Cage

According to modern concepts, each cell is a universal structural and functional unit of life. The cells of all living organisms have a similar structure. Cells reproduce only by division.

A cell (cellula) is an elementary ordered unit of life. It performs the functions of recognition, metabolism and energy, reproduction, growth and regeneration, adaptation to changing conditions of the internal and external environment. Cells are diverse in their shape, 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 a few micrometers (small lymphocyte) to 200 micrometers (egg cell).

The contents of each cell are separated from the environment and neighboring cells by the cytolemma (plasmolemma), which ensures the cell's relationship with the extracellular environment. The cell's constituent components, located inside the cytolemma, are the nucleus and cytoplasm, which consists of hyaloplasm and organelles and inclusions located in it.

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Cytolemma

The cytolemma, or plasmalemma, is a cell membrane 9-10 nm thick. It performs dividing and protective functions, and perceives environmental influences due to the presence of receptors (reception function). The cytolemma, performing exchange and transport functions, transfers various molecules (particles) from the environment surrounding the cell into the cell and in the opposite direction. The process of transfer into the cell is called endocytosis. Endocytosis is divided into phagocytosis and pinocytosis. During phagocytosis, the cell captures and absorbs large particles (particles of dead cells, microorganisms). During pinocytosis, the cytolemma forms protrusions that turn into vesicles, which involve small particles dissolved or suspended in tissue fluid. Pinocytotic vesicles mix the particles that are in them into the cell.

The cytolemma also participates in the removal of substances from the cell - exocytosis. Exocytosis is carried out with the help of vesicles, vacuoles, in which the substances removed from the cell first move to the cytolemma. The membrane of the vesicles merges with the cytolemma, and their contents enter the extracellular environment.

The receptor function is carried out on the surface of the cytolemma with the help of glycolipids and glycoproteins, which are capable of recognizing chemical substances and physical factors. Cell receptors can distinguish such biologically active substances as hormones, mediators, etc. Reception of the cytolemma is the most important link in intercellular interactions.

In the cytolemma, which is a semipermeable biological membrane, three layers are distinguished: outer, intermediate and inner. The outer and inner layers of the cytolemma, each about 2.5 nm thick, form an electron-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 - cytochrome, glycolipids, the carbohydrate chains of which are directed outward; in the inner monolayer facing the cytoplasm - cholesterol molecules, ATP synthetase. Protein molecules are located in the thickness of the cytolemma. Some of them (integral, or transmembrane) 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, and others are carriers of various substances, since they perform transport functions.

The outer surface of the cytolemma is covered with a thin-fibrillar layer (from 7.5 to 200 nm) of 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 lipids and proteins of the cytolemma.

The cytolemma on the surface of some cells forms specialized structures: microvilli, cilia, intercellular connections.

Microvilli (microvilli) are up to 1-2 µm long and up to 0.1 µm in diameter. They are finger-like outgrowths covered with cytolemma. In the center of the microvilli are bundles of parallel actin filaments attached to the cytolemma at the top of the microvilli and on its sides. Microvilli increase the free surface of the cells. In leukocytes and connective tissue cells, the microvilli are short, in intestinal epithelium they are long, and there are so many of them that they form the so-called brush border. Due to the actin filaments, the microvilli are mobile.

Cilia and flagella are also mobile, their movements are pendulum-shaped, wave-like. The free surface of the ciliated epithelium of the respiratory tract, vas deferens, and fallopian tubes is covered with cilia up to 5-15 μm long and 0.15-0.25 μm in diameter. In the center of each cilium there is an axial filament (axoneme) formed by nine peripheral double microtubules connected to each other, which 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 also consisting of microtubules. Flagella are similar in structure to cilia, they perform coordinated oscillatory movements due to the sliding of microtubules relative to each other.

The cytolemma is involved in the formation of intercellular connections.

Intercellular junctions are formed at the points of contact between cells, they provide intercellular interactions. Such junctions (contacts) are divided into simple, dentate and dense. A simple junction is the convergence of the cytolemmas of neighboring cells (intercellular space) at a distance of 15-20 nm. In a dentate junction, the protrusions (teeth) of the cytolemma of one cell enter (wedge) between the teeth of another cell. If the protrusions of the cytolemma are long, deeply enter between the same protrusions of another cell, then such junctions are called finger-like (interdigitations).

In special dense intercellular junctions, the cytolemma of neighboring cells is so close that they merge with each other. This creates a so-called blocking zone, impermeable to molecules. If a dense connection of the cytolemma occurs in a limited area, then a spot of adhesion (desmosome) is formed. A desmosome is a high-electron-density area with a diameter of up to 1.5 μm, which performs the function of mechanically connecting one cell with another. Such contacts are more common between epithelial cells.

There are also gap-like connections (nexuses), the length of which reaches 2-3 µm. Cytolemmas in such connections are spaced from each other by 2-3 nm. Ions and molecules easily pass through such contacts. Therefore, nexuses are also called conducting connections. For example, in the myocardium, excitation is transmitted from one cardiomyocyte to another through nexuses.

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Hyaloplasm

Hyaloplasm (hyaloplasma; from the Greek hyalinos - transparent) makes up approximately 53-55% of the total volume of cytoplasm, forming a homogeneous mass of complex composition. The hyaloplasm contains proteins, polysaccharides, nucleic acids, and enzymes. With the participation of ribosomes, proteins are synthesized in the hyaloplasm, and various intermediate exchange reactions occur. The hyaloplasm also contains organelles, inclusions, and the cell nucleus.

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Cell organelles

Organelles (organellae) are obligatory microstructures for all cells, performing certain vital functions. A distinction is made between membrane and non-membrane organelles. Membrane organelles, separated from the surrounding hyaloplasm by membranes, include the endoplasmic reticulum, the internal mesh apparatus (Golgi complex), lysosomes, peroxisomes, and mitochondria.

Membrane organelles of the cell

All membrane organelles are built from elementary membranes, the principle of organization of which is similar to the structure of cytolemmas. Cytophysiological processes are associated with constant adhesion, fusion and separation of membranes, while adhesion and unification of only topologically identical membrane monolayers are possible. Thus, the outer layer of any organelle membrane facing the hyaloplasm is identical to the inner layer of the cytolemma, and the inner layer facing the cavity of the organelle is similar to the outer layer of the cytolemma.

Membrane organelles of the cell

Non-membrane organelles of the cell

Non-membrane organelles of the cell include centrioles, microtubules, filaments, ribosomes and polysomes.

Non-membrane organelles of the cell

Transport of substances and membranes in the cell

Substances circulate in the cell, being packed in membranes ("movement of the contents of the cell in containers"). Sorting of substances and their movement are associated with the presence of special receptor proteins in the membranes of the Golgi complex. 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 expenditure, active transport is energy-dependent.

Transport of substances and membranes in the cell

Cell nucleus

The nucleus (s. karyon) is present in all human cells except erythrocytes and thrombocytes. The functions of the nucleus are to store and transmit hereditary information to new (daughter) cells. These functions are associated with the presence of DNA in the nucleus. The synthesis of proteins - ribonucleic acid RNA and ribosomal materials - also occurs in the nucleus.

Cell nucleus

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Cell division. Cell cycle

The growth of an organism occurs due to the increase in the number of cells through division. The main methods of cell division in the human body are mitosis and meiosis. The processes that occur during these methods of cell division proceed in the same way, but lead to different results.

Cell division: cell cycle

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