Subkingdom unicellular briefly. Single-celled organisms - list with names and examples

The phylum protozoa includes approximately 25 thousand species of single-celled animals that live in water, soil or the organisms of other animals and humans. Having morphological similarities in the structure of cells with multicellular organisms, protozoa differ significantly from them in functional terms.

If the cells of a multicellular animal perform special functions, then the cell of a protozoan is an independent organism, capable of metabolism, irritability, movement and reproduction.

Protozoa are organisms at the cellular level of organization. Morphologically, a protozoan is equivalent to a cell, but physiologically it is a whole independent organism. The vast majority of them are microscopically small in size (from 2 to 150 microns). However, some of the living protozoa reach 1 cm, and the shells of a number of fossil rhizomes have a diameter of up to 5-6 cm. The total number of known species exceeds 25 thousand.

The structure of protozoa is extremely diverse, but they all have features characteristic of the organization and function of the cell. What is common in the structure of protozoa are the two main components of the body - the cytoplasm and the nucleus.

Cytaplasm

The cytoplasm is bounded by an outer membrane, which regulates the flow of substances into the cell. In many protozoa it is complicated by additional structures that increase the thickness and mechanical strength of the outer layer. Thus, formations such as pellicles and membranes arise.

The cytoplasm of protozoa is usually divided into 2 layers - the outer one is lighter and denser - ectoplasm and internal, equipped with numerous inclusions, - endoplasm.

General cellular organelles are localized in the cytoplasm. In addition, a variety of special organelles may be present in the cytoplasm of many protozoa. Various fibrillar formations are especially widespread - supporting and contractile fibers, contractile vacuoles, digestive vacuoles, etc.

Core

Protozoa have a typical cell nucleus, one or more. The nucleus of protozoa has a typical two-layer nuclear envelope. Chromatin material and nucleoli are distributed in the nucleus. The nuclei of protozoa are characterized by exceptional morphological diversity in size, number of nucleoli, amount of nuclear juice, etc.

Features of the life activity of protozoa

Unlike somatic cells, multicellular protozoa are characterized by the presence of a life cycle. It consists of a number of successive stages, which are repeated with a certain pattern in the existence of each species.

Most often, the cycle begins with the zygote stage, corresponding to the fertilized egg of multicellular organisms. This stage is followed by single or multiple repeated asexual reproduction, carried out by cell division. Then sex cells (gametes) are formed, the pairwise fusion of which again produces a zygote.

An important biological feature of many protozoa is the ability to encystment. In this case, the animals become rounded, shed or retract the organelles of movement, secrete a dense shell on their surface and fall into a state of rest. In the encysted state, protozoa can tolerate sudden changes in the environment while maintaining viability. When conditions favorable for life return, the cysts open and the protozoa emerge from them in the form of active, mobile individuals.

Based on the structure of organelles of movement and characteristics of reproduction, the type of protozoa is divided into 6 classes. The main 4 classes: Sarcodaceae, Flagellates, Sporozoans and Ciliates.

Definition 1

Unicellular (protozoa) are organisms in which all the functions of living things are performed by one cell.

In addition to prokaryotes, these include unicellular eukaryotes, among which there are plants, animals, and fungi.

Features of single-celled organisms

The sizes of protozoa are microscopically small. The peculiarities of unicellular organisms include the fact that they perform all the functions of living things with the help of cellular organelles and are a separate independent organism, represented by only one cell. In structure and set of organelles, the cells of unicellular organisms are similar to the cells of multicellular organisms. Among unicellular eukaryotes, there are both simply constructed organisms (amoeba, chlorella) and quite complex ones (ciliates, acetabularia).

If the cells of multicellular organisms are characterized by differentiation of functions and the inability to perform all the functions of a living thing at once, then unicellular organisms retain this ability. The high level of their organization is cellular. The cell of unicellular organisms is an integral organism that has all the properties of a living thing: metabolism, irritability, growth, reproduction, and the like.

Their body consists of cytoplasm, in which there is an outer layer - ectoplasm, and an inner layer - endoplasm. In most species, the outside of the cell is covered with a membrane, which provides the single-celled animal with a permanent shape. Protozoa exhibit organelles that perform various functions:

• digestion (digestive vacuoles),
• secretions (contractile vacuoles),
• movements (flagella, cilia),
• perception of light (photosensitive eye)

and other organelles that provide all vital processes. According to the method of nutrition, these are heterotrophic organisms. Protozoa are characterized by irritability, which manifests itself in various movements - taxis. There are positive taxis - movements towards the stimulus, and negative taxis - movements away from the stimulus.

When exposed to unfavorable conditions, protozoa form cysts. Encystment is an important biological feature of protozoa. It not only provides the experience of unfavorable conditions, but also promotes widespread settlement.

Aquatic unicellular

Marine unicellular animals, such as foraminifera and radiolarians, have an external skeleton in the form of a calcareous shell. Highly organized unicellular animals include ciliates. The organelles of movement in them are cilia; the body is covered with a durable elastic shell, which gives it a constant shape. Most ciliates have two nuclei: large and small. Large vegetative nucleus - regulates the processes of movement, nutrition, excretion, as well as asexual reproduction, carried out by transverse division of the cell in half. The small nucleus is generative; it performs an important function in the sexual process.

Among aquatic unicellular organisms, mixotrophs are also distinguished - organisms that can feed both through photosynthesis and heterotrophy. For example, green euglena.

Euglena lives in freshwater bodies of water and swims using a single flagellum located at the front end of the body. In the cytoplasm of euglena there are chloroplasts containing chlorophyll, which allows euglena to feed phototrophically. If there is no light, it switches to heterotrophic nutrition. Thanks to this property, euglena combines the characteristics of a plant and an animal, which indicates the evolutionary unity of the plant and animal world.

Unicellular plants and fungi

Note 1

In nature there are many not only single-celled animals, but also single-celled plants and fungi. For example, among green algae, Chlamydomonas and Chlorella are unicellular representatives, and among fungi, yeast is unicellular.

Single-celled plants and animals are typical eukaryotic cells that have the corresponding organelles:

• surface membrane,
• core,
• mitochondria,
• Golgi apparatus,
• endoplasmic reticulum,
• ribosomes.

Differences in the structure of unicellular animals and unicellular plants are associated with differences in the way they feed. Plant cells are characterized by the presence of plastids, vacuoles, cell walls and other features associated with photosynthesis. Animal cells are characterized by the presence of a glycocalyx, digestive vacuoles and other features associated with heterotrophic nutrition.

In fungi, the cell has a cell wall, which shows the similarity of fungi with bacteria and plants. But mushrooms are heterotrophs, and this makes them similar to animals.

Single-celled eukaryotes reproduce predominantly asexually, but in some of them (for example, the slipper ciliates) a sexual process is observed - the exchange of genetic information, and in others (for example, chlamydomonas) sexual reproduction occurs. Asexual reproduction occurs by cell division in half through mitosis. During sexual reproduction, gametes are produced, which then fuse to form a zygote.

Note 2

Animals consisting of a single cell with a nucleus are called unicellular organisms.

They combine the characteristic features of a cell and an independent organism.

Unicellular animals

Animals of the subkingdom Unicellular or Protozoa live in liquid environments. Their external forms are varied - from amorphous individuals that do not have a definite outline, to representatives with complex geometric shapes.

There are about 40 thousand species of single-celled animals. The most famous include:

• amoeba;
• green euglena;
• ciliate-slipper.

Amoeba

It belongs to the rhizome class and is distinguished by its variable shape.

It consists of a membrane, cytoplasm, contractile vacuole and nucleus.

Nutrient absorption is carried out using the digestive vacuole, and other protozoa, such as algae and, serve as food. For respiration, amoeba requires oxygen dissolved in water and penetrating through the surface of the body.

Green euglena

It has an elongated fan-shaped shape. It feeds by converting carbon dioxide and water into oxygen and food products thanks to light energy, as well as ready-made organic substances in the absence of light.

Belongs to the class Flagellates.

Ciliate slipper

A class of ciliates, its outline resembles a shoe.

Bacteria serve as food.

Unicellular fungi

Fungi are classified as lower non-chlorophyll eukaryotes. They differ in external digestion and chitin content in the cell wall. The body forms a mycelium consisting of hyphae.

Unicellular fungi are systematized into 4 main classes:

• deuteromycetes;
• chytridiomycetes;
• zygomycetes;
• ascomycetes.

A striking example of ascomycetes is yeast, which is widespread in nature. The speed of their growth and reproduction is high due to their special structure. Yeast consists of a single round cell that reproduces by budding.

Unicellular plants

A typical representative of lower unicellular plants often found in nature are algae:

• chlamydomonas;
• chlorella;
• spirogyra;
• chlorococcus;
• Volvox.

Chlamydomonas differs from all algae in its mobility and the presence of a light-sensitive eye, which determines the places of greatest accumulation of solar energy for photosynthesis.

Numerous chloroplasts are replaced by one large chromatophore. The role of pumps that pump out excess fluid is performed by contractile vacuoles. Movement is carried out using two flagella.

Green algae, Chlorella, unlike Chlamydomonas, have typical plant cells. A dense shell protects the membrane, and the cytoplasm contains the nucleus and chromatophore. The functions of the chromatophore are similar to the role of chloroplasts in land plants.

The spherical algae Chlorococcus is similar to Chlorella. Its habitat is not only water, but also land, tree trunks growing in a humid environment.

Who discovered single-celled organisms

The honor of discovering microorganisms belongs to the Dutch scientist A. Leeuwenhoek.

In 1675, he examined them through a microscope of his own making. The name ciliates was assigned to the smallest creatures, and since 1820 they began to be called the simplest animals.

Zoologists Kelleker and Siebold in 1845 classified unicellular organisms as a special type of the animal kingdom and divided them into two groups:

• rhizomes;
• ciliates.

What does a single cell animal cell look like?

The structure of single-celled organisms can only be studied using a microscope. The body of the simplest creatures consists of a single cell that acts as an independent organism.

The cell contains:

• cytoplasm;
• organoids;
• core.

Over time, as a result of adaptation to the environment, certain species of unicellular organisms developed special organelles for movement, excretion and nutrition.

Who are the protozoa?

Modern biology classifies protozoa as a paraphyletic group of animal-like protists. The presence of a nucleus in a cell, unlike bacteria, includes them in the list of eukaryotes.

Cellular structures differ from those of multicellular organisms. In the living system of protozoa, digestive and contractile vacuoles are present; some have organelles similar to the oral cavity and anus.

Protozoan classes

In the modern classification based on characteristics, there is no separate rank and significance of unicellular organisms.

Labyrinthula

They are usually divided into the following types:

• sarcomastigophores;
• apicomplexans;
• myxosporidium;
• ciliates;
• labyrinthula;
• Ascestosporadia.

An outdated classification is considered to be the division of protozoans into flagellates, sarcodes, ciliates and sporozoans.

In what environments do unicellular organisms live?

The habitat of the simplest unicellular organisms is any humid environment. Common amoeba, green euglena and slipper ciliates are typical inhabitants of polluted fresh water sources.

Science has long classified opalines as ciliates, due to the external similarity of flagella to cilia and the presence of two nuclei. As a result of careful research, the relationship was refuted. Sexual reproduction of opalines occurs as a result of copulation, the nuclei are identical, and the ciliary apparatus is absent.

Conclusion

It is impossible to imagine a biological system without single-celled organisms, which are the source of nutrition for other animals.

The simplest organisms contribute to the formation of rocks, serve as indicators of pollution of water bodies, and participate in the carbon cycle. Microorganisms have found widespread use in biotechnology.

Unicellular or protozoan organisms are usually called those organisms whose bodies are one cell. It is this cell that carries out all the necessary functions for the life of the body: movement, nutrition, respiration, reproduction and removal of unnecessary substances from the body.

Subkingdom of Protozoa

Protozoa perform both the functions of a cell and an individual organism. There are about 70 thousand species of this Sub-Kingdom in the world, most of them are organisms of microscopic size.

2-4 microns is the size of small protozoa, and ordinary ones reach 20-50 microns; for this reason it is impossible to see them with the naked eye. But there are, for example, ciliates 3 mm long.

You can meet representatives of the Sub-Kingdom of Protozoa only in a liquid environment: in seas and reservoirs, in swamps and wet soils.

What are the types of unicellular organisms?

There are three types of unicellular organisms: sarcomastigophores, sporozoans and ciliates. Type sarcomastigophore includes sarcodes and flagellates, and the type ciliates- ciliated and sucking.

Structural features

A feature of the structure of unicellular organisms is the presence of structures that are characteristic exclusively of protozoa. For example, cell mouth, contractile vacuole, powder and cell pharynx.

Protozoa are characterized by the division of the cytoplasm into two layers: inner and outer, which are called ectoplasm. The structure of the inner layer includes organelles and endoplasm (nucleus).

For protection, there is a pellicle - a layer of cytoplasm characterized by compaction, and organelles provide mobility and some nutritional functions. Between the endoplasm and ectoplasm there are vacuoles that regulate the water-salt balance in a single cell.

Nutrition of unicellular organisms

In protozoa, two types of nutrition are possible: heterotrophic and mixed. There are three ways to absorb food.

Phagocytosis call the process of capturing solid food particles with the help of cytoplasmic outgrowths, which are found in protozoa, as well as other specialized cells in multicellular organisms. A pinocytosis represented by the process of fluid uptake by the cell surface itself.

Breath

Selection in protozoa it is carried out by diffusion or through contractile vacuoles.

Reproduction of protozoa

There are two methods of reproduction: sexual and asexual. Asexual represented by mitosis, during which division of the nucleus and then the cytoplasm occurs.

A sexual Reproduction occurs through isogamy, oogamy and anisogamy. Protozoa are characterized by alternating sexual reproduction and single or multiple asexual reproduction.

Class Flagellates

Structure. Flagellates have flagella that serve as organelles of movement and facilitate the capture of food. There may be one, two or many. The movement of the flagellum in the surrounding water causes a whirlpool, due to which small particles suspended in the water are carried to the base of the flagellum, where there is a small opening - the cell mouth, leading to the deep canal-pharynx.
Almost all flagellates are covered with a dense elastic membrane, which, along with developed cytoskeletal elements, determines the constant shape of the body.
Genetic apparatus in most flagellates it is represented by a single nucleus, but there are also binucleate (for example, Giardia) and multinucleate (for example, opalina) species.
Cytoplasm It is clearly divided into a thin outer layer - transparent ectoplasm and deeper endoplasm.
Method of nutrition. According to the method of feeding, flagellates are divided into three groups. Autotrophic organisms, as an exception in the animal kingdom, synthesize organic substances (carbohydrates) from carbon dioxide and water with the help of chlorophyll and solar radiation energy. Chlorophyll is found in chromatophores, similar in organization to plant plastids. Many flagellates with a plant type of nutrition have special devices that perceive light stimulation - stigmas.
Heterotrophic organisms (trypanosome, the causative agent of sleeping sickness) do not have chlorophyll and therefore cannot synthesize carbohydrates from inorganic substances. Mixotrophic organisms are capable of photosynthesis, but also feed on minerals and organic substances created by other organisms (green euglena).
Osmoregulatory and partly the excretory functions are performed in flagellates, like in sarcodidae, by contractile vacuoles, which are present in free-living freshwater forms.
Reproduction. In flagellates, sexual and asexual reproduction is observed. The usual form of asexual reproduction is longitudinal fission.
Habitat. Flagellates are widespread in fresh water bodies, especially small ones and polluted with organic residues, as well as in the seas. Many species parasitize various animals and humans and thereby cause great harm (tryponosomes, intestinal parasites, etc.).