Presentation of the diversity of unicellular organisms. The diversity of single-celled organisms and their role in human life and nature. Type roundworms

Topic: “SINGLE CELL ORGANISMS: PROKARYOTES AND EUKARYOTES”

Lesson 1 : Structure of eukaryotic cells".

Purpose of the lesson: give students a general idea of ​​the structure of eukaryotic cells, the features of their functions in connection with their structure.

Equipment and materials: diagram of the structure of a eukaryotic cell; photographs of organelles taken under a light and electron microscope.

Basic concepts and T terms:

Lesson concept: show the structure of eukaryotic cells (later, in comparison, give information about simpler prokaryotic cells). When talking about eukaryotes, use the knowledge that schoolchildren already have. Based on knowledge about eukaryotic cells, give (in comparison) information about simpler prokaryotic cells. Tell us in more detail about prokaryotes due to the fact that schoolchildren still do not have much information about these organisms.

STRUCTURE AND CONTENT OF THE LESSON:

I. Updating basic knowledge and motivation educational activities :

What organelles are characteristic of plant cells?

What organelles are characteristic of animal cells?

What functions do chloroplasts perform?

What do you know about mitochondria?

What is a cell wall for? Which cells have it?

II. LEARNING NEW MATERIAL

Opening remarks teachers.

PROKARYOTES.

Depending on the level of cell organization, organisms are divided into prokaryotes and eukaryotes.

Prokaryotes (from lat. about - before, instead of and Greek. karyon - core) - a superkingdom of organisms, which includes the kingdoms of Bacteria and Cyanobacteria (the outdated name is “blue-green algae”).

Prokaryotic cells are characterized by a simple structure: they do not have a nucleus and many organelles (mitochondria, plastids, endoplasmic reticulum, Golgi complex, lysosomes, cell center). Only some bacteria - inhabitants of water bodies or soil capillaries filled with water - have special gas vacuoles. By changing the volume of gases in them, these bacteria can move into aquatic environment with minimal energy consumption. The surface apparatus of prokaryotic cells includes plasma membrane, cell wall, Sometimes - mucous capsule.

(Fig. 1).

The cytoplasm of prokaryotes contains ribosomes, various inclusions, and one or more nuclear zones (nucleoids) containing hereditary material. Hereditary material prokaryotes are represented by a circular DNA molecule attached at a specific location to the inner surface of the plasma membrane (Fig. 1).

Ribosomes prokaryotes are similar in structure to ribosomes located in the cytoplasm and on the membranes of the endoplasmic reticulum of eukaryotic cells, but differ in smaller sizes. Plasma membrane prokaryotic cells can form smooth or folded protrusions directed into the cytoplasm. Enzymes and ribosomes can be located on folded membrane formations, and photosynthetic pigments can be located on smooth ones. In the cells of cyanobacteria, rounded closed membrane structures were found - chromatophores, in which photosynthetic pigments are located.

The cells of some bacteria have organelles of movement - one, several or many flagella. Prokaryotic flagella consist of one specific protein molecule with a tubular structure. Flagella can be several times longer than the cell itself, but their diameter is insignificant (10-25 nm), so they are not visible under a light microscope. In addition to flagella, the surface of bacterial cells often has filamentous and tubular formations consisting of proteins or polysaccharides. They provide cell attachment to the substrate or take part in transmission hereditary information during the sexual process.

Prokaryotic cells are small in size (do not exceed 30 microns, and there are species whose cell diameter is about 0.2 microns). Most prokaryotes are single-celled organisms, including colonial forms. Clusters of prokaryotic cells can take the form of threads, clusters, etc.; sometimes they are surrounded by: a common mucous membrane - capsule. In some colonial cyanobacteria, neighboring cells contact each other through microscopic tubules filled with cytoplasm.

The shape of prokaryotic cells is varied: spherical (cocci), rod-shaped (bacilli), curved (vibrios) or spirally twisted (spirilla) rods, etc. (Fig.2)

(Fig.2)

***

(student’s message – excerpt from the essay – up to 5 minutes)

Discovery of viruses and their place in the system of living nature. The existence of viruses was first proven by the Russian scientist D.I. Ivanovsky in 1892. While studying a tobacco disease - the so-called leaf mosaic, he tried to isolate the causative agent of this disease using microbiological filters. But even filters with the smallest pore diameter could not retain this pathogen, and the filtered juice of a diseased plant caused disease in healthy ones. The scientist suggested the existence of some unknown organism, significantly smaller in size than bacteria. Later, the existence of similar particles was proven to cause diseases in animals. All these particles, invisible under a light microscope, are collectively called viruses (from Lat. virus - I). However, the real study of viruses became possible only in the 30s of the XIX century. after the invention of the electron microscope. The science that studies viruses is called virology.

Features of the structure and functioning of viruses. The size of viral particles ranges from 15 to several hundred, sometimes up to 2 thousand (some plant viruses) nanometers. (Fig.3)

(Fig.3)

Life cycle viruses consists of two phases: extracellular and intracellular.

Each viral particle consists of a DNA molecule or a special RNA coated with a protein shell (respectively, they are called: DNA - or RNA-containing viruses). (Fig.4)

(Fig.4)

Both of these nucleic acids carry hereditary information about viral particles.

Viral nucleic acids have the form of one- or two-chain spirals, which, in turn, are linear, circular or secondarily twisted.

Depending on the structure and chemical composition enveloped viruses are divided into simple and complex.

Simple viruses have a shell consisting of the same type of protein formations (subunits) in the form of spiral or polyhedral structures (for example, tobacco mosaic virus) (Fig. 28). They have different shape- rod-shaped, filamentous, spherical, etc.

Complex viruses additionally covered with a lipoprotein membrane. It is part of the plasma membrane of the host cell and contains glycoproteins (smallpox viruses, hepatitis B, etc.). The latter serve to recognize specific receptors on the host cell membrane and attach the viral particle to it. Sometimes the virus membrane contains enzymes that ensure the synthesis of viral nucleic acids in the host cell and some other reactions.

In the extracellular phase, viruses are able to exist for a long time and withstand exposure to sunlight, low or high temperatures(and hepatitis B virus particles 1 - even short-term boiling). Poliomyelitis virus 2 in the external environment retains the ability to infect a host for several days, and smallpox virus for many months.

Mechanisms of virus penetration into the host cell. Most viruses specific: they infect only certain types of host cells in multicellular organisms (target cells) or certain types of single-celled organisms. Penetration into the host cell begins with the interaction of the viral particle with the cell membrane on which special receptor sites are located. The shell of the viral particle contains special proteins (attached) that “recognize” these areas, which ensures the specificity of the virus. If a viral particle attaches to a cell on the membrane of which there are no receptors sensitive to it, then infection does not occur. In simple viruses, attachment proteins are located in the protein shell; in complex viruses, they are located on needle-shaped or subulate-shaped projections of the surface membrane.

Viral particles enter the host cell in different ways. Many complex viruses - due to the fact that their envelope fuses with the membrane of the host cell (for example, like the influenza virus). Often the viral particle enters the cell by pinocytosis (eg, polio virus). Most plant viruses enter host cells at sites where cell walls are damaged.

It consists of an extended heads, the protein shell of which contains DNA, process, in the form of a case resembling an extended spring, inside of which there is a hollow rod, and tail filaments. Using these threads, the virus connects to the receptor sites of the host cell and attaches to its surface. The sheath then sharply contracts, causing the rod to pass through the bacterial shell and inject viral DNA inside it. The empty bacteriophage shell remains on the surface of the host cell.

(teacher’s summary – up to 1 min.)

EUKARYOTES.

(student’s message – excerpt from the essay – up to 5 minutes)

It is known that cells are very diverse. Their diversity is so great that at first, when examining cells under a microscope, scientists did not notice similar features and properties in them. But later it was discovered that behind all the diversity of cells there is hidden their fundamental unity, the common manifestations of life characteristic of them.

Why are cells the same?

The contents of any cell are separated from the external environment by a special structure - plasma membrane(plasmalemma). This separation allows you to create a completely special environment inside the cell, different from the one that surrounds it. Therefore, processes can occur in the cell that do not occur anywhere else. They are called life processes.

All contents of a cell, with the exception of the nucleus, are called cytoplasm. Since a cell must perform many functions, the cytoplasm contains various structures that ensure the performance of these functions. Such structures are called organelles(or organelles are synonyms, but organelles is a more modern term).

What are the main organelles of a cell?

The largest organelle of the cell is core, in which hereditary information is stored and rewritten. This is the metabolic control center of the cell; it controls the activities of all other organelles.

The core has nucleolus- This is the place where other important organelles involved in protein synthesis are formed. They are called ribosomes. But ribosomes are only formed in the nucleus, and they work (i.e., synthesize protein) in the cytoplasm. Some of them are free in the cytoplasm, and some are attached to membranes, which form a reticulum called the endoplasmic reticulum. Endoplasmic reticulum is a network of membrane-bounded tubules. There are two types of endoplasmic reticulum: smooth and rough. Ribosomes are located on the membranes of the rough endoplasmic reticulum, so protein synthesis and transport takes place there. And the smooth endoplasmic reticulum is the site of synthesis and transport of carbohydrates and lipids.

The synthesis of proteins, carbohydrates and fats requires energy, which is produced by the cell's energy stations - mitochondria. Mitochondria- double-membrane organelles in which the process takes place cellular respiration. Oxidizes on mitochondrial membranes food products and accumulates chemical energy in the form of special energy molecules.

There is also a place in the cage where organic compounds can accumulate and from where they can be transported. This Golgi apparatus- system of flat membrane bags. It takes part in the transport of proteins, lipids, carbohydrates, and renewal of the plasma membrane. The Golgi apparatus also produces organelles for intracellular digestion - lysosomes.

Lysosomes- single-membrane organelles, characteristic of animal cells, containing enzymes that can destroy proteins, carbohydrates, nucleic acids, lipids.

All cell organelles work together, taking part in metabolic and energy processes.

A cell may contain organelles that do not have a membrane structure.

Cytoskeleton- this is the musculoskeletal system of the cell, which includes microfilaments, cilia, flagella, cell center,

producing microtubules and centrioles.

There are organelles that are characteristic only of plant cells - plastids.

There are three types of plastids: chloroplasts, chromoplasts and leucoplasts. In chloroplasts, as you already know, the process of photosynthesis occurs. Plants also have vacuoles - these are waste products of the cell, which are reservoirs of water and compounds dissolved in it. (see Fig. 6,7,8)

Fig.6

Fig.7

Fig.8

(teacher’s summary – up to 1 min.)

(Work in pairs with flashcards and drawings )

The results of studying the eukaryotic cell can be summarized in a table.

Organelles of a eukaryotic cell

Organelle name	Structural features	Biological functions
	Largest double-membrane organelle in a cell	It is the information center of the cell, responsible for the processes of storage, change, transmission and implementation of hereditary information
Ribosomes	Non-membrane organelles, spherical structures with a diameter of 20 nm. These are the smallest cellular organelles	Ribosomes are where protein synthesis takes place in the cell.
Rough endoplasmic reticulum	A system of membranes that form tubules and cavities. Ribosomes are located on membranes	Protein synthesis and transport system
Smooth endoplasmic reticulum	A system of membranes that form tubules and cavities. There are no ribosomes on these membranes	System of synthesis and transport of carbohydrates and lipids
Golgi apparatus	Consists of cavities surrounded by membranes, stacked	Place of accumulation, sorting, packaging and further transport of substances throughout the cell
Lysosomes (characteristic of animal cells)	Single-membrane organelles, small vesicles containing enzymes	Capable of breaking down proteins, fats, carbohydrates and nucleic acids
Vacuoles (characteristic of plant cells)	Cavities surrounded by a membrane	Reservoirs of water and compounds dissolved in it maintain turgor pressure
Mitochondria	Double membrane organelles	Provides respiration processes in the cell
Plastids: chromoplasts, leucoplasts, chloroplasts	Double-membrane organelles: leucoplasts - colorless, chloroplasts - green, chromoplasts - colored (not green)	The process of photosynthesis occurs in chloroplasts, chromoplasts provide different colors of plant parts, and leucoplasts play a storage role
Cytoskeleton	Includes non-membrane organelles: microfilaments, cilia and flagella, cell center producing microtubules and centrioles	Provides cell movement, changes in cell shape, changes in the relative position of organelles inside the cell

III. Generalization, systematization and control of students' knowledge and skills.

Indicate the main structural elements (organelles) of plant and animal cells ON THE TEAM CARDS.

(work in pairs with flashcards)

(Samples of flashcards:

V. Homework :

§ 25, 26 of the textbook (pp. 100-107), - study; drawings - look at them.

§ 9, - repeat. Prepare for laboratory work.

LESSON 2 : "Structure of a prokaryotic cell."

Laboratory work : “Structure of cells of prokaryotes and eukaryotes.”

Purpose of the lesson: continue to form in students a general understanding of the structure of prokaryotic cells (in comparison with eukaryotes), about the features of their functions in connection with the structure.

Equipment and materials: diagram of the structure of prokaryotic and eukaryotic cells; permanent preparations of onion epidermal cells and epithelial tissue. For laboratory work: light microscope, cover glasses, tweezers, dissecting needles.

Basic concepts and T terms: organelles, eukaryotes, prokaryotes, nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, plasma membrane, membrane organelles, non-membrane organelles, cell center.

Lesson concept: based on knowledge about eukaryotic cells, give (in comparison) information about simpler prokaryotic cells. Tell us in more detail about prokaryotes due to the fact that schoolchildren still do not have much information about these organisms.

STRUCTURE AND CONTENT OF THE LESSON:

I. Updating basic knowledge and motivating learning activities:

What organelles are there in any cell?

Do all cells have a nucleus?

What functions does the nucleus perform in a cell?

Can there be nuclear-free cells?

II. Learning new material:

Working with a table.

Prokaryotes are single-celled organisms that do not have a formed nucleus and many other organelles. But since these are living organisms, they must perform all the functions of a living thing. How? With what? If they do not have those organelles that are characteristic of eukaryotes, then how do they manage without them? The differences in the characteristics of prokaryotes and eukaryotes are visible in the following table:

(Work in pairs with tables)

Characteristic	EUKARYOTES	PROKARYOTES
Cell sizes	Diameter up to 40 microns, cell volume 1000-10000 times greater than that of prokaryotes.	The average diameter is 0.5 – 5 microns
Form	Unicellular and multicellular	Unicellular
Presence of a kernel	There is a decorated core	There is a nuclear zone in which a circular DNA molecule is located, which acts as an information center
Presence of ribosomes	Present in the cytoplasm and on the rough ER	Found only in the cytoplasm, but much smaller in size
Where does protein synthesis and transport take place?	In the cytoplasm and on the membranes of the ER	Only in the cytoplasm
How do breathing processes proceed?	The process of aerobic respiration occurs in mitochondria	Aerobic respiration occurs on respiratory membranes; there are no special organelles for this process
How does the process of photosynthesis occur?	In chloroplasts	There are no special organelles. In some forms, photosynthesis occurs on photosynthetic membranes
Nitrogen fixation ability	Incapable of nitrogen fixation	Can fix nitrogen
Structure of cell walls	Plants have cellulose, fungi have chitin.	Basic structural component– murein
Presence of organelles	Many. Some are double membrane, others are single membrane	Few. Internal membranes are rare. If they exist, then the processes of respiration or photosynthesis occur on them

Laboratory work: “Structural features of prokaryotic and eukaryotic cells.”

PROGRESS:

Prepare the microscope for use.

At low magnification, examine a permanent preparation of cells (plants, fungi, animals). Then turn the microscope to high magnification and examine the preparations in more detail.

Compare drugs with each other. Sketch what you see.

Consider electron microscopic photographs of cells of various organisms. Find the cell wall, plasma membrane, nucleus, ER, Golgi apparatus, mitochondria, vacuoles on them.

4. Draw a conclusion.

III. Generalization, systematization and control of students’ knowledge and skills:

What are the main differences between eukaryotic and prokaryotic cells?

What are their similarities?

Which cells are more ancient?

What functions do they perform in a cell: nucleus, mitochondria, chloroplasts?

IV. Independent work students:

Name the parts with which prokaryotic cells perform vital functions.

V. Homework:

§ 26, - textbook (pp. 104-108), - repeat. Drawing No. 28 - examine and sketch.

Subkingdom Unicellular animals includes animals whose body consists of one cell. This cell is a complex organism with its own physiological processes: breathing, digestion, excretion, reproduction and irritation.

Their cell shapes are varied and can be constant(flagellates, ciliates) and fickle(amoeba). The organelles of movement are pseudopods, flagella And cilia. Protozoa eat autotrophic(photosynthesis) and heterotrophic(phagocytosis, pinocytosis). Reproduction in unicellular organisms asexual(nuclear division - mitosis, and then longitudinal or transverse cytokinesis, as well as multiple division) and sexual: conjugation (ciliates), copulation (flagellates).

About 30,000 species of unicellular organisms are grouped into several types. The most numerous are types of Sarcoflagellates And ciliate type.

Type of Ciliates totals more than 7,500 species. This is in highly organized protozoa that have a constant body shape.

A typical representative of the type is ciliate-slipper. The body of the ciliate is covered with a dense membrane. It has two cores: large ( macronucleus), which regulates all life processes, and small ( micronucleus), which plays a major role in reproduction. Ciliate slipper feeds on algae, bacteria, and some protozoa. The cilia of the ciliate oscillate, which “promotes” food into the mouth e, and then into the pharynx, at the bottom of which digestive vacuoles where food is digested and absorbed nutrients. Through powder– a special organ – undigested residues are removed. Selection functions are carried out contractile vacuoles. Reproduces ciliate-slipper, like amoeba, in asexual way(transverse division of the cytoplasm, the small nucleus divides mitotically, the large nucleus divides amitotically). Characteristic and sexual process– conjugation. This is a temporary connection between two individuals, between which a cytoplasmic bridge, through which they exchange separated small nuclei. The sexual process serves to update genetic information.

Ciliates are link in food chains. Living in the stomachs of ruminants, ciliates contribute to their digestion.

A typical representative is common amoeba.

Amoeba lives in freshwater bodies. Her body shape is not constant. The pseudopods also serve to capture food - bacteria, unicellular algae, and some protozoa. Undigested residues are thrown out from any place in the amoeba. The animal breathes with its entire body surface: oxygen dissolved in water penetrates into the amoeba’s body through diffusion, and oxygen formed during respiration in the cell carbon dioxide stands out. The animal is irritable. Amoeba reproduces division: First, the nucleus divides mitotically, and then the cytoplasm divides. Under unfavorable conditions it occurs encystment.

Typical presentation tel Zhgutikov - green euglena– has a spindle-shaped shape. A long thin flagellum extends from the front end of the euglena's body: by rotating it, the euglena moves, as if screwing into the water. In the cytoplasm of euglena there is a nucleus and several colored oval bodies - chromatophores(20 pieces) containing chlorophyll(in the light, euglena feeds autotrophically). Photosensitive peephole helps euglena find illuminated places. When kept in the dark for a long time, euglena loses its chlorophyll and switches to feeding on ready-made organic substances, which she absorbs from the water with the entire surface of her body. Euglena breathes through the entire surface of its body. Reproduction is carried out division in two(longitudinal).

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Type of Sarcomastigophora

More than $25,000 species of protozoa belong to the Sarcomastigophora type. The characteristic body structure of this type is the presence of flagella and pseudopodia. Sarcomastigophores inhabit marine and fresh water bodies. The Sarcomastigophora phylum includes three classes: Sarcodaceae, Flagellates, Radiolarians.

Class Sarcodae

The cells of sarcode organisms are covered with a plasma membrane; some species have a shell. They move using pseudopodia. Fresh forms have a contractile vacuole. They feed on algae, bacteria and many protozoa. They reproduce sexually and asexually.

Amoeba Proteus. Body size is about $0.5$ mm. Externally, the amoeba is covered with plasmalemma. Characteristic feature it is the absence of a permanent shape, since it forms outgrowths - pseudopodia. The pseudopodia contains a digestive vacuole, many food inclusions, a contractile vacuole, a nucleus and other organelles. Absorbs food by phagocytosis. Reproduction occurs through mitotic division, once every two days.

Also representatives of this class are amoeba dysentery and foraminifera.

Class Radiolaria, or Rayaceae

The class contains from $7$ to $8$ thousand marine species. The sizes range from $40$ microns to $1$ mm. All radiolarians are planktonic organisms. The membrane forms the central capsule and covers the endoplasm and nucleus. The walls of the capsule have pores through which the endoplasm is connected to the ectoplasm. Ectoplasm contains inclusions, for example, fat drops, mucus. Mucus and fat reduce friction and promote better movement of the protozoan's body in water. Radiolarians have a mineral skeleton. Pseudopodia pass through the pores of the skeleton.

Class Flagellates

A characteristic feature of this class is the presence of a flagellum. The number of flagella varies from $1 to several dozen. The flagellum consists of an outer and an inner part, provides movement and is formed by a basal body, respectively. Among flagellated organisms there are autotrophs, heterotrophs and those with a mixed type of nutrition.

Euglena green. Has one flagellum. Due to the presence of stigma, it exhibits positive phototaxis. Euglena's cytoplasm contains chloroplasts containing chlorophyll. Has a mixed type of nutrition.

Volvox. Colonial organism, forms colonies up to $0.9 mm, and includes several thousand cells. The shape of the colony is spherical. The cells produce mucus, which forms a colony. As daughter cells multiply in the colony, they grow so large that the mother cell ruptures and releases them outside.

Belongs to the class Flagellates trypanosoma is the causative agent of sleeping sickness, spread by the tsetse fly.

Single-celled animals are a category of organisms that stands outside the system. This means that they cannot be completely attributed to any specific kingdom. Single-celled organisms are distinguished by the absence of highly organized tissues. All animals belonging to this group do not have any common features. The only thing they have in common is simple structure.

Single-celled animals are usually so tiny that they can only be seen under a microscope. Their habitat is humid. This is soil and water, as well as the body of a person and animal. All of them, in one way or another, with the help of various devices, adapt to different conditions. First of all, it is the shape of the body. It may not have clear boundaries, constantly change, or, on the contrary, it may be streamlined, spindle-like or elongated. The types of symmetry also differ: radial, translational-rotational, bilateral. Some single-celled animals have shells on the outside, others, those that live deep under water, have unusual growths.

The cell that makes up the body of these organisms may contain from one to several nuclei. The shell is either just a membrane or a denser, more stretchable pellicle.

A single-celled organism moves with the help of various cilia, pseudopods, and flagella. They also react to the influence of such external factors as changes in temperature, lighting, presence chemicals.

Single-celled animals get food in different ways. Thus, during phagocytosis, cytoplasmic outgrowths capture solid food particles. Pinocytosis takes place in several stages: first, the surface of the entire cell captures liquid, and then absorbs the substances contained in it, processing them with the help of digestive enzymes that fill the vacuoles. Inside some protozoa (chlorella) there are chloroplasts, which, using photosynthesis, from inorganic substances can produce organic.

Also, the entire surface of the body of protozoa participates in gas exchange: decay products and excess water come out through it.

Single-celled animals reproduce both sexually and asexually. It depends on the conditions in which they exist. Asexual reproduction occurs like this. First, the nucleus is divided into several parts, then the cytoplasm is divided into the same number of parts. Thus, from one you get several (at least two).

Female and male individuals participate. Their structure and dimensions may differ, or they may be the same. As a result of their fusion, a zygote is formed, which then reproduces independently asexually. It happens that when individuals come into contact, they exchange nuclear particles. In this case, the zygote is not formed.

When conditions are not favorable for the normal functioning of protozoa, their body becomes round and covered with a dense shell. This is how a cyst is formed. As soon as conditions improve, the body is freed from the thick film and begins to lead the same lifestyle as before.

It is generally accepted that single-celled animals were the first to appear on Earth in the process of evolution. The most ancient are archaea and bacteria. They are similar in many ways (for example, the absence of a nucleus, the presence of a ring chromosome), for this reason they were previously classified as one group. But modern science proved that archaea have their own structural characteristics and evolved in a slightly different way. Although they are just as difficult to classify as before. The fact is that archaea have never been grown in laboratory conditions, but were discovered during the analysis of samples taken from the places where they live.

Single-celled organisms are a link without which it is impossible to imagine a full-fledged biocenosis. After all, they are eaten by many animals, which themselves serve as food for a number of other inhabitants of our planet.

Unicellular organisms are organisms whose body consists of a single cell. They can be prokaryotes (bacteria and blue-green algae, or cyanobacteria), i.e., do not have a formed nucleus (the function of the nucleus is performed by a nucleoid - a DNA molecule folded into a ring), but they can also be eukaryotes, i.e., have a formed core.

Unicellular eukaryotic organisms include many green and some other algae, as well as all representatives of the phylum Protozoa. General plan The structures and set of organelles in unicellular eukaryotes are similar to the cells of multicellular organisms, but the functional differences are very significant.

Unicellular organisms combine the properties of both cells and independent organism. Many single-celled organisms form colonies. From unicellular organisms in the process of evolution they evolved multicellular organisms.

The simplest structure is unicellular blue-green algae. Their cells do not have a nucleus or plastids; they are similar to bacterial cells. On this basis, they are classified as cyanobacteria. Pigments (chlorophyll, carotene) are dissolved in their outer layer of cytoplasm - chromatoplasm. These algae appeared in the Archean and were the first organisms on Earth to produce oxygen during photosynthesis. Blue-green algae can also form a multicellular form - filaments.

Among green algae, unicellular forms include Chlamydomonas, Chlorella, and Pleurococcus. Single-celled algae can form colonies (for example, Volvox).

Diatoms are also microscopic single-celled algae that can form colonies.

Single-celled algae most often live in water (Chlamydomonas in fresh water bodies, and Chlorella in both fresh and sea ​​water), but can also live in the soil (for example, chlorella, diatoms), and can live on the bark of trees (pleurococcus). Some algae live even on the surface of ice and snow (some Chlamydomonas, for example, Chlamydomonas snow). In Antarctica, diatoms form a dense brown coating on the underside of the ice.

Single-celled protozoa form the subkingdom Animalia. Most cells have one nucleus, but there are also multinucleate forms. On top of the membrane, many protozoa have a shell or shell. They move with the help of organelles of movement - flagella, cilia, and can form pseudopodia (pseudodepods).

Most protozoa are heterotrophs. Food particles are digested in digestive vacuoles. Osmotic pressure in the cell is regulated by contractile vacuoles: through them excess water is removed. Such vacuoles are characteristic of freshwater protozoa. Metabolic products are excreted from the body of protozoa along with water. However, the main function of excretion is carried out through the entire surface of the cell.

Protozoa have both asexual and sexual reproduction.

These single-celled organisms react to environmental influences: they have positive and negative taxis (for example, the slipper ciliate has negative chemotaxis - it moves away from a salt crystal placed in water).

Many protozoa are capable of encystation. Encystment allows you to experience unfavorable conditions and promotes the spread of protozoa.

The importance of unicellular algae in nature is directly related to their lifestyle. These organisms synthesize organic matter, release oxygen into the atmosphere, absorb carbon dioxide, are a link in the overall food chain, participate in soil formation, clean water bodies, and can enter into symbiosis with other organisms (for example, chlorella is a phycobiont of lichens). Dead diatom unicellular algae formed powerful