Paramecium – Classification, Structure, Function, And Characteristics

Paramecium is an important element of the biological world, and understanding the structure, function, and characteristics of this organism, as well as the ways in which it is classified, is an important element of mastering microbiology.

Paramecium - Classification, Structure, Function, And Characteristics

To help, we have put together the ultimate guide to help you understand paramecium; read on for all you need to know!

What Is Paramecium?

Paramecium is an organism that is assigned to the kingdom Protista, and is unicellular in structure, and is an organism that is largely found in freshwater habitats.

Its size ranges from 50 – 300um, depending on the species, and it is a genus of ciliate protozoa.

In addition, paramecium is also a member of the phylum (the taxonomic category to which an organism belongs) Ciliophor, and this means that the whole surface is covered in “cilia”, or small filaments, which aid movement and allow food to reach the oral cavity.

Classifying Paramecium

Paramecium is described as unicellular – that is, consisting of a single cell – and eukaryotic – that is, a cell that contains both organelles and a nucleus, both of which are enclosed within a plasma membrane.

This means that the organism is classified as a member of the kingdom Protista. They are ciliated protozoans belonging to the phylum Ciliophora.

Among the most common Paramecium species are:

  • Aurelia Paramecium
  • Caudatum Paramecium
  • Woodruff’s Paramecium
  • Trichium paramecium

Structure Of Paramecium

The first thing to note is the distinctive structure of paramecium, and this is as follows: 

Size And Shape

Paramecium is a unicellular protozoan that is microscopic in size, and ranges from between 170 and 290um, right up to between 300 and 350um.

Surprisingly, despite the small size, paramecium can be seen with the naked eye. The organism has a long shape which has been compared to a slipper – hence the moniker “slipper animalcule”.

The back of the body is pointed and thick, almost like a cone in shape, while the front is broader and more blunt, with the widest point located just below the midsection.

The body is asymmetrical, with a convex aboral or dorsal body surface and a well-defined oral or ventral surface.


The pellicle refers to the thin membrane or film that exists over an organism, and the body of paramecium is entirely covered by pellicles; these serve to support the cell membrane, and are made of gelatinous material, offering elastic properties.


Cilia are a series of small projections running across the entire body, which have been likened to hair. Cilia is organized in longitudinal rows that are uniform in length, and run across the entire surface – this is referred to as holotrichous.

A few longer strands are also found at the back end of the body, and this creates a caudal tuft of cilia, providing the name caudatum.

Cilia have the same structure as flagella: and this is a sheath that is made of plasma membrane, or protoplast, which features nine longitudinal fibrils – these are arranged in a ring.

The inner fibrils are much thinner than the outer ones, and each cilium is formed by a basal granule. Cilia are 0.2um in diameter and aid in locomotion.


A cytostome or cell mouth is a phagocytic component of a cell, and in order to understand this, we need to understand phagocytosis.

In the simplest terms, phagocytosis, a phrase derived from Ancient Greek (phagein) ‘to eat’ and o, (kytos) ‘cell’, is the process by which a cell uses its plasma membrane to engulf a large particle (0.5 m), to form an internal compartment that is known as the phagosome.

A phagocyte is a cell that performs phagocytosis.

Phagocytosis is a key mechanism in a multicellular organism’s immune system for removing pathogens and cell debris, keeping the cell clean and free from toxins and unwanted visitors.

The phagosome then digests the ingested material; bacteria, dead tissue cells, and small mineral particles are all examples of phagocytized objects.

Phagocytosis is used by some protozoa to obtain nutrients.

What, then, does this process have to do with paramecium? The cytostome is a key element of the structure of a paramecium, and this cytostome is responsible for the process of phagocytosis.

The cytostome contains a number of elements, and these include:


Also known as an oral groove, the peristome is a large, shallow depression on the bottom and sides of the body.

It continues into an area known as a vestibule via a short funnel shaped like a cone.

And this vestibule, in turn, continues into the cytostome first via an oval-shaped opening, then via a long opening known as the cytopharynx, and finally the esophagus, which leads to the food vacuole.

Cytopyge Or Cytoproct

The cytopyge, also known as a cytoproct, is found just to the back of the cytostome, on the ventral surface, and its function is to eliminate all undigested food.



This is an important substance that is connected to ectoplasm, and this has been compared to jelly in its texture.

It is closely linked to ectoplasm, a thin layer on the periphery of the cell.

It is a clear, dense layer with a granular inner mass consisting of endoplasm or semifluid plasmasol.

Essentially, the cytoplasm is the fluid that suspends and supports the organelles and cellular molecules.

These are the tiny cellular structures that are present within the cytoplasm, and these are responsible for performing specific functions within bacteria or prokaryotic cells, as well as eukaryotic cells found in humans, plants and animals. 

Cytoplasm also aids in the movement of molecules within cells, such as hormones, and dissolves any cellular waste that may accumulate.


Ectoplasm is composed of fibrillar structures, as well as the cilia we mentioned before, and trichocysts, which we will discuss shortly.

These elements form an outer layer which is thin, dense, and clear. This ectoplasm is then externally bound to the pellicle via a covering.


Endoplasm is a key element in any cytoplasm, containing a variety of granules. It has various elements such as vacuoles, mitochondria and nuclei.


Trichocysts are small bodies located within the cytoplasm.

Trichocysts contain a fluid that is dense and refractive, and which contains swollen substances. The head is cone-like, with a spike at the outer end.


As with any cell, the nucleus is a key element of paramecium and includes two smaller elements: the macronucleus, and the micronucleus.

The macronucleus is a feature that is ellipsoidal, or kidney-shaped, and it is tightly packed within the DNA (chromatin granules).

Also known as the vegetative nucleus, the macronucleus, is responsible for controlling all of the paramecium’s vegetative functions.

The micronucleus is located in close proximity to the macronucleus and is small, compact and spherical in structure.

Fine chromatin granules and threads are found across the cell with fairly uniform distribution – these take responsibility for the control and regulation  of the reproduction of that specific cell. 


Vacuoles are typically responsible for transporting materials in and out of a particular cell, and in paramecium, these come in two types: contractile vacuoles and food vacuoles.

Contractile Vacuole

These organisms also include two “contractile vacuoles”, and there is one at either end of the body.

Known as “temporary organs”, these vanish regularly, and are filled with various fluids.

There are between five and twelve radical canals attached to each contractile vacuole, which consist of three elements: an ampulla, an injector canal and a terminal part.

The injector canal feeds directly into the contractile vacuole, and transport liquid collected from the body – this increases the size of the vacuole, and allows the liquid to vacate the body.

Both contractile vacuoles contract in an irregular manner. Because the rear contractile vacuole is located close to the cytopharynx, it contracts more quickly as more water passes through.

Respiration, osmoregulation and excretion are all important roles of contractile vacuoles.

Food Vacuole

The size of the food-vacuole varies in the endoplasm, where it digests food particles, enzymes, and a small amount of fluid and bacteria.

These food vacuoles are linked to digestive granules, which aid in food digestion, and tend to be sphere-like in shape.

These are the main elements that make up the structure of paramecium, and understanding them will help you to gain a more comprehensive overview of the organism.

Characteristics Of Paramecium

Characteristics Of Paramecium

Now that we have taken a closer look at the structure of paramecium, it is time to consider the characteristics of the organism more closely, and there are a number of considerations to explore here.

Environment And Habitat

Paramecium is a free-living organism with a global distribution.

It is most commonly found in stagnant water, including ponds, areas of freshwater, pools, ditches, lakes, and any slow-flowing water that offers plenty of decaying organic matter.

In short, paramecium may be found in a rich multitude of locations, as long as the conditions are right.

Feeding And Movement

The outer body of paramecium is covered by cilia, which are tiny structures that are constantly moving, allowing the paramecium to move at a rate four times its body length every second. 

As the paramecium moves forward, it rotates around its own axis, and this aids in pushing the food into the gullet.

Paramecium can also move in the opposite direction by reversing the motion of its cilia, and this allows for greater freedom of movement.

As discussed, phagocytosis is the process by which food is pushed into the gullet by cilia and then into the food vacuoles.

Hydrochloric acid combines with enzymes to digest the food; when the digestion is finished, the remaining food content is quickly emptied into the cytoproct, or pellicles.

Water absorbed from the environment via osmosis is continuously removed from the body by contractile vacuoles located on the ends of the cell. 

Other microorganisms, such as bacteria and various yeasts, are also consumed by Paramecium.

It uses its cilia to gather food, using cilia to move and pull water, along with prey, into the mouth opening.

The food then enters the gullet through the mouth. Once enough food has been acquired, a vacuole forms within the cytoplasm and circulates through the cell.

Enzymes then use the cytoplasm to enter the cell and digest the food material.

When digestion is complete, the vacuole shrinks, and the digested nutrients enter the cytoplasm.

These then travel to the anal pore, where the nutrients that have been digested are expelled as waste material, and they leave the organism and enter the environment.


The mutual relationship between two organisms to benefit from each other is referred to as symbiosis.

Some paramecium species, such as P. bursaria and P. chlorelligerum, form symbiotic relationships with green algae, from which they not only obtain food and nutrients when needed but also some protection from predators such as Didinium nasutum.

Endosymbioses have been reported between green algae and paramecium, with one example being the bacteria known as Kappa particles, which give paramecium the ability to kill other paramecium strains that lack these bacteria.


Paramecium, like all other ciliates (organisms included in this phylum), has one or more diploid micronuclei, as well as a polyploid macronucleus, resulting in a dual nuclear apparatus.

The micronucleus’s function is to maintain genetic stability and ensure that the best quality genes are passed down through reproduction.

It is also known as the germline nucleus or the generative nucleus.

The macronucleus is involved in non-reproductive cell functions such as gene expression, which is required for the cell to function normally.

Asexual reproduction occurs in Paramecium via binary fission. During reproduction, the micronuclei divide via mitosis, whereas the macronuclei divides via amitosis.

After the cell divides transversely, each new cell contains a copy of both micronuclei and macronuclei. 

Binary fission reproduction can occur spontaneously.

Under certain conditions, it may also engage in autogamy (self-fertilization), or undergo a reproductive process which sees the genetic material exchanged through mating between two paramecia that are deemed suitable via a temporary fusion.

During conjugation, the micronuclei divide meiotically, resulting in haploid gametes that are passed on between cells.

The process destroys the original macronuclei, and the fusion of gametes from two organisms creates a diploid micronucleus.

If the conditions are unfavorable and food is scarce, Paramecium reproduces through autogamy or conjugation as an alternative.


During the asexual fission phase of the growth of paramecium, there is a slow loss of energy due to clonal aging during mitotic cell division.

P. tetraurelia is a well-researched species, which demonstrates the risk of a cell relying solely on asexual reproduction and cloning, rather than autogamy or conjugation- the former means that cells will die after 200 fissions.

Clonal aging sees a rise in damage to DNA, and specifically, damage to the macronucleus – this is the most common cause of aging in P. tetraurelia.

According to the DNA damage theory of aging, the entire aging process in single-celled protists echoes that of multicellular eukaryotes.


After the genome of the species P. tetraurelia was sequenced, researchers found strong evidence for three whole-genome duplications.

In some ciliates, such as Stylonychia and Paramecium, UAA and UAG are designated as sense codons, while UGA is designated as a stop codon.


Based on various experiments, some non-conclusive results have been obtained regarding the ability, or lack thereof, of the paramecium to exhibit learning behavior.

A 2006 study demonstrated that P. causatum could potentially be trained and taught to distinguish between brightness levels using a 6.5-volt electric current.

Though in the early stages, this finding is recognized as a strong possibility for cell memory, or possible epigenetic learning, in an organism without a nervous system.

Though it should be noted that further research is needed for conclusive results and evidence.

Significance Of Paramecium

Paramecium are also thought to play a key part in their wider environment and ecosystem, and this makes them significant to scientists.

According to experts, Paramecium can aid in the control of algae, bacteria, and other protists found in water.

They can also aid in the removal of small debris particles from the water, and eat small animals.

In addition, because the organisms are relatively transparent and have several visible organelles, paramecium is also used for teaching purposes in biological science classes.

Final Thoughts

Paramecium are a fascinating, complex organism, and they can also play a key part in maintaining our environments and ecosystems – though they may be small, these organisms have proven that they can be mighty!

Jennifer Dawkins