BOTANY WITH THE BASICS OF HYDROBOTANY - 2010

1. Fundamentals of Cell DOCTRINE (Cytology)

1.1. Forms of Life on Earth

The Cell is the basic Structural and functional unit of All living organisms, as well as an elementary living system.

There are approximately 5 billion different organisms in the biosphere. Most of them have a cellular Structure. Only Viruses represent non-cellular life forms. Viruses consist solely of a nucleic acid molecule (DNA or RNA) and a protein coat. They are the simplest, pre-cellular form of life.

In most living organisms, the body consists of a single cell (unicellular) or many Cells (multicellular). Based on structural features, cellular organisms are divided into two groups: PROKARYOTES AND EUKARYOTES. A third form has also been discovered—mesokaryotes—which are organisms with an intermediate type of genetic apparatus Organization.

Prokaryote means pre-nuclear. Such living organisms lack a clearly defined nucleus, and their genetic material is located in the form of a DNA molecule directly within the cell protoplasm, unprotected by a nuclear envelope. Prokaryotes include Bacteria and blue-green Algae.

Eukaryote, translated from Greek, means possessing a true nucleus. Eukaryotic cells contain a true nucleus separated from the Cytoplasm by a double membrane. Eukaryotes include all animals and higher plants, as well as unicellular and multicellular algae and Fungi.

The cell serves as the unit of structure and vital activity for all living organisms. It concentrates all manifestations of life. It assimilates substances and energy from the environment, breathes, responds to stimuli, grows, develops, and reproduces through division.

Unicellular organisms perform all Functions intrinsic to them. A characteristic feature of unicellular structures is their single-celled organization featuring a clearly segregated nucleus and other Organelles. Unicellular organisms include diatoms, green algae, euglenoids, and other algae.

In colonial structures, the Organism consists of numerous cells united by a common mucous mass. At the same time, the cells retain their independence and are not interconnected by METABOLISM. However, distinct groups of cells stand out among them (such as in Volvox), which are connected cytoplasmically and functionally. Such organisms occupy an intermediate position between typical colonial and multicellular structures characteristic of higher plants.

In Multicellular Organisms, groups of cells have adapted to perform one or more specific functions, executing them more thoroughly and perfectly. The cells of a multicellular organism are interconnected into a single system. Multicellular structures are characteristic of organisms that have reached a high level of development. Among them are those lacking body differentiation into true Tissues and vegetative Organs. These include charophytes, brown algae, and red algae. Meanwhile, higher spore-bearing and flowering plants are already differentiated into true tissues, vegetative organs, and Generative organs.

1.2. Cell Theory of Organism Structure

The structural and functional unit of a plant organism, like that of other living beings, is the cell. This term was first proposed in 1665 by the English scientist Robert Hooke.

The German botanist M. Schleiden (1838) proved that the cell and its organelles are constituent PARTS OF THE plant organism. The following year, his compatriot, the zoologist T. Schwann (1839), formulated the Cell Theory clearly and comprehensively for the first time in biology. Here are its main postulates:

- the cell is the basic structural and functional unit of living organisms;

- animal and plant cells share common structural principles;

- a cell arises from another cell;

- a living organism is a totality of cells interconnected through Various Forms of regulation.

The cell theory of organism structure had a profound impact on The Development of not only botany but natural science as a whole. It directed the efforts of biological scientists toward a more detailed study of cell contents and demonstrated the common Water/144.html">Origin of the organic world. Thus, the cell theory became the foundation of the evolutionary theory of the plant kingdom.

1.3. Main Differences Between Plant and Animal Cells

PLANT AND ANIMAL cells are similar to each other. However, despite sharing fundamental structures, they possess A number of significant differences associated with the structural features, vital activities, and lifestyles of plants and animals.

The main differences between a PLANT CELL AND an animal cell

are as follows:

- plant cells contain specialized organelles—Plastids—which are absent in animal cells;

- plant cells are invariably surrounded by a robust, rigid Cell wall that protects them against adverse environmental conditions, while providing structural support and mechanical strength;

- plant cells are characterized by a well-developed vacuolar system containing cell sap, which maintains cellular osmotic properties—namely turgor pressure and water uptake;

- plant cells are interconnected by cytoplasmic strands known as plasmodesmata, which traverse the rigid cell wall to integrate the organism's cells into a unified, functioning system;

- in many plant cells, reserve nutrients are accumulated in various forms, a process favored by the predominance of anabolic over catabolic processes in autotrophic plants.

Dead cells are also commonly present in the plant body, particularly in the stems of perennial plants, where they can make up to 80% of the tissue. In such cases, the cell refers simply to the remaining wall lacking living contents, yet performing vital functions (such as mechanical support or water conduction).

1.4. Shape and Size of Plant Cells

Plant cells exhibit A wide variety of shapes—spherical, cubic, prismatic, oval, fusiform, stellate, and others. This Morphology is determined by their surrounding environment and specialized functions.

Based on their shape, plant cells are divided into two main types: parenchymatous and prosenchymatous.

Parenchymatous cells have roughly equal dimensions across all three axes (length, width, and height), with their maximum diameter rarely exceeding the minimum by more than 2–3 times. These cells typically make up Meristems and certain permanent tissues, such as the epidermis, storage tissues, stem pith, and ground parenchyma. Tissues composed of parenchymatous cells are generally living.

Prosenchymatous cells are elongated and frequently feature tapered ends, with their length exceeding their width by 5 to 20 times or more. These cells are frequently dead and devoid of living protoplasm, forming the primary Structural components of conductive and mechanical (supportive) tissues.

Cell shape and structure depend directly on their spatial Location and physiological role. Cells of the apical meristem are isodiametric, but as they specialize and assume distinct physiological functions, they undergo significant morphological changes. Cells in the outer layer that give rise to the epidermis are prismatic or flat. Those in the central region of the apical meristem are polygonal and elongated, eventually forming vascular, ground, and other tissues. Epidermal cells are flattened with sinuous walls; conducting cells are tubular with thin cellulosic walls; and supportive cells develop heavily thickened cell walls.

Plant cells are microscopic in size, typically ranging from 10 to 100 µm in average diameter. Meristematic cells are generally smaller, around 5–8 µm, whereas cells of storage tissues in the pith and the flesh of succulent fruits (such as apples, watermelons, and citrus) are comparatively large, measuring 0.2–1 mm, and can be seen with the naked eye. Prosenchymatous cells, particularly fibers, achieve considerable length: flax fibers can reach up to 4 cm, cotton up to 5 cm, stinging nettle up to 8 cm, and ramie up to 22 cm. However, their transverse dimensions remain within typical cellular ranges (50–100 µm), rendering them invisible without magnification. The greatest lengths, reaching up to several meters, are found in the laticifers of woody plants.

1.5. Structure of the Plant Cell

A mature plant cell consists of three main components: a dense, elastic cell wall enclosing the exterior; the protoplast—the living content of the cell, pressed as a thin layer against The cell wall; and the vacuole—a central cavity filled with watery cell sap.

The protoplast, or living content of the cell, represents its core, most vital part and comprises an array of cellular organelles (nucleus, plastids, Mitochondria, Lysosomes, Ribosomes, etc.).

The cell wall and vacuoles are metabolic byproducts of the protoplast, derived from its activity and appearing at a specific stage of cellular development.

The protoplast is divided into two primary components: The Nucleus and the cytoplasm. The cytoplasm, in turn, consists of particulate and membranous organelles suspended in a ground substance known as the matrix, or Cytosol (Fig. 1.1).

Fig. 1.1. Diagram of leaf mesophyll Cell Structure

1— cell wall; 2 — middle lamella; 3 — Plasmalemma; 4 — cytoplasm; 5 — vacuole; 6 — nucleus; 7— nucleolus; 8 — nuclear envelope; 9 — Golgi apparatus; 10 — lipid droplet; 11 — Endoplasmic reticulum; 12 — plasmodesma; 13 — Microbodies; 14 — chloroplast; 15 — ribosome; 16 — mitochondrion

Protoplast and Cytoplasm. The protoplast is the colorless, transparent living content of the cell. Its viscosity varies across different plant species, averaging 12 to 20 times that of water, and only 5 to 6 times in aquatic plants.

The portion of the plant protoplast excluding the nucleus is termed the cytoplasm, which can be structurally differentiated into three distinct zones: the plasmalemma, the mesoplasm, and the tonoplast.

The outer layer adjacent to the cell wall—the plasmalemma—is extremely thin, lipid-rich, and contains densely packed, highly ordered micelles, devoid of organelles. The middle layer, or mesoplasm, is thicker and features a random arrangement of micelles, housing all major cellular organelles. The third, innermost layer—the tonoplast—surrounds the central vacuole and shares structural similarities with the plas

malemma.

Membranes. The protoplast is bounded both externally and internally by specialized biological membranes: the plasmalemma separates it from the cell wall, while the tonoplast separates it from the vacuole. Additional membranous structures include the nuclear envelope, mitochondrial and plastid membranes, Golgi sub-units, as well as the internal Cytoplasmic membranes of The endoplasmic reticulum, mitochondria, and Chloroplasts. Membranes represent highly organized cellular structures whose precise composition varies according to type and function, though they invariably contain Lipids and Proteins.

In 1972, English scientists S. Singer and G. Nicolson proposed the fluid-mosaic model of the membrane, according to which protein molecules within lipids form a mosaic-like structure (Fig. 1.2).

Membrane Lipids are represented by Phospholipids, Glycolipids, and sterols. Protein molecules likely permeate the lipid layer, potentially binding via their hydrophobic side chains to the internal hydrophobic portions of the lipid molecules. Granules in the inner regions of membranes are most commonly proteins anchored hydrophobically within the lipid matrix of the membrane.

Cellular membranes contain thousands of different proteins. Some are only partially embedded in the membrane, while others span it entirely. The hydrophobic regions of proteins interact with lipids, whereas the hydrophilic regions contact the aqueous content of the cell. Membrane lipids provide the necessary environment for these proteins to function.

In turn, the labile structure of membranes enables them to perform a wide variety of functions: barrier, transport, osmotic, electrical, structural, energetic, biosynthetic, secretory, receptor-regulatory, etc. Each organelle also possesses its own functions, carried out within a unique internal environment. This environment is established through selective permeability and other specific Properties of the membranes that surround the organelle and isolate it from the remaining protoplast compartments.

The cytoplasm matrix, or cytosol. The matrix is the fundamental substance of the cytoplasm, also referred to as the cytosol or hyaloplasm (from the Greek "hyalos" meaning Glass). It constitutes a colloidal system with a high water content (up to 90%). The matrix contains a vast number of enzyme proteins. All cellular organelles, including nuclei, are suspended within it.

The Functions of the matrix include uniting and interconnecting all organelles, transporting substances between them, and carrying out numerous enzymatic processes (Glycolysis, lipid synthesis, initial Stages of Protein Synthesis, etc.). The matrices of all cells are interconnected via plasmodesmata.

Cellular organelles. Organelles are structured cytoplasmic elements that perform specific functions in cells related to their vital activity.

Corpuscular organelles have a strictly defined shape (round, oval, disk-shaped, rod-shaped, thread-like, etc.). Corpuscular organelles include the nucleus, plastids, mitochondria, lysosomes, ribosomes, and microbodies.

Membranous organelles consist of a collection of membranes and lack a precisely defined shape (ER, plasmalemma, tonoplast). The Golgi apparatus is an organelle combining the characteristics of both corpuscular and membranous structures.

Among the listed cellular organelles, the majority are common to both plants and animals, though some are specialized and unique to plant organisms. The latter include plastids and certain microbodies (Peroxisomes, glyoxysomes). Protoplast waste products are also purely plant-specific cellular components (cell wall, vacuoles, stored nutrient reserves).

Membranous organelles. These include the plasmalemma, tonoplast, and endoplasmic reticulum.

Plant cell membranes, like all natural lipoprotein membranes, consist of a lipid bilayer in which protein globules are embedded.

The plasmalemma is The Plasma Membrane that externally surrounds the protoplast and adheres to the cell wall. The plasmalemma can form invaginations. The Main Functions of the plasmalemma are:

1) substance exchange between the cell and its environment;

2) Cellulose synthesis;

3) contribution to the cell's osmotic properties;

4) perception of stimuli;

5) cell-to-cell communication: the Plasma Membranes of all living plant cells are interconnected via plasmodesmata.

The tonoplast is the membrane surrounding the vacuole and separating it from the cytoplasm. In its structure, the tonoplast is largely similar to the plasmalemma: it has approximately the same thickness, utilizes both passive and Active Transport MECHANISMS, and can fuse with membrane-bound vesicles containing substances entering the vacuole (via exocytosis).

The endoplasmic reticulum, or ER (reticulum meaning network), is a complex three-dimensional membranous system represented by a network of tubules, vesicles, and cisternae bounded by lipoprotein membranes.

Rough and smooth endoplasmic reticulum (ER) are distinguished.

The surface of the rough endoplasmic reticulum features A large number of ribosomal granules. Rough ER is characteristic of cells undergoing intensive Protein Synthesis. Its functions are associated with The transport of proteins synthesized by ribosomes. During transport, the protein undergoes significant modifications, such as phosphorylation, Acetylation, conversion into Glycoproteins, etc.

Cells engaged in intensive lipid synthesis possess a different, highly branched smooth tubular ER. It lacks ribosomes On the surface of its membranes and predominantly consists of elongated tubular channels. Lipids and CARBOHYDRATES are primarily synthesized on the smooth membranes. Both types of ER may even coexist within the same cell, and a functional relationship exists between them.

Let us examine the functions of the ER in plant cells.

Compartmentalization. The ER divides the cell into discrete regions known as compartments. These compartments are reactive spaces enclosed by membranes, which is essential for carrying out various reactions and

metabolic processes within specific areas of the cell. The ER provides an extensive internal surface area that facilitates these diverse cellular reactions and processes.

Synthetic function. Synthetic processes take place on the ER membranes: protein synthesis occurs on the granular (rough) membranes, while Introduction/36.html">Carbohydrates and lipids are synthesized on the smooth membranes.

The ER tubules serve as a pathway for the Intracellular Transport of Substances, including proteins, lipids, and carbohydrates.

The ER participates in The formation of membranes for several organelles, such as the Golgi apparatus and spherosomes.

The ER networks of all living plant cells are interconnected into a single unified system via plasmodesmata.

Plasmodesmata are cytoplasmic strands that connect two adjacent cells. These strands pass through microscopic plasmodesmatal tubules that traverse the cell walls of neighboring cells. At the center of each tubule lies a desmotubule, which connects with the ER tubules of adjacent cells, effectively linking them. Thus, plasmodesmata integrate all living plant cells into a continuous living system known as the symplast. This system facilitates the intercellular movement of substances. In plants, another functional system can also be distinguished—the apoplast. This is a non-living system comprising cell walls, intercellular spaces, and the cavities of dead cells. Located outside the plasmalemma, the apoplast is likewise utilized by the plant for the Transport of substances.

The Golgi Apparatus. Among cytoplasmic structures of a membranous nature, the Golgi apparatus (or Golgi complex) plays a major role. It is a multi-tiered system of flattened, hollow, disc-shaped cisternae (dictyosomes) that widen at the periphery to form tubular extensions. A dictyosome (Golgi body) consists of a stack of 4 to 8 flattened cisternae made of smooth membranes, measuring 1-2 µm in diameter. These cisternae do not Touch one another, but are separated by an interval of approximately 10 nm.

Plant cells typically contain between 10 and 50 dictyosomes randomly dispersed throughout the cytoplasm, their number depending on the cell type and its metabolic activity. The cells of certain algae contain only a single dictyosome. The aggregate of all dictyosomes within a cell constitutes its Golgi apparatus.

A dictyosome is typically oriented such that its outer cisterna runs parallel to an ER tubule. Numerous small vesicles, known as transition vesicles, are found between them. It is hypothesized that these transition vesicles bud off from the ER membranes and fuse to form the dictyosome cisterna. This face of the dictyosome is referred to as the forming (cis) face. On the opposite side, the terminal cisterna breaks down into small vesicles—Golgi vesicles—which migrate toward and fuse with the plasmalemma or tonoplast. Golgi vesicles may also pinch off from the outermost cisterna or its peripheral tubular networks. This face of the dictyosome is called the maturing (trans) face. Consequently, the dictyosome is a dynamic structure: cisternae are continuously formed at one end from transition vesicles, gradually advance across the stack, and break down into Golgi vesicles at the other end (Fig. 1.3).

Fig. 1.3. Diagram of dictyosome structure:

1 — cisternae; 2 — tubules with swollen ends; 3 — Golgi vesicles

Let us outline several vital functions performed by the Golgi apparatus in plant cells.

The dictyosome cisternae synthesize matrix Polysaccharides of the cell wall, specifically hemicellulose and Pectins. Golgi vesicles then transport these compounds to the plasmalemma, integrate into it, and release their contents extracellularly. Cell wall matrix proteins, acting as Enzymes, are likewise delivered from the rough ER via transition vesicles. Within the dictyosomes, these proteins undergo modification—carbohydrate moieties are attached to convert them into glycoproteins—after which they are transported in Golgi vesicles to the plasmalemma.

Hydrolytic enzymes from the rough ER pass through the Golgi apparatus, where they are packaged into Golgi vesicles, transported to the tonoplast, and released into the vacuole to fulfill its lysosomal function.

Through the action of Golgi vesicles, Golgi membranes contribute to the planar growth of the plasmalemma and tonoplast: as vesicular contents are discharged into the vacuole or cell wall, the vesicle membrane incorporates into the tonoplast or plasmalemma, respectively.

The Golgi apparatus gives rise to lysosomes, which pinch off as vesicles from the cisternae at the maturing face or from peripheral tubular networks. These lysosomes contain hydrolytic enzymes that also originate from the rough ER membranes.

The Golgi apparatus mediates the transformation of intracellular membranes, converting ER membranes into the plasmalemma or tonoplast.

Dictyosomes are sites not only for the synthesis and modification of substances, but also for their sorting based on final destination. Cell wall matrix components are packaged into one set of Golgi vesicles directed toward the plasmalemma; hydrolytic enzymes are sorted into another set of vesicles that diffuse toward the tonoplast; while other vesicles accumulate high concentrations of hydrolytic enzymes to become cytoplasmic lysosomes.

The Endomembrane Concept. Formulated in the early 1970s, the endomembrane concept posits that all internal cellular membranes form a cohesive endomembrane system. Its components include the membranes of the ER, nucleus, Golgi apparatus, and Golgi vesicles, alongside the plasmalemma, tonoplast, and the outer membranes of chloroplasts and mitochondria. The inner membranes of chloroplasts and mitochondria are excluded from the endomembrane system because, According to the endosymbiotic theory of organelle origin, their inner membranes derived from the plasma membranes of prokaryotic ancestors that entered a host cell and established a symbiotic relationship.

The endomembrane system is highly dynamic, characterized by a continuous flux of membranes and their transformation along the pathway from the ER through the Golgi apparatus to the plasmalemma and tonoplast.

Corpuscular Organelles

Plastids are organelles exclusive to plant cells, present in the cells of all plant organs: stems, roots, leaves, and flowers. Based on their pigmentation, plastids are divided into three main groups: chloroplasts (green), chromoplasts (orange-red), and leucoplasts (colorless).

As a rule, individual cells contain only one type of plastid. All plastids share certain structural features: they are typically round, oval, or disc-shaped, and are bounded by an envelope consisting of two lipoprotein membranes. The interior contains a ground substance known as the matrix, referred to in plastids as the stroma. Plastids differ in the specific components suspended within this stroma; nevertheless, the matrix of virtually all plastids contains internal membrane structures, although their degree of development varies widely. Like mitochondria, plastids possess their own DNA. Proplastids and etioplasts, which serve as precursors to these organelles, are sometimes also classified as plastids.

Chloroplasts are green in color and carry out Photosynthesis. They are found in almost all cells of the aerial organs of plants where light penetrates. As a rule, they are absent in ROOT cells.

Chloroplasts are present in all green photosynthetic plant cells. They are absent only in blue-green algae (cyanobacteria), or more precisely, structurally defined green plastids are lacking. Their functions in blue-green algal cells are performed by membrane aggregates—

stray, ribbon-like, ribbed, cylindrical, etc.).

A chloroplast is enclosed by a double lipoprotein membrane—an envelope that regulates the metabolism between the organelle and the cytoplasm (Fig. 1.5). Inside the chloroplast is the ground substance known as the matrix, or stroma. Immersed in the stroma is a system of membranes called lamellae in chloroplasts. They form flat vesicles called thylakoids, which are stacked into grana. The grana are interconnected into a single system by tubular, elongated stroma thylakoids, or intergranal thylakoids.

sizes, and a cell contains only one or a few of them. The shape of chromatophores is unusual and bizarre (stellate-

Fig. 1.4. Chloroplasts in moss cells

Chloroplasts possess their own protein-synthesizing system complete with DNA, rRNA, and ribosomes. Chloroplast DNA is maternally inherited (via proplastids). It is not inherited from the parent plants through the pollen grain.

Fig. 1.5. Structure of a chloroplast:

1 — outer membrane; 2 — inner membrane; 3 — stroma;

4 — lipid droplets; 5 — granum;

6 — stroma thylakoids; 7 — starch grains; 8 — chloroplast envelope; 9 — ribosomes

Circular chloroplast DNA differs from nuclear DNA both in the information it carries and in its physicochemical properties. Chloroplasts contain their own DNA polymerase, which is why their DNA replicates within the chloroplasts themselves.

Chloroplasts of higher plants contain Two Types of green pigments: chlorophyll a and chlorophyll b. Chlorophyll strongly absorbs light in the red region of the spectrum (680–730 nm) and in the blue-violet region (470 nm and below), while transmitting yellow and green rays.

In addition to chlorophyll, chloroplasts contain orange pigments known as carotenoids, which include carotenes and xanthophylls. Carotenes have an orange color, whereas xanthophylls are yellow. Typically, the intense coloration of chlorophyll masks the carotenoids, giving chloroplasts and leaves their green appearance.

Chloroplasts contain numerous enzymes involved in carrying out various photosynthetic reactions as well as the synthesis of proteins, Nucleic Acids, and other molecules.

The primary function of chloroplasts is photosynthesis—The process of capturing solar energy and converting it into chemical bond energy.

The photosynthetic process is divided into light-dependent and light-independent (dark) reactions. During the light-dependent stage, light Energy is stored in ATP and the reducing agent NADPH + H⁺ is formed; these are then consumed in the dark stage to reduce CO₂ to the carbohydrate level. The Reactions of the light stage take place in the thylakoid lamellae of the grana, while those of the dark stage occur in the stroma.

Chloroplasts in cells develop from proplastids, which are small bodies (1–1.5 µm) enclosed by a double membrane.

Chloroplasts multiply by division. Once leaf growth ceases, chloroplast division stops.

Chloroplasts are typically positioned to receive diffused light. They avoid direct sunlight, which destroys chlorophyll.

Origin of chloroplasts. Because chloroplasts are relatively autonomous relative to the nucleus, being capable of division, growth, and differentiation, a hypothesis arose that in ancient times they were independent organisms. It is likely that chloroplasts formed As a result of a Symbiosis between an autotrophic organism (perhaps a cyanobacterium) capable of transforming solar energy and a heterotrophic cell. Plastids and photosynthetic prokaryotes share many similarities (circular DNA Structure, lack of Histones, ribosome sizes, etc.). From an evolutionary standpoint, this was mutually beneficial, which is why such a symbiotic union has been preserved in the form of the modern eukaryotic plant cell.

Chromoplasts are non-photosynthetic colored plastids that contain predominantly red, orange, and yellow pigments. They typically develop from chloroplasts, share approximately the same size and shape, and are quite similar in structure.

The interior of chromoplasts is filled with stroma, which occasionally retains remnants of the membrane system, such as isolated thylakoids or tubular aggregates. However, instead of a system of photosynthetic membranes, they contain structures rich in carotenoids.

Chromoplasts are found in the petals of certain flowers (buttercups, dandelions, etc.), in fruits (rose hips, peppers, rowan berries, tomatoes, watermelons), and more rarely in vegetative organs (carrots).

The physiological function of chromoplasts has not been studied in detail. Their bright coloration likely serves to attract insects for pollination and seed dispersal.

Leucoplasts are small, colorless plastids found predominantly in the cells of tissues that are not exposed to light (such as cotyledons, seed endosperm, rhizomes, tubers, and roots), and more rarely in tissues

exposed to light (such as the peel).

The primary function of leucoplasts is the storage of nutrients delivered to storage organs.

The inner membrane of leucoplasts forms single thylakoids. Leucoplasts contain their own DNA, ribosomes, and various enzymes. Leucoplasts that synthesize and store starch in non-green parts of plants are called amyloplasts, those storing plant oils are called elaioplasts, and those storing proteins are designated as proteinoplasts.

Amyloplasts are the most common type of leucoplast, accumulating reserve starch in the form of grains. They occur in storage tissues and organs, specifically in cotyledons, the endosperm, tubers, and the root cap.

Proteinoplasts contain reserve protein within their stroma in the form of crystals or an amorphous mass. Leucoplasts of this type are a rare occurrence (found in orchid root tubers).

Elaioplasts are even rarer. Fats (oils) accumulate within their stroma as droplets or plastoglobules (in monocots).

In the rhizomes of certain iris species, leucoplasts may accumulate either starch or fat.

Interconversion of plastids. All plastids—chloroplasts, chromoplasts, and leucoplasts—can develop directly from proplastids present in meristematic cells.

Mitochondria. Numerous mitochondria are scattered throughout the cytoplasm of cells, ranging from 1 to several µm in length and up to 0.5 µm in width. Mitochondria are organelles present in both plant and animal cells. A plant cell may contain anywhere from several dozen to several thousand mitochondria, their number depending on the cell's age and metabolic activity. Certain algae (such as Chlorella and Chlamydomonas) contain only a single large mitochondrion per cell.

Mitochondria are enclosed by a double lipoprotein membrane. The outer membrane differs from the inner one in its protein-to-lipid ratio, enzyme composition, and permeability. The inner mitochondrial membrane forms infoldings known as cristae, the size and number of which depend on the functional activity of the mitochondria (Fig. 1.6). The cristae provide a large surface area for reactions occurring on the inner membrane. The space between the cristae is filled with a ground substance called the matrix, which contains granular

and fibrillar components. The granules consist of ribosomes and calcium phosphate, while the fibrils represent DNA strands.

Fig. 1.6. Diagram of mitochondrial structure:

1 — outer membrane; 2 — inner membrane; 3 — cristae; 4 — intermembrane space; 5 — matrix

Similar to plastids, mitochondria are semiautonomous organelles, as protein synthesis within them occurs independently of the nucleus. The mitochondrial matrix houses its own protein-synthesizing system consisting of circular DNA molecules, RNA, and 70S ribosomes. The Mitochondrial Genome encodes up to 50 membrane and matrix proteins.

The primary function of mitochondria is bioenergetic. They are the site of cellular Respiration—The oxidation of nutrients, primarily carbohydrates, accompanied by the release of energy stored in the form of ATP. The breakdown of carbohydrates begins in the cytoplasmic matrix (glycolysis) and continues in the mitochondrial matrix (the Krebs cycle). ATP synthesis takes place on the inner mitochondrial membrane.

Mitochondria are constantly in motion, aggregating wherever energy demand is highest. Their lifespan is typically several days, and they reproduce within cells by division.

Detailed studies of mitochondrial structure and genome suggest that, much like chloroplasts, they originated from bacteria that established a symbiotic relationship within larger heterotrophic cells—the precursors of eukaryotes.

Lysosomes are small, rounded organelles ranging from 0.5 to 2.5 µm in diameter. They are generally considered to be more abundant in animal cells than in plant cells. Lysosomes are bounded by a single lipoprotein membrane enclosing a matrix filled with hydrolytic enzymes that break down various Organic compounds, such as proteins, nucleic acids, and polysaccharides. Lysosomes are formed from the cisternae of the Golgi apparatus or the smooth membranes of the ER. Their primary function is the Digestion of substances and specific Regions of the cytoplasm, a process known as local autolysis.

Lysosomes break down worn-out cellular components and organelles that have completed their functional lifespan and are no longer required by the cell. The products of their breakdown are recycled to build new substances and structures essential for the cell.

Lysosomes also degrade potentially harmful substances that enter the cell via endocytosis, and the resulting breakdown products are utilized by the cell.

During cell death, lysosomal membranes rupture, releasing their enzymes into the cytoplasm. This triggers autolysis of all living contents, resulting in the clearing of the lumens of dead cells (such as xylem tracheae and tracheids, and mechanical tissue cells).

Ribosomes are tiny cellular organelles that can only be visualized using an Electron microscope. They possess an oval shape. Each ribosome is composed of two subunits—a large one and a small one. Ribosomes dissociate into their two constituent subunits when the concentration of Mg²⁺ ions in the medium drops.

Ribosomes consist of approximately equal amounts of RNA and Protein. When the subunits join to form an intact ribosome, a cleft remains between them through which Messenger RNA passes, while the large subunit features an additional groove that accommodates and guides the synthesized protein molecule.

Ribosomes serve as the protein-synthesis centers within the cell. They facilitate the arrangement and linking of Amino Acids into a polypeptide chain according to genetic instructions received from the nucleus via messenger RNA. Protein synthesis takes place both on individual ribosomes and on clusters of them bound to mRNA, known as polyribosomes.

Ribosomes are widespread throughout the cell: they are attached to the endoplasmic reticulum membranes (granular membranes) and the outer nuclear membrane, and are found in the cytoplasmic matrix, nucleus, chloroplasts, and mitochondria.

The number of ribosomes in a cell can reach tens of millions. It depends on the intensity of protein synthesis, the physiological state of the cell, and environmental factors such as light, Temperature, humidity, and mineral Nutrition conditions.

rRNA is synthesized in the nucleolus, whereas ribosomal proteins are produced in the cytoplasm, from which a portion of them is transported into the nucleus. Ribosome assembly begins in the nucleolus and is completed in the cytoplasm.

Microbodies, or microparticles, are small, spherical or thread-like cellular organelles. The former include peroxisomes, glyoxysomes, and spherosomes, while the latter comprise microtubules and microfilaments.

Peroxisomes are found in the cells of photosynthetic tissues. Here, Photorespiration takes place—a process functionally linked to chloroplasts and mitochondria. During photorespiration, peroxisomes generate and break down hydrogen peroxide, hence their name.

Glyoxysomes occur in the seeds of oilseed plants and function during their germination. They mediate The conversion of reserve fats into carbohydrates (glucose), which are transported to the seedling to nourish it until it emerges from the soil.

Spherosomes are fat- (oil-) storing organelles. Present in the seeds of oilseed crops, they contain enzymes that synthesize fats and accumulate them during seed maturation.

Microtubules are delicate cylindrical structures with a diameter of 24–25 nm and a variable length (up to several µm). Microtubules perform several vital functions in the cell:

- they form the achromatic spindle during nuclear division and the phragmoplast during Cell Division;

- they lie beneath the plasmalemma, determining the orientation of cellulose microfibrils within the cell wall;

- they direct the movement of Golgi vesicles carrying cell-wall matrix substances to the regions of the plasmalemma where cellulose is actively synthesized;

- in motile cells, microtubules serve as components of flagella and cilia.

Microfilaments are filamentous structures composed of Actin protein subunits, sharing properties similar to Muscle actin. Microfilaments drive the movement of the cytoplasm and its organelles.

Owing to their relatively rigid structure, microtubules and microfilaments fulfill yet another crucial cellular role—they form the cytoplasmic Cytoskeleton.

The nucleus is the largest and most prominent particulate organelle in eukaryotic cells. It is a spherical body, 5–10 µm in diameter, containing the majority of Genetic information in the form of long DNA strands. This organelle is present in all plant cells except for the enucleate elements of phloem sieve tubes.

In a young cell, the nucleus typically occupies a central position. As the cell grows and specializes, when its central area becomes taken up by a vacuole, the nucleus, along with the cytoplasm, is pushed toward the cell wall.

In parenchymatous cells, the shape of the nucleus is most often spherical or disk-like, whereas in prosenchymatous cells it is elongated, spindle-shaped, or thread-like. In some cells, the nuclear shape is lobed or even branched.

The nucleus performs two paramount functions:

- it serves as the repository for storing and replicating genetic information passed from the mother cell to daughter cells during division;

- it regulates cellular activity, growth, and development primarily through mRNA, which is synthesized in the nucleus and carries instructions regarding the cell's protein composition at any given moment. The Complement of enzyme proteins dictates metabolic pathways and, consequently, The properties of both the cells and the organism as a whole. Removal of the nucleus results in cell death.

The nucleus concentrates nearly all of the cellular DNA (99%) and a much smaller fraction of RNA. The remaining 1% corresponds to the DNA of chloroplasts and mitochondria.

The core structural Components of the nucleus are the nuclear envelope, nuclear matrix, Chromatin, and nucleoli.

The nuclear envelope consists of two elementary membranes separated by a cavity known as the perinuclear space. The outer membrane is continuous with the endoplasmic reticulum membranes, and the perinuclear space communicates with the cavities of its tubules and cisternae.

The main substance of the nucleus, the nuclear matrix, is called the nucleoplasm or karyoplasm. The nucleoplasm is a structureless mass containing ribosome-like granules. It is connected to the cytoplasmic matrix through nuclear pores. The functions of the nucleoplasm include interconnecting all structural components of the nucleus and driving various enzymatic reactions.

Chromatin is a cluster of fine filaments immersed in a matrix. The filament component is called euchromatin, and the granular component is heterochromatin. Chromatin represents a structural modification of Chromosomes. In interphase and metabolically active nuclei, chromosomes undergo extensive despiralization and Hydration, forming a faintly visible network of euchromatin within the matrix. Heterochromatin granules are regions where chromosome coiling and packaging are preserved even in non-dividing nuclei. Chromatin is characteristic of cells during active metabolic phases, whereas during cell division, it organizes into densely packed chromosomes. Chromatin concentrates almost all the nuclear DNA and the enzymes responsible for its synthesis.

The functions of chromatin are as follows:

- the synthesis of organism-specific nucleic acids that direct the Synthesis of specific proteins;

- the transmission of hereditary traits from the mother cell to daughter cells, for which chromatin filaments are packaged into chromosomes during cell division.

The nucleolus is a spherical body, clearly visible under a microscope, with a diameter of 1-3 µm. It forms at a region of chromatin called the nucleolar organizer. The function of the nucleolus is to synthesize rRNA and form early ribosomal precursors by combining rRNA with proteins imported from the cytoplasm.

Reserve nutrients. Large amounts of these accumulate in seeds, fruits, and vegetative organs (roots, tubers, rhizomes, bulbs). The primary reserve nutrients in plants are carbohydrates, proteins, and fats (oils).

Starch is the main reserve carbohydrate in plants. Assimilation (primary) starch is formed in chloroplasts during photosynthesis. However, it is rapidly hydrolyzed into sugars, which flow out into storage organs. There, reserve (secondary) starch is deposited as grains within amyloplasts. Starch grains (amyloplasts) vary in shape and size: rounded, oval, polyhedral, ranging from 2 to 150 µm. The smallest starch grains are found in rice and buckwheat seeds, while the largest occur in potato tubers.

Starch constitutes a major portion of cereal seeds, legumes, and potato tubers (30-85%).

Reserve Proteins are most commonly deposited as aleurone (protein) grains, which are predominantly found in seeds.

Plant reserve proteins play a vital role in Human nutrition. Nowadays, 70–80% of human Dietary Protein Requirements are met by plant-based proteins. Cereal seeds contain up to 20% protein. Legume seeds are the richest in reserve protein, containing 30-35%, while soybeans contain up to 40%.

Plant fats are liquid at room temperature and are therefore commonly referred to as oils. Cells of seeds, fruits, stem pith and bark, and rhizomes are the richest in oils. The majority of plants (up to 90% of all families) accumulate oils as reserve nutrients. The seeds of many plants contain up to 50% or more oil (sunflower, mustard, soybean, safflower, camelina, peanut, etc.). Oils serve as an advantageous reserve nutrient for plants because they are richer in energy than starch and proteins.

Vacuoles and their functions. A vacuole is a region within the protoplast enclosed by a membrane and filled with cell sap. The vacuolar membrane, as previously noted, is called the tonoplast.

Meristematic cells typically lack vacuoles or contain very small ones. As the cell grows and develops, these small vacuoles gradually fuse into a single large central vacuole characteristic of mature plant cells. Consequently, the protoplast is displaced and pressed against the cell wall.

Plant cell vacuoles perform three main functions:

1. Storage. They store reserve nutrients (carbohydrates, proteins, organic acids, etc.) and Metabolic waste products whose functions are diverse and not yet fully understood.

2. Osmotic. Due to a relatively high concentration of solutes in the cell sap, osmotic pressure is generated: this creates a suction force that ensures water uptake into the cell and its movement from Cell to Cell; it also generates turgor, which provides structural support to cells and soft organs, giving them rigidity and spatial orientation.

3. Lysosomal. Hydrolytic enzymes present in the cell sap break down polymeric substances and cellular structures that have completed their functional lifespan and entered the vacuole via tonoplast invagination. Simple compounds (sugars, amino acids) resulting from this degradation diffuse back into the cytoplasm and are reused in metabolism.

Cell sap and its chemical composition. Cell sap is an aqueous solution of various organic and Mineral Substances. Water is its primary component (up to 98%) and is present in the vacuoles of all cells. Other substances in vacuoles may exist as true solutions, colloids, emulsions, Suspensions, or solid deposits.

The cell sap of various plants contains carbohydrates, proteins, amino acids, organic acids, Glycosides, Tannins, Alkaloids, Terpenes, pigments, mineral salts, and more.

Carbohydrates—specifically sugars (Monosaccharides and Disaccharides) and certain soluble polysaccharides—are present in the cell sap of almost all plants.

Reserve proteins are found in the cell sap of a number of plants, particularly in mature seeds (wheat, peas, castor bean, etc.). In addition to proteins, various Amino acids are present in vacuoles.

Organic acids are typically present in the cell sap in significant quantities, either in free form or as salts. Their presence dictates the acidic reaction of the cell sap. The most common are oxalic, acetic, malic, tartaric, and citric acids.

Glycosides often have a bitter taste and a specific odor. For example, mustard contains the glycoside sinigrin, which imparts its characteristic Smell and Taste.

Tannins accumulate in cell cavities following the death of their protoplasts. Significant amounts (up to 20–30% or more) are found in tree bark (oak, pine, willow, eucalyptus, etc.), leaves (sumac, tea), and fruits (persimmon, sloe, etc.). Tannins exhibit astringent and antiseptic properties.

Alkaloids are heterocyclic nitrogen-containing compounds. They exhibit alkaline properties and form salts with organic acids that dissolve in the cell sap. Alkaloids exert a powerful physiological effect on animal and human organisms. In small doses, they are used as medicinal drugs, whereas in large doses they act as poisons (e.g., morphine, codeine, and papaverine from the opium poppy, cocaine from coca leaves, caffeine from tea leaves, coffee beans, and cocoa beans, as well as atropine from belladonna and thorn apple, among others).

Terpenes are unsaturated Hydrocarbons. This group of compounds, found in the cell sap of certain plants, includes rubber and gutta-percha. On an industrial scale, rubber is obtained from the para rubber tree (Hevea) and used to manufacture rubber goods, as well as medical plasters and mustard plasters in medicine. Gutta-percha is extracted from guayule and used as an insulating material for coating submarine cables.

Cell sap is often colored due to pigments dissolved in it. Among the pigments of the cell sap, anthocyanins and anthoxanthins are the most common.

Anthocyanins are vital plant pigments that impart various shades of blue, pink, red, and purple to flowers, fruits, and leaves. Anthocyanins determine the coloration of the flowers of hyacinths, roses, forget-me-nots, and poppies, the fruits of black currants, cherries, and plums, and, less frequently, vegetative organs (such as the roots of garden beets and the leaves of red cabbage).

Anthoxanthins are yellow pigments that primarily color flower petals (such as toadflax and primrose) and fruits (lemons, oranges).

Solid deposits found in the cell sap are most commonly insoluble calcium salts, particularly calcium oxalate.

1.6. Osmotic Properties of Plant Cells

All plant cells are separated from their environment by a cell wall and a plasma membrane capable of regulating not only the quantity and type of substances passing through them, but often also the direction of their movement. To sustain all vital processes, water and nutrients must enter the cell from the external environment.

Diffusion is a spontaneous process that causes any substance to move from one region to another where the concentration of that substance is lower.

A plant cell represents an osmotic system in which one solution is the external environment (or the vacuole of an adjacent cell) and the other is the cell sap of its own vacuole. The membranes of living cells are capable of transporting only certain molecules or ions of dissolved substances, exhibiting selectivity that depends on The Nature of the membrane. Therefore, such membranes are called selectively permeable, or semipermeable, and the Diffusion of Water across these membranes is termed osmosis. Since water serves as the solvent in all biological systems, osmosis for them is essentially the diffusion of water through a selectively permeable membrane. The plasmalemma and tonoplast function as semipermeable membranes.

When discussing osmosis, primary attention is given to osmotic pressure—the force that must be applied to prevent the Movement of water through a semipermeable membrane toward the solution with the higher concentration. The higher the concentration of a solution, the higher its osmotic pressure. The greater the difference in concentration between solutions separated by a semipermeable barrier, the more intensively water will flow toward the more concentrated solution.

The cell wall protects the protoplast from rupture under the Influence of the vacuole's hydrostatic pressure.

The rate of water penetration into a cell depends on the Osmotic Pressure of the fluid within the vacuole and in the external environment, or more precisely, on the difference in osmotic pressure inside and outside the cell. The force with which water penetrates into the vacuole of a living cell is referred to as suction pressure.

Regarding the concentration of cell sap, external solutions can be divided into three types: isotonic, or isoosmotic, whose osmotic pressure equals that of the cell; hypotonic, whose osmotic pressure is lower than that of the cell; and hypertonic, whose osmotic pressure is higher than that of the cell.

If a plant cell is placed in an isotonic solution, no net movement of water will occur either into or out of the cell. However, if the cell is placed in a hypotonic solution, water will flow into its vacuole. As the cell absorbs water, the volume of its vacuole increases, exerting pressure on the protoplast and, through it, on the cell wall. The cell wall stretches and develops tension, in turn beginning to push back against the protoplast. The pressure exerted by the stretched cell wall on the protoplast is called turgor pressure, and the resulting tensed state of the cell is known as turgor.

The more concentrated the solution, the higher its osmotic pressure. Certain substances, particularly inorganic salts, undergo electrolytic dissociation in water, causing the concentration of particles to increase. Consequently, the osmotic potential of such a solution increases accordingly.

Knowledge of osmotic potential is essential when conducting various ecological studies, which help determine a plant's ability to absorb water from the soil and retain it regardless of environmental conditions.

Osmotic potential typically ranges between 0.1 and 20 MPa. It is lowest in aquatic plants, reaching 0.1 MPa, whereas in many halophytes it can be as high as 20 MPa. In mesophytes, osmotic potential generally ranges from 0.5 to 3 MPa.

Under The Influence of various factors, the value of osmotic potential can fluctuate even between adjacent cells.

When a plant loses water due to soil moisture deficiency and high evaporation, cell turgor decreases, cell walls shrivel, and wilting occurs: leaves droop and herbaceous stems sag.

Experiments have proven that the concentration of cell sap, and consequently the osmotic pressure, is higher in the cells of upper leaves than in lower ones, and greater in root cells than in the surrounding soil.

Osmotic properties play a crucial role in the structure and vital activity of plants:

Osmosis governs the uptake of Water and Its transport throughout the plant.

Osmotic properties provide turgor to the plant, particularly its softer organs (leaves, herbaceous stems), giving them shape and rigidity while allowing them to maintain their spatial orientation.

Plasmolysis. When a cell comes into contact with a hypertonic solution, water begins to leave the cell via osmosis across the plasma membrane. Initially, water is lost by the cytoplasm, and subsequently through

tonoplast water also exits the vacuole. The volume of the intracellular contents decreases, the protoplast surrounded by the plasma membrane shrinks, and eventually the plasmalemma detaches from the cell wall, which is known as plasmolysis.

Plasmolysis is the detachment of the peripheral layer of cytoplasm from the rigid cell wall of a plant cell.

Plasmolysis occurs exclusively in living cells as a result of protoplast compression under the influence of a plasmolytic that is hypertonic relative to the cell sap. With slow plasmolysis, cells remain alive for a long time. In the Presence of water available to the cell, they easily restore their turgor state. Prolonged plasmolysis leads to cell death. The phenomenon of plasmolysis is used to determine osmotic potential, cytoplasmic viscosity, cell membrane permeability, and so on.

This process is reversible. If a plasmolyzed cell is transferred into pure water or a low-concentration solution, water will enter the cell again, resulting in deplasmolysis.

Imbibition. Alongside osmotic forces, imbibition forces exist within cells. In certain plant parts, water is absorbed exclusively through imbibition, such as in seeds. Water penetrates the cell during imbibition via diffusion. The imbibed state of the protoplast is crucial for the intensity of overall metabolism, as the hydration of protoplasmic proteins is necessary to maintain the ultrastructure and functional activity of organelles.

Pinocytosis is the invagination of the surface membrane, resulting in the uptake of liquid droplets.

1.7. Cell Wall

All plant cells possess a rigid, elastic cell wall, which is a metabolic product of the protoplast. The presence of a cell wall is a key feature distinguishing plant cells from animal cells.

The cell wall determines the cell's shape.

Cell walls, especially rigid and thickened ones, provide mechanical support to PLANT CELLS AND organs.

The cell wall performs various protective functions: it shields the protoplast from various forms of damage—mechanical injury, pests, and animals—as well as from the invasion of pathogenic microorganisms and from

water loss and desiccation.

The wall contributes to the cell's osmotic properties by limiting protoplast expansion and preventing it from rupturing under the hydrostatic pressure of the vacuole.

The cell wall participates in the transport of water and dissolved substances that must cross the cell wall before entering or upon exiting the cytoplasm.

The Composition and Structure of cell walls vary depending on the cell type and plant species. They also change during the individual Development of the cell.

COMPOSITION OF THE cell wall. The primary substances of the cell wall are carbohydrates (cellulose, hemicellulose, pectins), proteins, and water. Cellulose is an essential component of all Plant Cell Walls. It forms the framework, or Skeletal structure, of the cell wall. Hemicellulose, pectins, and proteins make up the matrix of the wall, which surrounds the cellulosic framework.

Cellulose, or fiber, is a carbohydrate polymer. In the cell wall, cellulose molecules are assembled into bundles called microfibrils, in which they lie parallel to one another.

Cellulose has very wide Practical Applications in the pulp and paper, chemical, and other industries.

Hemicellulose is a less stable, amorphous component of the cell wall. It is part of the matrix, filling the spaces between microfibrils. Hemicellulose is abundant in the walls of young cells.

Pectins, or pectic substances, also have a carbohydrate nature. Existing in a colloidal state, pectins impart elasticity to the cell wall.

Pectic substances possess gelling properties, which form the basis for preparing jams, preserves, fruit jellies, marmalade, caramel fillings, and similar products.

Proteins account for 5–10% of the cell wall's mass. Their composition primarily includes the structural protein extensin. Additionally, certain enzymatic proteins have been detected in the wall.

Water in the cell wall acts as a solvent and facilitates the transport of substances across the cell wall: the higher the water content in the wall, the greater its permeability. It forms a colloidal solution of pectic substances.

STRUCTURE OF THE cell wall. During cytokinesis, between two

daughter cells, a pectic middle lamella is first formed, after which a primary wall is deposited on it from the side of each cell. In some tissues, this wall persists throughout the entire life of the cells (meristems, root hairs, leaf mesophyll), while in others—after cell growth ceases—a secondary wall is deposited over the primary wall (mechanical tissues, xylem).

The Primary and secondary cell walls differ in their Chemical Composition and ultrastructure.

The primary cell wall is very thin, measuring about 0.1–0.5 µm. It is composed of cellulose (5–15%), hemicellulose (up to 30%), pectins (up to 5%), proteins (5–10%), and a high proportion of water (60–90%). Due to the relatively low cellulose content, the microfibrils in the primary wall are loosely arranged and interconnected by matrix molecules. Modern researchers have proposed several molecular models for the primary cell wall architecture. It is generally accepted that the cross-linked network of cellulose microfibrils and hemicellulose molecules provides structural tensile strength, while pectic substances contribute to its elasticity.

The primary wall varies in thickness across its surface. It features thinner regions where the microfibrils are particularly sparse, allowing plasmodesmata to pass through and connect the cytoplasms of adjacent cells. These specialized regions are known as primary pit fields, which later overlie the pits formed in the secondary wall.

Meristematic cells undergoing continuous division, as well as many specialized mature cells (such as leaf mesophyll cells), possess only primary cell walls. Such cells retain the capacity to alter their shape, resume division, and differentiate into new cell types, playing a crucial role in plant wound healing and tissue regeneration. Although primary cell walls vary in thickness, they invariably contain thin areas—primary pit fields—traversed by plasmodesmata.

In most cells, however, additional layers of cellulose are deposited onto the inner surface (facing away from the plasma membrane) to form a secondary cell wall. The cellulose content in the secondary wall can reach up to 60%; its fibrils within each layer lie parallel to one another, while those in adjacent layers are oriented at an angle. The matrix also becomes impregnated with various secondary compounds, such as Lignin, suberin, cutin, and Waxes. This grants the secondary wall remarkable rigidity and strength while causing it to lose its elasticity and halting further cell expansion. Lignin is the primary component of this rigid framework, typical of secondary walls in woody tissues; it binds cellulose microfibrils together and locks them in place. Xylem elements (vessels, tracheids) and mechanical sclerenchyma cells undergo intense lignification. Suberin, combined with waxes, causes the suberization of the cell wall, significantly reducing its permeability. Cutin forms an external cuticular layer that renders the wall nearly impermeable.

Functions of the cell wall. The cell wall is a secretory product of the protoplast and is generally considered non-living in mature cells. Nevertheless, it contains various proteins, some of which exhibit enzymatic activity.

The cell wall provides mechanical strength and structural support to individual cells and the plant as a whole. It determines the size, shape, and stability of plant cells, protecting the plasma membrane from rupture under the high hydrostatic (turgor) pressure generated within the cell. Furthermore, the cell wall acts as a barrier against infections and is involved in the uptake, transport, and secretion of substances. Cytoplasmic strands known as plasmodesmata traverse the Pores in the cell walls, interconnecting the contents of adjacent cells and integrating all protoplasts into a single functional continuum called the symplast.

Evidence suggests that carbohydrate components of the cell wall can interact with phytohormones to trigger physiological changes within the cell.

Pits. The secondary cell wall is not deposited over primary pit fields, leaving them recessed instead. These depressions in the secondary wall are called pits. Pits typically occur in pairs, as those in the walls of adjacent cells are positioned directly opposite one another. Based on their channel morphology, pits are classified as either simple or bordered.

Simple pits feature a straight cylindrical channel with a relatively uniform diameter throughout its length, and are characteristic of parenchyma and mechanical tissues.

In bordered pits, the channel is funnel-shaped, tapering toward the cell interior due to the progressive thickening of the secondary wall. Bordered pits are typical of water-conducting elements (xylem). Xylem elements are dead cells that have lost their living protoplasts; concurrently, the plasmodesmata within the pit membranes degenerate, and hemicelluloses and pectins disappear, leaving behind a porous mesh of cellulose microfibrils that facilitates water flow. In many conifers, a central thickening known as a torus develops within the closing membrane of bordered pits.

Occasionally, the closing membrane of a pit ruptures, forming a continuous opening known as a perforation. Multiple smaller perforations may also develop across a single pit membrane.

Physicochemical modifications of the cell wall. In some cells, the walls remain unchanged and purely cellulosic throughout their lifespan. However, during the cellular differentiation of many tissues, the cell walls undergo various physicochemical modifications: lignification, suberization, cutinization, mucilaginous transformation, and mineralization.

Lignification is the most widespread cell wall modification in mature tissues of higher plants. It occurs through the impregnation of the wall matrix with lignin, a complex phenolic polymer that is highly stable and sparingly soluble. Lignification imparts hardness and rigidity to the cell wall, albeit significantly reducing its elasticity. Cells with lignified walls lose their living contents and eventually die. Lignification primarily affects mechanical tissues and the vascular xylem.

Suberization involves the accumulation of suberin—a resistant, lipid-like polymer—within the cell walls. Suberized walls become impermeable to water and gases, leading to the death of the cell protoplast. Suberization typically occurs in the cell walls of protective (dermal) tissues and in wound-healing tissues that seal off damaged organ surfaces.

Cutinization is the deposition of the lipid-like substance cutin within the cell wall. Cutin is generally secreted onto the outer walls of epidermal cells to form a continuous film known as the cuticle. The cuticle is impermeable to water and gases, thereby protecting leaves and young stems from excessive Transpiration. Additionally, it acts as a physical barrier preventing The entry of pathogenic fungi and other microorganisms. The thickness of the cuticle varies, reaching its maximum development in the leaves of xerophytic plants adapted to arid environments.

In some plants, waxes are incorporated into the cuticle and cuticular layers. This epicuticular wax layer further enhances the protective properties of the cuticle.

Mucilaginous transformation of seed cell walls facilitates their anchorage in the soil, enhances water absorption, and assists the embryo in breaking out of the seed coat during germination. Mucilaginous polysaccharides are found in the cell walls of seeds, root caps, root hairs, and the leaves of certain xerophytes. The mucilage secreted on the surface

of the root cap and root hairs helps the root navigate through the soil, ensures intimate contact with soil particles, and AIDS in the uptake of water and mineral nutrients.

1.8. Nuclear and Cellular Division

Mitosis is the most common and universal mode of nuclear division in both plant and animal cells. As a result of mitosis, daughter cells receive the exact same complement of chromosomes as the parent cell.

The Cell Cycle—defined as the lifespan of a mitotically competent cell from the division that produced it until its own completion of division into two daughter cells—consists of two main phases: a prolonged interphase (between divisions) and a short phase of nuclear and cellular division (mitosis).

The duration of the cell cycle in plants depends on the cell and tissue type, as well as environmental factors (such as temperature and mineral nutrition). However, interphase is invariably longer than mitosis, lasting anywhere from 8–10 hours to several days, whereas mitosis typically takes from a few minutes to 2–3 hours.

Interphase encompasses the preparatory processes for nuclear and cell division and is conventionally divided into three distinct periods designated as G1, S, and G2.

G1 represents the post-mitotic period, characterized by an increase in cytoplasm volume and organelle proliferation, along with preparation for DNA Replication. S is the synthesis phase, during which DNA is replicated. G2 is the post-synthetic phase, involving the synthesis of RNA, cellular proteins, and the structural components required for mitosis.

Mitosis is a continuous process of nuclear division in which the genetic material is distributed equally between the nuclei of the daughter cells. Mitosis is conventionally divided into four phases: prophase, metaphase, anaphase, and telophase.

Prophase is The first phase of mitosis. The nucleus increases in size, and chromosomes form from chromatin threads. During this stage, the threads coil, shorten, and condense, becoming visible under a Light Microscope. Chromosomes are filamentous nucleoprotein structures. Each chromosome in prophase consists of two closely positioned chromatids running parallel to each other. The chromatids within a chromosome are joined at a single point by a narrow region called the centromere, which divides the chromosome into two arms of unequal length.

Microtubules assemble around the nucleus. Initially arranged in a random fashion, they soon organize into bundles near the poles of the nucleus. This marks the early stage of Formation of the achromatic spindle

(spindle apparatus). The nucleolus gradually loses its distinct outline and disappears entirely. By the end of prophase, the nuclear envelope also breaks down.

Metaphase is characterized by the full development of the achromatic spindle. The chromosomes, each consisting of two chromatids, begin to move. They migrate to the equatorial region of the cell and attach via their centromeres to the half-spindle fibers extending from opposite poles. By the end of metaphase, the chromosomes align at the equator of the achromatic spindle, ready for division.

Anaphase begins with the division of the centromere. The chromatids separate completely from one another and are now referred to as daughter chromosomes. The daughter chromosomes begin to be pulled toward opposite poles of the cell. By the end of anaphase, two identical groups of daughter chromosomes gather at the cell poles.

Telophase is the final phase of mitosis, during which daughter nuclei form at the poles of the parent cell. The chromosomes uncoil, hydrate, swell, and elongate. Gradually, they lose their distinct contours, merge into a mass, and form an inconspicuous, fine chromatin network. Nucleoli reappear within the nuclei. Simultaneously, a nuclear envelope reforms around each group of daughter chromosomes from elements of the endoplasmic reticulum.

Cytokinesis is the division of the cytoplasm and the formation of two daughter cells following nuclear division.

Meiosis is a specialized type of indirect nuclear division that occurs in all sexually reproducing organisms. During meiosis, a reduction occurs—meaning the chromosome number is halved. Therefore, meiosis is also known as reductional division.

Meiosis consists of two successive nuclear divisions occurring in rapid succession. In the First Division, termed heterotypic, the chromosome number is reduced as homologous chromosomes segregate to opposite poles of the cell, resulting in distinct haploid daughter nuclei. In the Second Division, termed homeotypic—which closely resembles standard mitosis—identical haploid nuclei are formed. Each of the two meiotic divisions is conventionally divided into the same four phases characteristic of mitosis: prophase, metaphase, anaphase, and telophase.

In plants, meiosis occurs during the formation of pollen grains and the embryo sac. It marks the beginning of the gametophytic phase of the plant life cycle.

Selection/41.html">Review Questions and Exercises

1. Define a cell. How do plant cells differ from animal cells?

2. What determines the shape and size of plant cells? Which cells are referred to as parenchymatous and which as prosenchymatous?

3. Describe the composition of a plant cell. Which organelles are corpuscular and which are membranous?

4. What are the STRUCTURE AND FUNCTIONS of the plasmalemma and tonoplast?

5. Characterize the endoplasmic reticulum. What functions does it perform?

6. Describe the structure and significance of plasmodesmata.

7. Discuss the structure, functions, and origin of chloroplasts.

8. Compare the Three types of plastids.

9. What do you know about the structure and functions of mitochondria?

10. What is The Role of lysosomes and local autolysis in The plant cell?

11. What is your knowledge of the Golgi apparatus and its role in forming the endomembrane system?

12. Describe the microbodies of the plant cell.

13. What substances are stored as reserves in plant cells? What are their storage forms?

14. Describe the formation and functions of vacuoles.

15. Describe the composition of cell sap.

16. Describe the composition of the cell wall and its functions.

17. Compare the composition and structure of primary and secondary cell walls.

18. What are pits? What types of pits do you know?

19. What physicochemical modifications of the cell wall do you know? What is their significance for plants?

20. What types of nuclear division exist? What is their significance?

21. What are the Specific features of plant cell division?