Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter I. Cytology
1.3. Structure of the Eukaryotic Cell

A Introduction/5.html">Eukaryotic Cell consists of a protoplast and its metabolic products. The protoplast comprises the Cytoplasm and its Organelles, including membranes, The Endoplasmic reticulum, The Nucleus, Plastids, Mitochondria, Ribosomes, the Golgi apparatus (Golgi complex), spherosomes, Lysosomes, microtubules, microfilaments, Microbodies, and others. The metabolic products of The Cell include The Cell wall, vacuoles with cell sap, reserve CARBOHYDRATES, Proteins, fats, Vitamins, phytoncides, Antibiotics, phytohormones, organic acids, and various amorphous and crystalline inclusions (Table 1).

Class="center">Table 1. Components of a plant cell

Cytoplasm is the internal semi-fluid environment of the cell and a colloidal system that houses all cellular organelles. It facilitates their interaction and serves as the medium for various Chemical Reactions and the translocation of substances. The cytoplasm consists of the matrix (hyaloplasm), Cytoskeleton, and organelles.

Hyaloplasm is the colorless colloidal System of the cell. It contains proteins, RNA, Polysaccharides, Lipids, Water, and mineral salts, and serves as the medium where organelles, membrane structures, the cytoskeleton, and inclusions are located.

The cytoskeleton is a network of microtubules and microfilaments that provide structural support and perform several other Functions.

Organoids or organelles are permanent cellular structures that perform specific functions and drive various vital cellular processes.

The cytoplasm of Eukaryotic Cells is divided by numerous internal membranes into reaction spaces known as compartments. Chemical reactions occur simultaneously and independently within these enclosed spaces.

The presence of Cytoplasmic membranes is a key characteristic exclusive to eukaryotic cells. They are subdivided into the Plasmalemma, mesoplasm, and tonoplast. All membranes share a similar structural Organization and chemical composition (Fig. 1).

The plasmalemma is the most stable, fundamental, and universal membrane found in all cells. Under an Electron microscope, it appears as a delicate film primarily composed of orderly arranged phospholipid and protein molecules held together by non-covalent bonds. The plasmalemma consists of a lipid bilayer with nonpolar hydrophobic tails oriented toward the interior of the membrane, and polar heads facing both the intracellular and extracellular environments. The lipid layer is continuous in places yet interspersed with protein molecules that form hydrophilic pores, allowing water-soluble substances to pass through. Other protein molecules are located on both surfaces of the membrane. Carbohydrate molecules reside on the outer surface, forming complexes with proteins or lipids.

According to current models, the membrane adopts a fluid-mosaic Structure: it consists of loosely packed globular protein layers with lipid molecules filling the spaces between them. Some proteins associate with polar lipid groups on the exterior of The Lipid Bilayer; others are partially or fully embedded within the lipid layer or span the membrane entirely, while over 70% of proteins remain unbound to lipids and essentially "float" within the "lipid sea." Consequently, not only lipid molecules but also proteins move rapidly along this fluid lipoprotein layer. Proteins can cluster or disperse across different Regions of the membrane to fulfill its various functions.

The plasmalemma bounds the cytoplasm, protects it from external environmental factors, and participates in metabolic processes. It forms outgrowths, invaginations, and folds that significantly increase its surface area. The plasmalemma determines cell size. Compounds and ions vital for cellular activity cross the membrane via diffusion, active and passive transport, as well as through its pores.

Diffusion is the process by which substances pass freely through membrane pores without Energy Expenditure, moving along a concentration gradient—that is, driven by the difference in concentration between the outside and inside of the cell. Cellular membranes are characterized by semipermeability: The transport of specific substances depends on how permeable the membrane is to them.

The selective permeability of substances across the membrane is facilitated by passive transport. Similar to diffusion, substances move down a concentration gradient with virtually no energy expenditure. There are several mechanisms of passive transport: the translocation of substances via carrier proteins that bind a molecule on one surface of the membrane and release it on the other; and the Transport of substances driven by conformational shifts in proteins spanning the membrane. Carrier molecules anchored within the membrane can form a trans-membrane chain, passing a specific substance sequentially from one link to the next. Passive transport continues until the concentration of substances equalizes on both sides of the membrane.

Active Transport of substances across Biological Membranes requires energy expenditure. This energy can be derived from ATP Hydrolysis or from the ion concentration gradient established across the membranes. Additionally, substances can be translocated via phagocytosis and pinocytosis. Phagocytosis is the active uptake of solid particles by the membrane, typical of certain animal cells. Pinocytosis is the engulfment and uptake of fluids by the cell.

In summary, the plasmalemma performs barrier, transport, and receptor functions, mediates intercellular contacts, participates in Cell Growth and Division, and facilitates communication between the Cell Cytoplasm and its surrounding environment. The integration of these functions ensures METABOLISM/37.html">Cellular Homeostasis—the maintenance of a stable internal environment. Water is absorbed by the cell primarily through osmosis. Osmosis is the Diffusion of Water across a semipermeable membrane driven by a concentration gradient between the cell interior and the external environment. If a cell is placed in a hypotonic solution, water flows into the cell. This Movement of water driven by a concentration gradient can be counteracted by applying a specific external pressure known as osmotic pressure. It arises from the tendency of water molecules to cross the semipermeable membrane and equalize concentrations on both sides. Because the concentration of ions and molecules is higher inside a plant cell than in its surroundings (such as the soil), the cell develops a suction pressure that drives water uptake. Water influx generates an internal hydrostatic pressure called turgor pressure. This rigid, taut state of the cell is known as turgor. When a cell is placed in a hypertonic solution, the plasmalemma pulls away from the cell wall—a phenomenon termed plasmolysis—whereas the restoration of the cell to its original state is called deplasmolysis. Situated between the plasmalemma and the tonoplast is the mesoplasm or hyaloplasm (matrix, Cytosol), a transparent, heterogeneous colloidal solution of organic and Inorganic Compounds where water accounts for 50–90% of its composition. Proteins and free Amino Acids predominate among the Organic compounds, alongside carbohydrates, lipids, Ribonucleic Acids, and individual NUCLEOTIDES. The hyaloplasm contains metal cations, carbonate and phosphate anions, chlorine, dissolved oxygen, and other gases. Hyaloplasm can exist in either a liquid (sol) or gelatinous (gel) state.

As the internal environment of the cell, hyaloplasm integrates all Cellular Structures and ensures their interaction. It facilitates the transport of substances, hosts a portion of anabolism and Catabolism—including the preparatory and anaerobic phases of dissimilation accompanied by a certain ATP yield—and during Protein Synthesis, it harbors amino acid–tRNA complexes and ribosomal complexes linked by mRNA molecules, while also storing metabolic waste and reserve nutrients.

The tonoplast is a thin biological membrane (8–10 nm) that bounds the vacuole. It exhibits selective permeability and is capable of active substance transport. The tonoplast may also participate in The formation of lysosomes. Together with other cellular components, the tonoplast determines the cell's capacity for osmoregulation and acts as a regulatory interface between the cytoplasm and the vacuoles.

A crucial feature of the cytoplasm is its capacity for continuous movement, which facilitates Intracellular Transport, maintains constant communication among organelles, and drives metabolic processes. In plant cells, various types of cytoplasmic streaming can be observed: cytoplasmic streaming proper, circular, rotational, and fountain streaming. Streaming occurs in cells where the cytoplasm occupies a parietal position as well as in trans-vacuolar strands, and is typical of most angiosperm cells. In circular movement, the cytoplasm circulates in a single direction, which is more common in algal cells. Rotational movement is characteristic of aquatic plant cells, while fountain streaming is observed in the ROOT hairs of certain aquatic plants.

The nucleus is an essential component of The Eukaryotic Cell. Most cells contain a single nucleus, although certain Algae and Fungi may be multinucleated. Its position within the cell is not static; in young cells, the nucleus resides in the center, whereas displacement toward one side occurs as the cell differentiates and ages. The nucleus is typically spherical or oval, and its size varies considerably (Fig. 2).

The nucleus consists of a nuclear envelope (surface apparatus) and an internal environment. The surface apparatus is spatially and functionally connected to the membranes of the Endoplasmic reticulum and is composed of two membranes: outer and inner. A perinuclear space lies between these membranes, which are perforated by nuclear pores. Ribosomes may populate The surface of the outer membrane. During nuclear division, the nuclear envelope dissolves. Much like the cytoplasm, the nucleus exhibits colloidal properties and a more viscous consistency. Its composition includes Nucleoproteins, Lipoproteins, all types of ribonucleic acids, enzymes, and Mineral Substances. Unlike the cytoplasm, the nucleus contains DNA, which consists of two antiparallel, helically coiled strands made up of nucleotides. DNA is capable of self-Replication in the presence of polymerase enzymes and through THE PRINCIPLE OF complementary base pairing. The enzyme unwinds the polynucleotide strands, and each strand acts as a template to synthesize a complementary counterpart. This process occurs during or immediately preceding nuclear division. DNA molecules encode the Genetic information inherited by the cell. mRNA is synthesized on the DNA template and exported to ribosomes, where it directs protein synthesis. The internal environment of the nucleus (the nuclear matrix) comprises the nuclear sap, nucleoli, ribonucleoprotein complexes, and chromatin threads. The nuclear sap (karyoplasm) consists of soluble complex proteins—nucleoproteins—and enzymes involved in protein and NUCLEIC ACID METABOLISM. In a metabolically active nucleus, chromatin appears as a fine network or granules, whereas in a dividing nucleus, it condenses into Chromosomes. Every eukaryotic cell possesses a characteristic set of chromosomes known as the karyotype. The peculiarities of an individual's karyotype within a given species are defined by the number, size, and shape of the chromosomes. The Stability of the karyotype is precisely what ensures the persistence of the species.

Chromosomes are nuclear structures composed of genes. Each chromosome consists of two chromatids joined together at the primary constriction (centromeric region), which divides the chromosome into two arms of equal or unequal size and shape. Situated within the primary constriction is the centromere, a plate-like disc structure to which the spindle fibers attach. Some chromosomes also feature a secondary constriction and a satellite (Fig. 3).

Chromosome numbers vary among species: the potato has 48 chromosomes, the male fern (Dryopteris filix-mas) has 104, maize has 20, and bread wheat has 42. The chromosomal Complement of a nucleus can be haploid, diploid, or polyploid. In a haploid set, all chromosomes are non-identical; in a diploid set, each chromosome has a homolog matching it in size, shape, and Gene composition. Chromosomes belonging to the same pair are termed homologous. When the number of homologous chromosomes in a cell exceeds two, the set is referred to as polyploid.

In plants, a haploid set of chromosomes is characteristic of spores in lower vascular plants and Gametes across all plant species. Conversely, somatic cells typically possess a diploid set.

A chromosome splits into two chromatids, each consisting of two spirally coiled chromonemata (which are primarily composed of DNA molecules). Prior to nuclear division, the chromonemata undergo duplication. Through the latter, parental traits are exchanged between individuals. Each pair of chromosomes forms a chromatid. Thus, a chromosome consists of two chromatids and four chromonemata.

Nucleoli are rounded, highly dense regions of the nucleus that lack a surrounding membrane. Their shape, size, and number depend on the functional state of the nucleus: the larger the nucleolus, the higher the nuclear activity. A nucleus may contain anywhere from one to ten nucleoli. Nucleoli are composed of approximately 80% proteins (both simple and complex), 10—15% RNA, along with trace amounts of DNA and other chemical substances. During nuclear division, nucleoli break down. Under the control of the nucleoli, RNA Synthesis takes place and ribosomal precursors are formed. The primary Functions of the nucleus are the preservation of hereditary information and its transmission to daughter cells during division. Various types of RNA molecules are synthesized on DNA templates. Ribosomal subunits are formed in the nucleus with the participation of nucleoli, after which they pass into the cytoplasm to take part in protein synthesis. Consequently, by executing the hereditary information encoded in The nucleotide sequence of DNA molecules, the nucleus regulates the biochemical, physiological, and morphological processes within the cell.

Plastids are characteristic organelles of plant cells. Three MAIN TYPES OF plastids are distinguished: Chloroplasts, chromoplasts, and leucoplasts. Chloroplasts are the most widespread in plants. Chloroplasts (chromatophores) were first described in 1676 by Antoni van Leeuwenhoek, while the various types of plastids were categorized by the German botanist Andreas Schimper in 1882.

Chloroplasts are typically spherical or granular in shape, and their number per cell varies enormously: from 30—60 in cereal cells to as many as 1,000 in tobacco palisade tissue cells. The ultrastructure of chloroplasts has been investigated using Electron Microscopy. Chloroplasts consist of the following structures: a double-membrane envelope separated by a periplastid space, stroma, grana, lamellae, and ribosomes. The primary structural subunit of chloroplasts is the lipoprotein lamella (a bilayer plate or tubule) which bears Photosynthetic Pigments, namely chlorophylls. In places, lamellae form flattened vesicular disks known as thylakoids, which are grouped into grana. The grana thylakoids are interconnected into a unified system by intergranal thylakoids. Chloroplasts contain the primary photosynthetic pigments—chlorophylls a and b, accessory pigments such as carotenoids (carotene and xanthophyll), and the enzymes required to drive the biochemical processes of the Light-dependent phase of Photosynthesis (Fig. 4).

Thylakoid membranes are capable of capturing light and directing it toward chlorophyll. The stroma of chloroplasts contains DNA molecules and ribosomes, which facilitate Protein Biosynthesis. The chloroplast envelope contains pores that mediate the exchange of substances between the stroma, the cytoplasm, and other organelles. The presence of DNA in chloroplasts indicates a degree of cellular autonomy. Chloroplasts are the site of photosynthesis and photosynthetic phosphorylation (ATP generation driven by solar energy), as well as the synthesis of Amino Acids and Fatty acids.

Chromoplasts are plastids pigmented in various colors (red, yellow, orange, brown); their coloration is predominantly due to carotene (a red pigment) and xanthophyll (lemon-yellow), which accumulate in varying amounts. They are found in carrot root crops, certain fruits, flower petals, and leaves. Chromoplasts either lack an internal membrane entirely or possess only isolated thylakoids. Their shape is variable, ranging from spindle-shaped and sickle-shaped to rod-shaped and beyond.

Leucoplasts are colorless, morphologically diverse plastids found in tubers, fruits, and rhizomes. They serve as sites for the synthesis of reserve starch, proteins, and fats. They also lack a lamellar system, though occasionally the inner membrane invaginates to form sparse thylakoids. The leucoplast matrix contains DNA, ribosomes, and enzymes that drive starch and PROTEIN SYNTHESIS AND hydrolysis. Depending on their storage product, leucoplasts storing starch are termed amyloplasts, those storing proteins proteinoplasts, and those storing lipids elaioplasts.

All types of plastids share a common origin, developing from proplastids. Proplastids are small, double-membraned vesicles. Furthermore, one type of plastid can differentiate into another. For instance, when exposed to light, an internal membrane system develops within proplastids, chlorophyll is synthesized, and they transform into chloroplasts. Leucoplasts can likewise develop into chloroplasts or chromoplasts under illumination. Chromoplasts represent The final stage of plastid development and do not convert into other types.

Mitochondria are discrete, protein-lipid subcellular bodies of spherical or rod-like shape, bounded by a double membrane (outer and inner) with a thickness of 75—100 Å. The inner membrane forms incomplete transverse inward folds known as cristae, which significantly increase the active surface area (Fig. 5).

The cristae bear polyenzyme systems required for ATP synthesis, along with ribosomes and other components. The intermembrane spaces are filled with the matrix. The matrix contains DNA molecules, RNA, ribosomes, and granules composed of calcium and magnesium salts; it is also the site of synthesis for proteins incorporated into the inner membrane. The primary function of mitochondria is cellular Respiration, during which organic compounds undergo aerobic breakdown (oxidation), releasing a vast amount of energy (far exceeding that of anaerobic pathways).

Part of the released energy is dissipated as heat, while the remainder is channeled into the synthesis of adenosine triphosphate (ATP), the universal energy currency of living cells. Consequently, mitochondria are referred to as cellular respiration centers, or energy powerhouses. Like chloroplasts, mitochondria replicate by fission. Their Abundance within a cell depends directly on its metabolic activity.

The endoplasmic reticulum is a continuous network of channels, cisternae, tubules, and vesicles that connects the cytoplasm with the nucleus and neighboring cells, while participating in the transport and synthesis of diverse substances. Two Types of endoplasmic reticulum are distinguished: rough (granular) and smooth (agranular). The membranes of the rough endoplasmic reticulum are studded with ribosomes and are involved in protein synthesis. The Newly synthesized proteins pass through its cavities to various PARTS OF THE cell. The membranes of the smooth endoplasmic reticulum are the site of steroid hormone, lipid, and carbohydrate synthesis (Fig. 6).

Ribosomes are submicroscopic, spherical or mushroom-shaped non-membranous bodies located on the endoplasmic reticulum, as well as in the nucleus, mitochondria, and chloroplasts, where they establish local protein-synthesizing systems. Ribosomes occur either freely or assembled into chains (polyribosomes). A ribosome consists of two unequal subunits (large and small)—dimers—each possessing a constant chemical composition (Fig. 7).

Figure 7.

The core of the subunits is formed by ribosomal RNA and structural proteins that interact to form a unified ribonucleoprotein complex. Messenger RNA, which carries the code for synthesizing specific proteins, runs between the two dimers. Protein biosynthesis proceeds as follows: initially, the small dimer binds to an mRNA molecule on the membranes of the rough endoplasmic reticulum, after which the large dimer joins the complex. Meanwhile, Amino acids are activated in the cytoplasm by attaching to specific tRNA molecules, which feature an amino acid binding site at one end and a corresponding nucleotide triplet (anticodon) at the other. When a tRNA approaches an mRNA and their respective triplets prove complementary, The amino acid detaches from the tRNA and slots into its proper position within the polypeptide chain. Once Protein synthesis is complete, the ribosomal subunits dissociate.

The Golgi apparatus morphologically consists of dictyosomes, which are stacks (ranging from 5 to 20 or more) of flattened cisternae arranged one above the other, accompanied by associated vesicles and tubules (Fig. 8).

The Golgi apparatus plays a crucial role in the secretion of oils and mucus, the synthesis of Glycoproteins and polysaccharides, the Formation of the primary cell wall and the endoplasmic reticulum, the accumulation of secretory products, and the Regulation of cellular water balance. Secretory vesicles migrate to the periphery and fuse with The Plasma Membrane, supplying essential membrane material to the plasmalemma or contributing to vacuoles. Terminal metabolic products accumulate within the dictyosomes.

Sphaerosomes are small protein-lipid bodies with a diameter of 0.8—1.5 µm. They possess a single membrane and a granular stroma, exhibiting greater electron density and light-refracting capacity than water. Sphaerosomes consist of roughly three-quarters proteins and lipids. They contain lipid-synthesizing enzymes (lipase) and fats, which is why sphaerosomes are commonly known as lipid-producing bodies.

Lysosomes are spherical vesicles with a diameter of 0.2–0.8 мкм. They possess a single-membrane envelope and a granular stroma. In terms of chemical composition, they are similar to spherosomes: their stroma consists of ¾ proteins and lipids, along with hydrolytic enzymes. Characteristic enzymes include acid phosphatase, deoxyribonuclease, Ribonuclease, and cathepsin. Lytic enzymes are used to digest foreign bodies entering the cell. If the membrane is ruptured, these enzymes dissolve proteins, Nucleic Acids, and phosphorus-containing compounds, leading to cell necrosis; hence, they are often referred to as the cell's "suicide" weapons. Lysosomes perform local autolysis, which helps ensure cell survival during periods of nutrient scarcity.

Microtubules are thin, cylindrical cytoplasmic structures with a diameter of 25 нм and a length of 0.5–3.5 мкм, composed of spherical subunits of a protein called tubulin. Each subunit is formed by 13 longitudinal filaments surrounding a central lumen. Microtubules form a dynamic system within the cell: genetically older ones disappear and are replaced by new ones, restoring the System and Its functional activity. The synthesis of cellular substances is also associated with specific organizing centers of these structures.

In plant cells, microtubules perform crucial functions. In young, growing cells, they are located in the parietal cytoplasm, where they regulate cell growth, dimensions, shape, and wall development. They participate in the formation and aggregation of Cellulose microfibrils, as well as their incorporation into the growing cell wall. The direction of cell elongation is determined by the orientation of cellulose microfibrils within the cell wall. They assist in transporting Golgi microvesicles to the cell wall. Furthermore, they ensure the spatial arrangement and movement of organelles to sites of physiological activity, as well as the segregation of chromosomes to opposite poles during nuclear division. These structures form primary cell plates between daughter cells during cytokinesis and also serve as components of flagella, cilia, centrioles, and spindle fibers.

Microfilaments are cellular organelles with a diameter of 5–7 нм. While structurally similar to microtubules, they are much longer and thinner. These structures consist of individual protein subunits grouped into spiraled, ribbon-like formations. Microfilaments are essential cytoplasmic components that form a network of cytoplasmic fibers. Through the contraction of microfilaments and their shifting or displacement in opposite directions, cytoplasmic streaming is initiated in the cell. They drive various types of movement in the cytoplasm and organelles, with their pathways guided by the microfilament system. Together with microtubules, microfilaments form a labile reticular system known as the cell cytoskeleton.

Microbodies are organelles found in both PLANT AND ANIMAL cells. They frequently occur near internal membranes—such as mitochondrial cristae, the endoplasmic reticulum, and other structures. They are categorized into Peroxisomes and glyoxysomes. These are spherical bodies measuring 0.15–1.5 мкм. They consist of a fine-grained stroma, or matrix, differentiated into an amorphous central region or an ordered substructure, enclosed by a peripheral membrane. Occasionally, they contain crystalline protein inclusions. Structurally, they originate from cisternae of the endoplasmic reticulum, from which they detach or remain connected. The stroma contains catalase and several Other Enzymes that facilitate carbohydrate oxidation. Microbodies participate in energy production and metabolism, the maintenance of Anaerobic Metabolism, and glucose neogenesis.

Cellular locomotory organelles. These are not obligatory organelles in PLANT CELLS AND occur infrequently. They include pseudopodia, flagella, and cilia. Pseudopodia are temporary cytoplasmic protrusions formed As a result of cytoplasmic streaming, characteristic of animal cells. Flagella and cilia appear as fine cellular extensions covered by the plasma membrane; they contain inner microtubules and are anchored by basal bodies. In plants, flagella and cilia are found in certain green and euglenoid algae.

Cell wall. Plant cells possess a relatively rigid envelope that provides structural support and defined shape. The cell wall is a product of protoplast activity and is composed of cellulose, hemicellulose, and pectic substances (Fig. 9).

Cellulose forms micelles containing 40–60 glucose residues. Micelles aggregate into microfibrils, which in turn form macrofibrils that create a loose, three-dimensional meshwork. The spaces between fibrils are filled with pectic substances. Primary, secondary, and tertiary cell walls are distinguished. Wall thickening can be outward or inward. Between the cell walls of adjacent cells lies a three-layered middle lamella composed of pectic substances. Through certain thinner regions of the primary wall run plasmodesmatal tubules. These areas, known as pit fields, facilitate communication between adjacent cells. During Cell Differentiation, a secondary wall develops; it is significantly thicker and also consists of three layers, the innermost of which is sometimes referred to as the tertiary wall. The secondary wall is not continuous; it contains pits, or pit canals, which arise at pit field regions. The pits or pit canals of adjacent cells lie opposite one another, traversed by cytoplasmic strands called plasmodesmata, which mediate intercellular substance exchange. Pits are categorized as simple or bordered. Simple pits feature a cylindrical pit canal within the secondary wall and are characteristic of ground and meristematic Tissues. Bordered pits are more common in Vascular Tissues: overhanging borders develop on both sides above the primary wall. The primary wall thickens in the middle to form a torus, which can seal the pit aperture. Under high fluid pressure, the torus deflects, allowing water to flow in only one direction. Semi-bordered pits occasionally occur in vascular tissues adjacent to storage parenchyma, where a simple pit forms on the parenchymal side and a bordered pit on the vascular side. With age, the cell wall undergoes chemical modifications: lignification, suberization, cutinization, mucilaginous degeneration, and mineralization.

During lignification, the cell wall is impregnated with Lignin. This process begins in the middle lamella, where pectic substances actively undergo lignification. The Nature and extent of cell wall lignification vary—ranging from isolated encrustations in intermicellar spaces to heavy deposits of lignin. In cases of severe lignification, the cell protoplast dies, as seen in the xylem of tree trunks.

During suberization, the cell wall is impregnated with suberin, a lipid-like substance that renders the wall impermeable to water and gases. Suberized cells quickly die, become filled with air, and perform a protective function.

Cutinization involves the deposition of a lipid-like substance called cutin predominantly on the outer surface of the outer cell wall, forming a cuticle. Located in epidermal cells, the cuticle prevents excessive water loss and protects against fungal spore penetration and other hazards.

During mineralization, the cell wall is impregnated with calcium, silicon, and magnesium salts. Various pigments and Tannins may also be found within it. The dark coloration of most seed coats is due to tannins, which fully or partially impregnate the walls of protective tissues. Mineralization of epidermal cell walls is observed in the stems and branches of horsetails, as well as the stems and leaves of sedges.

In some plants, mucilaginous degeneration of cell walls occurs. This phenomenon involves The conversion of cellulose or starch into highly complex carbohydrates known as mucilages. This process can be observed in the seed coats of flax, pumpkin, melon, and watermelon, and it facilitates better seed germination.

Cell wall growth occurs via two mechanisms: apposition, which involves the deposition of new cellulose layers onto existing ones; and intus-susception, where the wall grows through the insertion of cellulose molecules among previously formed ones. The cell wall is likewise a product of protoplast metabolic activity.

Plant cell metabolic products. During cellular activity, the protoplast produces various substances, some of which are consumed in organelle synthesis, while others are stored as reserves or represent End products of Metabolism. Reserve nutrients accumulate in the form of formed (solid) and unformed inclusions. The primary reserve nutrients in cells are carbohydrates, proteins, and lipids.

Carbohydrates quantitatively predominate over other organic compounds. Simple carbohydrates, such as glucose and fructose, participate in cellular respiration; ribose and deoxyribose are incorporated into the cytoplasm as essential Components of nucleic acids. Plant cell carbohydrates include vacuolar carbohydrates, reserve starch, cellulose, cell wall hemicellulose, Pectins, mucilage, and others. Most of these carbohydrates are integral components of cellular structures and play a vital role in the Molecular organization of the cell.

Among reserve carbohydrates, an important role belongs to starch, which is stored in leucoplasts. Unlike assimilatory or primary starch formed in chloroplasts, reserve starch is of secondary origin. It is deposited in seeds, roots, root crops, and tubers in the form of starch grains. Starch grains vary in shape—spherical, oval, Kidney-shaped, or polyhedral—and are characterized by layering formed around the grain's origin center. This layering results from the alternation of more hydrated and less hydrated strata. In spherical and oval starch grains, layering can be eccentric or concentric. For example, starch grains in wheat endosperm exhibit concentric layering, whereas those in potato tubers show eccentric layering. Starch grains may be simple, compound, or semi-compound.

Simple grains always possess a single origin center, whereas compound grains develop in the plastid stroma when two or more origin centers form within it. Semi-compound grains arise when simple grains approximate and become enclosed by shared outer layers. The shape of starch grains is specific to each plant species: beans always produce simple grains, oats produce compound grains, and potatoes produce both.

Starch consists of two components—amylose and amylopectin—along with small amounts of potassium, sodium, silicon, phosphorus, and other elements.

Reserve Proteins are deposited in the endosperm or cotyledons of many plant seeds in the form of aleurone grains. Additionally, they are found in the parenchyma of root crops and other Organs where reserve nutrients are stored. Aleurone grains originate from vacuoles that, upon accumulating nutrients, lose water and solidify into oval or spherical granules. These grains, composed of amorphous proteins, frequently contain one or more crystalloids and globoids. Simple aleurone grains consist of a single protein, whereas compound grains comprise a protein, a globoid, and a crystalloid.

Lipids perform both structural and storage functions in the cell. They are constituents of most organelles, while reserve lipids accumulate in cellular spherosomes, predominantly in fruits and seeds. In some plants, such as sunflowers, lipid content reaches 52–55%, in peanuts 50%, and in wheat grains only 2%. Chemically, fatty oils are classified as esters of glycerol and fatty acids, most commonly oleic, palmitic, and stearic acids.

Essential Oils, which accumulate less frequently than fatty oils and are characteristic only of certain plant families, are a group of compounds containing carbon, hydrogen, and oxygen. They accumulate in glandular epidermal trichomes (e.g., in members of the Geraniaceae family), intercellular spaces, and schizogenous or lysigenous cavities.

Cellular activity gives rise to vacuoles filled with cell sap. In young cells, vacuoles are numerous and appear as tiny droplets bounded by a tonoplast. As the cell grows and ages, these vacuoles fuse to form a few large vacuoles or a single large central vacuole. Consequently, the nucleus is displaced to the periphery, and the cytoplasm is restricted to a thin parietal layer.

The functions of the vacuole include maintaining cellular turgor—which helps preserve the cell's fixed shape—and storing nutrient reserves.

Cell sap is a solution rich in metabolic products whose chemical composition varies across a very wide range. It primarily consists of carbohydrates, Glycosides, organic acids and their salts, Alkaloids, pigments, and tannins.

Carbohydrates in the cell sap are represented by sugars, predominantly Monosaccharides such as glucose and fructose (formula C₆H₁₂O₆), which accumulate in ripe fruits, stems, and leaves of certain plants. Disaccharides (C₁₂H₂₂O₁₁) in the cell sap are most commonly represented by sucrose, which is particularly abundant in sugar beet root crops, sugarcane stems, watermelons, and melons. Polysaccharides occur less frequently and include inulin, which is soluble in hot water and accumulates in the tubers of Jerusalem artichokes, dahlias, and some other plants of the Asteraceae family.

Glycosides are derivatives of monosaccharides, mostly combinations of glucose with alcohols, aldehydes, or phenols. Some glycosides are poisonous, such as amygdalin (found in apricot and almond seeds), or solanine, which forms in potato tubers exposed to light or in potato fruits. The bitterness of onions is also due to the presence of glycosides. In the air, glycosides break down under The Influence of certain enzymes, forming Aromatic Compounds. Examples include tea leaves, coffee beans, and vanilla flowers.

The glycosides of many plants, such as lily of the valley (Convallaria majalis) and large-flowered foxglove, are used in pharmacology.

Glycosides also include most tannins. These are complex, nitrogen-free compounds found in tree bark, tea leaves, and the roots and leaves of badan (bergenia), etc.

Cell sap contains a significant amount of organic acids, the most characteristic of which for plants are oxalic, malic, citric, and tartaric acids. These are abundant in the fruits of lemons, oranges, grapes, apples, and many other plants.

Cell sap also contains alkaloids—nitrogenous salts of organic acids that form in the leaves, stems, fruits, and seeds of many plants. Alkaloids include quinine, strychnine, morphine, papaverine, nicotine, and others. It is believed that they perform a protective role in plants because they are toxic to animals and humans. Some alkaloids affect The Nervous system and are used in medicine, most commonly as analgesics, as well as in experimental genetics to produce polyploids, such as the alkaloid colchicine.

Among the pigments found in cell sap, the most widespread are anthocyanins, anthochlor, and anthophaein. Anthocyanins are a group of substances that change color depending on the pH reaction of the cell sap: in an acidic environment, they acquire various shades of red, while in an alkaline environment, they turn blue. Anthocyanin is precisely what determines the color of most flower corollas. In addition to flowers, anthocyanins can also form in stem cells, leaf petioles (such as rhubarb and beet), root crops like radish and beet, and the pericarp of cherries, plums, grapes, cranberries, and others. Chemically, anthocyanins are glycosides in which a glucose residue is bound to a colored aglycone belonging to the anthocyanidin group. When combined with metals, the latter change color depending on the environment.

Anthochlor is a yellow pigment found in the petals of buttercups, primroses, and toadflaxes.

Anthophaein is a brown or dark brown pigment that rarely occurs as spots on the petals of bean flowers and certain orchids.

Among mineral salts, cell sap contains phosphoric, hydrochloric, calcium, magnesium, and other mineral salts. Sometimes the salts of these acids form various crystals, most frequently crystals of oxalic acid salts. These form within vacuoles and exhibit diverse shapes: solitary rhombic or cubic crystals; raphides—needle-like crystals mostly gathered in bundles; and druses—star-shaped crystal clusters formed by intergrowth. Gypsum or magnesium oxalate crystals occur less frequently.

Other metabolic products in the cell include enzymes, phytohormones, and vitamins.

Enzymes or ferments are protein-based substances that function as biocatalysts. Crucial processes such as assimilation, respiration, photosynthesis, and the Synthesis and Breakdown of substances occur under the action of specific enzymes. Enzymes consist of simple or complex proteins. The latter comprise two components—the protein carrier, which is the main bearer, and a prosthetic group, or coenzyme. Characteristic Features of enzymes include their Specificity (they can only transform corresponding substrates and catalyze certain reactions of a single type), high activity, and lability (their action depends on Temperature, trace impurities, and hydrogen ion concentration). All Enzymes are classified into Hydrolases, phosphorylases, Cleavage enzymes, oxidoreductases, transferases (phrases), and isomerases.

Phytohormones are BIOLOGICALLY ACTIVE SUBSTANCES that accelerate Cell Division, growth, and the formation of tissues and organs. Many different hormones are known. Some act as stimulants, while others act as inhibitors (suppressing enzyme activity). The best-known among them are Auxins, which are produced by the growing tips of stems and roots, promoting their elongation. Cytokinins, which stimulate plant metabolism and accelerate cell division, are also synthesized in young rootlets. Artificially created growth hormones include heteroauxin and gibberellin.

Vitamins are complex organic compounds of diverse chemical composition that play a vital role in metabolic processes. They interact closely with enzymes and phytohormones. For example, B-complex vitamins, first discovered in Yeast cells, stimulate root system development even at minimal concentrations. Vitamins are most abundant in plant fruits, leaves, seeds, and roots. They are designated by capital letters of the Latin alphabet and divided into specific groups. The most common vitamins produced in plant organs are: Vitamin C (ascorbic acid)—found in lemons, cabbage leaves, currants, and other plants; and provitamin A (carotene), which is abundant in carrot roots, sweet peppers, and tomatoes. The Skin of darkly pigmented fruits contains group P vitamins (bioflavonoids).

Vitamins are essential for the normal functioning of All living organisms. A deficiency of these substances leads to a condition known as hypovitaminosis in humans.

The cells of many higher plants produce specific volatile compounds of diverse chemical nature known as phytoncides. They possess antibiotic activity. Phytoncides are particularly abundant in the needles of pine and fir, onions, garlic, walnut leaves, and the fragrant flowers of most plants. Allicin, a phytoncide extracted from onions, is widely used in medicine.

Cells of fungi and certain cyanobacteria secrete antibiotics that also exhibit bactericidal action.