Many believe that the cell represents the lowest level of organization of living matter. However, in reality, the cell is a complex organism, the development of which from a primitive form that first appeared on Earth and resembled the current virus took hundreds of billions of years. The figure below is a diagram showing the relative sizes of: (1) the smallest known virus; (2) a large virus; (3) rickettsiae; (4) bacteria; (5) nucleated cell. The figure shows that the cell diameter is 10 and the volume is 10 times the size of the smallest virus.
Features of the structure and function of cells are many times more complex than those of viruses.
The lifeblood of the virus lies in nucleic acid molecule coated with a protein coat. Nucleic acid, as in mammalian cells, is either DNA or RNA, which, under certain conditions, are capable of self-copying. Thus, the virus, like human cells, reproduces from generation to generation, maintaining its "kind".
As a result of evolution into the composition of the body along with nucleic acids and simple proteins entered other substances, and various sections of the virus began to perform specialized functions. A membrane formed around the virus, a liquid matrix appeared. The substances formed in the matrix began to perform special functions, enzymes appeared that were able to catalyze a number of chemical reactions, which ultimately determine the vital activity of the organism.
At the next stages of development, in particular at the stages rickettsia and bacteria, intracellular organelles appear, with the help of which individual functions are performed more efficiently than with the help of substances diffusely distributed in the matrix.
Finally, in a nucleated cell more complex organelles arise, the most important of which is the nucleus itself. The presence of a nucleus distinguishes this cell type from lower life forms; the nucleus exercises control over all the functions of the cell and organizes the process of division in such a way that the next generation of cells turns out to be almost identical to the precursor cell.
Comparative sizes of pre-nuclear structures with a cell of the human body.Endocytosis- uptake of substances by the cell. A living, growing and dividing cell must receive nutrients and other substances from the surrounding fluid. Most of the substances penetrate the membrane by diffusion and active transport. Diffusion is a simple random transfer of substance molecules through the membrane, which penetrate into the cell more often through pores, and fat-soluble substances directly through the lipid bilayer.
active transport- is the transport of substances through the thickness of the membrane with the help of a carrier protein. The mechanisms of active transport are extremely important for cell activity.
Large particles enter the cell by a process called endocytosis. The main types of endocytosis are pinocytosis and phagocytosis. Pinocytosis is the capture and transfer of small vesicles with extracellular fluid and microparticles into the cytoplasm. Phagocytosis ensures the capture of large elements, including bacteria, whole cells, or fragments of damaged tissues.
pinocytosis. Pinocytosis occurs constantly, and in some cells it is very active. Thus, in macrophages, this process occurs so intensively that in 1 min about 3% of the total membrane area is converted into vesicles. However, the size of the bubbles is extremely small - only 100-200 nm in diameter, so they can only be seen with electron microscopy.
pinocytosis- the only way by which most macromolecules can enter the cell. The intensity of pinocytosis increases when such molecules come into contact with the membrane.
Typically, proteins attach to surface receptors membranes, which are highly specific for absorbable protein species. Receptors are concentrated mainly in the region of the smallest depressions on the outer surface of the membrane, which are called bordered pits. The bottom of the pits from the side of the cytoplasm is lined with a reticular structure of the fibrillar protein clathrin, which, like other contractile proteins, contains actin and myosin filaments. The attachment of a protein molecule to the receptor changes the shape of the membrane in the fovea due to contractile proteins: its edges close, the membrane sinks more and more into the cytoplasm, capturing protein molecules along with a small amount of extracellular fluid. Immediately after the edges close, the vesicle separates from the outer membrane of the cell and a pinocytic vacuole forms inside the cytoplasm.
It is not yet clear why the deformation occurs membranes necessary for the formation of bubbles. It is known that this process is energy dependent; requires macroergic substance ATP, the role of which is discussed below. The presence of calcium ions in the extracellular fluid, in all likelihood, is also necessary for interaction with the bordered pits with contractile filaments lying in the bottom area, which create the force necessary to split the vesicles from the outer cell membrane.
Rice. 18. Phagocytosis and pinocytosis.
macromolecular structure. 2. Beginning of phagocytosis. 3.Phagosome. 4.Phagolysosome (secondary lysosome). 5. Granular EPS. 6. Mitochondria. 7. Primary lysosome. 8.Endosome. 9. Golgi apparatus. 10. Fragment of the core. 11. Plasma membrane. 12. Residual body.
There are several ways of endocytosis (from Greek endon - inside, kytos - cell: pinocytosis (from Greek pino - I drink) and phagocytosis (from Greek phagos - devouring). During pinocytosis, the cell captures liquid colloidal particles, and during phagocytosis - dense particles (macromolecular complexes, cell parts, bacteria, etc.) Plasmolemma (11) and glycocalyx play an active role in these processes.
During pinocytosis and phagocytosis, the particles taken up by the cell interact with the plasmalemma and are surrounded by it (2). Liquid particles are additionally bordered by the protein clathrin (bordered vesicles) (12). Subsequently, both during pinocytosis and during phagocytosis, the substances captured by the cell interact with lysosomes.
Phagocytosis is characteristic of macrophage cells, which are located in loose connective tissue in each organ, neutrophils, etc. During phagocytosis of bacteria, cell parts by specialized cells, the phagocytosed particle interacts with receptors on the cell surface and phagocytosis is activated with a change in intracellular calcium content. This leads to a change in the polymerization of thin microfilaments and microtubules, causes the formation of a protrusion of the cytolemma (pseudopodia) with the immersion of a large particle inside the cell (formation of a phagosome). In the future, endocytic vesicles (endosomes) (8) can merge with each other and inside the vesicles, in addition to the absorbed substances, hydrolytic enzymes are found that come from lysosomes. Enzymes break down biopolymers to monomers, which, as a result of active transport through the vesicle membrane, pass into the hyaloplasm. Thus, the absorbed molecules inside the membrane vacuoles formed from the elements of the plasmolemma undergo intracellular digestion.
These two processes, which occur with the absorption of energy, ensure that even larger particles enter the cell than those that penetrate through the pores of membranes of the fourth type.
A. Pinocytosis. In pinocytosis, the membrane (usually the first type of membrane) forms invaginations, which eventually transform into vesicles.
In this way, the penetration of molecules through the membrane, the size of which is too large for them to diffuse in the usual way, especially proteins, is carried out. Due to pinocytosis, substances that were outside the cell are inside it and vice versa.
B. Phagocytosis. Due to phagocytosis, which has a certain similarity with pinocytosis, even larger particles move. Thus, it has been clearly shown by electron microscopy that solid particles pass through the cell membranes of capillaries in mammals, and the entire surface of the capillary can apparently be used for this purpose. Enzymes and hormones are often squeezed out of cells in the form of bubbles enclosed in a lipid membrane. It is in this way that the five hydrolytic proenzymes of the pancreas are extruded all together in the form of the so-called "zymogen granules". This is also the origin of the vesicles in which ACh is secreted by nerve endings, as well as the granules in the form of which norepinephrine is released from the adrenal medulla.
More on Pinocytosis and Phagocytosis:
- ACQUIRED PHAGOCYTOSIS DISORDERS AND POSSIBLE REASONS FOR THEIR DEVELOPMENT
Endocytosis is a reaction of cells aimed at the absorption and digestion of soluble macromolecular compounds, as well as foreign or structurally altered own cells. The term "endocytosis" is generalizing for two close, but, nevertheless, independent processes - pinocytosis and phagocytosis. The first of them is characterized by the absorption and intracellular destruction of macromolecular compounds, such as proteins, nucleic acids, polysaccharides, lipoproteins, protein complexes. At the same time, phagocytosis is the phenomenon of absorption and digestion by a cell (macrophages, neutrophils) of corpuscular material (bacteria, large viruses, dying own cells of the body or foreign cells, such as, for example, erythrocytes of various types).
The object of pinocytosis as a factor of nonspecific immune defense are, in particular, the toxins of microorganisms.
On fig. B.1 shows the successive stages of capture and intracellular digestion of soluble macromolecules located in the extracellular space. The adhesion of such molecules on the cell can be carried out in two ways: non-specific - as a result of a random meeting of molecules with the cell, and specific, which depends on pre-existing
Rice. IN 1. Endocytosis of macromolecules by phagocytes.
PM - soluble macromolecules; RC - receptor; PP - pinocytic vesicle; PS - pinosoma
receptors on the surface of the pinocytic cell. In the latter case, extracellular substances act as ligands interacting with the corresponding receptors. Adhesion
nation (invagination) of the membrane, culminating in the formation of a pinocytic vesicle of a very small size (approximately OD c). Several merged vesicles form a larger formation - shsosome. At the next stage, pinosomes fuse with lysosomes containing hydrolytic enzymes that break down polymer molecules into monomers. In those cases when the process of pinocytosis is realized through the receptor apparatus, in pinosomes, before merging with lysosomes, detachment of the captured molecules from receptors is observed, which in combination
Phagocytosis as a factor of nonspecific protection manifests itself when pathogenic microbes enter the body. Random or receptor-mediated contact of a microbial cell with a phagocyte (macrophage, neutrophil) leads to the formation of membrane outgrowths - pseudopodia surrounding the foreign cell. The formed vacuole (phagosome) is 10-20 times larger than the pinosome. It enters the cell, where, after fusion with lysosomes, it forms a phagolysosome. It is in it that, due to the activity of hydrolytic enzymes, complete or
the microbial cell is removed into the extracellular environment, the other remains on the surface of the phagocytic cell (Fig. B.2). 
Rice. AT 2. phagocytosis of bacteria.
B - bacteria; P - pseudopodia; FS - phagosome; FLS - phagalisosome
Many cells of the immune system have the property of phagocytosis. Phagocytosis is a phenomenon in which one cell “devours” another.
The ability of some shreds to phagocytize others was discovered by II Mechnikov, who placed the spores of the fungus into the body of the daphnia crustacean and observed how the spores were attacked by the cells of the crustacean, absorbed and digested.
Phagocytosis consists of 8 stages:
The approach of a phagocyte to a microbial cell, which is possible due to chemotaxis - movement along a chemical trail.
Adhesion of the phagocyte to the object of absorption. Perhaps this is due to the presence on the surface of the phagocyte of specific receptors for a particular object, that is, peculiar chemical locks, with the help of which the microorganism or its part is "fastened" to the phagocyte.
After the object sticks, the phagocyte membrane must prepare for its absorption, this happens under the influence of the C-protein kinase enzyme.
After the phagocyte membrane becomes ready, the object is immersed in the cytoplasm.
When immersed, the part of the phagocyte membrane that is in contact with the object bends into the inside of the cell, gradually enveloping the object, as a result of which a shell of the phagocyte membrane is formed around the object. The object surrounded by a shell is called a phagosome.
The resulting phagosome merges with lysosomes, which are microscopic vesicles containing many enzymes that break down proteins, fats and carbohydrates. As a result of this merger,
Splitting an object.
Phagocytosis ends with the release of the digested remains of the object, which will no longer bring any harm to the body.
The object of phagocytosis can be bacteria, viruses, fungi, and other particles that are not genetically related to the organism.
If the object splits, then phagocytosis is called complete, if the object survived, then incomplete.
pinocytosis (from the Greek. Foam - I drink, I absorb) - the capture and absorption of liquid by the cell along with the compounds dissolved in them. The process of pinocytosis is similar to phagocytosis, but occurs mainly due to membrane penetration. Pinocytosis is observed in cells of various organisms.
10)Exocytosis(from Greek Έξω - external and κύτος - cell) - at eukaryote cellular process in which intracellular vesicles (membrane vesicles) fuse with the outer cell membrane. During exocytosis, the contents of secretory vesicles (exocytic vesicles) are released outside, and their membrane merges with the cell membrane. Almost all macromolecular compounds (proteins, peptide hormones, etc.) are released from the cell in this way.
Exocytosis can perform three main tasks:
delivery to the cell membrane of lipids necessary for cell growth;
release of various compounds from the cell, such as toxic products metabolism or signal molecules ( hormones or neurotransmitters);
delivery to the cell membrane of functional membrane proteins, such as receptors or transporter proteins. In this case, the part of the protein that was directed inside the secretory vesicle turns out to be protruding on the outer surface of the cell