What is a cell and what is its structure. Significant differences between plant and animal cells

The cell is the basic elementary unit of all living things, therefore it has all the properties of living organisms: a highly ordered structure, receiving energy from the outside and using it to perform work and maintain order, metabolism, an active response to irritations, growth, development, reproduction, duplication and transmission of biological information to descendants, regeneration (restoration of damaged structures), adaptation to the environment.

The German scientist T. Schwann in the middle of the 19th century created the cellular theory, the main provisions of which indicated that all tissues and organs consist of cells; cells of plants and animals are fundamentally similar to each other, they all arise in the same way; the activity of organisms is the sum of the vital activities of individual cells. Great influence on further development Cell theory and the theory of cells in general were influenced by the great German scientist R. Virchow. He not only brought together all the numerous disparate facts, but also convincingly showed that cells are a permanent structure and arise only through reproduction.

Cell theory in its modern interpretation includes the following main provisions: the cell is a universal elementary unit of living things; The cells of all organisms are fundamentally similar in their structure, function and chemical composition; cells reproduce only by dividing the original cell; multicellular organisms are complex cellular assemblies that form integral systems.

Thanks to modern research methods, it was revealed two main cell types: more complexly organized, highly differentiated eukaryotic cells (plants, animals and some protozoa, algae, fungi and lichens) and less complexly organized prokaryotic cells (blue-green algae, actinomycetes, bacteria, spirochetes, mycoplasmas, rickettsia, chlamydia).

Unlike a prokaryotic cell, a eukaryotic cell has a nucleus bounded by a double nuclear membrane and a large number of membrane organelles.

ATTENTION!

The cell is the basic structural and functional unit of living organisms, carrying out growth, development, metabolism and energy, storing, processing and implementing genetic information. From a morphological point of view, a cell is a complex system of biopolymers, separated from external environment plasma membrane (plasmolemma) and consisting of a nucleus and cytoplasm in which organelles and inclusions (granules) are located.

What types of cells are there?

Cells are diverse in their shape, structure, chemical composition and nature of metabolism.

All cells are homologous, i.e. have a number of common structural features on which the performance of basic functions depends. Cells are characterized by unity of structure, metabolism (metabolism) and chemical composition.

At the same time, different cells also have specific structures. This is due to their performance of special functions.

Cell structure

Ultramicroscopic cell structure:

1 - cytolemma (plasma membrane); 2 - pinocytotic vesicles; 3 - centrosome, cell center (cytocenter); 4 - hyaloplasm; 5 - endoplasmic reticulum: a - membrane of the granular reticulum; b - ribosomes; 6 - connection of the perinuclear space with the cavities of the endoplasmic reticulum; 7 - core; 8 - nuclear pores; 9 - non-granular (smooth) endoplasmic reticulum; 10 - nucleolus; 11 - internal reticular apparatus (Golgi complex); 12 - secretory vacuoles; 13 - mitochondria; 14 - liposomes; 15 - three successive stages of phagocytosis; 16 - connection of the cell membrane (cytolemma) with the membranes of the endoplasmic reticulum.

Chemical composition of the cell

The cell contains more than 100 chemical elements, four of them account for about 98% of the mass, these are organogens: oxygen (65–75%), carbon (15–18%), hydrogen (8–10%) and nitrogen (1.5–3.0%) . The remaining elements are divided into three groups: macroelements - their content in the body exceeds 0.01%); microelements (0.00001–0.01%) and ultramicroelements (less than 0.00001).

Macroelements include sulfur, phosphorus, chlorine, potassium, sodium, magnesium, calcium.

Microelements include iron, zinc, copper, iodine, fluorine, aluminum, copper, manganese, cobalt, etc.

Ultramicroelements include selenium, vanadium, silicon, nickel, lithium, silver and more. Despite their very low content, microelements and ultramicroelements play a very important role important role. They mainly affect metabolism. Without them it is impossible normal functioning each cell and the organism as a whole.

The cell consists of inorganic and organic matter. Among inorganic greatest number water. The relative amount of water in the cell is between 70 and 80%. Water is a universal solvent; all biochemical reactions in the cell take place in it. With the participation of water, thermoregulation is carried out. Substances that dissolve in water (salts, bases, acids, proteins, carbohydrates, alcohols, etc.) are called hydrophilic. Hydrophobic substances (fats and fat-like substances) do not dissolve in water. Other inorganic substances (salts, acids, bases, positive and negative ions) range from 1.0 to 1.5%.

Among organic substances, proteins (10–20%), fats or lipids (1–5%), carbohydrates (0.2–2.0%), and nucleic acids (1–2%) predominate. The content of low molecular weight substances does not exceed 0.5%.

A protein molecule is a polymer that consists of a large number of repeating units of monomers. Amino acid protein monomers (20 of them) are connected to each other by peptide bonds, forming a polypeptide chain (the primary structure of the protein). It twists into a spiral, forming, in turn, the secondary structure of the protein. Due to the specific spatial orientation of the polypeptide chain, the tertiary structure of the protein arises, which determines the specificity and biological activity of the protein molecule. Several tertiary structures combine with each other to form a quaternary structure.

Proteins perform essential functions. Enzymes are biological catalysts that increase speed chemical reactions hundreds of thousands of millions of times in a cell are proteins. Proteins, being part of all cellular structures, perform a plastic (construction) function. Cell movements are also carried out by proteins. They provide transport of substances into the cell, out of the cell and within the cell. The protective function of proteins (antibodies) is important. Proteins are one of the sources of energy. Carbohydrates are divided into monosaccharides and polysaccharides. The latter are built from monosaccharides, which, like amino acids, are monomers. Among the monosaccharides in the cell, the most important are glucose, fructose (contains six carbon atoms) and pentose (five carbon atoms). Pentoses are part of nucleic acids. Monosaccharides are highly soluble in water. Polysaccharides are poorly soluble in water (in animal cells glycogen, in plant cells - starch and cellulose. Carbohydrates are a source of energy, complex carbohydrates combined with proteins (glycoproteins), fats (glycolipids) are involved in the formation cell surfaces and cell interactions.

Lipids include fats and fat-like substances. Fat molecules are built from glycerol and fatty acids. Fat-like substances include cholesterol, some hormones, and lecithin. Lipids, which are the main components of cell membranes, thereby perform a construction function. Lipids - the most important sources energy. So, if with complete oxidation of 1 g of protein or carbohydrates 17.6 kJ of energy is released, then with complete oxidation of 1 g of fat - 38.9 kJ. Lipids carry out thermoregulation and protect organs (fat capsules).

DNA and RNA

Nucleic acids are polymer molecules formed by nucleotide monomers. A nucleotide consists of a purine or pyrimidine base, a sugar (pentose) and a residue phosphoric acid. In all cells, there are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), which differ in the composition of bases and sugars.

Spatial structure of nucleic acids:

(according to B. Alberts et al., with modification). I - RNA; II - DNA; ribbons - sugar phosphate backbones; A, C, G, T, U are nitrogenous bases, the lattices between them are hydrogen bonds.

DNA molecule

A DNA molecule consists of two polynucleotide chains twisted around one another in the form of a double helix. The nitrogenous bases of both chains are connected to each other by complementary hydrogen bonds. Adenine combines only with thymine, and cytosine - with guanine (A - T, G - C). DNA contains genetic information that determines the specificity of the proteins synthesized by the cell, that is, the sequence of amino acids in the polypeptide chain. DNA transmits by inheritance all the properties of a cell. DNA is found in the nucleus and mitochondria.

RNA molecule

An RNA molecule is formed by one polynucleotide chain. There are three types of RNA in cells. Informational, or messenger RNA tRNA (from the English messenger - “intermediary”), which transfers information about the nucleotide sequence of DNA to ribosomes (see below). Transfer RNA (tRNA), which carries amino acids to ribosomes. Ribosomal RNA (rRNA), which is involved in the formation of ribosomes. RNA is found in the nucleus, ribosomes, cytoplasm, mitochondria, and chloroplasts.

Composition of nucleic acids:

All cellular life forms on earth can be divided into two superkingdoms based on the structure of their constituent cells - prokaryotes (prenuclear) and eukaryotes (nuclear). Prokaryotic cells are simpler in structure; apparently, they arose earlier in the process of evolution. Eukaryotic cells are more complex and arose later. The cells that make up the human body are eukaryotic.

Despite the variety of forms, the organization of cells of all living organisms is subject to common structural principles.

Prokaryotic cell

Eukaryotic cell

Structure of a eukaryotic cell

Surface complex of an animal cell

Consists of glycocalyx, plasma membranes and the cortical layer of cytoplasm located underneath. The plasma membrane is also called plasmalemma, the outer membrane of the cell. This is a biological membrane, about 10 nanometers thick. Provides primarily a delimiting function in relation to the environment external to the cell. In addition, she performs transport function. The cell does not waste energy to maintain the integrity of its membrane: the molecules are held together according to the same principle by which fat molecules are held together - it is thermodynamically more advantageous for the hydrophobic parts of the molecules to be located in close proximity to each other. The glycocalyx is molecules of oligosaccharides, polysaccharides, glycoproteins and glycolipids “anchored” in the plasmalemma. The glycocalyx performs receptor and marker functions. The plasma membrane of animal cells mainly consists of phospholipids and lipoproteins interspersed with protein molecules, in particular surface antigens and receptors. In the cortical (adjacent to the plasma membrane) layer of the cytoplasm there are specific cytoskeletal elements - actin microfilaments ordered in a certain way. The main and most important function of the cortical layer (cortex) is pseudopodial reactions: ejection, attachment and contraction of pseudopodia. In this case, the microfilaments are rearranged, lengthened or shortened. The shape of the cell (for example, the presence of microvilli) also depends on the structure of the cytoskeleton of the cortical layer.

Cytoplasmic structure

The liquid component of the cytoplasm is also called cytosol. Under a light microscope, it seemed that the cell was filled with something like liquid plasma or sol, in which the nucleus and other organelles “floated”. Actually this is not true. The internal space of a eukaryotic cell is strictly ordered. The movement of organelles is coordinated with the help of specialized transport systems, the so-called microtubules, which serve as intracellular “roads” and special proteins dyneins and kinesins, which play the role of “motors”. Individual protein molecules also do not diffuse freely throughout the entire intracellular space, but are directed to the necessary compartments using special signals on their surface, recognized by the cell’s transport systems.

Endoplasmic reticulum

In a eukaryotic cell, there is a system of membrane compartments (tubes and cisterns) passing into each other, which is called the endoplasmic reticulum (or endoplasmic reticulum, ER or EPS). That part of the ER, to the membranes of which ribosomes are attached, is referred to as granular(or rough) endoplasmic reticulum, protein synthesis occurs on its membranes. Those compartments that do not have ribosomes on their walls are classified as smooth(or agranular) ER, which takes part in lipid synthesis. The internal spaces of the smooth and granular ER are not isolated, but pass into each other and communicate with the lumen of the nuclear envelope.

Golgi apparatus

Core

Cytoskeleton

Centrioles

Mitochondria

Comparison of pro- and eukaryotic cells

Most important difference Eukaryotes were distinguished from prokaryotes by the presence of a formed nucleus and membrane organelles for a long time. However, by the 1970-1980s. it became clear that this was only a consequence of deeper differences in the organization of the cytoskeleton. For some time it was believed that the cytoskeleton is characteristic only of eukaryotes, but in the mid-1990s. proteins homologous to the main proteins of the cytoskeleton of eukaryotes have also been discovered in bacteria.

It is the presence of a specifically structured cytoskeleton that allows eukaryotes to create a system of mobile internal membrane organelles. In addition, the cytoskeleton allows endo- and exocytosis to occur (it is assumed that it was thanks to endocytosis that intracellular symbionts, including mitochondria and plastids, appeared in eukaryotic cells). Another important function of the eukaryotic cytoskeleton is to ensure division of the nucleus (mitosis and meiosis) and body (cytotomy) of the eukaryotic cell (the division of prokaryotic cells is organized more simply). Differences in the structure of the cytoskeleton also explain other differences between pro- and eukaryotes - for example, the constancy and simplicity of the forms of prokaryotic cells and the significant diversity of shape and the ability to change it in eukaryotic cells, as well as the relatively large size of the latter. Thus, the sizes of prokaryotic cells average 0.5-5 microns, the sizes of eukaryotic cells average from 10 to 50 microns. In addition, only among eukaryotes are there truly giant cells, such as the massive eggs of sharks or ostriches (in a bird egg, the entire yolk is one huge egg), neurons of large mammals, the processes of which, strengthened by the cytoskeleton, can reach tens of centimeters in length.

Anaplasia

The destruction of cellular structure (for example, in malignant tumors) is called anaplasia.

History of cell discovery

The first person to see cells was the English scientist Robert Hooke (known to us thanks to Hooke's law). In the year, trying to understand why the cork tree floats so well, Hooke began to examine thin sections of cork using a microscope he had improved. He discovered that the cork was divided into many tiny cells, which reminded him of monastery cells, and he called these cells cells (in English cell means “cell, cell, cage”). In the same year, the Dutch master Anton van Leeuwenhoek (-) used a microscope for the first time to see “animals” - moving living organisms - in a drop of water. Thus, already by early XVIII centuries, scientists knew that under high magnification plants have a cellular structure, and they saw some organisms that were later called unicellular. However, the cellular theory of the structure of organisms was formed only in the middle of the 19th century, after more powerful microscopes appeared and methods for fixing and staining cells were developed. One of its founders was Rudolf Virchow, but his ideas contained a number of errors: for example, he assumed that cells were weakly connected to each other and each existed “on its own.” Only later was it possible to prove the integrity of the cellular system.

Cells are the basic units from which all living organisms are built. To a modern reader who considers such a statement trivial, it may seem surprising that the recognition of the universality of the cellular structure of all living things occurred just over 100 years ago.

For the first time cell theory was formulated in 1839 by botanist Matthias Jakob Schleiden and zoologist Theodor Schwann; these researchers came to it independently of each other, as a result of studying plant and animal tissues. Soon after, in 1859, Rudolf Virchow confirmed the exclusive role of the cell as the container of “living matter”, showing that all cells come only from pre-existing cells: “Omnis cellula e cellula” (each cell from a cell). Since cells are very concrete objects that are easy to observe, after all these discoveries, the experimental study of the cell replaced theoretical discussions about “life” and dubious scientific research, based on such vague concepts as the concept of "protoplasm".

Over the next hundred years, scientists who studied the cell approached this object from two completely different positions. Cytologists, using continually improving microscopes, continued to develop the microscopic and submicroscopic anatomy of an intact cell. Starting with the idea of ​​a cell as a lump of jelly-like substance in which nothing could be distinguished,

in addition to the gelatinous cytoplasm covering it outside the shell and located in the center of the nucleus, they were able to show that the cell is a complex structure differentiated into various organelles, each of which is adapted to perform one or another vital function. With the help electron microscope cytologists began to distinguish the individual structures involved in performing these functions on molecular level. Thanks to this, in recent times the research of cytologists has merged with the work of biochemists, who began with the ruthless destruction of the delicate structures of the cell; By studying the chemical activity of the material obtained as a result of such destruction, biochemists were able to decipher some of the biochemical reactions occurring in the cell that underlie life processes, including the processes of creating the very substance of the cell.

It is the current intersection of these two areas of cell research that has necessitated devoting an entire issue of Scientific American to the living cell. Nowadays the cytologist tries to explain at the molecular level what he sees with the help of his various microscopes; thus the cytologist becomes a "molecular biologist". The biochemist turns into a “biochemical cytologist” who studies equally both the structure and biochemical activity of the cell. The reader will be able to see that morphological or biochemical research methods alone do not give us the opportunity to penetrate into the secrets of the structure and function of the cell. In order to achieve success, it is necessary to combine both research methods. However, the understanding of life phenomena achieved through the study of cells fully confirmed the opinion of 19th century biologists, who argued that living matter has a cellular structure, just as molecules are built from atoms.

Discussion functional anatomy living cell, perhaps, we should start with the fact that in nature there is no certain typical cell. We know a wide variety of single-celled organisms, and brain cells or muscle cells differ just as much from each other in their structure as in their functions. However, despite all their diversity, they are all cells - they all have a cell membrane, a cytoplasm containing various organelles, and in the center of each of them there is a nucleus. In addition to a certain structure, all cells have a number of interesting common functional features. First of all, all cells are capable of using and converting energy, which is ultimately based on the use of solar energy by the cells of green plants and its conversion into the energy of chemical bonds. Various specialized cells are capable of converting the energy contained in chemical bonds into electrical and mechanical energy and even back into visible light energy. The ability to convert energy has a very important for all cells, since it gives them the opportunity to maintain the constancy of their internal environment and the integrity of their structure.

A living cell is different from its surroundings inanimate nature because it contains very large and extremely complex molecules. These molecules are so unique that, having encountered them in the inanimate world, we can always be sure that these are the remains of dead cells. IN early periods During the development of the Earth, when life first arose on it, there apparently was a spontaneous synthesis of complex macromolecules from smaller molecules. In modern conditions, the ability to synthesize large molecules from simpler substances is one of the main distinctive features living cells.

Proteins are among these macromolecules. In addition to the fact that proteins constitute the bulk of the “solid” matter of the cell, many of them (enzymes) have catalytic properties; this means that they are capable of greatly increasing the rate of chemical reactions occurring in the cell, in particular the rate of reactions associated with energy conversion. The synthesis of proteins from simpler units - amino acids, of which there are more than 20, is regulated by deoxyribonucleic and ribonucleic acids (DNA and RNA); DNA and RNA are perhaps the most complex of all macromolecules in a cell. For recent years and even months it has been established that DNA located in the cell nucleus directs the synthesis of RNA, which is contained both in the nucleus and in the cytoplasm. RNA, in turn, provides a specific sequence of amino acids in protein molecules. The role of DNA and RNA can be compared to the role of an architect and a civil engineer, as a result of whose joint efforts a beautiful house grows from a pile of bricks, stones and tiles.

At one stage or another in life, every cell divides: the mother cell grows and gives rise to two daughter cells as a result of very fine process, described in the article by D. Maziy. Still on the threshold of the 20th century. biologists understood that the most important feature of this process was the uniform distribution between daughter cells of special bodies contained in the nucleus of the mother cell; these bodies were called chromosomes, since it turned out that they were stained with certain dyes. It has been suggested that chromosomes serve as carriers of heredity; Thanks to the accuracy with which their self-reproduction and distribution occurs, they transmit to the daughter cells all the properties of the mother cell. Modern biochemistry has shown that chromosomes consist mainly of DNA, and one of important tasks molecular biology is to find out how genetic information is encoded in the structure of this macromolecule.

In addition to the ability to convert energy, biosynthesis and reproduction through self-reproduction and division, the cells of highly organized animals and plants have other features due to which they are adapted to the complex and coordinated activity that is the life of an organism. Development from a fertilized egg, which is one single cell, multicellular organism occurs not only as a result of cell division, but also as a result of differentiation of daughter cells into various specialized types, from which different tissues are formed. In many cases, after differentiation and specialization, cells stop dividing; there is a kind of antagonism between differentiation and growth by cell division.

In an adult organism, the ability to reproduce and maintain the population of the species at a certain level depends on the egg and sperm. These cells, called gametes, arise, like all other cells of the body, during the process of fragmentation of a fertilized egg and subsequent differentiation. However, in all those parts of the adult body where wear and tear of cells constantly occurs (in the skin, intestines, etc.) bone marrow where they are produced shaped elements blood), cell division remains a very common event.

For embryonic development Differentiating cells of the same type exhibit the ability to recognize each other. Cells belonging to the same type and similar to each other combine to form a tissue that is inaccessible to cells of all other types. In this mutual attraction and repulsion of cells, the main role apparently belongs to the cell membrane. This membrane is, in addition, one of the main cellular components with which the function of muscle cells is associated (providing the body with the ability to move), nerve cells(creating connections necessary for the coordinated activity of the body) and sensory cells (perceiving irritations from the outside and from the inside).

Although in nature there is no cell that could? considered typical, we think it would be useful to create some kind of model of it, a so-called “collective” cell, which would combine morphological characteristics expressed to one degree or another in all cells.

Even in a cell membrane with a thickness of some 100 angstroms (1 angstrom is equal to one ten-millionth of a millimeter), which under a conventional microscope looks like just a boundary line, electron microscopic examination reveals a certain structure. True, we still know almost nothing about this structure, but the very presence of cell membrane complex structure agrees well with everything we know about its functional properties. For example, the membranes of red blood cells and nerve cells are able to distinguish sodium ions from potassium ions, although these ions have similar sizes and the same electric charge. The membrane of these cells helps potassium ions penetrate the cell, but it “resists” sodium ions, and this does not depend on permeability alone; in other words, the membrane has the ability to “actively transport ions.” In addition, the cell membrane mechanically draws large molecules and macroscopic particles into the cell. The electron microscope also made it possible to penetrate into the fine structure of organelles located in the cytoplasm, which in a conventional microscope look like grains. The most important organelles are the chloroplasts of green plant cells and the mitochondria, found in both animal and plant cells. These organelles are the “powerhouses” of all life on Earth. Their fine structure is adapted to a specific function: in chloroplasts - to bind the energy of sunlight during photosynthesis, and in mitochondria - to extract energy (embedded in the chemical bonds of nutrients entering the cell) in the process of oxidation and respiration. These “power stations” supply the energy necessary for various processes occurring in the cell, so to speak, in “convenient packaging” - in the form of the energy of phosphate bonds of one chemical compound, adenosine triphosphate (ATP).

An electron microscope makes it possible to clearly distinguish mitochondria with their complex fine structure from other bodies of approximately the same size - from lysosomes. As de Duve showed, lysosomes contain digestive enzymes that break down large molecules, such as fats, proteins and nucleic acids, into smaller components that can be oxidized by mitochondrial enzymes. The membrane of lysosomes isolates the digestive enzymes contained in these bodies from the rest of the cytoplasm. Rupture of the membrane and release of enzymes contained in lysosomes quickly leads to cell lysis (dissolution).

The cytoplasm contains many other inclusions that are less widespread in cells various types. Among them, centrosomes and kinetosomes are of particular interest. Centrosomes can be seen with a regular microscope only at the time of cell division; they play a very important role, forming the poles of the spindle - the apparatus that pulls chromosomes between two daughter cells. As for kinetosomes, they can be found only in those cells that move with the help of special cilia or flagella; At the base of each cilium or flagellum lies a kinetosome. Both centrosomes and kinetosomes are capable of self-reproduction: each pair of centrosomes during cell division gives rise to another pair of these bodies; Whenever a new cilium appears on the cell surface, it receives a kinetosome, resulting from the self-duplication of one of the existing kinetosomes. In the past, some cytologists have expressed the opinion that the structure of these two organelles is largely similar, despite the fact that their functions are completely different. Electron microscopic studies confirmed this assumption. Each organelle consists of 11 fibers; two of them are located in the center, and the remaining nine are located on the periphery. This is exactly how all cilia and all flagella are arranged. The exact purpose of this structure is unknown, but it is undoubtedly associated with the contractility of cilia and flagella. It is possible that the same principle of “monomolecular muscle” underlies the action of the kinetosome and centrosome, which have completely different functions.

The electron microscope made it possible to confirm another assumption of cytologists of past years, namely the assumption of the existence of a “cytoskeleton” - an invisible structure of the cytoplasm. In most cells, using an electron microscope, one can detect a complex system of internal membranes that is invisible when observed with a conventional microscope. Some of these membranes have a smooth surface, while others have one of the surfaces rough due to the tiny granules covering it. IN different cells these membrane systems are developed in to varying degrees; in amoeba they are very simple, and in specialized cells in which intensive protein synthesis occurs (for example, in the cells of the liver or pancreas), they are very highly branched and distinguished by significant granularity.

Electron microscopy specialists evaluate all these observations differently. The most widely accepted point of view is that of K. Porter, who proposed the name “endoplasmic reticulum” for this membrane system; in his opinion, movement occurs through the network of tubules formed by the membranes various substances from the outer cell membrane to the nuclear membrane. Some researchers consider the inner membrane to be a continuation of the outer membrane; According to these authors, thanks to the deep depressions in the inner membrane, the surface of contact of the cell with the liquid washing it greatly increases. If the role of the membrane is really so important, then we would expect that the cell has a mechanism that allows it to continuously create a new membrane. J. Palad suggested that such a mechanism is the mysterious Golgi apparatus, first discovered by the Italian cytologist C. Golgi at the end of the last century. An electron microscope made it possible to establish that the Golgi apparatus consists of a smooth membrane, which often serves as a continuation of the endoplasmic reticulum.

The nature of the granules covering the “inner” surface of the membrane is beyond any doubt. These granules are especially pronounced in cells that synthesize large amounts of protein. As T. Kaspersson and the author of this article showed about 20 years ago, such cells are different high content RNA. Recent studies have revealed that these granules are extremely rich in RNA and, accordingly, are very active in protein synthesis. Therefore, they are called ribosomes.

The inner boundary of the cytoplasm is formed by the membrane surrounding the cell nucleus. There is still a lot of disagreement about the structure of this membrane, which we observe in an electron microscope. In appearance, it is a double film, in the outer layer of which there are rings or holes opening towards the cytoplasm. Some researchers consider these rings to be pores through which large molecules pass from the cytoplasm to the nucleus or from the nucleus to the cytoplasm. Since the outer layer of the membrane is often in close contact with the endoplasmic reticulum, it has also been suggested that the nuclear envelope is involved in the formation of the membranes of this reticulum. It is also possible that fluids flowing through the tubules of the endoplasmic reticulum accumulate in the space between the two layers of the nuclear envelope.

The nucleus contains the most important structures of the cell - chromatin threads, which contain all the DNA contained in the cell. When a cell is at rest (that is, during the period of growth between two divisions), chromatin is scattered throughout the nucleus. Thanks to this, DNA acquires the maximum surface of contact with other substances of the nucleus, which probably serve as material for the construction of RNA molecules and for self-reproduction. As a cell prepares for division, chromatin is assembled and compacted to form chromosomes, after which it is evenly distributed between both daughter cells.

Nucleoli are not as elusive as chromatin; these spherical bodies are clearly visible in the nucleus when observed under a conventional microscope. An electron microscope allows us to see that the nucleolus is filled with small granules, similar to the ribosomes of the cytoplasm. The nucleoli are rich in RNA and appear to be active centers for protein and RNA synthesis. To complete the description of the functional anatomy of the cell, we note that chromatin and nucleoli float in an amorphous protein-like substance - nuclear juice.

Creating a modern picture of the structure of the cell required the development of sophisticated equipment and more advanced research methods. The ordinary light microscope continues to be an important tool today. However, for research internal structure Cells using this microscope usually have to kill the cell and stain it with various dyes that selectively reveal its main structures. To view these structures in an active state in a living cell, various microscopes have been developed, including phase contrast, interference, polarization, and fluorescence; all these microscopes are based on the use of light. IN lately The electron microscope becomes the main research tool for cytologists. The use of an electron microscope “is complicated, however, by the need to expose the objects under study complex processes processing and recording, which inevitably entails a violation of the original paintings associated with various distortions and artifacts. Nevertheless, we are making progress and are getting closer to examining a living cell at high magnification.

The history of the development of technical equipment in biochemistry is no less remarkable. The creation of centrifuges with ever-increasing rotation speeds makes it possible to divide the contents of the cell into ever larger and larger number separate factions. These fractions are further separated and subdivided using chromatography and electrophoresis. Classic methods analysis has now been able to be adapted to study quantities and volumes 1000 times smaller than those that could be determined previously. Scientists have acquired the ability to measure the respiration rate of several amoebas or several eggs sea ​​urchin or determine the content of enzymes in them. Finally, autoradiography, a method that uses radioactive tracers, allows one to observe at the subcellular level the dynamic processes occurring in an intact living cell.

All other articles in this collection are devoted to the successes achieved through the merging of these two most important directions in cell research, and to the further prospects that open up for biology. In conclusion, it would seem useful to me to show how a combination of cytological and biochemical approaches is used to solve one problem - the problem of the role of the nucleus in the life of the cell. Removing the nucleus from a unicellular organism does not entail the immediate death of the cytoplasm. If you divide the amoeba into two halves, leaving the nucleus in one of them, and subject both halves to starvation, then both of them will live for about two weeks; in a single-celled protozoan, the slipper, the beating of cilia can be observed for several days after removal of the nucleus; Nuclear-free fragments of the giant unicellular algae acetabularia live for several months and are even capable of quite noticeable regeneration. Thus, many of the basic life processes of a cell, including (in the case of Acetabularia) processes of growth and differentiation, can occur with complete absence genes and DNA. Nuclear-free fragments of acetabularia are capable, for example, of synthesizing proteins and even specific enzymes, although it is known that protein synthesis is regulated by genes. However, the ability of these fragments to synthesize gradually fades. Based on these data, we can conclude that in the nucleus, under the influence of DNA, some substance is formed, which is released into the cytoplasm, where it is gradually used. From such experiments, carried out with the simultaneous use of cytological and biochemical methods, a number of important conclusions emerge.

Firstly, the nucleus should be considered the main center for the synthesis of nucleic acids (both DNA and RNA). Secondly, nuclear RNA (or part of it) enters the cytoplasm, where it plays the role of an intermediary, transmitting genetic information from DNA to the cytoplasm. Finally, experiments show that the cytoplasm, and in particular ribosomes, serves as the main arena for the synthesis of specific proteins such as enzymes. It should be added that the possibility of independent RNA synthesis in the cytoplasm cannot be considered excluded and that such synthesis can be detected in nuclear-free fragments of acetabularia under appropriate conditions.

This brief outline of current data clearly shows that the cell is not only a morphological but also a physiological unit.

The most valuable thing a person has is his own life and the lives of his loved ones. The most valuable thing on Earth is life in general. And at the basis of life, at the basis of all living organisms, are cells. We can say that life on Earth has a cellular structure. That's why it's so important to know how cells are structured. The structure of cells is studied by cytology - the science of cells. But the idea of ​​cells is necessary for all biological disciplines.

What is a cell?

Definition of the concept

Cell is a structural, functional and genetic unit of all living things, containing hereditary information, consisting of a membrane membrane, cytoplasm and organelles, capable of maintenance, exchange, reproduction and development. © Sazonov V.F., 2015. © kineziolog.bodhy.ru, 2015..

This definition of a cell, although brief, is quite complete. It reflects 3 sides of the cell’s universality: 1) structural, i.e. as a structural unit, 2) functional, i.e. as a unit of activity, 3) genetic, i.e. as a unit of heredity and generational change. An important characteristic of a cell is the presence of hereditary information in it in the form of nucleic acid - DNA. The definition also reflects the most important feature of the cell structure: the presence of an outer membrane (plasmolemma), separating the cell and its environment. AND, finally, 4 most important signs of life: 1) maintaining homeostasis, i.e. constancy of the internal environment in conditions of its constant renewal, 2) exchange with the external environment of matter, energy and information, 3) the ability to reproduce, i.e. to self-reproduction, reproduction, 4) the ability to develop, i.e. to growth, differentiation and morphogenesis.

A shorter but incomplete definition: Cell is the elementary (smallest and simplest) unit of life.

A more complete definition of a cell:

Cell is an ordered, structured system of biopolymers bounded by an active membrane, forming the cytoplasm, nucleus and organelles. This biopolymer system participates in a single set of metabolic, energy and information processes that maintain and reproduce the entire system as a whole.

Textile is a collection of cells similar in structure, function and origin, jointly performing common functions. In humans, in the four main groups of tissues (epithelial, connective, muscle and nervous), there are about 200 various types specialized cells [Faler D.M., Shields D. Molecular biology of cells: A guide for doctors. / Per. from English - M.: BINOM-Press, 2004. - 272 p.].

Tissues, in turn, form organs, and organs form organ systems.

A living organism begins from a cell. There is no life outside the cell; outside the cell only the temporary existence of life molecules is possible, for example, in the form of viruses. But for active existence and reproduction, even viruses need cells, even foreign ones.

Cell structure

The figure below shows the structure diagrams of 6 biological objects. Analyze which of them can be considered cells and which cannot, according to two options for defining the concept “cell”. Present your answer in the form of a table:

Cell structure under an electron microscope

Membrane

The most important universal structure of the cell is cell membrane (synonym: plasmalemma), covering the cell in the form of a thin film. The membrane regulates the relationship between the cell and its environment, namely: 1) it partially separates the contents of the cell from the external environment, 2) connects the contents of the cell with the external environment.

Core

The second most important and universal cellular structure is the nucleus. It is not present in all cells, unlike the cell membrane, which is why we put it in second place. The nucleus contains chromosomes containing double strands of DNA (deoxyribonucleic acid). Sections of DNA are templates for the construction of messenger RNA, which in turn serve as templates for the construction of all cell proteins in the cytoplasm. Thus, the nucleus contains, as it were, “blueprints” for the structure of all the proteins of the cell.

Cytoplasm

It's semi-liquid internal environment cells divided into compartments by intracellular membranes. It usually has a cytoskeleton to maintain a certain shape and is in constant motion. The cytoplasm contains organelles and inclusions.

In third place we can put all other cellular structures that can have their own membrane and are called organelles.

Organelles are permanent, necessarily present cell structures that perform specific functions and have a specific structure. Based on their structure, organelles can be divided into two groups: membrane organelles, which necessarily include membranes, and non-membrane organelles. In turn, membrane organelles can be single-membrane - if they are formed by one membrane and double-membrane - if the shell of the organelles is double and consists of two membranes.

Inclusions

Inclusions are non-permanent structures of the cell that appear in it and disappear during the process of metabolism. There are 4 types of inclusions: trophic (with a supply of nutrients), secretory (containing secretions), excretory (containing substances “to be released”) and pigmentary (containing pigments - coloring substances).

Cellular structures, including organelles ( )

Inclusions . They are not classified as organelles. Inclusions are non-permanent structures of the cell that appear in it and disappear during the process of metabolism. There are 4 types of inclusions: trophic (with a supply of nutrients), secretory (containing secretions), excretory (containing substances “to be released”) and pigmentary (containing pigments - coloring substances).

1. (plasmolemma).
2. Nucleus with nucleolus .
3. Endoplasmic reticulum : rough (granular) and smooth (agranular).
4. Golgi complex (apparatus) .
5. Mitochondria .
6. Ribosomes .
7. Lysosomes . Lysosomes (from the gr. lysis - “decomposition, dissolution, decay” and soma - “body”) are vesicles with a diameter of 200-400 microns.
8. Peroxisomes . Peroxisomes are microbodies (vesicles) 0.1-1.5 µm in diameter, surrounded by a membrane.
9. Proteasomes . Proteasomes are special organelles for breaking down proteins.
10. Phagosomes .
11. Microfilaments . Each microfilament is a double helix of globular actin protein molecules. Therefore, the actin content even in non-muscle cells reaches 10% of all proteins.
12. Intermediate filaments . They are a component of the cytoskeleton. They are thicker than microfilaments and have a tissue-specific nature:
13. Microtubules . Microtubules form a dense network in the cell. The microtubule wall consists of a single layer of globular subunits of the protein tubulin. A cross section shows 13 of these subunits forming a ring.
14. Cell center .
15. Plastids .
16. Vacuoles . Vacuoles are single-membrane organelles. They are membrane “containers”, bubbles filled with aqueous solutions of organic and inorganic substances.
17. Cilia and flagella (special organelles) . They consist of 2 parts: a basal body located in the cytoplasm and an axoneme - a growth above the surface of the cell, which is covered on the outside with a membrane. Provide cell movement or movement of the environment above the cell.

The cell is the basic structural and functional unit of all living organisms, except viruses. It has a specific structure, including many components that perform specific functions.

What science studies the cell?

Everyone knows that the science of living organisms is biology. The structure of a cell is studied by its branch - cytology.

What does a cell consist of?

This structure consists of a membrane, cytoplasm, organelles, or organelles, and a nucleus (absent in prokaryotic cells). The structure of cells of organisms belonging to different classes differs slightly. Significant differences are observed between the cell structure of eukaryotes and prokaryotes.

Plasma membrane

The membrane plays a very important role - it separates and protects the contents of the cell from the external environment. It consists of three layers: two protein layers and a middle phospholipid layer.

Cell wall

Another structure that protects the cell from exposure external factors, located on top plasma membrane. Present in the cells of plants, bacteria and fungi. In the first it consists of cellulose, in the second - from murein, in the third - from chitin. In animal cells, a glycocalyx is located on top of the membrane, which consists of glycoproteins and polysaccharides.

Cytoplasm

It represents the entire cell space limited by the membrane, with the exception of the nucleus. The cytoplasm includes organelles that perform the main functions responsible for the life of the cell.

Organelles and their functions

The structure of a cell of a living organism involves a number of structures, each of which performs a specific function. They are called organelles, or organelles.

Mitochondria

They can be called one of the most important organelles. Mitochondria are responsible for the synthesis of energy necessary for life. In addition, they are involved in the synthesis of certain hormones and amino acids.

Energy in mitochondria is produced due to the oxidation of ATP molecules, which occurs with the help of a special enzyme called ATP synthase. Mitochondria are round or rod-shaped structures. Their number in an animal cell, on average, is 150-1500 pieces (this depends on its purpose). They consist of two membranes and a matrix - a semi-liquid mass that fills the internal space of the organelle. The main components of the shells are proteins; phospholipids are also present in their structure. The space between the membranes is filled with liquid. The mitochondrial matrix contains grains that accumulate certain substances, such as magnesium and calcium ions, necessary for energy production, and polysaccharides. Also, these organelles have their own protein biosynthesis apparatus, similar to that of prokaryotes. It consists of mitochondrial DNA, a set of enzymes, ribosomes and RNA. The structure of a prokaryotic cell has its own characteristics: it does not contain mitochondria.

Ribosomes

These organelles are composed of ribosomal RNA (rRNA) and proteins. Thanks to them, translation is carried out - the process of protein synthesis on an mRNA (messenger RNA) matrix. One cell can contain up to ten thousand of these organelles. Ribosomes consist of two parts: small and large, which combine directly in the presence of mRNA.

Ribosomes, which are involved in the synthesis of proteins necessary for the cell itself, are concentrated in the cytoplasm. And those with the help of which proteins are produced that are transported outside the cell are located on the plasma membrane.

Golgi complex

It is present only in eukaryotic cells. This organelle consists of dictosomes, the number of which is usually approximately 20, but can reach several hundred. The Golgi apparatus is included in the cell structure of only eukaryotic organisms. It is located near the nucleus and performs the function of synthesis and storage of certain substances, for example, polysaccharides. Lysosomes are formed in it, which will be discussed below. This organelle is also part excretory system cells. Dictosomes are presented in the form of stacks of flattened disc-shaped cisterns. At the edges of these structures, vesicles form, containing substances that need to be removed from the cell.

Lysosomes

These organelles are small vesicles containing a set of enzymes. Their structure has one membrane covered with a layer of protein on top. The function performed by lysosomes is the intracellular digestion of substances. Thanks to the enzyme hydrolase, with the help of these organelles, fats, proteins, carbohydrates, and nucleic acids are broken down.

Endoplasmic reticulum (reticulum)

The cell structure of all eukaryotic cells also implies the presence of EPS (endoplasmic reticulum). The endoplasmic reticulum consists of tubes and flattened cavities with a membrane. This organelle comes in two types: rough and smooth network. The first is distinguished by the fact that ribosomes are attached to its membrane, the second does not have this feature. The rough endoplasmic reticulum performs the function of synthesizing proteins and lipids that are required for the formation of the cell membrane or for other purposes. Smooth takes part in the production of fats, carbohydrates, hormones and other substances, except proteins. The endoplasmic reticulum also performs the function of transporting substances throughout the cell.

Cytoskeleton

It consists of microtubules and microfilaments (actin and intermediate). The components of the cytoskeleton are polymers of proteins, mainly actin, tubulin or keratin. Microtubules serve to maintain the shape of the cell; they form organs of movement in simple organisms, such as ciliates, Chlamydomonas, euglena, etc. Actin microfilaments also play the role of a scaffold. In addition, they are involved in the process of organelle movement. Intermediates in different cells are built from different proteins. They maintain the shape of the cell and also secure the nucleus and other organelles in a constant position.

Cell center

Consists of centrioles, which have the shape of a hollow cylinder. Its walls are formed from microtubules. This structure is involved in the process of division, ensuring the distribution of chromosomes between daughter cells.

Core

In eukaryotic cells it is one of the most important organelles. It stores DNA, which encrypts information about the entire organism, its properties, proteins that must be synthesized by the cell, etc. It consists of a shell that protects the genetic material, nuclear sap (matrix), chromatin and nucleolus. The shell is formed from two porous membranes located at some distance from each other. The matrix is ​​represented by proteins; it forms a favorable environment inside the nucleus for storing hereditary information. The nuclear sap contains filamentous proteins that serve as support, as well as RNA. Also present here is chromatin, an interphase form of chromosome existence. During cell division, it turns from clumps into rod-shaped structures.

Nucleolus

This is a separate part of the nucleus responsible for the formation of ribosomal RNA.

Organelles found only in plant cells

Plant cells have some organelles that are not characteristic of any other organisms. These include vacuoles and plastids.

Vacuole

This is a kind of reservoir where reserve nutrients are stored, as well as waste products that cannot be removed due to the dense cell wall. It is separated from the cytoplasm by a specific membrane called the tonoplast. As the cell functions, individual small vacuoles merge into one large one - the central one.

Plastids

These organelles are divided into three groups: chloroplasts, leucoplasts and chromoplasts.

Chloroplasts

These are the most important organelles of a plant cell. Thanks to them, photosynthesis occurs, during which the cell receives the nutrients it needs. nutrients. Chloroplasts have two membranes: outer and inner; matrix - the substance that fills the internal space; own DNA and ribosomes; starch grains; grains. The latter consist of stacks of thylakoids with chlorophyll, surrounded by a membrane. It is in them that the process of photosynthesis occurs.

Leukoplasts

These structures consist of two membranes, a matrix, DNA, ribosomes and thylakoids, but the latter do not contain chlorophyll. Leukoplasts perform a reserve function, accumulating nutrients. They contain special enzymes that make it possible to obtain starch from glucose, which, in fact, serves as a reserve substance.

Chromoplasts

These organelles have the same structure as those described above, however, they do not contain thylakoids, but there are carotenoids that have a specific color and are located directly next to the membrane. It is thanks to these structures that flower petals are painted a certain color, allowing them to attract pollinating insects.