PLANT ANATOMY - Yu.I. Korniyevsky - 2017

Lecture Notes

Lecture No. 1. THE PLANT CELL

Lecture Outline

1. Structure OF THE plant Cell.

2. Products of protoplast METABOLISM.

2.1. Cell wall.

2.2. Cell inclusions.

2.3. Vacuoles and cell sap.

Botany (from Greek botane – herb, green plant) is a comprehensive biological science dedicated to plants, occupying a prominent place in The history of our understanding of nature and encompassing a wide range of problems. Botany studies the external and Internal Structure of plants, their life processes and physiological Functions, interactions with the environment, geographical distribution, potential pathways for the rational economic use of the phytosphere, as well as its conservation and protection.

Plant development, Nutrition, Respiration, and metabolism are investigated by physiology, while chemical processes occurring within the plant Organism are studied by plant biochemistry. Plant diseases caused by Viruses, Bacteria, and Fungi fall under the domain of phytopathology. The purview of plant ecology is to establish the relationships and mutual influences between plant organisms and their environment. Plant distribution across the Earth's surface is elucidated by phytogeography, whereas phytocenology explores plant communities. Paleobotany analyzes fossil plants, which is of great significance for historical geology, Taxonomy, Morphology, and plant anatomy. Applied botany, crop science, and botanical resource science accumulate knowledge regarding the useful properties of PLANTS AND THEIR domestication. Relatively young disciplines include genetics—the science of plant heredity and Variability—and space botany, which investigates plant behavior under conditions of weightlessness. Bioethics emerged as a response to the rapid advancement of leading technologies involving manipulations of living organisms, addressing the ethical dimension of balancing human needs with the capacities of the living world.

1. Structure of The plant cell

The cell is the fundamental Structural and functional unit of All living organisms. As defined by American scientists A. Levy and F. Siekevitz:

"The Cell is a unit of biological activity, bounded by a semipermeable membrane and capable of self-reproduction in a medium free of other living systems."

During the first half of the 19th century, a substantial body of reliable information concerning The structure of various cell types was accumulated. Building upon the works of the English botanist R. Brown (1773–1858), the Czech histologist J. Purkinje (1787–1869), and the German botanist M. Schleiden (1804–1881), the German zoologist T. Schwann (1810–1882) synthesized these findings and, in 1838, formulated the foundational tenets of the Cell Theory. According to this theory, all Living organisms are composed of Cells; animal and plant cells are similar in chemical Composition and Structure, and they drive the GROWTH AND DEVELOPMENT of organisms. Alongside these undisputed postulates, it contained a flawed assertion that new cells arise from a cell-free, unstructured substance (cytoblastema). The discoveries of Academician K.M. Baer regarding the origin and reproduction of cells, together with crucial additions made by the German physician R. Virchow (1858–1859), enabled the final formulation of the core principles of the cell theory: cells of living organisms are similar in origin, structure, chemical composition, and basic manifestations of life; every new cell is formed by the Reproduction of a mother cell through division; in Multicellular Organisms, different cell types are formed via specialization during the individual development of an organism and constitute Tissues; tissues comprise Organs that are closely interrelated.

Depending on their age and function, cells may be living or dead, and their sizes range from microscopically small (10–60 µm) to visible to the naked eye. Cell shapes vary greatly, yet There are two primary morphological types: parenchymatous cells, which have approximately equal dimensions in all spatial directions, and prosenchymatous cells, in which the length exceeds the width by more than five times, and frequently by tens, hundreds, or sometimes thousands of times.

The plant cell consists of the living content, the protoplast, and the products of protoplast metabolism—The cell wall, the vacuole containing cell sap, and inclusions.

A plant cell differs from an animal cell by the presence of Plastids, a carbohydrate cell wall, plasmodesmata, a central vacuole with cell sap, and crystalline inclusions.

The protoplast is composed of The Nucleus and the Cytoplasm with membrane structures and Organelles.

✵ Cytoplasmic organelles include: the smooth and rough Endoplasmic reticulum (ER), which ensures the synthesis of Proteins, CARBOHYDRATES, and Lipids, as well as their Intracellular Transport; Ribosomes, which synthesize proteins; the Golgi apparatus, which participates in the synthesis, accumulation, and export of various substances from cells, and The formation of cell wall components; Lysosomes, which hydrolyze proteins, Nucleic Acids, and Other Compounds; Mitochondria, which facilitate energy release and ATP formation; and plastids.

✵ The nucleus is not classified among the cytoplasmic organelles. Examination of the plant cell using an Electron microscope reveals that the cytoplasm is separated from the cell wall by the Plasmalemma.

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✵ The connection between plant protoplasts and Metabolic exchange between them are ensured by fine cytoplasmic threads passing through Pores in the cell wall, known as plasmodesmata.

✵ When a hypertonic salt solution is added to plant cells, the withdrawal of the cytoplasm from the cell wall is observed—a phenomenon known as plasmolysis.

✵ Plant cell organelles responsible for the concentration, dehydration, and compaction of substances of endogenous and exogenous origin include the Golgi apparatus, which is a system of dictyosomes and vesicles.

✵ In plant cells, lysosomes—cytoplasmic organelles—perform a protective function.

✵ In plant cells, cell sap compartments separated from the cytoplasm by the tonoplast accumulate Water, reserve and ergastic substances, and maintain osmotic pressure and cell turgor; these are vacuoles.

✵ The Endoplasmic reticulum and the Golgi apparatus participate in the formation of vacuoles.

✵ The membrane adjacent to the vacuole is called the tonoplast. Vacuoles are separated from the cytoplasm by a protein-lipoid vacuolar membrane.

✵ The cell sap of citrus pericarp contains yellow pigments that color the fruit and participate in oxidation-reduction reactions—anthochlors.

✵ Microscopic and histochemical analysis of petal epidermis has established that the cell sap contains a purple pigment—anthocyanin.

✵ The cell sap of garlic and onion bulbs contains biologically active secondary metabolites that protect plants from pests and diseases and serve as natural Antibiotics—phytoncides.

Plastids are relatively large double-membrane organelles found exclusively in plant cells.

Depending on their structure, coloration, and functions, higher plant plastids are subdivided into Chloroplasts, chromoplasts, and leucoplasts; algal plastids are known as chromatophores.

Chloroplasts are green plastids that carry out Photosynthesis. They are disc-shaped, enclosed by a double protein-lipoid membrane with internal outgrowths called lamellae or thylakoids, and contain photosynthetic chlorophylls a and b along with accessory carotenoid pigments. The main substance, the stroma or matrix of the chloroplast, contains Enzymes, lipids, and sugars, and drives the dark reactions of photosynthesis.

The complex-forming ion of chlorophyll is Mg²⁺; primary assimilation starch is synthesized in chloroplasts.

Chromoplasts are plastids colored yellow, orange, or red due to the presence of carotenoids—α-, β-, and γ-carotenes, as well as xanthophylls—and are found in the tissues of petals, fruits, seeds, and ROOT crops. They serve as provitamin A precursors and antioxidants.

Leucoplasts are colorless plastids lacking pigments within a protein-lipoid stroma. They act as light filters and synthesize and store reserve substances:

amyloplasts synthesize secondary starch in the form of starch grains;

proteinoplasts produce Reserve Proteins;

oleoplasts accumulate fatty oils.

Algal chromatophores have diverse yet species-specific shapes, containing chlorophylls a, b, c, and d, carotenes, and specific pigments (phycocyanins, phycoerythrins), along with a protein complex known as the pyrenoid. Spirogyra is characterized by a ribbon-like chromatophore.

2. Protoplast metabolites

The cell wall, or cell membrane of a plant cell, provides a definite shape to the cell, limits and protects the protoplast, and participates in the absorption, conduction, and excretion of substances. During Cell Division, a cell plate is formed first, followed by the middle lamella (or intercellular substance), and then the primary wall, which features the thinnest primary pit fields containing plasmodesmata—thread-like cytoplasmic strands.

As cells grow and specialize, the cell wall undergoes chemical changes, becoming less elastic. It thickens through the deposition of secondary wall Cellulose fibrils on the inside of the cell, leaving unthickened areas known as pits.

The structural and supporting units of the secondary cell wall are cellulose molecules combined into chains called micelles. Bundles of micelles form microfibrils, which in turn assemble into fibrous macrofibrils. The spaces between micelles are filled with a plastic matrix composed of hemicellulose.

Pits

Simple pits, consisting of a pit aperture and a pit canal, include straight, oblique, slit-like, and branched pits. Complex pits include bordered and half-bordered pits.

3. Cell wall

Detailed structure of the cell wall: A — cell region; B — microfibril; C — macrofibril; D — region of the macrofibril containing ordered micelle structures; E — micelle fragment.

Secondary modifications of the cell wall

Lignification is the impregnation of the cell wall with Lignin, a resistant yellow phenolic substance that is insoluble in water and common Solvents. Lignin exerts an antiseptic and preservative effect. Lignification fixes the shape of the cell, leads to protoplast death, reduces the elasticity of cell walls, and increases their hardness, strength, and durability. Lignin is detected using qualitative microreactions.

Suberization is the impregnation of the cell walls of protective tissue (cork) with suberin, a high-molecular-weight lipid-like substance. In the process, cells die, lose their elasticity, become impermeable to water and gases, resistant to decay, and insoluble even in sulfuric acid.

Cutinization is the deposition of a lipid-like substance, cutin, into the outer wall of epidermal base cells, along with the formation of an outer wax-like layer known as the cuticle. Cutinized cells remain living; their walls have low permeability to water and gases, providing reliable protection against overheating, hypothermia, microbial penetration, and other environmental factors.

Mineralization of the cell wall is caused by amorphous or crystalline mineral deposits, most commonly silica (found in the stems and leaves of grasses, sedges, and horsetails), and occasionally carbonates. Mineralized walls become hard, yet brittle and fragile. Silica in the cell wall can be identified using specific Reagents.

Mucilaginous degeneration (mucilage formation) involves metabolic processes of isomeric transformation of wall or cytoplasmic Polysaccharides, resulting in The production of mucilage. The mucilaginous transformation of root Hair surfaces and the root cap AIDS in water and nutrient absorption, thermal protection, and anchorage in the substrate. Qualitative detection of mucilage is performed using reagents.

Gummosis is a pathological, post-traumatic mucilaginous degeneration of wood or pith cells, in which the cell walls and contents transform into Gums. These are complex polysaccharides containing calcium and magnesium salts of uronic acids esterified with neutral Monosaccharides.

Table 1. Histochemical reactions for certain Components of the secondary cell wall

Maceration is The breakdown of the intercellular substance, leading to cell dissociation. Naturally, maceration occurs when protopectin converts into pectin during the ripening of succulent fruits.

4. Cell inclusions

Non-permanent components of plant cells. They are temporarily removed from metabolic processes, accumulated, and utilized during life activity. They are located in vacuoles, hyaloplasm, or organelles. They can be solid (granular, crystalline) or liquid; nutritive (starch and aleurone grains) or excretory—End products of Secondary Metabolism (crystals of oxalates, carbonates, silicates, gypsum, etc.). Soluble inclusions include mono- and Disaccharides, polysaccharides (inulin, Glycogen), fats, Essential Oils, resins, Alkaloids, Glycosides, polyphenols, and other BIOLOGICALLY ACTIVE SUBSTANCES.

Reserve nutrients

Carbohydrades: starch (Amylum). Starch grains consist of water-soluble amylase (15–25%) and water-insoluble amylopectin (75–85%).

Depending on the Location and mode of formation, starches are classified as follows

Starch grains of certain plants

A – simple eccentric, compound, and semi-compound starch grains of potato; B – simple concentric grains of wheat; C – compound grains of oats; D – rice; E – simple concentric grains of corn; F – compound grains of buckwheat; 1 – simple grain; 2 – compound grain; semi-compound.

Starch grains have The ability to swell in cold water and form a paste when treated with hot water. Upon Treatment with iodine-containing reagents, starch grains turn dark purple, while rice grains turn blue.

Inulin is a soluble polysaccharide found in vacuolar cell sap, which can be detected using specific reagents. It is characteristic of members of the Asteraceae family.

Glycogen is a soluble polysaccharide found in the cells of cyanobacteria and fungi.

Aleurone or protein grains are classified into simple and compound.

Simple aleurone grains consist of a Protein Membrane and amorphous protein, occasionally containing a rounded or botryoidal globoid (a calcium and magnesium salt of phosphoric acid).

In compound aleurone grains, In addition to the components mentioned above, protein crystal hydrates—crystalloids—are formed.

Fats (Olea pinguia). Chemically, plant oils are triglycerides of high-molecular-weight acids. They accumulate in the hyaloplasm as lipid droplets or are synthesized in oleoplasts. They are non-volatile and leave grease spots on paper that expand upon heating. They undergo saponification with alkalis and stain orange with Sudan III.

Essential oils are a mixture of volatile, liquid, Aromatic Compounds of plant cells. They accumulate in secretory structures, glands, ducts, and receptacles (found in families such as Lamiaceae, Asteraceae, Rutaceae, and Pinaceae).

Table 2. Histochemical reactions for reserve inclusions in plant cells

Table 3. Histochemical reactions for reserve inclusions in fungal and cyanobacterial cells

Glycogen, or animal starch, serves as reserve inclusions in the cells of fungi and cyanobacteria. The cell wall of fungi contains Chitin. In fungal cells, pigments are localized in the cytoplasm and cell wall. The vegetative body of a fungus consists of individual filaments called hyphae, the aggregation of which forms a pseudotissue known as plectenchyma.

5. Crystalline inclusions – End Products of Metabolism

The end products of metabolism include calcium oxalate. It accumulates in vacuoles as crystal hydrates of specific shapes, which depend on the number of molecules of crystallization water.

Metabolic end products accumulate in plant organs and parts that are periodically shed or discarded by the plant, such as leaves, bark, pericarp, and seed coats. In Aging cells, crystals are more numerous and larger.

Calcium carbonate and silica crystals.

They are deposited in cell walls, vacuoles, or form the body of a cystolith.

A cystolith is an internal outgrowth of the cell wall in specialized cells called lithocysts. It may be differentiated into a cellulosic stalk and a body consisting of aggregated crystals of calcium carbonate or silica.

Leaf cystoliths: A — Ficus; B — stinging nettle; C — mulberry; D — fig; E — hop; F — hemp: 1 — cystolith stalk (cell wall outgrowth); 2 — cystolith body (CaCO3); 3 — lithocyst cell; 4 — epidermal trichome with a cystolith.

6. Vacuoles and Cell Sap

A vacuole is a cell sap compartment bounded by a vacuolar membrane, or tonoplast. Young cells feature dense cytoplasm devoid of vacuoles, but as they grow, numerous small vacuoles appear. Their formation involves dictyosomes, Golgi vesicles, cisternae, and agranular vesicles of The endoplasmic reticulum. In a mature cell, these vacuoles merge into a single central vacuole that presses the cytoplasm against the cell wall. Substances synthesized in the cytoplasm selectively enter the vacuoles to form a complex mixture known as cell sap. It is more viscous than water, optically inactive, and exhibits a weakly acidic, neutral, or less frequently alkaline reaction. The Chemical Composition and concentration of cell sap vary depending on the age, type, function, and physiological state of Cells and Tissues, as well as the plant species and its growing conditions. Cell sap consists of water (90%), true and colloidal solutions of various mineral and Organic compounds, and mineral acid salts dissociated into ions. In certain tissues, specific vacuolar inclusions also form, such as calcium oxalate crystal hydrates and aleurone grains. Salts of organic acids, along with mineral ions, play a vital role in cellular osmotic processes. During histochemical analysis of micropreparations, the detection of cell sap compounds is facilitated by the ability of some of them to bind specific stains.

The functions of vacuoles include the storage of water, reserve nutrients, and ergotic substances. They maintain osmotic pressure and cellular turgor, allowing succulent plant parts to retain their shape and spatial orientation, resist mechanical stress, and exhibit frost and heat tolerance.

The concentration of ions and sugars in the cell sap is higher than that in the cell wall. The tonoplast slows down the diffusion of these substances out of the vacuole while remaining permeable to water. Consequently, water saturates the cell wall and enters the vacuole via diffusion. This unidirectional Diffusion of Water across a membrane semipermeable to solutes is called osmosis. The water entering the vacuole exerts pressure on the protoplast and, in turn, on the cell wall, generating a state of tension known as cell turgor. Placing a cell in a hypertonic salt or sugar solution induces plasmolysis—the osmotic efflux of water from the vacuole, a decrease in its volume, the retraction of the cytoplasm from the cell wall, and the loss of cell turgor.

The reverse process, deplasmolysis, occurs when the cytoplasm returns to its original position as the plasmolysed cell absorbs water.