Paramecium - Classification, Structure, Function and Characteristics (2024)

Paramecium is a unicellular organism with a shape resemblingthe sole of a shoe. It ranges from 50 to 300um in size which varies from species to species. It is mostly found in a freshwaterenvironment.

It is a single-celledeukaryote belonging to kingdom Protista and is a well-known genus of ciliateprotozoa.

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. Themain function of this cilia is to help both in locomotion as well as draggingthe food to its oral cavity.

Classification of Paramecium

Paramecium can be classifiedinto the following phylum and sub-phylum based ontheir certain characteristics.

• Phylum Protozoa
• Sub-Phylum Ciliophora
• Class Ciliates
• Order Hymenostomatida
• Genus Paramecium
• Species Caudatum

Being awell-known ciliate protozoan, paramecium exhibits a high-level cellular differentiation containing several complexorganelles performing a specific function to make its survival possible.

Besides a highly specialized structure, it also has a complexreproductive 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

P.cadatum is amicroscopic, unicellular protozoan. Its size ranges from 170 to 290um or up to300 to 350um. Surprisingly, paramecium is visible to the naked eye and has anelongated slipper like shape, that’s the reason it’s also referred to as aslipper animalcule.

The posterior end of the body is pointed, thick and cone-like while the anterior part is broad andblunt. The widest part of the body is below the middle. The body of a paramecium is asymmetrical. It has awell-defined ventral or oral surface and has a convex aboral or dorsal bodysurface.

2. Pellicle

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's made up of a gelatinous substance.

3. Cilia

Cilia refers to the multiple,small hair-like projections that cover the whole body. It is arranged in longitudinal rows with a uniform lengththroughout the body of the animal. This condition is called holotrichous. There are also a few longer ciliapresent at the posterior end of the body forming a caudal tuft of cilia, thusnamed caudatum.

The structure of ciliais the same as flagella, a sheath made of protoplast or plasma membrane withlongitudinal nine fibrils in the form of a ring. The outer fibrils are muchthicker than the inner ones with each cilium arising from a basal granule.Cilia have a diameter of 0.2um and helpsin its locomotion.

4. Cytostome

It contains the following parts:

• Oral groove: Thereis a large oblique shallow depression on the ventrio-lateralside of the body called peristome or an oral grove. This oral groove gives anasymmetrical appearance to the animal. It further extends into a depressioncalled a vestibule through a short conical funnel. This vestibule furtherextends into the cytostome through an oval-shapedopening, through a longopening called a cytopharynx and then the esophagusleads to the food vacuole.
• Cytopyge: Lyingon the ventral surface, just behind the cytostome is the cytopyge also called a cytoproct. All theundigested food gets eliminated through the cytopyge.
• Cytoplasm:Cytoplasm is a jelly-like substancefurther 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:Ectoplasm forms a thin, dense and clearouter layer containing cilia, trichocysts, andfibrillar structures. This ectoplasm is further bound to pellicle externallythrough a covering.
• Endoplasm:Endoplasm is one of the most detailed partsof the cytoplasm. It contains several different granules. It contains differentinclusions and structures like vacuoles, mitochondria, nuclei, food vacuole,contractile vacuole etc.
• Trichocysts:Embedded in the cytoplasm are small spindle-likebodies called trichocysts. Trichocysts are filled with a dense refractivefluid containing swelled substances. There is a conical head on the spike atthe outer end. Trichocysts are perpendicular to the ectoplasm.

5. Nucleus

The nucleus further consists of a macronucleusand a micronucleus.

• Macro Nucleus: Macronucleus is kidney like or ellipsoidal inshape. It's densely packed within the DNA (chromatin granules). The macronucleus controls all the vegetativefunctions 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 inshape. The fine chromatin threads and granules are uniformly distributedthroughout the cell and controlreproduction of the cell. The numberin a cell varies from species to species. There is no nucleolus present in caudatum.

6. Vacuole

Paramecium consists of twotypes of vacuoles: contractile vacuole andfood vacuole.

Contractile vacuole:

Thereare two contractile vacuoles present close to the dorsal side, one on each endof the body. They are filled with fluids and are present at fixed positionsbetween the endoplasm and ectoplasm. They disappear periodically and henceare called temporary organs.

Each contractile vacuole is connected to atleast five to twelve radical canals. These radical canals consist of a long ampulla, a terminal part and an injector canal which is short insize and opens directly into the contractile vacuole. These canals pour all the liquid collected from the wholebody of paramecium into the contractile vacuole which makes the vacuole increase in size. This liquid is discharged to the outside through a permanentpore.

The contraction ofboth the contractile vacuoles is irregular. The posterior contractile vacuoleis close to the cytopharynx and hence contract more quickly because of morewater passing through. Some of the main functions of contractile vacuoles include osmoregulation, excretion, and respiration.

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.

Characteristics

1.Habit and Habitat

Paramecium has a worldwide distribution and is a free-living organism. It usually lives in thestagnant water of pools, lakes, ditches, ponds, freshwater and slow flowingwater that is rich in decaying organic matter.

2.Movement and Feeding

Its outer body is covered by the tiny hair-likestructures called cilia. These cilia are in constant motion and help it move with a speed that isfour 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 thegullet. By reversing the motion of cilia, paramecium can move in the reversedirection as well.

Through a process known as phagocytosis, the food ispushed 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 emptiedinto cytoproct also known as the pellicles.

The water absorbed from thesurroundings through osmosis is continuously expelled from the body with thehelp of the contractile vacuoles present on either end of the cell. P. bursariais one of the species which forms a symbiotic relationship withphotosynthetic algae.

In this case, theparamecium provides a safe habitat for the algae to grow and live in its owncytoplasm, however, in return theparamecium might use this algae as asource of nutrition in case there is a scarcityof food in the surroundings.

Paramecium also feeds on other microorganismslike yeasts and bacteria. Togather the food it makes use of its cilia, making quick movementswith cilia to draw the water along with its prey organisms inside the mouthopening through its oral groove.

The food further passes into the gulletthrough the mouth. Once there is enough food accumulated a vacuole is formedinside the cytoplasm, circulating through the cellwith enzymes entering the vacuole through the cytoplasm to digest the foodmaterial.

Once the digestion is completed the vacuole starts to shrink and thedigested nutrients enter into the cytoplasm. Once the vacuole reaches the analpore with all of its digested nutrients it ruptures and expels all of itswaste material into the environment.

3.Symbiosis

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 likeDidinium 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.

4.Reproduction

Just like all the otherciliates, paramecium also consists of one or more diploid micronuclei and apolypoid macronucleus hence containing a dual nuclear apparatus.

The functionof the micronucleus is to maintain thegenetic stability and making sure that the desirable genes are passed to thenext generation. It is also called the germline or generative nucleus.

The macronucleus plays a role in non-reproductivecell functions including the expression of genes needed for the everydayfunctioning of the cell.

Paramecium reproducesasexually through binary fission. The micronucleiduring reproduction undergo mitosis whilethe macronuclei divide through amitosis. Each new cell, in the end, contains acopy of macronuclei and micronuclei after the cell undergoes a transversedivision. Reproduction through binary fission may occur spontaneously.

It may also undergo autogamy (self-fertilization) under certainconditions. It may also follow a sexual reproduction process in which there is an exchange of genetic material because of matingbetween two paramecia who are compatible for mating through a temporaryfusion.

There is a meiotic division of the micronucleiduring the conjugation which results in haploid gametes and is further passed on from cell to cell. The oldmacronuclei are destroyed and formationof a diploid micronuclei takes placewhen gametes of two organisms fuse together.

Paramecium reproduces throughconjugation and autogamy when conditions are not favorable and there is a scarcityof food.

5.Aging

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.

6.Genome

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.

7.Learning

There have been some ambiguous results yielded, based ondifferent experiments regarding whetheror not paramecium exhibits the learning behavior.

There was a study published in 2006 which showed that P. causatum can betrained to differentiate between levels of brightness through a 6.5 voltselectric current. For an organismwith no nervous system, this type offinding is cited as a strong possible instance for epigenetic learning or cellmemory.

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