BIOCHEMISTRY FOR TEACHERS - F.F. BOIECHKO - 1985

FORMATION OF LIVING SYSTEMS AND THEIR STRUCTURAL ORGANIZATION

CHARACTERISTICS OF THE MAIN STRUCTURAL COMPONENTS OF THE CELL

Membrane systems are vital structural components for various types of PLANT AND ANIMAL Cells.

The cellular membrane system encompasses the outer Cell/33.html">Plasma Membrane, as well as a complex network of internal membranes (endomembranes) and the membranes of cell Organelles such as Mitochondria, Plastids, and, in eukaryotes, the Cell Nucleus. Each membrane is a rather complex formation with a characteristic Structure, composition, and specific Functions.

Membrane thickness is only 6—12 nm, yet they possess high mechanical strength, resistance, flexibility, and lability. In these respects, biological membranes surpass many natural, artificial, and synthetic engineering Materials.

The total surface area of membranes in the body's Organs and Tissues is quite large. On average, 1 g of Liver Endoplasmic reticulum protein accounts for 50 m² of membrane surface. Despite the microscopic size of The Cell, such a vast membrane area effectively Supports diverse metabolic processes.

Membranes of different cells and organs vary somewhat in both Chemical Composition and the relative content of their components, which plays an essential role in providing a wide range of physiological activities. However, regardless of their morphological Structure and Chemical composition, membranes serve as a highly effective means of localizing intracellular processes.

Each membrane separates the cytoplasmic space from the non-cytoplasmic one, participates in The formation of cell compartments—regions with diverse metabolic activity—and ensures that various biochemical reactions can proceed simultaneously within a single cell. By dividing the cell into numerous isolated compartments, membranes preserve specific conditions and physicochemical parameters within each of them, such as solution pH, Temperature, solute concentration, and electrical potential.

Cell membranes are highly dynamic systems that remain in constant motion within the cell. They break down very rapidly and easily repair themselves over damaged areas, fuse with one another, and stretch or compress during movement and changes in cell shape.

Membranes play a critically important role in cellular activity. They regulate a vast number of metabolic processes both inside the cell and on its surface. Membranes participate in shaping The structure of the cell and its organelles. They enclose The Nucleus, form the framework of mitochondria and the Structure of METABOLISM/14.html">Chloroplasts, and constitute the Endoplasmic reticulum and the Golgi apparatus. Due to their physicochemical, biological, and structural features, membranes perform a whole range of vital functions, such as regulating the Transport of Molecules and ions, ensuring specific reception, cell "recognition," and determining antigenic Specificity.

Membranes also take part in the cell's energy processes, Nerve Impulse transmission, and photosynthetic reactions. Owing to their submicroscopic structure, an electrical potential difference is generated, established, and maintained between the outer and inner sides of the membranes, playing a crucial role in facilitating numerous metabolic reactions. Membranes exert a significant influence on various intracellular metabolic processes by coordinating certain enzymatic reactions. Membranes harbor an entire system of diverse Enzymes, each operating in precise coordination with the others. Furthermore, membranes substantially modulate The activity of various enzymes; Some enzymes are active only when in close contact with membranes, whereas others exhibit maximum catalytic activity upon release into the Cytoplasm. The body contains a multitude of membranes that, beyond their core functions, perform specialized tasks—such as the absorption and Digestion of food, Muscle contraction and relaxation, and the transformation of various stimuli into electrical impulses, among others.

The primary Structural components of most membranes are Proteins, Lipids, and CARBOHYDRATES. Each of these structural components possesses specific characteristic features and properties that dictate its contribution to the assembly of cellular structures.

The first model of a biological membrane was developed in the 1930s by American researchers. According to this model, protein molecules are located on the outer and inner surfaces of a bimolecular lipid layer. This creates a protein-lipid "sandwich" structure, with protein molecules on the top and bottom and lipids in the middle. This sandwich model of the membrane was long considered universal for all living systems. However, with the advent of new Research Methods, A number of new facts emerged that could not be explained by this Membrane Structure. Consequently, the sandwich model was gradually discarded and replaced by the fluid-mosaic model, which is currently universally accepted.

According to current understanding, The cell membrane is a heterogeneous structure consisting of two-dimensionally oriented solutions of Globular proteins and their aggregates (clusters) within a bimolecular layer of Complex Lipids. Membranes are often depicted as a lipid "sea" in which protein "icebergs" float (Fig. 3).

STRUCTURE AND FUNCTIONS of Membrane Lipids. The primary components of biological membranes are complex lipid molecules, which exhibit pronounced structural Asymmetry. Lipids consist of two parts: a polar HEAD bearing electrical charges, and a non-polar tail.

The polar portion accounts for no more than 1/3 of the molecule and contains phosphates, sugars, Choline, and colamine, which confer its charge. In the vast majority of cases, polar lipid heads carry a negative charge or are electroneutral.

The non-polar part of the molecule contains uncharged residues of Fatty acids and Other Compounds. The linking bridge between the polar and non-polar PARTS OF THE molecules is usually a glycerol residue (glycerolipids) or the amino alcohol sphingosine (Sphingolipids). The non-polar tails of lipids differ from one another in their degree of saturation, branching, and number of carbon atoms. The presence of double bonds in the hydrocarbon chains of the non-polar tails of lipid molecules is crucial for the normal functioning of biological membranes in living systems. It has been found that The rate of permeation of various substances through biological membranes depends on the degree of saturation of membrane lipids.

In addition to Phospholipids, many membranes—particularly in animal cells—also contain Glycolipids and Cholesterol. Cholesterol is present in exceptionally large amounts in outer membranes, notably in the Plasma Membranes of liver cells, where its content reaches approximately 30%.

The lipid content within membranes varies and depends on their type and function. Membranes with a high lipid content are formed where a distinct boundary with the external environment needs to be established—On the surface of cells, The endoplasmic reticulum, and vacuoles. The formation of lipid layers in membranes is biologically adaptive under adverse cellular conditions, or when creating insulating (dielectric) interlayers along The pathway of electron transport.

Structure and functions of Membrane Proteins. The vast majority of membrane protein molecules are voluminous structures—globules—and, much like lipids, are characterized by structural asymmetry. Due to the presence of Amino Acids with polar and non-polar radicals in their polypeptide chains, a protein molecule in an aqueous environment orients itself such that its non-polar regions, avoiding contact with Water, reside within the interior of the globule, while the polar regions are located on its surface. The parts of the molecules located on The surface of the globules protrude from the membrane and bear fixed charges. On the outer surface, the net charge is negative.

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Fig. 3. Mosaic model of the cell membrane.

All membrane proteins participate in diffusive movement, meaning their distribution on the membrane surface at any given moment is random. It is believed that proteins float within the lipid layer like icebergs. However, the speed of such movement varies among different proteins and is quite low; it depends not only on membrane viscosity but also on the depth to which the proteins are embedded in The Lipid Bilayer.

The protein content in various membranes is non-uniform and depends on the functions they perform. Protein content is high in membranes where numerous enzymatic reactions take place. For instance, the inner membranes of mitochondria contain over 75% protein. The cytoplasmic membrane also features a high protein content (50%). In contrast, in the myelin sheath membranes, which primarily serve an insulating role, the protein content accounts for only 20% of the total membrane mass.

Depending on the protein content within membranes, their Organization also varies. Protein molecules are distributed within the lipid structure in a mosaic pattern when present in small amounts. However, when they constitute a significant proportion, they form a perforated network whose meshes are filled with lipids.

Plasma membrane. The presence of a distinct cellular boundary—The Plasma Membrane—is crucial for the normal functioning of plant and animal organisms, as it regulates metabolism between the cell and its external environment. The thickness of the plasma membrane predominantly ranges from 6— 8 nm, making it visible only under an Electron microscope. The chemical composition of plasma membranes in plant and animal organisms is practically identical. It comprises proteins, lipids, Polysaccharides, enzymes, and various ions—essentially all the compound groups that make up a cell.

The plasma membrane acts as a unique regulator of metabolic processes within the cell and plays a vital role for the Organism as a whole. It is the site of diverse biochemical reactions upon which the functioning of the organism as a complex, highly organized living system depends. Membrane components directly participate in The transport of various substances, molecules, and ions into the cell, where they are utilized. These processes are frequently accompanied by the synthesis of macroergic compounds and the release of energy necessary to drive metabolic processes.

With the involvement of the plasma membrane, processes of specific reception, intercellular interaction, and information transfer between individual cells, organs, and body systems take place.

The plasma membrane contains a set of specific enzymes and receptors, The Nature of which is determined by the structural features and Functions of the cells.

Since cells assimilate substances from the external environment to sustain their vital activities, the barrier function of the plasma membrane—its ability to selectively allow certain substances to pass while restricting others—is of paramount importance.

The cellular plasma membrane is a heterogeneous structure. It consists of various regions that differ from one another in both Molecular structure and chemical composition. In some areas, the plasma membrane has a smooth surface, while in others it may be covered with microvilli or exhibit various folds depending on the functions performed by the cell. These formations can be temporary, arising under specific conditions (during the uptake or secretion of certain substances), or permanent.

The plasma membranes of specialized cells sometimes possess a unique structure. For instance, in intestinal epithelial cells, the plasma membrane forms invaginations known as microvilli, which significantly increase the surface area of the cell and enhance the efficiency of Metabolic exchange between the internal environment (cytoplasm) and the extracellular space. Deep invaginations of the plasma membrane in the renal tubules (the basal labyrinth) perform an important secretory function.

In bacterial cells, the fusion of concave membrane regions forms pockets containing lamellar, tubular, or vesicular bodies known as mesosomes. The internal space of mesosomes is partially in contact with the extracellular space. The functions of mesosomes are not yet fully understood, though they are believed to serve as attachment sites for the nuclear zone DNA.

Membrane-associated extracellular components significantly influence the Surface Properties, functions, and vital activity of the membrane. In many cases, these formations can only be observed under an electron microscope. These include, in particular, the extracellular coat found on the surface of animal cells, known as the glycocalyx.

The glycocalyx contains complex carbohydrate-protein compounds called Glycoproteins. Glycoproteins are arranged on the membrane surface in such a way that the C-terminus of the polypeptide chain protrudes from the inner side of the membrane, while the N-terminus, featuring numerous branched polysaccharide side chains, is located on the outer membrane surface.

Among the glycoprotein Components of the glycocalyx, erythrocyte membrane Glycophorin is one of the most thoroughly studied; it provides antigenic specificity and mediates the binding of various Viruses and Lectins. Glycophorin consists of approximately 60% carbohydrates. The functions of the glycocalyx are quite diverse. It is believed to play a critical role in reception, recognition, and intercellular interaction processes. The eggs of most animals feature a specialized type of glycocalyx on their surface known as the vitelline envelope. In mammals, there is additionally a thick, transparent outer coat separated from the plasma membrane by a fluid-filled space (the perivitelline space) containing carbohydrates, proteins, and glycoproteins. The vitelline envelope plays a crucial role in ensuring the specific functions of these cells, as its surface bears specific receptors that bind spermatozoa.

The surface of the plasma membrane in many cells often contains other extracellular substances that are metabolic byproducts of the cytoplasm (such as mucus, Chitin, and mineral salts), most of which play a vital role in cellular activity. The mucous secretions of many Algae facilitate Hydration processes, while the rigid chitinous exoskeletons of insects provide protective functions. Extracellular components also impart elasticity, structural resilience, and cellular adhesiveness, without which the existence of Multicellular Organisms would be impossible. In Bacteria, the outer layer of the cell envelope also determines immunological type and virulence.

In addition to the plasma membrane, plant cells possess a well-defined and relatively thick Cell wall—a cellulosic sheath. This structure supports the protoplast and maintains its specific shape. Consequently, unlike animal cells, most plant and bacterial cells are unable to change their shape.

Membrane transport. The plasma membrane plays a crucial role in fulfilling a barrier function, which involves regulating the influx of various substances into the cell and their efflux into the extracellular space. Under normal physiological conditions, a continuous exchange of various substances and ions takes place between the cell and its environment, meaning membranes cannot be entirely impermeable. Even when a membrane is at rest and its channels are closed, a fraction of substances can still diffuse across it. For instance, water molecules, small molecules of other substances, and certain ions readily cross the membrane.

The plasma membrane exhibits osmotic properties. Osmosis is the unidirectional diffusion of a solvent into a solution through a semipermeable membrane. It occurs when a semipermeable membrane separates two solutions of differing concentrations, causing the solvent to move from the lower-concentration solution to the higher-concentration solution. When cells are placed in pure water, it enters the cell following The water potential gradient (φ). As a result of osmotic water uptake, the cells swell and eventually rupture (hemolysis). This behavior is observed in cells placed in solutions whose osmotic pressure is lower than that of the intracellular contents (hypotonic solutions). In solutions with an osmotic pressure higher than that of the intracellular contents (hypertonic solutions), plasmolysis occurs, causing the cells to shrink due to water efflux.

Solutions whose osmotic pressure equals that of the cell contents are called isotonic; they do not alter the volume of the intracellular contents and are used for intravenous and interstitial administration in various Metabolic Disorders.

Various substances can penetrate the cytoplasmic membrane via passive transport along a concentration gradient, or through Facilitated Diffusion mediated by membrane carriers.

The transport of relatively large particles and liquid droplets across the plasma membrane involves Endocytosis and Exocytosis.

The mechanisms of membrane transport for various substances are not yet fully understood. It is widely believed that passive transport is driven by the kinetic energy of the transported molecules. Lipophilic substances undergo transport most readily, as they can dissolve in membrane lipids and diffuse in accordance with the laws of diffusion. Passive transport of hydrophilic substances occurs through submicroscopic pores (transient voids)—unstable structural formations that arise from temporary, localized reorganizations of the dynamic membrane's molecular architecture.

The transport of charged molecules and ions depends not only on the concentration gradient, but also on the electrical potential difference. Furthermore, ion diffusion proceeds in the direction of the Electrochemical Potential.

Numerous experiments have demonstrated that membrane permeability to various molecules and ions largely depends on specific channel (tunnel) proteins. These proteins are directly involved in forming the channels through which cations, anions, or uncharged molecules pass.

The Transmembrane Movement of substances for which the membrane is practically impermeable (amino acids, sugars, NUCLEOTIDES) occurs via facilitated diffusion, mediated by specific transport proteins. Substances characterized by facilitated diffusion are typically highly water-soluble and lipid-insoluble. Upon forming a substance-carrier complex, The properties of the transported molecules change slightly, enabling them to dissolve readily in the membrane lipids.

Carriers differ from one another in both their MECHANISM OF ACTION and substrate specificity. Based on their mechanism of action, carriers are divided into two groups. The first group comprises carriers that operate via a shuttle mechanism: they transport specific substances across the membrane, after which the substance-carrier complex dissociates and the cycle repeats. In some cases, the carrier may transport different substances in the reverse direction.

The second group includes carriers embedded directly within the membrane structure. The protomers of these oligomeric transport proteins form a hydrophilic pore-channel equipped with a valve, through which the transported substance is passed molecule by molecule, almost like a Relay race. The transmembrane transfer of substances is driven by Conformational Changes in the substrate-binding domains, which open specific channels within the membrane to facilitate transport.

Carrier-mediated Transport of substances often depends on the presence of other substances, resulting in so-called coupled transport. When a single carrier transports two different substrates in the same direction, it is called parallel transport, or symport. An example is the cotransport of protons and lactose in Escherichia coli. If the transport of certain substances is coupled with The transfer of

others in the opposite direction, antiparallel coupled transport, or antiport, occurs. Examples include the counter-transport of Na+ and K⁺ ions across plasma membranes, the exchange of ATP and ADP across The inner mitochondrial membrane, etc.

Active Transport of molecules and ions across the membrane. Most plant and animal cells maintain distinct ionic compositions on either side of the plasma membrane. For instance, K⁺ ions predominate inside the cell with a significantly lower concentration of Na+ ions, whereas the extracellular space shows the reverse. Thus, K⁺ are intracellular ions, while Na⁺ are extracellular. Although the plasma membrane restricts passive ion permeation, diffusion gradients—in the absence of proper counteraction—would eventually equalize the concentrations and ionic compositions inside and outside the cell. Therefore, to maintain required ion concentrations alongside free and facilitated diffusion, a mechanism exists that drives Active ion transport against the electrochemical gradient via so-called Na⁺—K⁺ and Na⁺—Ca²⁺ pumps. Because ion pumping occurs against concentration gradients, the process is energy-dependent and coupled with exergonic (energy-yielding) reactions.

The primary function of the Na⁺—K⁺ pump is to extrude Na+ ions from the cell in exchange for K⁺ ions. Typically, for every three Na⁺ ions pumped out, two K⁺ ions are brought in. Because this translocation goes against the concentration gradient, it can proceed only through the Utilization of ATP energy, consuming one ATP molecule per transport cycle.

The functioning of the sodium-potassium pump is vital for the metabolic processes of the organism. Inhibiting this pump for just 5–10 minutes in isolated Neurons alters their ionic composition by 40–50 %. Consequently, any disruption in biological pump activity leads to cell death.

Pump operation is regulated by Na+ and K⁺ ions. A Na+ binding site faces the inner surface of the membrane, while a K⁺ binding site faces the exterior. Simultaneous binding of these ions activates the pump, a process mediated by the (Na⁺, K⁺)-dependent ATPase enzyme. Ion transport occurs in several stages. First, in the presence of ATP and Mg²⁺ ions on the inner side of the membrane, the enzyme captures a Na⁺ ion and translocates it to the extracellular space. In doing so, the ATP molecule transfers its high-energy bond to the enzyme and is converted into ADP. Following Na⁺ translocation, the high-energy bond reverts to a low-energy state due to energy dissipation, while the ATPase captures a K⁺ ion and transports it into the cell. This step is accompanied by ATP synthesis—the transfer of a high-energy bond from the ATPase to ADP. An important feature of the Na⁺—K⁺ pump is its reversibility and the dependence of the Na⁺/K⁺ coupling coefficient on oppositely directed gradients. The reversibility of the pump is evidenced by the existence of a specific energetic threshold beyond which the extrusion of Na+ ions from the cell ceases.

Ion transport driven by biological pumps plays a crucial role in utilizing chemical energy and converting it into electrical energy. If the rate of Na+ ion efflux exceeds the rate of K⁺ ion influx, a potential difference develops across the membrane, which powers a range of metabolic processes such as nerve impulse transmission and muscle contraction.

Evidence suggests that specific membrane channel proteins—namely, adjacent sodium and potassium channels—play a critical role in the functioning of the Na⁺—K⁺ pump. The binding of Na+ ions alters the conformation of the protein molecules, disrupting the Hydrogen bond network and converting the α-Helix into a looser α-helix configuration containing 4.4 amino acid residues per turn instead of 3.6. This conformational shift opens an internal channel that permits the passage of Na+ ions while remaining too narrow for K⁺ ions. Following Na⁺ transit, another conformational change converts the π-helix back into an α-helix; this closes the sodium channel while parting the walls of the adjacent potassium channel to allow K+ ion entry.

Currently, researchers are actively studying another ion pump—the Na⁺—Ca²⁺ exchanger—which mediates the reciprocal exchange of Ca²⁺ and Na+ ions. This mechanism pumps Ca²⁺ ions against their concentration gradient. Each cycle involves either the influx of Ca²⁺ and efflux of Na+ or, conversely, the extrusion of Ca²⁺ coupled with the uptake of extracellular Na+ ions. The pump operates via a carrier protein that forms a membrane channel and utilizes ATPase activity to drive ion transport.

The Role of the calcium pump is to maintain a precise intracellular concentration of Ca²⁺ ions, which are essential for triggering numerous intracellular processes. For example, a surge in intracellular Ca²⁺ concentration serves as the trigger for muscle fiber contraction. Furthermore, Ca²⁺ ions initiate Cell Division, cellular motility, and the secretion of various BIOLOGICALLY ACTIVE SUBSTANCES.

The plasma membrane not only regulates The entry of molecules and ions into the cell but also mediates the uptake of Solid and liquid substances via pinocytosis and phagocytosis. From a physiological standpoint, these two processes are equivalent, enabling the cell to ingest large substances that cannot cross the plasma membrane independently.

The term "phagocytosis" is used when a cell engulfs solid particles, whereas "pinocytosis" refers to the uptake of fluid droplets. Pinocytosis is further divided into endocytosis (the ingestion and internalization of fluid) and exocytosis (the extrusion of synthesis and metabolic products through the plasma membrane).

In unicellular organisms, pinocytosis is associated with intracellular digestion and serves as a major pathway for nutrient uptake; this is known as amoeboid pinocytosis. In contrast, cells whose functions are specialized for absorption typically exhibit submicroscopic pinocytosis.

Endocytosis unfolds in several stages, beginning with the adsorption of the target substances onto the surface of the plasma membrane. The membrane then invaginates to form channels filled with the substance to be ingested, after which the edges of the invagination fuse to form an endocytic vesicle that migrates into the cytoplasm. Once inside, lysosomal enzymes dissolve the vesicle membrane, releasing the entrapped substance.

The MOLECULAR MECHANISMS OF endocytosis are not yet fully understood. It is believed that certain stages of this process require metabolic energy, utilizing ATP as an energy source.

The endocytosis of protein-based substances occurs via specific receptors, a process known as receptor-mediated pinocytosis. This cellular entry mechanism is characteristic of various Hormones (such as Insulin), IMMUNOGLOBULINS, and Lipoproteins.

Exocytosis ensures the transport and secretion of synthesized substances—such as proteins, hormones, and enzymes—out of the cell.

Like endocytosis, exocytosis proceeds in multiple stages. First, synthesized substances reach the cisternae of the Golgi apparatus, where they are modified and packaged into specialized secretory exocytic vesicles. These vesicles are then transported to the cytoplasmic membrane via intracellular cytoskeletal systems. Upon direct contact with the plasma membrane, the vesicles fuse with it, releasing their cargo into the extracellular space.

Substances can also be secreted from the cell via reverse pinocytosis, whereby components are engulfed by the plasma membrane and pinched off to form vesicles. Exocytosis is particularly characteristic of lipid-based substances that exhibit a high affinity for Cytoplasmic membranes.

Phagocytosis was first discovered in the late 19th century by the outstanding Russian biologist Ilya Metchnikoff, who established that white Blood Cells—leukocytes—are capable of engulfing bacterial cells. Soon after, similar properties were discovered in other cell types. This engulfment process was termed phagocytosis. During phagocytosis, bacteria or other solid particles are embraced by cytoplasmic extensions and pulled into the cell, where they are digested by hydrolytic enzymes, allowing the breakdown products to be assimilated by the cell. Phagocytosis plays a vital role in the body's defense mechanisms; specialized leukocytes (phagocytes) capture and neutralize pathogenic bacteria. Thus, biological membranes are highly dynamic, as phagocytosis exemplifies all forms of biomembrane transformations—remodeling, displacement, fusion, and more.

The cytoplasm is the primary component of the intracellular contents. It represents a living colloidal system (a viscoelastic thixotropic gel) possessing an ordered submicroscopic structure. Cytoplasm consists of 75–85 % water, 10–12 % proteins, 4–6 % carbohydrates, 2–3 % lipids, and 1 % inorganic substances.

Owing to its viscoelastic properties, the cytoplasm simultaneously exhibits the characteristics of a viscous liquid and a solid. It is both fluid and elastic. As a thixotropic gel, the cytoplasm can alter its viscosity from a liquid state (sol) to a jelly-like state (gel). Transitions between these states are triggered by various intracellular and external factors.

In many cells, the outer layer of the cytoplasm (ectoplasm), which contains numerous organelles, exists in a gel state, whereas the inner layer (endoplasm) is thixotropic. Microtubules and microfilaments—thin, elongated, filamentous non-membranous protein structures—are thought to play a key role in maintaining cytoplasmic thixotropy. Microtubules form an intracellular network known as the Cytoskeleton. They typically lie in the ectoplasm parallel to the plasma membrane, facilitating specialized cellular movements. Microfilaments are arranged in bundles directly beneath the plasma membrane, determining intracellular motility and cytoplasmic fluidity.

In most Eukaryotic cells, cytoplasmic streaming occurs at a velocity of 1–6 cm/h, ensuring the optimal distribution of organelles. This movement promotes biochemical reactions and the clearance of metabolic waste, as cellular organelles move along with the cytoplasm. In Protozoa, cytoplasmic streaming drives locomotion. For example, in amoebae, the contraction of Actin and Myosin microfilaments causes a localized shift of the endoplasm from one part of the cell to another, propelling the cell forward. Cytoplasmic fluidity also enables the light-dependent translocation of chloroplasts, as well as the movement of cell nuclei and vesicles. In plant cells, circular cytoplasmic streaming (cyclosis) can occur around the central vacuole.

The homogeneous substance of the cytoplasm located between microfilaments is called the matrix. The matrix contains water, dissolved mineral and organic substances, metabolic intermediates, and enzymes that drive essential carbohydrate metabolic pathways such as Glycolysis and the Pentose Phosphate Pathway within the cell.

The cytoplasmic matrix, along with certain lightweight structures that do not precipitate upon ultracentrifugation, is known as the Cytosol, although within the cell, the matrix exists as a viscoelastic thixotropic gel.

The cytoplasm of plant and animal organisms contains various organelles. They serve as essential structural components of the intracellular environment and drive numerous metabolic reactions without which the functioning of a living system is unthinkable. All of them possess a definite shape, size, and chemical composition, and perform specific functions. The Importance of intracellular

organelles is evidenced by the fact that upon their removal, the cell loses its ability to function normally and eventually perishes.

The most vital cell organelles include the nucleus, mitochondria, Ribosomes, plastids, and vacuoles.

The Nucleus. Given the Functional Significance of the nucleus in supporting metabolic and genetic processes, it is referred to as the control center or the information center of the cell. The primary functions of the nucleus consist of storing and transmitting information to the cytoplasm via Transcription, and passing Genetic information to daughter cells through Replication during cell division.

The Cell Theory was formulated as early as the 1930s of the last century; however, the role of The Nucleus as the information center of the cell—housing Chromosomes that carry hereditary information in the form of DNA—was established much later.

Most commonly, the nucleus is located in the center of the cell and may be spherical, oval, lenticular, or egg-shaped, much like the cells that enclose it. The average diameter of the nucleus is 5 µm, though this value can range from 0.5 µm (in Fungi) to 500 µm (in egg cells). In plant cells that contain a central vacuole, the nucleus is located in the exoplasm and has a flattened, lenticular shape.

The presence of a membrane-bounded nucleus in a cell is the primary characteristic that distinguishes eukaryotes from prokaryotes. Among certain eukaryotic cells, enucleated cells can be found. Such cells (such as erythrocytes and sieve tube elements in higher plants) are short-lived and incapable of division. Some cells may contain multiple nuclei. These multinucleated cells often form as a result of the fusion of several cells (striated Muscles, certain plant vessels). Unicellular Eukaryotes can be binucleated: one nucleus serves as the source of genetic information (the micronucleus), while the other manages metabolic processes (the macronucleus). Nevertheless, the vast majority of cells in plant and animal organisms are uninucleate.

For every cell type, There is a specific ratio between the size of the nucleus and the volume of the cytoplasm. Regardless of their shape and number within a cell, all nuclei consist of nucleoplasm, chromosomes (Chromatin), nucleoli, and the nuclear envelope. The nuclear envelope features two membranes, each 6–8 nm thick, separated by a perinuclear space 10–40 nm wide. The outer nuclear membrane is continuous with the endoplasmic reticulum and forms at the end of cell division through the fusion of endoplasmic reticulum cisternae with fragments of the old nuclear envelope broken down during division.

The nuclear membranes contain pores (visible via Electron Microscopy) that facilitate the passage of relatively large particles from the nucleus into the cytoplasm. In most cases, these pores have an octagonal shape and occupy approximately 5% of the nuclear surface. It is believed that ribosomal proteins, enzymes, and Histones enter the nucleus from the cytoplasm through these pores, whereas smaller molecules and ions pass from the interior of the endoplasmic reticulum into the perinuclear space, and subsequently cross the inner nuclear membrane via active transport systems.

The interior of the nucleus contains the nucleoplasm (karyolymph), which consists of a liquid phase—the nuclear matrix—and various inclusions. In terms of chemical composition, the nuclear matrix is similar to the cytoplasmic matrix, containing enzymes and metabolic intermediates. Among the inclusions, granular or filamentous ribonucleoprotein particles and nuclear bodies composed of Introduction/36.html">Carbohydrates and lipids are most frequently observed. The nucleus also reveals a network of fine threads interspersed with a relatively dense, stained mass known as chromatin, from which chromosomes form during mitosis.

Chromosomes are elongated nucleoprotein particles that can be quite easily isolated from both prokaryotic and eukaryotic cells. The number, shape, and size of chromosomes are characteristic of each plant and animal species. The entire pool of genetic information within the cell nucleus—The Genome—is distributed among a specific chromosome content ($n$). This chromosome number ($n$) is species-specific: 10 in maize, 23 in humans, and up to 600 in certain algae. Haploid cells contain a single set of chromosomes, diploid cells contain two, and polyploid cells contain several.

Somatic cells of Higher Plants and animals are diploid and contain one paternal and one maternal set of chromosomes. Germ Cells are haploid. Haploid cells are formed from diploid cells through Meiosis, whereas diploid cells arise from haploid cells as a result of Fertilization.

The chromatin of Prokaryotic Cells consists solely of DNA, whereas in eukaryotic cells it comprises four components: DNA, RNA, basic low-molecular-weight histone proteins, and acidic non-histone proteins. Within chromatin, DNA forms a rather stable nucleoprotein complex with histones, known as a nucleosome. On average, chromatin is composed of 40% DNA, 40% histones, 20% non-histone proteins, and a minor amount of RNA. Non-histone proteins perform diverse functions: they drive chromosome movement (actin and myosin), exhibit enzymatic properties (polymerases, Kinases, methylases), and regulate the activity of specific genes. Chromosomal DNA contains hereditary information and ensures its transmission to newly formed cells. In addition, the nucleus synthesizes mRNA, which serves as a template for protein molecule synthesis. Throughout the Cell Cycle, chromosomes shift between two physiological forms: transport (during cell division) and functional (during interphase). Transport chromosomes are compact and rod-shaped, whereas functional chromosomes are decondensed and filamentous. Chromosome decondensation is a prerequisite for transcription—the synthesis of mRNA. A continuous double helix of DNA runs the entire length of the chromosome, which in higher organisms may contain $1\cdot10^{8}$ Base Pairs. Linear functional regions called genes are located along the DNA molecule, accounting for up to 25% of its mass and carrying the code for mRNA synthesis. The average Gene length is 1000 base pairs.

In addition to chromatin, the nucleus contains rounded, relatively dense regions known as nucleoli. Their number can vary from one to seven. Nucleoli are attached to a specific region of one of the chromosomes called the nucleolar organizer. The Internal Structure of the nucleolus consists of a loose network containing ribonucleoprotein fibrils, granular structures—ribonucleoprotein granules—and a bulk matrix composed of proteins and RNA. Nucleoli are composed of 80% protein and 15% RNA. Furthermore, they contain a minor fraction of nucleolar chromatin enveloped by ribonucleoprotein fibrils.

A crucial process takes place within the nucleoli: the synthesis of ribosomal RNA. During mitosis, nucleoli disappear and ribosomal RNA Synthesis ceases. Upon the completion of division, they re-form at specific regions of decondensed chromatin.

The Nuclear Equivalent in Prokaryotic Cells. Prokaryotic unicellular organisms lack a segregated nucleus. Instead, they contain its equivalent—a polynucleotide that is not sharply demarcated from the surrounding cytoplasm. It consists of a double-stranded DNA molecule, twisted and closed into a ring, which is attached to the plasma membrane. Unlike eukaryotic nuclear DNA, prokaryotic DNA is not bound to histones or other Nuclear Proteins. It contains linearly arranged

regions—genes—that ensure replication. This is how the nuclear equivalent is organized in the bacterium Escherichia coli, while other bacteria may harbor DNA in the form of tiny Plasmids similar to extranuclear DNAs in eukaryotes.

Plasmids are short, double-stranded DNA molecules closed into rings that exist freely, unintegrated into the genome. On average, they comprise 100,000 base pairs and a few genes, and their replication occurs independently of the main genetic material. Plasmids frequently transfer from one cell to another. They have been detected in prokaryotic cells as well as in the mitochondria of eukaryotic cells.

Ribosomes are mandatory, independent cell organelles. They appear as mushroom-shaped bodies with a diameter of 0.015–0.02 µm, consisting of two unequal subunits (Fig. 4). These organelles are crucial for Protein Synthesis, serving as the direct site of genetic Translation. During protein synthesis, individual ribosomes aggregate, sometimes grouping together to form Polysomes. The structural backbone of ribosomes consists of specific Ribosomal RNAs and proteins. The highest concentration of ribosomes is found in the cytoplasm of cells undergoing active protein synthesis—such as growing cells, embryonic cells, and meristematic cells. Additionally, ribosomes are present in other cellular compartments, such as chloroplasts and mitochondria, where they likewise participate in the synthesis of protein molecules.

Mitochondria and Plastids are present in the majority of eukaryotic cells. They share similarities in function, morphological features, and origin. These organelles drive the principal stages of energy conversion within the cell, which is why they are referred to as the cellular power plants. At the same time, they possess a number of features indicating a certain degree of Structural and functional autonomy. They contain replication-competent DNA, various types of RNA, and ribosomes—a kind of protein-synthesizing machinery that differs somewhat from its nuclear and cytoplasmic counterparts. In its properties, this machinery approaches that of prokaryotic cells, such as bacteria and blue-green algae. Evidence suggests that a portion of the proteins and RNAs in these organelles are synthesized on their own ribosomes, meaning the organelles exert partial control over The production of their own components. Moreover, mitochondria and plastids arise through the transverse fission of pre-existing organelles or develop from small vesicles that possess double membranes and a dense matrix, budding off from mature mitochondria or plastids. Consequently, there is a hypothesis regarding the symbiotic origin of these organelles. It is believed that they descend from prokaryotes—similar to bacteria and blue-green algae—that invaded the cell via phagocytosis as "aggressors," eventually establishing a symbiotic association vital to cellular function through evolution. According to other hypotheses, mitochondria originated from invaginations of the plasma membrane that once surrounded fragments of a primitive genome.

Fig. 4. Liver ribosomes (after Nonomura, 1971).

Mitochondria are constituents of most cells, with the exception of bacteria, blue-green algae, mature red blood cells, and certain parasitic protozoa that derive their energy through Fermentation. In prokaryotes, all energy-supplying processes take place across the plasma membrane and its invaginations, known as thylakoids. The number and shape of mitochondria depend on the cell type. On average, a cell contains 150 to 1,500 mitochondria, although certain protozoa with intense metabolism may house up to 500,000.

Plant cells contain significantly fewer mitochondria than animal cells. For the most part, they are rod-, sphere-, lens-, or thread-shaped.

The mitochondrial surface is bounded by a double membrane, with a perimitochondrial space located between its layers. A significant portion of the enzymes residing in this space facilitates the phosphorylation of various substrates utilizing ATP. The thickness of the membranes ranges from 7 to 10 nm. The outer membrane has a smooth surface, whereas the inner one forms numerous protrusions, folds, partitions, and tubules known as cristae. These cristae permeate the internal space of the mitochondria, forming distinct compartments (chambers) that substantially increase the surface area of the inner membrane. Between the cristae lies the ground substance—the matrix—whose composition closely resembles that of the cytoplasm.

The ratio of matrix to cristae in different mitochondria depends on their specific functions and level of biological activity. Mitochondria engaged in intensive biosynthetic processes are characterized by a predominant matrix and very few cristae (such as liver mitochondria), whereas mitochondria specialized in energy production contain a greater Abundance of cristae. The matrix contains granules composed of proteins, phospholipids, various ions (Ca²⁺, Mg²⁺), phosphates, DNA, various types of RNA, and ribosomes. Mitochondrial DNA encodes mitochondrial transfer and ribosomal RNAs, as well as certain Proteins of the inner mitochondrial membrane, namely cytochrome b and cytochrome c oxidase subunits.

The outer and inner mitochondrial membranes differ in chemical composition and possess distinct structural features. The outer membranes contain a high concentration of complex lipids and cholesterol, and their chemical composition is similar to that of the plasma membrane. Due to numerous Pores in the outer membrane formed by tunnel proteins, relatively large molecules—such as amino acids, sugars, and ATP—pass through quite readily. The outer mitochondrial membrane is the site of localization for enzymes involved in Phospholipid Metabolism, fatty acid activation, and monoamine oxidase. Conversely, the inner mitochondrial membranes contain up to 75% protein and only minimal lipid content, with cardiolipin predominating among the lipids. Unlike the outer membrane, the inner membrane is virtually impermeable to most metabolites present in the matrix, including glucose, Pyruvate, amino acids, ATP, ADP, and phosphates. Consequently, the transport of these substances relies on specific transport proteins and requires ATP energy (active transport).

The inner mitochondrial membrane and its cristae harbor enzyme systems that drive numerous reactions associated with ENERGY GENERATION AND storage—transforming the chemical bonds of nutrients into the high-energy bonds of ATP. Integrated proteins within this membrane form a complex of enzymes participating in electron transport (the Respiratory Chain). The peripheral proteins comprise various dehydrogenases that take part in The oxidation of respiratory substrates and transfer hydrogen removed from the substrate to the respiratory chain.

Using an electron microscope, mushroom-like structures can be observed on the inner membranes of mitochondria and cristae; these are membrane ATPases responsible for ATP synthesis, alongside linking factors that ensure the energetic coupling between Respiration and Oxidative Phosphorylation. The energy stored in the high-energy bonds of ATP is subsequently utilized to drive diverse metabolic processes, including growth, development, and synthesis.

Plastids are unique to PLANT CELLS AND represent the largest cytoplasmic intracellular structures after the nucleus. All of them develop from a single common type of colorless proplastids, which subsequently differentiate—depending on the type of tissue in which they ultimately function—into green chloroplasts, chromoplasts, or leucoplasts. Chloroplasts are the most widespread type of plastid in plant cells and serve as the primary organelles where the complex conversion of light energy into the chemical energy of Organic compounds takes place. This unique and still incompletely understood process is known as Photosynthesis. Chloroplasts consist of 50–70% water, with the dry matter comprising 36–50% protein, 29–30% lipids, 6–8% minerals, 1% chlorophyll, 8–10% carbohydrates, and other components such as Vitamins, Nucleic Acids, and pigments.

A high concentration of chloroplasts is found in the cells of tissues exposed to light, such as leaves, stems, and young fruits. A leaf surface area of 1 mm² contains approximately 400,000 chloroplasts. Algal cells typically contain 2 to 3 chloroplasts, whereas higher plant cells harbor significantly more, ranging from 20 to 50. Algal chloroplasts may appear as a network, a stellate plate, or a spiral ribbon.

The structure of chloroplasts in higher plants is considerably more complex. Much like the mitochondria of animal cells, they possess outer and inner membranes enclosing a colorless ground substance known as the matrix or stroma.

Associated with the inner membrane inside chloroplasts is an extensive system of double-membrane structures called lamellae. The ends of the lamellae are connected in pairs such that every two adjacent lamellae form hollow membranous pockets, local accumulations of which are termed thylakoids. This pairwise connection of lamellae allows for the distinction of their inner and outer surfaces. Thylakoids are grouped into densely packed discs, or grana, measuring 0.5–0.8 µm in size. A single chloroplast typically contains 10 to 100 grana, which are interconnected by intergranal lamellae traversing the stroma. The grana lamellae have a thickness of 4–6 nm, while the intergranal lamellae measure 2–3 nm. Thus, a lamellar-granular architecture is established within the chloroplasts.

The fundamental structural layout of chloroplasts is consistent across all higher plants, although chloroplasts from different species may vary in size and lamellar abundance. Lamellar membranes contain up to 50% protein. Lipids play a crucial role in membrane formation and in establishing their hydrophobic dielectric layers. Proteins penetrate the membrane matrix and, together with the green pigment chlorophyll, form spheroid particles known as Photosystems, which host the vital reactions involved in light energy absorption.

The stroma of chloroplasts contains a closed circular DNA molecule that carries genetic material. Like mitochondrial DNA, chloroplast DNA is not associated with histone proteins; however, it is larger in size and encodes more genetic information. It specifies various types of RNA (tRNA, RNA polymerases), certain ribosomal proteins, plastid Cytochromes, and a significant number of enzymes involved in specific Phases of Photosynthesis.

Leucoplasts are colorless plastids of spherical, oval, or fusiform shape, located in the cytoplasm of non-illuminated plant tissues. They are most abundant in the cells of rhizomes, seeds, epidermis, tubers, and stem pith. Leucoplasts differ markedly from chloroplasts both in size (being significantly smaller) and in internal architecture. They almost entirely lack thylakoids—the invaginations and folds characteristic of the inner membrane. Their formation is either genetically arrested, as in ROOT and epidermal cells, or suppressed by the absence of light (in tuber cells). In the latter case, excessive illumination can trigger a reorganization of the leucoplast internal structure, transforming them into chloroplasts; this is why potato tubers frequently turn green upon prolonged exposure to light. Aside from isolated thylakoids, leucoplasts contain starch grains, DNA, plastoglobules, and a plastid center (prolamellar body).

Starch grains serve as storage sites for reserve carbohydrates in the form of starch, which is synthesized by a specific enzyme system from excess glucose produced during photosynthesis. Because starch grains vary in shape and internal structure among different plant species, they serve as an important taxonomic feature.

The plastid center of leucoplasts consists of an intertwined network of branching tubules or various bodies and vesicles. It is precisely from this plastid center that thylakoids can develop under The Influence of light.

Chromoplasts determine the color spectrum of fruits, flowers, and other plant parts. They typically originate from chloroplasts or leucoplasts through The breakdown of chlorophyll and thylakoids accompanied by the release or de novo synthesis of carotenoids. This phenomenon is readily observed during the ripening of fruits and vegetables or the autumnal color change in tree foliage. In the petals of certain flowers (such as lamiales), chromoplasts function as independent organelles.

Chromoplasts predominantly contain yellow pigments known as carotenoids, represented by some 50 different species. These pigments are deposited within plastoglobules—tubular or filamentous protein structures. The internal organization of chromoplasts is poorly developed; aside from plastoglobules, they contain starch grains and protein crystalloids. Thylakoids are virtually absent, as is the plastid center. Consequently, chromoplasts lack metabolic functional activity, and their primary role is to impart coloration to specific plant organs.

Although various structural forms and functional modifications of plastids exist within cells, only chlorophyll-containing plastids possessing a thylakoid (lamellar-granular) type of organization are capable of driving The conversion of light energy into the chemical bond energy of organic compounds.

In addition to the plasma membrane, the cell harbors an extensive system of endomembranes, including the endoplasmic reticulum, Golgi apparatus, Lysosomes, Microbodies, and vacuoles. All of these delineate membrane-bounded intracellular compartments characterized by specific functional properties.

Endoplasmic Reticulum. All Eukaryotic cells contain an endomembrane network composed of highly branched and interwoven membranes. The membranes of the endoplasmic reticulum form a system of channels, tubules, and cisternae of diverse Sizes and Shapes that are frequently interconnected, with their internal space continuous with the perinuclear region of the cell. The degree of Development of the endoplasmic reticulum and its components varies among different cell types, reflecting their metabolic activity. In plant cells, tubular cisternae can even penetrate The cell wall to reach neighboring cells.

Upon cell homogenization, the endoplasmic reticulum undergoes fragmentation, yielding vesicles of specific sizes known as microsomes. The membranes of endoplasmic reticulum cisternae are rich in glycerophosphatides, which can account for up to 95% of their lipid content. The membrane thickness is approximately 6 nm.

Two MAIN TYPES OF endoplasmic reticulum are distinguished based on the presence or absence of specific cellular organelles—ribosomes—on their surface. The absence of ribosomes yields a smooth-surfaced endoplasmic reticulum (agranular reticulum), whereas the presence of small granular ribosomes studding the surface defines the rough (granular) endoplasmic reticulum.

The granular endoplasmic reticulum is highly developed in cells engaged in intensive protein synthesis. Ribosomes frequently assemble into spiral structures known as polysomes, attaching to the outer surface of the endoplasmic reticulum membranes via their larger subunits and directly participating in protein synthesis. Newly synthesized proteins pass from the ribosomes across the membrane into the cisternae of the endoplasmic reticulum, from where they are exported to the extracellular space through tubular elements. Thus, the endoplasmic reticulum coordinates the synthesis and transport of vital Biopolymers, establishing an Intracellular Transport system that segregates synthesized products and distributes them to various cellular locations or the extracellular milieu.

The agranular endoplasmic reticulum forms tubular elements known as canaliculi of varying lengths, with tubule lumen diameters ranging from 0.025 to 0.03 µm. This architecture of hollow, porous tubules creates an expansive surface that facilitates sequential or simultaneous Chemical Reactions.

The tubular, porous elements of the smooth endoplasmic reticulum participate in the synthesis and transport of various substances both within and outside the cell. They also mediate successive stages of carbohydrate, lipid, and steroid metabolism. The agranular endoplasmic reticulum is particularly well-developed in cells that synthesize lipids and membrane Steroids, such as cholesterol. It is found in large quantities in the Cells of the Testes and Adrenal Glands, where Steroid Hormones are synthesized. Furthermore, the membranes of the agranular endoplasmic reticulum are associated with enzymes that support carbohydrate and Lipid Metabolism, as well as the enzyme systems of the respiratory chain.

A close functional relationship exists within the cell between the elements of the granular and agranular reticulum. They represent parts of a single, highly labile membrane system capable of intracellular differentiation and specific modifications.

The Golgi apparatus. Almost all eukaryotic cells contain a membrane system similar to the endoplasmic reticulum, which was named after the scientist who first described it. As a rule, the Golgi apparatus consists of several isolated Golgi complexes, or dictyosomes. A dictyosome is composed of 3—12 disk-shaped, enclosed Golgi cisternae arranged in parallel and enveloped by a membrane. Golgi cisternae are quite frequently curved, forming a convex outer and a concave inner surface. The number of cisternae varies among cells depending on their type. Fine tubular or fibrillar elements often interweave between the cisternae, branching out in various directions throughout the cell. The fusion of all dictyosomes produces a granular-stellate structure, which is sometimes positioned like a cap directly in the nuclear zone of the cell, forming the Golgi system or apparatus. Small vesicles filled with secretion gradually separate from mature dictyosomes and migrate to the periphery of the system. In addition to vesicles, the Golgi apparatus also includes voluminous vacuoles formed by the expansion of cisternae. The formed vesicles and vacuoles approach the plasma membrane and fuse with it, thereby releasing the secretion into the extracellular space (exocytosis). Occasionally, vesicles may also fuse with secretory granules inside the cell. Consequently, the primary function of the Golgi apparatus is the accumulation, packaging, and Vesicular Transport of intracellular metabolic products—synthesized substances, breakdown products, and toxins.

It is believed that the Golgi apparatus is a derivative of the endoplasmic reticulum. The fusion of individual parts of the endoplasmic reticulum (vesicles and fragments) leads to the formation of new Golgi cisternae. Concurrently, the membranes of the endoplasmic reticulum are remodeled into the denser membrane of the Golgi apparatus, which possesses a different lipid and protein composition; thus, a close functional relationship exists between these two membrane systems.

Vacuoles are characteristic of mature plant cells; they represent vesicles filled with an aqueous content—cell sap—and surrounded by a single membrane, the tonoplast. They originate from the endoplasmic reticulum or Golgi vesicles.

Vacuoles play a vital role in regulating the turgor of plant cells and participate in the processes of growth, movement, accumulation, and storage of the End products of cellular metabolism.

Depending on the functions they perform within cells, several types of vacuoles are distinguished. Freshwater protozoa are characterized by contractile (pulsating) vacuoles that ensure osmotic regulation. Through the action of pulsating vacuoles, excess water—which enters from a hypotonic surrounding solution or is taken up via pinocytosis—is periodically expelled from the cell. Vacuoles contract with the participation of elastic fibers located within the vacuolar membrane.

In the cells of storage tissues in plants, vacuoles frequently occur in which nutrient reserves, such as fats and proteins, are accumulated.

Quite common in plant cells, especially meristematic ones, is the central vacuole, which is formed by the fusion of smaller vacuoles. The central vacuole often occupies 90 % of the intracellular space, pushing the cytoplasmic components and the nucleus toward the cell wall, thereby increasing the intensity of exchange between the cytoplasm and the external environment.

Cell sap is contained inside the vacuole. The vacuolar membrane resembles the membranes of the endoplasmic reticulum in its properties, yet it is thinner, less dense, and more permeable. Because the internal content of the vacuole is hypertonic relative to the cytoplasm and the surrounding medium, water is pumped into the vacuole quite intensively, generating turgor pressure. Turgor pressure stretches the elastic cell wall and maintains specific cell dimensions and rigidity, which is especially important for growing young plant tissues.

The central vacuole performs essential functions in plant cells and serves a variety of purposes. Intermediate metabolic products—such as sugars, organic acids, and amino acids—are accumulated and isolated here. The vacuole also stores various End Products of Metabolism, notably various pigments responsible for the coloration of flowers and fruits (anthocyanins, flavones), as well as toxic compounds such as Alkaloids and polyphenols. Furthermore, vacuoles can act as a lysosomal compartment, as they contain a certain amount of lysosomal enzymes delivered via vesicles from the Golgi apparatus.

Lysosomes are small spherical formations, cytoplasmic granules 0.2—0.5 µm in size, surrounded by a single membrane. They contain hydrolytic enzymes and ensure the digestion of various substances within the cell. Altogether, a single lysosome contains about 30 different enzymes.

Lysosomes are characterized by the phenomena of hetero- and autophagy. Heterophagy is the digestion, with the participation of heterolysosomes (phagolysosomes), of substances entering the cell through endocytosis and phagocytosis. Autophagy is the breakdown, within autolysosomes (cytolysosomes), of intracellular reserve substances, as well as macromolecules or organelles that have lost functional activity, particularly mitochondria. Moreover, due to the damage of lysosomal membranes, cellular Aging, or pathological changes, enzymes are released into the cell interior and begin to digest intracellular contents. Autolysis, or lysis (self-digestion of the protoplast), ensues, leading to cell death.

Lysosomes are formed from Golgi cisternae or the agranular endoplasmic reticulum. Newly formed primary lysosomes appear as smooth vesicles containing a set of hydrolytic enzymes synthesized on the granular endoplasmic reticulum. Upon substrate uptake, primary lysosomes transform into functionally active secondary lysosomes. The substrate may be internalized via autophagy—the active incorporation of macromolecules from the surrounding cytoplasm. Additionally, lysosomes can fuse with endocytic or phagocytic vesicles (vacuoles).

Sometimes, numerous small lysosomes fuse to form a larger primary lysosome capable of entirely engulfing various organelles or pinocytic vesicles. Such resulting secondary lysosomes are referred to as multivesicular bodies.

Upon completion of the digestion of internalized substances, secondary lysosomes transform into residual bodies containing indigestible residues devoid of any enzymes. Residual bodies are either eliminated from the cell via exocytosis or dissolved in the cytoplasm. Thus, lysosomes can be regarded as a system for removing various metabolic products entering from the environment, as well as eliminating foreign bodies and structural cellular elements that have lost their functional significance.

Microbodies, much like lysosomes, are surrounded by a membrane and contain specific enzymes. These are small, short-lived vesicles (0.1—1.5 µm) with smooth walls and a finely granular

matrix containing amorphous crystalloid inclusions. The primary component of the microbody matrix is protein. The main function of these organelles is to anchor enzymes that catalyze specific biochemical reactions at defined cellular sites, thereby ensuring their simultaneous occurrence within the cell. It is believed that microbodies originate from expanded, enzyme-filled cisternae of the endoplasmic reticulum.

Thus, the membrane systems examined constitute compartmentalized membrane-bound Regions of the cell specialized According to the functions they perform. A close interrelationship exists between the membranes of the endoplasmic reticulum and other membrane systems within the cell. It is widely held that the membranes of various cellular components are derived from the membranes of the endoplasmic reticulum. Despite The Diversity of the membrane system, all constituting membranes share a common structural plan, although the content of specific constituent components may vary.