differentiate between levels of brightness through a 6.5 volts
electric current. For an organism
with no nervous system, this type of
finding is cited as a strong possible instance for epigenetic learning or cell
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