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Last reviewed: 23.04.2024
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According to modern ideas, each cell is a universal structural-functional unit of the living. The cells of all living organisms have a similar structure. The cells multiply only by division.
Cell (cellula) is the elementary ordered unit of the living. It performs the functions of reviewing (recognition), metabolism and energy, reproduction, growth and regeneration, adaptation to the changing conditions of the internal and external environment. Cells are diverse in form, structure, chemical composition and functions. In the human body there are flat, spherical, ovoid, cubic, prismatic, pyramidal, stellate cells. There are cells ranging in size from several micrometers (small lymphocyte) to 200 micrometers (egg).
From the environment and neighboring cells, the contents of each cell are separated by a cytolemma (plasmolemma), which ensures the relationship of the cell with the extracellular environment. The constituent components of the cell located inside the cytolemma are the nucleus and cytoplasm, which consists of the hyaloplasm and the organelles and inclusions located in it.
[Cytolemma
Cytolemma (cytolemma), or plasmolemma, is a cell membrane 9-10 nm thick. It performs separation and protective functions, perceives environmental influences due to the presence of receptors (reception function). The cytolemma, carrying out metabolic, transport functions, carries out the transfer of various molecules (particles) from the environment surrounding the cell to the interior of the cell and in the opposite direction. The process of transfer into the cell is called endocytosis. Endocytosis is divided into phagocytosis and pinocytosis. When phagocytosis, the cell captures and absorbs large particles (particles of dead cells, microorganisms). In pinocytosis, the cytolemma forms protrusions that turn into vesicles in which small particles are dissolved, dissolved or suspended in the tissue fluid. Pinocytosis vesicles mix the particles in them into the cell.
The cytolemma is also involved in excretion of substances from the cell - exocytosis. Exocytosis is carried out with the help of vesicles, vacuoles, in which the substances withdrawn from the cell move first to the cytolemma. The vesicle envelope merges with the cytolemma, and their contents enter the extracellular environment.
The receptor function is performed on the surface of the cytolemma with the help of glycolipids and gl and to proteins, which are capable of recognizing chemicals and physical factors. The receptors of a cell can distinguish such biologically active substances as hormones, mediators, etc. Cytolemma receptor is the most important link in intercellular interactions.
In the cytolemma, which is a semipermeable biological membrane, three layers are distinguished: external, intermediate and internal. The outer and inner layers of the cytolemma, about 2.5 nm in thickness, form an electronically dense lipid double layer (bilayer). Between these layers is an electron-light hydrophobic zone of lipid molecules, its thickness is about 3 nm. In each monolayer of the lipid bilayer there are different lipids: in the outer layer - cytochrome, glycolipids, carbohydrate chains of which are directed to the outside; in the inner monolayer facing the cytoplasm, the molecules of cholesterol, ATP synthetase. Molecules of protein are located in the thickness of the cytolemma. Some of them (integral, or transmembrannye) pass through the entire thickness of the cytolemma. Other proteins (peripheral, or external) lie in the inner or outer monolayer of the membrane. Membrane proteins perform various functions: some are receptors, others are enzymes, others are carriers of various substances, since they perform transport functions.
The outer surface of the cytolemma is covered with a fine-fibrillar layer (from 7.5 to 200 nm) of the glycocalyx. Glycocalyx (glycocalyx) is formed by the side carbohydrate chains of glycolipids, glycoproteins and other carbohydrate compounds. Carbohydrates in the form of polysaccharides form branching chains connected by slipids and cytolemma proteins.
The cytolemma forms specialized structures on the surface of some cells: microvilli, cilia, intercellular connections.
Microvilli (microvilli) with a length of up to 1 -2 microns and a diameter of up to 0.1 microns is a digitally covered finger-shaped growth. In the center of the microvilli there are bundles of parallel ac-tine filaments attached to the cytolemma at the tip of the microvillus and along its sides. Microvilli increase the free surface of cells. In leukocytes and cells of connective tissue, microvilli are short, in the intestinal epithelium - long, and there are so many of them that they form the so-called brush border. Thanks to actin filaments, microvilli are mobile.
Cilia and flagella are also mobile, their movements pendulum-shaped, wavy. The free surface of the ciliated epithelium of the respiratory tract, the vas deferens, the fallopian tubes is covered with cilia up to 5-15 μm in length and 0.15-0.25 μm in diameter. In the center of each cilium there is an axial filament (axoneme) formed by nine interconnected peripheral double microtubules that surround the axoneme. The initial (proximal) part of the microtubule ends in the form of a basal body located in the cytoplasm of the cell and consisting also of microtubules. Flagellum is similar in structure to cilia, they perform coordinated oscillatory movements due to slip of microtubules relative to each other.
The cytolemma is involved in the formation of intercellular compounds.
Intercellular connections are formed at the points of contact of cells with each other, they provide intercellular interactions. Such connections (contacts) are divided into simple, dentate and dense. A simple connection is the cytolemma of neighboring cells (intercellular space) approaching a distance equal to 15-20 nm. When the jagged connection protrusions (crenellations) of the cytolemma of one cell come (wedged in) between the teeth of another cell. If the protuberances of the cytolemma are long, go deep between the same protuberances of another cell, then such compounds are called finger-like (interdigitation).
In special dense intercellular connections, the cytolemma of neighboring cells is so close that they merge with each other. This creates a so-called locking zone, impermeable to molecules. If a dense junction of the cytomegma occurs in a restricted area, an adhesion spot (desmosome) is formed. The desmosome is a site of high electron density up to 1.5 μm in diameter, performing the function of mechanical coupling of one cell to another. Such contacts are more often found between epithelial cells.
Slit-like compounds (nexus), whose length reaches 2-3 microns, also occur. The cytolemmas of such compounds are spaced from each other by 2-3 nm. Through such contacts, ions and molecules pass easily. Therefore, nexus is also called a conductive compound. So, for example, in the myocardium through neksusy excitation is transmitted from one cardiomyocyte to another.
Hyaloplasm
Hyaloplasm (hyaloplasma, from the Greek hyalinos - transparent) is approximately 53-55% of the total cytoplasm volume, forming a homogeneous mass of complex composition. In the hyaloplasm there are proteins, polysaccharides, nucleic acids, enzymes. With the participation of ribosomes, proteins are synthesized in the hyaloplasm, various reactions of intermediate metabolism occur. In the hyaloplasm there are also organelles, inclusions and the cell nucleus.
Cell Organelles
Organelles (organellae) are mandatory microstructures for all cells performing certain vital functions. There are membrane and non-membrane organelles. Membrane organelles, separated from the surrounding membrane by the hyaloplasm, include the endoplasmic reticulum, the inner reticular apparatus (Golgi complex), lysosomes, peroxisomes, mitochondria.
Membrane cell organelles
All membrane organelles are constructed from elementary membranes, the principle of organization of which is similar to the structure of cytolemmas. Cytofiziologicheskie processes are associated with the constant adhesion, fusion and separation of membranes, while sticking and unification of only topologically identical monolayers of membranes is possible. Thus, the outer layer of any membrane of the organelle facing the hyaloplasm is identical to the inner layer of the cytolemma, and the inner layer facing the organelle is similar to the outer layer of the cytolemma.
Membrane cell organelles
Non-membrane organelles of the cell include centrioles, microtubules, filaments, ribosomes and polysomes.
Transport of substances and membranes in a cell
Substances circulate in the cell, being packed into membranes ("movement of the cell's contents in containers"). Sorting of substances and their movement are associated with the presence in the membranes of the Golgi complex of special receptor proteins. Transport through membranes, including through the plasma membrane (cytolemma), is one of the most important functions of living cells. There are two types of transport: passive and active. Passive transport does not require energy costs, active transport is volatile.
Transport of substances and membranes in a cell
Cell nucleus
The nucleus (nucleus, s. Karyon) is present in all human cells, except for erythrocytes and platelets. Kernel functions - storage and transfer to the new (child) cells of hereditary information. These functions are related to the presence of DNA in the nucleus. In the nucleus there is also a synthesis of proteins - ribonucleic acid RNA and ribosomal materials.
Cell division. Cell cycle
The growth of the body occurs due to the increase in the number of cells by division. The main methods of cell division in the human body are mitosis and meiosis. The processes occurring in these methods of cell division proceed in the same way, but lead to different results.