What is better for nature: to buy a live Christmas tree or an artificial one? This is a very complex question, which we will try to answer as simply as possible, based on the "carbon footprint" that a living and artificial tree leaves, that is, the amount of greenhouse gases emitted during the life cycle of a product. The Carbon Trust estimates that a two-meter-sized live Christmas tree leaves the equivalent of 16 kg of carbon dioxide if discarded after use, and only 3.5 kg if burned. Rotting causes the production of methane, which creates a much stronger greenhouse effect than carbon dioxide itself. According to the same data, a two-meter artificial Christmas tree leaves a "carbon footprint" of 40 kg of CO 2 . So if you choose this option, try to use this tree longer. Why do people hiccup? Not only humans hiccup: these convulsive spasms of the diaphragm from time to time torment all mammals and many other animals that use pulmonary respiration. Short and strong movements create sharp breaths that are interrupted by a sudden occlusion of the airways, creating the characteristic sound of hiccups. This involuntary reaction is supposed to help expel air from the stomach. But it can also be the result of accidental irritation of the vagus nerve, which passes through the same opening in the diaphragm as the esophagus. Therefore, hiccups can cause too hasty absorption of food. There is also a hypothesis that hiccups are a relic left to us from dizzyingly distant times. At any rate, amphibians breathe through very similar spasms that allow their gills to be washed over. Can you get a cold from your dog?
Unlikely: most cases of SARS occur due to rhinoviruses, which, as a rule, specialize in infecting a particular species. Animals have their cold strains, we have ours. On the other hand, the influenza virus is more flexible, and cases of transmission of "swine" or "bird" flu to humans are rare, but widely known. But bacteria - pathogens are much more universal, and streptococci or tubercle bacilli that cause angina can both infect your dog and become infected from it. Is it possible to harm a person by pointing a thousand laser pointers at him? The laser in the pointer can damage vision, but it is not felt by the skin. Usually these are systems with a power of less than 5 mW, according to GOST they must be marked with warning labels, but their sales are not limited. If you came up with the idea to destroy the enemy with laser pointers, then hundreds will not be enough. University of Texas physicist Rebecca Thompson has calculated that for a beam that enters the eye to be able to penetrate and damage the brain, it would take at least 1 kW of power, which means at least 200,000 pointers focused at one point. Theoretically, they can be placed on a large parabolic "dish", concentrating radiation on the victim. Which body cells do not have DNA?
Initially, DNA is present in all our cells, but during the adult stages of life, some of them lose the nucleus and the chromosomes contained in it. Thus, keratinized keratinocytes of the upper layers of the skin end their lives without a nucleus and major organelles. Platelets also do not have a nucleus - pieces of cytoplasm that have separated from megakaryocyte cells. The best-known example is the oxygen-carrying red blood cells, which, as a result, are drastically reduced in size and can move through thin capillaries. Mature erythrocytes do not even have mitochondria that could contain extranuclear DNA.
How do astronauts live on the ISS? Sunrises and sunsets on the ISS occur every hour and a half. The sun can no longer set a comfortable rhythm of sleep and wakefulness. But astronauts observe the same habitual cyclicality. 24 hours are broken down into 6.5 hours of working time, 2.5 hours of training on simulators, an hour for lunch, the rest is rest and sleep. Usually the rise is announced at 6:00, work begins at 8:00 and ends at 19:00, lights out at 21:30. Time is counted according to Greenwich Mean Time, that is, four hours behind Moscow. Why is Pluto not a planet?
Almost half a century after the first observation of Pluto, its dimensions remained exactly unknown. Only in 1978, when the satellite Charon was discovered, was it possible to determine the mass, and then the diameter of Pluto, which was only 2370 km. For comparison, the diameter of the Moon is 3475 km. In the Kuiper belt, where Pluto is located, there are many bodies comparable in size, and Eris is even heavier. The discovery of Eris in 2005 was the last straw: it was necessary either to rank it as a planet, and dozens of bodies similar to Pluto, or to exclude Pluto itself from their number. The decision was made in 2006: a solar system planet is now considered a body orbiting the Sun, not a satellite of one of the planets, massive enough to take a rounded shape and clear the vicinity of its orbit. Pluto, as well as Eris, Ceres and many others do not satisfy the third condition and do not reach full-fledged planets.
The biology of the cell in general terms is known to everyone from the school curriculum. We invite you to remember what you once studied, as well as discover something new about it. The name "cell" was proposed as early as 1665 by the Englishman R. Hooke. However, it was only in the 19th century that it began to be studied systematically. Scientists were interested, among other things, in the role of the cell in the body. They can be part of many different organs and organisms (eggs, bacteria, nerves, erythrocytes) or be independent organisms (protozoa). Despite all their diversity, there is much in common in their functions and structure.
Cell functions
All of them are different in form and often in function. Cells of tissues and organs of one organism can also differ quite strongly. However, the biology of the cell highlights the functions that are inherent in all their varieties. This is where protein synthesis always takes place. This process is controlled. A cell that does not synthesize proteins is essentially dead. A living cell is one whose components change all the time. However, the main classes of substances remain unchanged.
All processes in the cell are carried out using energy. These are nutrition, respiration, reproduction, metabolism. Therefore, a living cell is characterized by the fact that energy exchange takes place in it all the time. Each of them has a common most important property - the ability to store energy and spend it. Other functions include division and irritability.
All living cells can respond to chemical or physical changes in their environment. This property is called excitability or irritability. In cells, when excited, the rate of decay of substances and biosynthesis, temperature, and oxygen consumption change. In this state, they perform the functions peculiar to them.
Cell structure
Its structure is quite complex, although it is considered the simplest form of life in such a science as biology. Cells are located in the intercellular substance. It provides them with breathing, nutrition and mechanical strength. The nucleus and cytoplasm are the main components of every cell. Each of them is covered with a membrane, the building element for which is a molecule. Biology has established that the membrane is made up of many molecules. They are arranged in several layers. Thanks to the membrane, substances penetrate selectively. In the cytoplasm are organelles - the smallest structures. These are the endoplasmic reticulum, mitochondria, ribosomes, cell center, Golgi complex, lysosomes. You will better understand what cells look like by studying the drawings presented in this article.
Membrane
Endoplasmic reticulum
This organoid was named so because it is located in the central part of the cytoplasm (from the Greek the word "endon" is translated as "inside"). EPS is a very branched system of vesicles, tubules, tubules of various shapes and sizes. They are separated from membranes.
There are two types of EPS. The first is granular, which consists of tanks and tubules, the surface of which is dotted with granules (grains). The second type of EPS is agranular, that is, smooth. Grans are ribosomes. Curiously, granular EPS is mainly observed in the cells of animal embryos, while in adult forms it is usually agranular. Ribosomes are known to be the site of protein synthesis in the cytoplasm. Based on this, it can be assumed that granular EPS occurs mainly in cells where active protein synthesis occurs. The agranular network is believed to be represented mainly in those cells where active lipid synthesis occurs, that is, fats and various fat-like substances.
Both types of EPS not only take part in the synthesis of organic substances. Here these substances accumulate and are also transported to the necessary places. EPS also regulates the exchange of substances that occurs between the environment and the cell.
Ribosomes
Mitochondria
Energy organelles include mitochondria (pictured above) and chloroplasts. Mitochondria are the original powerhouses of every cell. It is in them that energy is extracted from nutrients. Mitochondria have a variable shape, but most often they are granules or filaments. Their number and size are not constant. It depends on the functional activity of a particular cell.
If we consider an electron micrograph, we can see that mitochondria have two membranes: inner and outer. The inner one forms outgrowths (cristae) covered with enzymes. Due to the presence of cristae, the total surface of mitochondria increases. This is important for the activity of enzymes to proceed actively.
In mitochondria, scientists have found specific ribosomes and DNA. This allows these organelles to reproduce on their own during cell division.
Chloroplasts
As for chloroplasts, in shape it is a disk or a ball with a double shell (inner and outer). Inside this organoid there are also ribosomes, DNA and grana - special membrane formations associated both with the inner membrane and with each other. Chlorophyll is found in the membranes of the gran. Thanks to him, the energy of sunlight is converted into the chemical energy of adenosine triphosphate (ATP). In chloroplasts, it is used for the synthesis of carbohydrates (formed from water and carbon dioxide).
Agree, you need to know the information presented above not only in order to pass a biology test. The cell is the building material that makes up our body. And all living nature is a complex collection of cells. As you can see, they have many components. At first glance, it may seem that studying the structure of a cell is not an easy task. However, if you look, this topic is not so complicated. It is necessary to know it in order to be well versed in a science such as biology. The composition of the cell is one of its fundamental themes.
The cell is the basic unit of life. The cell is delimited from other cells or from the external environment by a special membrane and has a nucleus or its equivalent, in which the main part of the chemical information that controls heredity is concentrated. Cytology deals with the study of the structure of the cell, and physiology deals with the functioning. The science that studies the cells of tissue is called histology.
There are unicellular organisms, the body of which consists entirely of one cell. This group includes bacteria and protists (protozoa and unicellular algae). Sometimes they are also called acellular, but the term unicellular is used more often. True multicellular animals (Metazoa) and plants (Metaphyta) contain many cells.
unicellular organism
The vast majority of tissues are made up of cells, but there are some exceptions. The body of slime molds (myxomycetes), for example, consists of a homogeneous, non-celled substance with numerous nuclei. Some animal tissues, in particular the heart muscle, are organized in a similar way. The vegetative body (thallus) of fungi is formed by microscopic filaments - hyphae, often segmented; each such thread can be considered the equivalent of a cage, albeit of an atypical form.
Some structures of the body that are not involved in metabolism, such as shells, pearls, or the mineral base of bones, are formed not by cells, but by their secretion products. Others, such as wood, bark, horns, hair, and the outer layer of skin, are not of secretory origin, but are formed from dead cells.
Small organisms, such as rotifers, consist of only a few hundred cells. For comparison: in the human body, there are approx. 1014 cells, in it every second 3 million erythrocytes die and are replaced by new ones, and this is only one ten millionth of the total number of body cells.
Usually, the sizes of plant and animal cells range from 5 to 20 microns in diameter. A typical bacterial cell is much smaller - approx. 2 µm, and the smallest known is 0.2 µm.
Some free-living cells, such as protozoa such as foraminifera, can be several centimeters long; they always have many nuclei. The cells of thin plant fibers reach a length of one meter, and the processes of nerve cells reach several meters in large animals. With such a length, the volume of these cells is small, and the surface is very large.
The largest cells are unfertilized bird eggs filled with yolk. The largest egg (and, therefore, the largest cell) belonged to an extinct huge bird - epiornis (Aepyornis). Presumably its yolk weighed approx. 3.5 kg. The largest egg in living species belongs to the ostrich, its yolk weighs approx. 0.5 kg.
As a rule, the cells of large animals and plants are only slightly larger than the cells of small organisms. An elephant is larger than a mouse, not because its cells are larger, but mainly because the cells themselves are much larger. There are groups of animals, such as rotifers and nematodes, in which the number of cells in the body remains constant. Thus, although large nematode species have a larger number of cells than small ones, the main difference in size is due in this case to the large cell size.
Within a given cell type, their sizes usually depend on ploidy, i.e. on the number of sets of chromosomes present in the nucleus. Tetraploid cells (with four sets of chromosomes) are 2 times larger in volume than diploid cells (with a double set of chromosomes). The ploidy of a plant can be increased by injecting the herbal preparation colchicine into it. Because exposed plants have larger cells, they are also larger. However, this phenomenon can only be observed on polyploids of recent origin. In evolutionarily ancient polyploid plants, cell sizes are subject to "reverse regulation" towards normal values despite an increase in the number of chromosomes.
Cell structure.
At one time, the cell was considered as a more or less homogeneous droplet of organic matter, which was called protoplasm or living substance. This term became obsolete after it became clear that the cell consists of many clearly separated structures, called cellular organelles ("small organs").
Chemical composition. Usually 70-80% of the cell mass is water, in which various salts and low molecular weight organic compounds are dissolved. The most characteristic components of a cell are proteins and nucleic acids. Some proteins are structural components of the cell, others are enzymes, i.e. catalysts that determine the speed and direction of chemical reactions occurring in cells. Nucleic acids serve as carriers of hereditary information, which is realized in the process of intracellular protein synthesis.
Cells often contain a certain amount of reserve substances that serve as a food reserve. Plant cells primarily store starch, the polymeric form of carbohydrates. In the cells of the liver and muscles, another carbohydrate polymer, glycogen, is stored. Fat is also among the commonly stocked foods, although some fats perform a different function, namely, they serve as the most important structural components. Proteins in cells (with the exception of seed cells) are usually not stored.
It is not possible to describe the typical composition of a cell, primarily because there are large differences in the amount of stored food and water. The liver cells contain, for example, 70% water, 17% proteins, 5% fats, 2% carbohydrates and 0.1% nucleic acids; the remaining 6% are salts and low molecular weight organic compounds, in particular amino acids. Plant cells usually contain less protein, significantly more carbohydrates, and somewhat more water; the exception is cells that are at rest. A resting cell of a wheat grain, which is a source of nutrients for the embryo, contains approx. 12% protein (mostly stored protein), 2% fat, and 72% carbohydrate. The amount of water reaches a normal level (70–80%) only at the beginning of grain germination.
"TYPICAL" ANIMAL CELL - schematically depicts the main cellular structures.
"TYPICAL" PLANT CELL - schematically depicts the main cellular structures.
Some cells, mostly plant and bacterial, have an outer cell wall. In higher plants, it consists of cellulose. The wall surrounds the cell itself, protecting it from mechanical influences. Cells, especially bacterial ones, can also secrete mucous substances, thereby forming a capsule around them, which, like the cell wall, performs a protective function.
It is with the destruction of cell walls that the death of many bacteria under the action of penicillin is associated. The fact is that inside the bacterial cell the concentration of salts and low-molecular compounds is very high, and therefore, in the absence of a reinforcing wall, the influx of water into the cell caused by osmotic pressure can lead to its rupture. Penicillin, which prevents the formation of its wall during cell growth, just leads to rupture (lysis) of the cell.
Cell walls and capsules are not involved in metabolism and can often be detached without killing the cell. Thus, they can be considered as external auxiliary parts of the cell. In animal cells, cell walls and capsules are usually absent.
The cell itself consists of three main parts. Beneath the cell wall, if any, is the cell membrane. The membrane surrounds a heterogeneous material called the cytoplasm. A round or oval nucleus is immersed in the cytoplasm. Below we will consider in more detail the structure and functions of these parts of the cell.
cell membrane
The cell membrane is a very important part of the cell. It holds together all cellular components and delimits the internal and external environment. In addition, modified cell membrane folds form many of the cell's organelles.
The cell membrane is a double layer of molecules (bimolecular layer, or bilayer). Basically, these are molecules of phospholipids and other substances close to them. Lipid molecules have a dual nature, manifested in the way they behave in relation to water. The heads of the molecules are hydrophilic, i.e. have an affinity for water, and their hydrocarbon tails are hydrophobic. Therefore, when mixed with water, lipids form a film on its surface, similar to an oil film; at the same time, all their molecules are oriented in the same way: the heads of the molecules are in the water, and the hydrocarbon tails are above its surface.
There are two such layers in the cell membrane, and in each of them the heads of the molecules are turned outward, and the tails are turned inside the membrane, one to the other, thus not touching the water. The thickness of this membrane is approx. 7 nm. In addition to the main lipid components, it contains large protein molecules that are able to “float” in the lipid bilayer and are located so that one of their sides is turned inside the cell, and the other is in contact with the external environment. Some proteins are located only on the outer or only on the inner surface of the membrane, or are only partially immersed in the lipid bilayer.
The main function of the cell membrane is to regulate the transport of substances into and out of the cell. Since the membrane is physically similar to oil to some extent, substances soluble in oil or organic solvents, such as ether, easily pass through it. The same applies to gases such as oxygen and carbon dioxide. At the same time, the membrane is practically impermeable to most water-soluble substances, in particular, to sugars and salts. Due to these properties, it is able to maintain a chemical environment inside the cell that differs from the outside. For example, in the blood, the concentration of sodium ions is high, and potassium ions are low, while in the intracellular fluid, these ions are present in the opposite ratio. A similar situation is typical for many other chemical compounds.
Obviously, the cell, however, cannot be completely isolated from the environment, since it must receive the substances necessary for metabolism and get rid of its end products. In addition, the lipid bilayer is not completely impermeable even for water-soluble substances, but the so-called “layers” penetrating it. "Channel-forming" proteins create pores, or channels, which can open and close (depending on the change in protein conformation) and in the open state conduct certain ions (Na+, K+, Ca2+) along the concentration gradient. Consequently, the difference in concentrations inside the cell and outside cannot be maintained solely due to the low permeability of the membrane. In fact, it contains proteins that perform the function of a molecular "pump": they transport certain substances both into the cell and out of it, working against the concentration gradient. As a result, when the concentration of, for example, amino acids is high inside the cell and low outside, amino acids can still be transferred from the outside to the inside. Such transfer is called active transport, and energy supplied by metabolism is expended on it. Membrane pumps are highly specific: each of them is able to transport either only ions of a certain metal, or an amino acid, or sugar. Membrane ion channels are also specific.
Such selective permeability is physiologically very important, and its absence is the first evidence of cell death. This can be easily illustrated with the example of beets. If a live beet root is immersed in cold water, it retains its pigment; if the beets are boiled, then the cells die, become easily permeable and lose the pigment, which turns the water red.
Large molecules such as protein cells can "swallow". Under the influence of certain proteins, if they are present in the fluid surrounding the cell, an invagination occurs in the cell membrane, which then closes, forming a bubble - a small vacuole containing water and protein molecules; after that, the membrane around the vacuole breaks, and the contents enter the cell. This process is called pinocytosis (literally "cell drinking"), or endocytosis.
Larger particles, such as food particles, can be absorbed in a similar way during the so-called. phagocytosis. As a rule, the vacuole formed during phagocytosis is larger, and the food is digested by the enzymes of the lysosomes inside the vacuole until the membrane surrounding it ruptures. This type of nutrition is typical for protozoa, for example, for amoebas that eat bacteria. However, the ability to phagocytosis is characteristic of both intestinal cells of lower animals and phagocytes, one of the types of white blood cells (leukocytes) of vertebrates. In the latter case, the meaning of this process is not in the nutrition of the phagocytes themselves, but in the destruction of bacteria, viruses and other foreign material harmful to the body.
The functions of vacuoles may be different. For example, protozoa living in fresh water experience a constant osmotic influx of water, since the concentration of salts inside the cell is much higher than outside. They are able to secrete water into a special excreting (contractile) vacuole, which periodically pushes its contents out.
In plant cells, there is often one large central vacuole that occupies almost the entire cell; the cytoplasm forms only a very thin layer between the cell wall and the vacuole. One of the functions of such a vacuole is the accumulation of water, which allows the cell to rapidly increase in size. This ability is especially needed at a time when plant tissues are growing and forming fibrous structures.
In tissues, in places of tight junction of cells, their membranes contain numerous pores formed by proteins penetrating the membrane - the so-called. connectons. The pores of adjacent cells are arranged opposite each other, so that low molecular weight substances can move from cell to cell - this chemical communication system coordinates their vital activity. One example of such coordination is the more or less synchronous division of neighboring cells observed in many tissues.
CELL MEMBRANE MODEL showing the position of protein molecules relative to the double layer of lipid molecules. The proteins of most cells located on the surface of the lipid bilayer or immersed in it can shift somewhat in the lateral direction. Cholesterol is also present in the cell membrane of higher organisms.
Cytoplasm
In the cytoplasm there are internal membranes similar to the outer ones and forming organelles of various types. These membranes can be thought of as folds of the outer membrane; sometimes the inner membranes form an integral whole with the outer one, but often the inner fold is laced up, and contact with the outer membrane is interrupted. However, even if contact is maintained, the inner and outer membranes are not always chemically identical. In particular, the composition of membrane proteins in different cell organelles differs.
Endoplasmic reticulum. A network of tubules and vesicles extends from the cell surface to the nucleus. This network is called the endoplasmic reticulum. It has often been noted that the tubules open on the cell surface, and the endoplasmic reticulum thus plays the role of a microcirculatory apparatus through which the external environment can directly interact with all the contents of the cell. Such an interaction has been found in some cells, in particular in muscle cells, but it is not yet clear whether it is universal. In any case, the transport of a number of substances through these tubules from one part of the cell to another actually occurs.
Tiny bodies called ribosomes cover the surface of the endoplasmic reticulum, especially near the nucleus. Ribosome diameter approx. 15 nm, they are half proteins, half ribonucleic acids. Their main function is the synthesis of proteins; matrix (information) RNA and amino acids associated with transfer RNA are attached to their surface. Areas of the reticulum covered with ribosomes are called rough endoplasmic reticulum, and those without them are called smooth. In addition to ribosomes, various enzymes are adsorbed or otherwise attached to the endoplasmic reticulum, including enzyme systems that ensure the use of oxygen for the formation of sterols and for the neutralization of certain poisons. Under unfavorable conditions, the endoplasmic reticulum rapidly degenerates, and therefore its condition serves as a sensitive indicator of cell health.
Golgi apparatus. The Golgi apparatus (Golgi complex) is a specialized part of the endoplasmic reticulum, consisting of stacked flat membrane sacs. It is involved in the secretion of proteins by the cell (the packing of secreted proteins into granules occurs in it) and therefore is especially developed in cells that perform a secretory function. The important functions of the Golgi apparatus also include the attachment of carbohydrate groups to proteins and the use of these proteins to build the cell membrane and lysosome membrane. In some algae, cellulose fibers are synthesized in the Golgi apparatus.
Lysosomes are small vesicles surrounded by a single membrane. They bud from the Golgi apparatus and possibly from the endoplasmic reticulum. Lysosomes contain a variety of enzymes that break down large molecules, in particular proteins. Due to their destructive action, these enzymes are, as it were, "locked" in lysosomes and are released only as needed. So, during intracellular digestion, enzymes are released from lysosomes into digestive vacuoles. Lysosomes are also necessary for cell destruction; for example, during the transformation of a tadpole into an adult frog, the release of lysosomal enzymes ensures the destruction of tail cells. In this case, this is normal and beneficial for the body, but sometimes such cell destruction is pathological. For example, when asbestos dust is inhaled, it can enter the cells of the lungs, and then lysosomes rupture, cells are destroyed, and lung disease develops.
Mitochondria and chloroplasts. Mitochondria are relatively large sac-like formations with a rather complex structure. They consist of a matrix surrounded by an inner membrane, an intermembrane space, and an outer membrane. The inner membrane is folded into folds called cristae. Accumulations of proteins are located on the cristae. Many of them are enzymes that catalyze the oxidation of carbohydrate breakdown products; others catalyze the reactions of synthesis and oxidation of fats. Auxiliary enzymes involved in these processes are dissolved in the mitochondrial matrix.
In mitochondria, the oxidation of organic substances occurs, coupled with the synthesis of adenosine triphosphate (ATP). The breakdown of ATP with the formation of adenosine diphosphate (ADP) is accompanied by the release of energy, which is spent on various life processes, such as the synthesis of proteins and nucleic acids, the transport of substances into and out of the cell, the transmission of nerve impulses, or muscle contraction. Mitochondria, therefore, are energy stations that process "fuel" - fats and carbohydrates - into a form of energy that can be used by the cell, and therefore the body as a whole.
Plant cells also contain mitochondria, but the main source of energy for their cells is light. Light energy is used by these cells to form ATP and synthesize carbohydrates from carbon dioxide and water.
Chlorophyll, a pigment that accumulates light energy, is found in chloroplasts. Chloroplasts, like mitochondria, have an inner and outer membrane. From the outgrowths of the inner membrane in the process of development of chloroplasts, so-called. thylakoid membranes; the latter form flattened bags, collected in piles like a column of coins; these stacks, called grana, contain chlorophyll. In addition to chlorophyll, chloroplasts contain all the other components necessary for photosynthesis.
Some specialized chloroplasts do not carry out photosynthesis, but carry out other functions, for example, they provide storage of starch or pigments.
relative autonomy. In some respects, mitochondria and chloroplasts behave like autonomous organisms. For example, just like cells themselves, which arise only from cells, mitochondria and chloroplasts form only from pre-existing mitochondria and chloroplasts. This was demonstrated in experiments on plant cells, in which the formation of chloroplasts was inhibited by the antibiotic streptomycin, and on yeast cells, where the formation of mitochondria was inhibited by other drugs. After such influences, the cells never restored the missing organelles. The reason is that mitochondria and chloroplasts contain a certain amount of their own genetic material (DNA) that codes for part of their structure. If this DNA is lost, which is what happens when organelle formation is suppressed, then the structure cannot be recreated. Both types of organelles have their own protein-synthesizing system (ribosomes and transfer RNAs), which is somewhat different from the main protein-synthesizing system of the cell; it is known, for example, that the protein-synthesizing system of organelles can be suppressed by antibiotics, while they do not affect the main system.
Organelle DNA is responsible for the bulk of extrachromosomal, or cytoplasmic, inheritance. Extrachromosomal heredity does not obey Mendelian laws, since during cell division, organelle DNA is transmitted to daughter cells in a different way than chromosomes. The study of mutations that occur in the DNA of organelles and the DNA of chromosomes has shown that organelle DNA is responsible for only a small part of the structure of organelles; most of their proteins are encoded in genes located on chromosomes.
The partial genetic autonomy of the organelles under consideration and the features of their protein-synthesizing systems served as the basis for the assumption that mitochondria and chloroplasts originated from symbiotic bacteria that settled in cells 1–2 billion years ago. A modern example of such a symbiosis is the small photosynthetic algae that live inside the cells of some corals and molluscs. Algae provide their hosts with oxygen, and from them they receive nutrients.
fibrillar structures. The cytoplasm of a cell is a viscous fluid, so surface tension would expect the cell to be spherical except when the cells are tightly packed. However, this is usually not observed. Many protozoa have dense integuments or membranes that give the cell a specific, non-spherical shape. Nevertheless, even without a membrane, cells can maintain a non-spherical shape due to the fact that the cytoplasm is structured with numerous, rather rigid, parallel fibers. The latter are formed by hollow microtubules, which consist of protein units organized in a spiral.
Some protozoa form pseudopodia - long thin cytoplasmic outgrowths with which they capture food. Pseudopodia retain their shape due to the rigidity of the microtubules. If the hydrostatic pressure increases to about 100 atmospheres, the microtubules disintegrate and the cell takes on a droplet shape. When the pressure returns to normal, microtubules reassemble and the cell forms pseudopodia. Many other cells react similarly to changes in pressure, which confirms the participation of microtubules in maintaining the shape of the cell. The assembly and disintegration of microtubules, necessary for the cell to change shape rapidly, also occurs in the absence of pressure changes.
Microtubules also form fibrillar structures that serve as organs of cell movement. Some cells have whip-like outgrowths called flagella, or cilia - their beating ensures the movement of the cell in the water. If the cell is immobile, these structures drive water, food particles, and other particles toward or away from the cell. The flagella are relatively large, and usually the cell has only one, occasionally several flagella. Cilia are much smaller and cover the entire surface of the cell. Although these structures are characteristic mainly of protozoa, they may also be present in highly organized forms. In the human body, all respiratory tracts are lined with cilia. Small particles that enter them are usually caught by mucus on the cell surface, and the cilia move them out along with the mucus, thus protecting the lungs. The male germ cells of most animals and some lower plants move with the help of a flagellum.
There are other types of cellular movement. One of them is amoeboid movement. Amoeba, as well as some cells of multicellular organisms, "flow" from place to place, i.e. move due to the current of the contents of the cell. A constant current of matter also exists inside plant cells, but it does not entail the movement of the cell as a whole. The most studied type of cellular movement is the contraction of muscle cells; it is carried out by sliding fibrils (protein threads) relative to each other, which leads to shortening of the cell.
Core
The nucleus is surrounded by a double membrane. A very narrow (about 40 nm) space between two membranes is called perinuclear. The membranes of the nucleus pass into the membranes of the endoplasmic reticulum, and the perinuclear space opens into the reticular. Typically, the nuclear membrane has very narrow pores. Apparently, large molecules are transferred through them, such as messenger RNA, which is synthesized on DNA and then enters the cytoplasm.
The main part of the genetic material is located in the chromosomes of the cell nucleus. Chromosomes consist of long chains of double-stranded DNA, to which basic (i.e., alkaline) proteins are attached. Sometimes chromosomes have several identical strands of DNA lying next to each other - such chromosomes are called polytene (multifilamentous). The number of chromosomes in different species is not the same. Diploid cells of the human body contain 46 chromosomes, or 23 pairs.
In a nondividing cell, the chromosomes are attached at one or more points to the nuclear membrane. In the normal non-spiralized state, the chromosomes are so thin that they are not visible under a light microscope. At certain loci (areas) of one or more chromosomes, a dense body present in the nuclei of most cells is formed - the so-called. nucleolus. In the nucleolus, RNA is synthesized and accumulated, which is used to build ribosomes, as well as some other types of RNA.
cell division
Although all cells come from the division of the preceding cell, not all of them continue to divide. For example, nerve cells in the brain, once having arisen, no longer divide. Their number is gradually decreasing; damaged brain tissue is not able to recover by regeneration. If the cells continue to divide, then they are characterized by a cell cycle consisting of two main stages: interphase and mitosis.
The interphase itself consists of three phases: G1, S and G2. Below is their duration, typical for plant and animal cells.
G1 (4–8 h). This phase begins immediately after the birth of the cell. During the G1 phase, the cell, with the exception of chromosomes (which do not change), increases its mass. If the cell does not divide further, it remains in this phase.
S (6–9 h). The mass of the cell continues to increase, and doubling (duplication) of chromosomal DNA occurs. However, the chromosomes remain single in structure, albeit doubled in mass, since the two copies of each chromosome (chromatids) are still connected to each other along their entire length.
G2. The mass of the cell continues to increase until it is approximately twice the initial mass, and then mitosis occurs.
Mitosis
After the chromosomes have doubled, each of the daughter cells must receive a complete set of chromosomes. Mere cell division cannot achieve this - this result is achieved through a process called mitosis. Without going into details, the beginning of this process should be considered the alignment of chromosomes in the equatorial plane of the cell. Then each chromosome splits longitudinally into two chromatids, which begin to diverge in opposite directions, becoming independent chromosomes. As a result, at the two ends of the cell is located on the full set of chromosomes. Then the cell divides into two, and each daughter cell receives a complete set of chromosomes.
The following is a description of mitosis in a typical animal cell. It is usually divided into four stages.
I. Prophase. A special cellular structure - the centriole - doubles (sometimes this doubling occurs in the S-period of interphase), and the two centrioles begin to diverge towards opposite poles of the nucleus. The nuclear membrane is destroyed; at the same time, special proteins combine (aggregate), forming microtubules in the form of filaments. Centrioles, now located at opposite poles of the cell, have an organizing effect on microtubules, which as a result line up radially, forming a structure that resembles an aster flower (“star”) in appearance. Other threads of microtubules stretch from one centriole to another, forming the so-called. division spindle. At this time, the chromosomes are in a spiralized state, resembling a spring. They are clearly visible under a light microscope, especially after staining. In prophase, the chromosomes split, but the chromatids still remain bonded in pairs in the zone of the centromere, a chromosomal organelle similar in function to the centriole. Centromeres also have an organizing effect on the spindle threads, which now stretch from centriole to centromere and from it to another centriole.
II. Metaphase. Chromosomes, up to this point randomly arranged, begin to move, as if drawn by spindle threads attached to their centromeres, and gradually line up in one plane in a certain position and at an equal distance from both poles. Lying in the same plane, centromeres together with chromosomes form the so-called. equatorial plate. The centromeres connecting pairs of chromatids divide, after which the sister chromosomes are completely separated.
III. Anaphase. The chromosomes of each pair move in opposite directions towards the poles, as if being dragged by spindle threads. In this case, threads are also formed between the centromeres of paired chromosomes.
IV. Telophase. As soon as the chromosomes approach opposite poles, the cell itself begins to divide along the plane in which the equatorial plate was located. As a result, two cells are formed. The spindle fibers break down, the chromosomes unwind and become invisible, and a nuclear membrane forms around them. Cells return to the G1 phase of interphase. The entire process of mitosis takes about an hour.
The details of mitosis vary somewhat in different cell types. In a typical plant cell, a spindle is formed, but there are no centrioles. In fungi, mitosis occurs within the nucleus, without prior disintegration of the nuclear membrane.
The division of the cell itself, called cytokinesis, is not strictly related to mitosis. Sometimes one or more mitoses pass without cell division; as a result, multinucleated cells are formed, often found in algae. If the nucleus is removed from the sea urchin egg by micromanipulation, the spindle then continues to form and the egg continues to divide. This shows that the presence of chromosomes is not a necessary condition for cell division.
Reproduction by mitosis is called asexual reproduction, vegetative reproduction, or cloning. Its most important aspect is genetic: with such reproduction, there is no divergence of hereditary factors in the offspring. The resulting daughter cells are genetically exactly the same as the parent. Mitosis is the only mode of self-reproduction in species that do not have sexual reproduction, such as many unicellular organisms. However, even in sexually reproducing species, body cells divide by mitosis and originate from a single cell, the fertilized egg, and are therefore all genetically identical. Higher plants can reproduce asexually (using mitosis) by seedlings and whiskers (a famous example is strawberries).
MITOSIS, the process of cell division, is divided into four stages. Between mitotic divisions, the cell is in the interphase stage.
Meiosis
Sexual reproduction of organisms is carried out with the help of specialized cells, the so-called. gametes - ova (eggs) and sperm (spermatozoa). Gametes fuse to form one cell, the zygote. Each gamete is haploid, i.e. has one set of chromosomes. Within the set, all chromosomes are different, but each chromosome of the egg corresponds to one of the chromosomes of the sperm. The zygote, therefore, already contains a pair of such chromosomes corresponding to each other, which are called homologous. Homologous chromosomes are similar because they have the same genes or their variants (alleles) that determine specific features. For example, one of the paired chromosomes may have a gene that codes for blood group A, and the other a variant of it that codes for blood group B. The zygote chromosomes originating from the egg are maternal, and those originating from the sperm are paternal.
As a result of multiple mitotic divisions, either a multicellular organism or numerous free-living cells arise from the formed zygote, as occurs in sexually reproducing protozoa and in unicellular algae.
During the formation of gametes, the diploid set of chromosomes that the zygote had had should be reduced by half (reduced). If this did not happen, then in each generation the fusion of gametes would lead to a doubling of the set of chromosomes. Reduction to the haploid number of chromosomes occurs as a result of reduction division - the so-called. meiosis, which is a variant of mitosis.
MEIOSIS provides for the formation of male and female gametes. It is common to all plants and animals that reproduce sexually.
splitting and recombination. A feature of meiosis is that during cell division, the equatorial plate is formed by pairs of homologous chromosomes, and not doubled individual chromosomes, as in mitosis. Paired chromosomes, each of which remained single, diverge to opposite poles of the cell, the cell divides, and as a result, the daughter cells receive a half set of chromosomes compared to the zygote.
For example, suppose that the haploid set consists of two chromosomes. In the zygote (and, accordingly, in all cells of the organism that produces gametes), maternal chromosomes A and B and paternal A "and B" are present. During meiosis, they can separate as follows:
The most important in this example is the fact that when the chromosomes diverge, the initial maternal and paternal set is not necessarily formed, but recombination of genes is possible, as in the gametes AB "and A" B in the above diagram.
Now suppose that the pair of chromosomes AA" contains two alleles - a and b - of the gene that determines blood types A and B. Similarly, the pair of chromosomes BB" contains alleles m and n of another gene that determines blood types M and N. Separating these alleles can go like this:
Obviously, the resulting gametes can contain any of the following combinations of alleles of two genes: am, bn, bm or an.
If there are more chromosomes, then allele pairs will split independently in the same way. This means that the same zygotes can produce gametes with different combinations of gene alleles and give rise to different genotypes in the offspring.
meiotic division. Both of these examples illustrate the principle of meiosis. In fact, meiosis is a much more complex process, as it involves two successive divisions. The main thing in meiosis is that the chromosomes are duplicated only once, while the cell divides twice, resulting in a reduction in the number of chromosomes and the diploid set becomes haploid.
During the prophase of the first division, homologous chromosomes conjugate, that is, they come together in pairs. As a result of this very precise process, each gene is opposite its homologue on the other chromosome. Both chromosomes then double, but the chromatids remain connected to each other by a common centromere.
In metaphase, the four joined chromatids line up to form the equatorial plate, as if they were one duplicated chromosome. In contrast to what happens during mitosis, centromeres do not divide. As a result, each daughter cell receives a pair of chromatids still connected by the cetromere. During the second division, the chromosomes, already individual, line up again, forming, as in mitosis, an equatorial plate, but their doubling does not occur during this division. The centromeres then divide and each daughter cell receives one chromatid.
division of the cytoplasm. As a result of two meiotic divisions of a diploid cell, four cells are formed. During the formation of male germ cells, four sperm of approximately the same size are obtained. During the formation of eggs, the division of the cytoplasm occurs very unevenly: one cell remains large, while the other three are so small that they are almost entirely occupied by the nucleus. These small cells, the so-called. polar bodies, serve only to accommodate the excess of chromosomes formed as a result of meiosis. The main part of the cytoplasm necessary for the zygote remains in one cell - the egg.
Generation alternation
Primitive cells: prokaryotes
All of the above applies to cells of plants, animals, protozoa and unicellular algae, collectively called eukaryotes. Eukaryotes evolved from a simpler form, prokaryotes, which are now bacteria, including archaebacteria and cyanobacteria (the latter were formerly called blue-green algae). Compared to eukaryotic cells, prokaryotic cells are smaller and have fewer cell organelles. They have a cell membrane but no endoplasmic reticulum, and ribosomes float freely in the cytoplasm. Mitochondria are absent, but oxidative enzymes are usually attached to the cell membrane, which thus becomes the equivalent of mitochondria. Prokaryotes are also deprived of chloroplasts, and chlorophyll, if present, is present in the form of very small granules.
Prokaryotes do not have a membrane-enclosed nucleus, although the location of the DNA can be identified by its optical density. The equivalent of a chromosome is a strand of DNA, usually circular, with a much smaller number of attached proteins. A chain of DNA at one point is attached to the cell membrane. Mitosis is absent in prokaryotes. It is replaced by the following process: DNA doubles, after which the cell membrane begins to grow between adjacent attachment points of two copies of the DNA molecule, which as a result gradually diverge. The cell eventually divides between the attachment points of the DNA molecules, forming two cells, each with its own copy of the DNA.
cell differentiation
Multicellular plants and animals evolved from single-celled organisms whose cells remained together after division, forming a colony. Initially, all cells were identical, but further evolution gave rise to differentiation. First of all, somatic cells (i.e. body cells) and germ cells differentiated. Further, differentiation became more complicated - more and more different cell types arose. Ontogeny - the individual development of a multicellular organism - repeats in general terms this evolutionary process (phylogenesis).
Physiologically, cells differentiate partly by strengthening one or another feature common to all cells. For example, contractile function is increased in muscle cells, which may be the result of an improvement in the mechanism that performs amoeboid or other types of movement in less specialized cells. A similar example is thin-walled root cells with their processes, the so-called. root hairs, which serve to absorb salts and water; to one degree or another, this function is inherent in any cells. Sometimes specialization is associated with the acquisition of new structures and functions - an example is the development of a locomotor organ (flagellum) in spermatozoa.
Differentiation at the cellular or tissue level has been studied in some detail. We know, for example, that sometimes it proceeds autonomously, i.e. one type of cell can transform into another, regardless of what type of cells the neighbors belong to. However, the so-called. embryonic induction is a phenomenon in which one type of tissue stimulates cells of another type to differentiate in a given direction.
In the general case, differentiation is irreversible, i.e. highly differentiated cells cannot transform into another type of cell. However, this is not always the case, especially in plant cells.
Differences in structure and function are ultimately determined by what types of proteins are synthesized in the cell. Since genes control the synthesis of proteins, and the set of genes in all cells of the body is the same, differentiation must depend on the activation or inactivation of certain genes in different types of cells. Regulation of gene activity occurs at the level of transcription, i.e. formation of messenger RNA using DNA as a template. Only transcribed genes produce proteins. Synthesized proteins can block transcription, but sometimes activate it. Also, because proteins are products of genes, some genes can control the transcription of other genes. Hormones, in particular steroid hormones, are also involved in the regulation of transcription. Very active genes can be duplicated (doubled) many times to produce more messenger RNA.
The development of malignancies has often been regarded as a special case of cellular differentiation. However, the appearance of malignant cells is the result of a change in the DNA structure (mutation), and not the processes of transcription and translation into normal DNA protein.
Methods for studying the cell
Light microscope. In the study of cell shape and structure, the first instrument was the light microscope. Its resolution is limited to dimensions comparable to the wavelength of light (0.4–0.7 µm for visible light). However, many elements of the cellular structure are much smaller in size.
Another difficulty is that most cellular components are transparent and their refractive index is almost the same as that of water. To improve visibility, dyes are often used that have different affinities for different cellular components. Staining is also used to study the chemistry of the cell. For example, some dyes bind predominantly to nucleic acids and thereby reveal their localization in the cell. A small part of the dyes - they are called intravital - can be used to stain living cells, but usually the cells must be pre-fixed (using substances that coagulate the protein) and only then can they be stained.
Before testing, cells or pieces of tissue are usually embedded in paraffin or plastic and then cut into very thin sections using a microtome. This method is widely used in clinical laboratories to detect tumor cells. In addition to conventional light microscopy, other optical methods for studying cells have also been developed: fluorescence microscopy, phase-contrast microscopy, spectroscopy, and X-ray diffraction analysis.
Electron microscope. The electron microscope has a resolution of approx. 1–2 nm. This is sufficient for the study of large protein molecules. It is usually necessary to stain and contrast the object with metal salts or metals. For this reason, and also because objects are examined in a vacuum, only dead cells can be studied with an electron microscope.
Autoradiography. If a radioactive isotope absorbed by cells during metabolism is added to the medium, then its intracellular localization can then be detected using autoradiography. In this method, thin sections of cells are placed on film. The film darkens under those places where there are radioactive isotopes.
Centrifugation. For the biochemical study of cellular components, cells must be destroyed - mechanically, chemically or by ultrasound. The released components are in suspension in the liquid and can be isolated and purified by centrifugation (most often in a density gradient). Typically, such purified components retain high biochemical activity.
Cell cultures. Some tissues can be divided into individual cells in such a way that the cells remain alive and are often able to reproduce. This fact finally confirms the idea of a cell as a unit of life. A sponge, a primitive multicellular organism, can be divided into cells by rubbing through a sieve. After a while, these cells recombine and form a sponge. Animal embryonic tissues can be made to dissociate using enzymes or other means that weaken the bonds between cells.
The American embryologist R. Harrison (1879–1959) was the first to show that embryonic and even some mature cells can grow and multiply outside the body in a suitable environment. This technique, called cell culture, was perfected by the French biologist A. Carrel (1873-1959). Plant cells can also be grown in culture, but compared to animal cells, they form larger clusters and are more strongly attached to each other, so tissue is formed during culture growth, rather than individual cells. In cell culture, a whole adult plant, such as a carrot, can be grown from a single cell.
Microsurgery. With the help of a micromanipulator, individual parts of the cell can be removed, added, or modified in some way. A large amoeba cell can be divided into three main components - the cell membrane, cytoplasm and nucleus, and then these components can be reassembled and a living cell is obtained. In this way, artificial cells can be obtained, consisting of components of different types of amoebas.
Considering that it is possible to synthesize some cellular components artificially, experiments on the assembly of artificial cells may be the first step towards the creation of new life forms in the laboratory. Since each organism develops from a single cell, the method of obtaining artificial cells in principle allows the construction of organisms of a given type, if at the same time using components that are slightly different from those found in currently existing cells. In reality, however, complete synthesis of all cellular components is not required. The structure of most, if not all, components of a cell is determined by nucleic acids. Thus, the problem of creating new organisms is reduced to the synthesis of new types of nucleic acids and their replacement of natural nucleic acids in certain cells.
Cell fusion. Another type of artificial cells can be obtained by fusion of cells of the same or different types. To achieve fusion, the cells are exposed to viral enzymes; in this case, the outer surfaces of two cells stick together, and the membrane between them collapses, and a cell is formed in which two sets of chromosomes are enclosed in one nucleus. You can merge cells of different types or at different stages of division. Using this method, it was possible to obtain hybrid cells of a mouse and a chicken, a human and a mouse, a human and a toad. Such cells are hybrid only initially, and after numerous cell divisions they lose most of the chromosomes of either one or another type. The end product becomes, for example, essentially a mouse cell, where human genes are absent or present only in small quantities. Of particular interest is the fusion of normal and malignant cells. In some cases, the hybrids become malignant, in others they do not; both properties can appear both as dominant and as recessive. This result is not unexpected, since malignancy can be caused by various factors and has a complex mechanism.
General information
Cell theory is a fundamental theory for biology, formulated in the middle 19th century, which provided a basis for understanding the patterns of the living world and for the development evolutionary doctrine. Matthias Schleiden And Theodor Schwann formulated cell theory based on many studies on cage (1838 ). Rudolf Virchow later ( 1858 ) supplemented it with the most important provision (every cell comes from another cell).
Schleiden and Schwann, summarizing the available knowledge about the cell, proved that the cell is the basic unit of any organism. Cells animals, plants And bacteria have a similar structure. Later, these conclusions became the basis for proving the unity of organisms. T. Schwann and M. Schleiden introduced the fundamental concept of the cell into science: there is no life outside the cells. The cellular theory was supplemented and edited every time.
Provisions of the cell theory of Schleiden-Schwann
All animals and plants are made up of cells.
Plants and animals grow and develop through the formation of new cells.
A cell is the smallest unit of life, and the whole organism is a collection of cells.
The main provisions of modern cell theory
Cell is an elementary, functional unit of the structure of all living things. (Except viruses that do not have a cellular structure)
Cell- a single system, it includes many naturally interconnected elements, representing a holistic formation, consisting of conjugated functional units - organelles.
Cells of all organisms homologous.
The cell occurs only by dividing the mother cell.
A multicellular organism is a complex system of many cells united and integrated into systems of tissues and organs connected with each other.
Cells of multicellular organisms totipotent.
A cell can only arise from a previous cell.
Additional Provisions of Cell Theory
In order to bring the cellular theory more fully into line with the data of modern cell biology, the list of its provisions is often supplemented and expanded. In many sources, these additional provisions differ, their set is quite arbitrary.
Cells prokaryotes And eukaryote are systems of different levels of complexity and are not completely homologous to each other (see below).
The basis of cell division and reproduction of organisms is the copying of hereditary information - nucleic acid molecules ("each molecule from a molecule"). The provisions on genetic continuity apply not only to cage in general, but also to some of its smaller components - to mitochondria, chloroplasts, genes And chromosomes.
A multicellular organism is a new system, a complex ensemble of many cells, united and integrated in a system of tissues and organs, connected to each other with the help of chemical factors, humoral and nervous (molecular regulation).
Multicellular cells are totipotent, that is, they have the genetic potencies of all cells of a given organism, are equivalent in genetic information, but differ from each other in different expression (work) of various genes, which leads to their morphological and functional diversity - to differentiation.
17th century
1665 - English physicist R. Hooke in the work "Micrographia" describes the structure of cork, on thin sections of which he found correctly located voids. Hooke called these voids "pores, or cells." The presence of a similar structure was known to him in some other parts of plants.
1670s - Italian medic and naturalist M. Malpighi and English naturalist N. Gru described "sacs or vesicles" in different organs of plants and showed the wide distribution of the cellular structure in plants. Cells were depicted in their drawings by a Dutch microscopist A. Levenguk. He was the first to discover the world of unicellular organisms - he described bacteria and protists (ciliates).
The researchers of the 17th century, who showed the prevalence of the "cellular structure" of plants, did not appreciate the significance of the discovery of the cell. They imagined cells as voids in a continuous mass of plant tissue. Grew considered cell walls as fibers, so he introduced the term "tissue", by analogy with textile fabric. Studies of the microscopic structure of animal organs were of a random nature and did not provide any knowledge about their cellular structure.
18th century
In the 18th century, the first attempts were made to compare the microstructure of plant and animal cells. K. F. Wolf in his Theory of Generation (1759) he tries to compare the development of the microscopic structure of plants and animals. According to Wolf, the embryo in both plants and animals develops from a structureless substance in which movements create channels (vessels) and voids (cells). The facts cited by Wolff were erroneously interpreted by him and did not add new knowledge to what was known to the seventeenth-century microscopists. However, his theoretical ideas largely anticipated the ideas of the future cell theory.
19th century
In the first quarter of the 19th century, there was a significant deepening of ideas about the cellular structure of plants, which is associated with significant improvements in the design of the microscope (in particular, the creation achromatic lenses).
Link and Moldnhower establish the presence of independent walls in plant cells. It turns out that the cell is a kind of morphologically isolated structure. In 1831 G. Mol proves that even seemingly non-cellular structures of plants, like aquifers, develop from cells.
F. Meyen in "Phytotomy" (1830) describes plant cells that "are either solitary, so that each cell is a separate individual, as is found in algae and fungi, or, forming more highly organized plants, they combine into more and less significant masses". Meyen emphasizes the independence of the metabolism of each cell.
In 1831 Robert Brown describes core and suggests that it is a permanent part of the plant cell.
Purkinje School
In 1801, Vigia introduced the concept of animal tissues, but he isolated tissues on the basis of anatomical preparation and did not use a microscope. The development of ideas about the microscopic structure of animal tissues is associated primarily with the research of Purkinje, who founded his school in Breslau.
Purkinje and his students (G. Valentin should be especially noted) revealed in the first and most general form the microscopic structure of tissues and organs of mammals (including humans). Purkinje and Valentin compared individual plant cells with individual microscopic animal tissue structures, which Purkinje most often called "seeds" (for some animal structures, the term "cell" was used in his school).
In 1837 Purkinje delivered a series of lectures in Prague. In them, he reported on his observations on the structure of the gastric glands, the nervous system, etc. In the table attached to his report, clear images of some cells of animal tissues were given. Nevertheless, Purkinje could not establish the homology of plant cells and animal cells:
firstly, by grains he understood either cells or cell nuclei;
secondly, the term "cell" was then understood literally as "a space bounded by walls."
Purkinje compared plant cells and animal "seeds" in terms of analogy, not homology of these structures (understanding the terms "analogy" and "homology" in the modern sense).
Müller school and Schwann's work
The second school where the microscopic structure of animal tissues was studied was the laboratory Johannes Müller in Berlin. Müller studied the microscopic structure of the dorsal string (chord); his student Henle published a study on the intestinal epithelium, in which he gave a description of its various types and their cellular structure.
Here the classic studies of Theodor Schwann were carried out, laying the foundation for the cell theory. Schwann's work was strongly influenced by the Purkinje school and Henle. Schwann found the correct principle for comparing plant cells and the elementary microscopic structures of animals. Schwann was able to establish homology and prove correspondence in the structure and growth of the elementary microscopic structures of plants and animals.
The significance of the nucleus in the Schwann cell was prompted by the research of Matthias Schleiden, who in 1838 published the work Materials on Phytogenesis. Therefore, Schleiden is often called a co-author of the cell theory. The basic idea of the cell theory - the correspondence of plant cells and the elementary structures of animals - was alien to Schleiden. He formulated the theory of new cell formation from a structureless substance, according to which, first, the nucleolus condenses from the smallest granularity, and a nucleus is formed around it, which is the cell's former (cytoblast). However, this theory was based on incorrect facts.
In 1838, Schwann published 3 preliminary reports, and in 1839 his classic work “Microscopic studies on the correspondence in the structure and growth of animals and plants” appeared, in the very title of which the main idea of the cell theory is expressed:
In the first part of the book, he examines the structure of the notochord and cartilage, showing that their elementary structures - cells develop in the same way. Further, he proves that the microscopic structures of other tissues and organs of the animal organism are also cells, quite comparable with the cells of cartilage and chord.
The second part of the book compares plant cells and animal cells and shows their correspondence.
The third part develops theoretical provisions and formulates the principles of cell theory. It was Schwann's research that formalized the cell theory and proved (at the level of knowledge of that time) the unity of the elementary structure of animals and plants. Schwann's main mistake was his opinion, following Schleiden, about the possibility of the emergence of cells from a structureless non-cellular substance.
At the dawn of the development of life on Earth, all cellular forms were represented by bacteria. They sucked organic matter dissolved in the primordial ocean through the surface of the body.
Over time, some bacteria adapted to produce organic substances from inorganic ones. To do this, they used the energy of sunlight. The first ecological system emerged in which these organisms were producers. As a result, oxygen released by these organisms appeared in the Earth's atmosphere. With it, you can get much more energy from the same food, and use the additional energy to complicate the structure of the body: dividing the body into parts.
One of the important achievements of life is the separation of the nucleus and cytoplasm. The nucleus contains hereditary information. A special membrane around the core made it possible to protect against accidental damage. As necessary, the cytoplasm receives commands from the nucleus that direct the vital activity and development of the cell.
Organisms in which the nucleus is separated from the cytoplasm formed the super-kingdom of the nuclear (these include plants, fungi, animals).
Thus, the cell - the basis of the organization of plants and animals - arose and developed in the course of biological evolution.
Even with the naked eye, and even better under a magnifying glass, you can see that the pulp of a ripe watermelon consists of very small grains, or grains. These are cells - the smallest "bricks" that make up the bodies of all living organisms, including plants.
The life of a plant is carried out by the combined activity of its cells, creating a single whole. With the multicellularity of plant parts, there is a physiological differentiation of their functions, specialization of various cells depending on their location in the plant body.
A plant cell differs from an animal cell in that it has a dense shell that covers the inner contents from all sides. The cell is not flat (as it is usually portrayed), it most likely looks like a very small vial filled with slimy contents.
The structure and functions of a plant cell
Consider a cell as a structural and functional unit of an organism. Outside, the cell is covered with a dense cell wall, in which there are thinner sections - pores. Under it is a very thin film - a membrane that covers the contents of the cell - the cytoplasm. In the cytoplasm there are cavities - vacuoles filled with cell sap. In the center of the cell or near the cell wall is a dense body - the nucleus with the nucleolus. The nucleus is separated from the cytoplasm by the nuclear envelope. Small bodies, plastids, are distributed throughout the cytoplasm.
The structure of a plant cell
The structure and functions of plant cell organelles
| Organoid | Drawing | Description | Function | Peculiarities |
Cell wall or plasma membrane | Colourless, transparent and very durable | Passes into the cell and releases substances from the cell. | The cell membrane is semi-permeable |
|
Cytoplasm | Thick viscous substance | It contains all other parts of the cell. | Is in constant motion |
|
Nucleus (important part of the cell) | round or oval | Ensures the transfer of hereditary properties to daughter cells during division | Central part of the cell |
|
Spherical or irregular shape | Takes part in protein synthesis | |||
 | A reservoir separated from the cytoplasm by a membrane. Contains cell sap | Spare nutrients and waste products that are unnecessary to the cell accumulate. | As the cell grows, small vacuoles merge into one large (central) vacuole |
|
plastids | Chloroplasts | Use the light energy of the sun and create organic from inorganic | The shape of discs separated from the cytoplasm by a double membrane |
|
Chromoplasts | Formed as a result of the accumulation of carotenoids | Yellow, orange or brown |
||
 | Leucoplasts | Colorless plastids | ||
nuclear envelope | Consists of two membranes (outer and inner) with pores | Separates the nucleus from the cytoplasm | Enables exchange between nucleus and cytoplasm |
The living part of the cell is a membrane-limited, ordered, structured system of biopolymers and internal membrane structures involved in the totality of metabolic and energy processes that maintain and reproduce the entire system as a whole.
An important feature is that there are no open membranes with free ends in the cell. Cell membranes always limit cavities or areas, closing them from all sides.
Modern generalized diagram of a plant cell
plasmalemma(outer cell membrane) - an ultramicroscopic film 7.5 nm thick., Consisting of proteins, phospholipids and water. This is a very elastic film that is well wetted by water and quickly restores integrity after damage. It has a universal structure, i.e. typical for all biological membranes. Plant cells outside the cell membrane have a strong cell wall that creates an external support and maintains the shape of the cell. It is made up of fiber (cellulose), a water-insoluble polysaccharide.
Plasmodesmata of a plant cell, are submicroscopic tubules penetrating the membranes and lined with a plasma membrane, which thus passes from one cell to another without interruption. With their help, intercellular circulation of solutions containing organic nutrients occurs. They also transmit biopotentials and other information.
Poromy called holes in the secondary membrane, where the cells are separated only by the primary membrane and the middle plate. The areas of the primary membrane and the middle plate that separate the adjacent pores of adjacent cells are called the pore membrane or the closing film of the pore. The closing film of the pore is pierced by plasmodesmenal tubules, but a through hole is usually not formed in the pores. Pores facilitate the transport of water and solutes from cell to cell. In the walls of neighboring cells, as a rule, one against the other, pores are formed.
Cell wall has a well-defined, relatively thick shell of a polysaccharide nature. The plant cell wall is a product of the cytoplasm. The Golgi apparatus and the endoplasmic reticulum take an active part in its formation.
The structure of the cell membrane
The basis of the cytoplasm is its matrix, or hyaloplasm, a complex colorless, optically transparent colloidal system capable of reversible transitions from sol to gel. The most important role of hyaloplasm is to unite all cellular structures into a single system and ensure interaction between them in the processes of cellular metabolism.
Hyaloplasm(or the matrix of the cytoplasm) makes up the internal environment of the cell. It consists of water and various biopolymers (proteins, nucleic acids, polysaccharides, lipids), of which the main part is proteins of various chemical and functional specificities. The hyaloplasm also contains amino acids, monosugars, nucleotides and other low molecular weight substances.
Biopolymers form a colloidal medium with water, which, depending on the conditions, can be dense (in the form of a gel) or more liquid (in the form of a sol), both in the entire cytoplasm and in its individual sections. In the hyaloplasm, various organelles and inclusions are localized and interact with each other and with the environment of the hyaloplasm. Moreover, their location is most often specific to certain cell types. Through the bilipid membrane, the hyaloplasm interacts with the extracellular environment. Consequently, hyaloplasm is a dynamic environment and plays an important role in the functioning of individual organelles and the vital activity of cells as a whole.
Cytoplasmic formations - organelles
Organelles (organelles) are the structural components of the cytoplasm. They have a certain shape and size, are mandatory cytoplasmic structures of the cell. In their absence or damage, the cell usually loses the ability to continue to exist. Many of the organelles are capable of division and self-reproduction. They are so small that they can only be seen with an electron microscope.
Core
The nucleus is the most visible and usually the largest organelle of the cell. It was first studied in detail by Robert Brown in 1831. The nucleus provides the most important metabolic and genetic functions of the cell. It is quite variable in shape: it can be spherical, oval, lobed, lenticular.
The nucleus plays a significant role in the life of the cell. A cell from which the nucleus has been removed no longer secretes a shell, stops growing and synthesizing substances. The products of decay and destruction intensify in it, as a result of which it quickly dies. The formation of a new nucleus from the cytoplasm does not occur. New nuclei are formed only by fission or crushing of the old one.
The internal content of the nucleus is karyolymph (nuclear juice), which fills the space between the structures of the nucleus. It contains one or more nucleoli, as well as a significant number of DNA molecules connected to specific proteins - histones.
The structure of the nucleus
nucleolus
The nucleolus, like the cytoplasm, contains mainly RNA and specific proteins. Its most important function is that the formation of ribosomes takes place in it, which carry out the synthesis of proteins in the cell.
golgi apparatus
The Golgi apparatus is an organoid that has a universal distribution in all types of eukaryotic cells. It is a multi-tiered system of flat membrane sacs, which thicken along the periphery and form vesicular processes. It is most often located near the nucleus.
golgi apparatus
The Golgi apparatus necessarily includes a system of small vesicles (vesicles), which are laced from thickened cisterns (discs) and are located along the periphery of this structure. These vesicles play the role of an intracellular transport system of specific sectoral granules and can serve as a source of cellular lysosomes.
The functions of the Golgi apparatus also consist in the accumulation, separation and release of intracellular synthesis products, decay products, and toxic substances outside the cell with the help of bubbles. The products of the synthetic activity of the cell, as well as various substances that enter the cell from the environment through the channels of the endoplasmic reticulum, are transported to the Golgi apparatus, accumulate in this organoid, and then enter the cytoplasm in the form of droplets or grains and are either used by the cell itself or excreted. . In plant cells, the Golgi apparatus contains enzymes for the synthesis of polysaccharides and the polysaccharide material itself, which is used to build the cell wall. It is believed that it is involved in the formation of vacuoles. The Golgi apparatus was named after the Italian scientist Camillo Golgi, who first discovered it in 1897.
Lysosomes
Lysosomes are small vesicles, limited by a membrane, the main function of which is the implementation of intracellular digestion. The use of the lysosomal apparatus occurs during the germination of the plant seed (hydrolysis of reserve nutrients).
The structure of the lysosome
microtubules
Microtubules are membrane, supramolecular structures consisting of protein globules arranged in spiral or straight rows. Microtubules perform a predominantly mechanical (motor) function, providing mobility and contractility of cell organelles. Located in the cytoplasm, they give the cell a certain shape and ensure the stability of the spatial arrangement of organelles. Microtubules facilitate the movement of organelles to locations that are determined by the physiological needs of the cell. A significant number of these structures are located in the plasmalemma, near the cell membrane, where they are involved in the formation and orientation of cellulose microfibrils of plant cell membranes.
Microtubule structure
Vacuole
The vacuole is the most important component of plant cells. It is a kind of cavity (reservoir) in the mass of the cytoplasm, filled with an aqueous solution of mineral salts, amino acids, organic acids, pigments, carbohydrates and separated from the cytoplasm by a vacuolar membrane - the tonoplast.
The cytoplasm fills the entire internal cavity only in the youngest plant cells. With the growth of the cell, the spatial arrangement of the initially continuous mass of the cytoplasm changes significantly: small vacuoles filled with cell sap appear in it, and the entire mass becomes spongy. With further cell growth, individual vacuoles merge, pushing the cytoplasmic layers to the periphery, as a result of which there is usually one large vacuole in the formed cell, and the cytoplasm with all organelles are located near the membrane.
Water-soluble organic and mineral compounds of vacuoles determine the corresponding osmotic properties of living cells. This solution of a certain concentration is a kind of osmotic pump for controlled penetration into the cell and the release of water, ions and metabolite molecules from it.
In combination with the cytoplasm layer and its membranes, which are characterized by semipermeability properties, the vacuole forms an effective osmotic system. Osmotically determined are such indicators of living plant cells as osmotic potential, suction force and turgor pressure.
The structure of the vacuole
plastids
Plastids are the largest (after the nucleus) cytoplasmic organelles, inherent only in plant cells. They are not found only in fungi. Plastids play an important role in metabolism. They are separated from the cytoplasm by a double membrane membrane, and some of their types have a well-developed and ordered system of internal membranes. All plastids are of the same origin.
Chloroplasts- the most common and most functionally important plastids of photoautotrophic organisms that carry out photosynthetic processes that ultimately lead to the formation of organic substances and the release of free oxygen. Chloroplasts of higher plants have a complex internal structure.
The structure of the chloroplast
The sizes of chloroplasts in different plants are not the same, but on average their diameter is 4-6 microns. Chloroplasts are able to move under the influence of the movement of the cytoplasm. In addition, under the influence of illumination, an active movement of amoeboid-type chloroplasts to the light source is observed.
Chlorophyll is the main substance of chloroplasts. Thanks to chlorophyll, green plants are able to use light energy.
Leucoplasts(colorless plastids) are clearly marked bodies of the cytoplasm. Their sizes are somewhat smaller than the sizes of chloroplasts. More uniform and their shape, approaching the spherical.
The structure of the leucoplast
They are found in the cells of the epidermis, tubers, rhizomes. When illuminated, they very quickly turn into chloroplasts with a corresponding change in the internal structure. Leucoplasts contain enzymes, with the help of which starch is synthesized from excess glucose formed during photosynthesis, the bulk of which is deposited in storage tissues or organs (tubers, rhizomes, seeds) in the form of starch grains. In some plants, fats are deposited in leukoplasts. The reserve function of leukoplasts occasionally manifests itself in the formation of storage proteins in the form of crystals or amorphous inclusions.
Chromoplasts in most cases they are derivatives of chloroplasts, occasionally - leukoplasts.
The structure of the chromoplast
Ripening of rose hips, peppers, tomatoes is accompanied by the transformation of chloro- or leukoplasts of pulp cells into carotenoids. The latter contain predominantly yellow plastid pigments - carotenoids, which, upon maturation, are intensively synthesized in them, forming colored lipid drops, solid globules or crystals. Chlorophyll is destroyed.
Mitochondria
Mitochondria are organelles found in most plant cells. They have a variable shape of sticks, grains, threads. They were discovered in 1894 by R. Altman using a light microscope, and the internal structure was later studied using an electronic one.
The structure of the mitochondria
Mitochondria have a two-membrane structure. The outer membrane is smooth, the inner one forms outgrowths of various shapes - tubules in plant cells. The space inside the mitochondria is filled with semi-liquid content (matrix), which includes enzymes, proteins, lipids, calcium and magnesium salts, vitamins, as well as RNA, DNA and ribosomes. The mitochondrial enzyme complex accelerates the work of a complex and interconnected mechanism of biochemical reactions, as a result of which ATP is formed. In these organelles, cells are provided with energy - the energy of chemical bonds of nutrients is converted into high-energy bonds of ATP in the process of cellular respiration. It is in the mitochondria that the enzymatic breakdown of carbohydrates, fatty acids, amino acids occurs with the release of energy and its subsequent conversion into ATP energy. The accumulated energy is spent on growth processes, on new syntheses, etc. Mitochondria reproduce by division and live for about 10 days, after which they are destroyed.
Endoplasmic reticulum
Endoplasmic reticulum - a network of channels, tubules, vesicles, cisterns located inside the cytoplasm. Opened in 1945 by the English scientist K. Porter, it is a system of membranes with an ultramicroscopic structure.
The structure of the endoplasmic reticulum
The entire network is integrated into a single whole with the outer cell membrane of the nuclear envelope. Distinguish ER smooth and rough, carrying ribosomes. On the membranes of the smooth EPS there are enzyme systems involved in fat and carbohydrate metabolism. This type of membrane prevails in seed cells rich in reserve substances (proteins, carbohydrates, oils), ribosomes are attached to the membrane of the granular ER, and during the synthesis of a protein molecule, the polypeptide chain with ribosomes is immersed in the ER channel. The functions of the endoplasmic reticulum are very diverse: the transport of substances both inside the cell and between neighboring cells; division of a cell into separate sections in which various physiological processes and chemical reactions take place simultaneously.
Ribosomes
Ribosomes are non-membrane cellular organelles. Each ribosome consists of two unequal-sized particles and can be divided into two fragments that continue to retain the ability to synthesize protein after combining into a whole ribosome.
The structure of the ribosome
Ribosomes are synthesized in the nucleus, then leave it, passing into the cytoplasm, where they are attached to the outer surface of the membranes of the endoplasmic reticulum or are located freely. Depending on the type of protein synthesized, ribosomes can function alone or combine into complexes - polyribosomes.