MEDICAL BOTANY - A.G. Serbin - 2003

SECTION 1. ANATOMY

THE PLANT CELL

The Cell is the basic Structural and functional unit of PLANT AND ANIMAL organisms. As defined by American scientists Loewy and Siekevitz, "the cell is a unit of biological activity, bounded by a semipermeable membrane and capable of self-reproduction in a medium lacking living systems."

Depending on age and function, Cells can be either alive or devoid of living contents—dead. Their sizes range from microscopically small to visible to the naked eye. Cell shapes are quite diverse, driven by their specific Functions. However, two morphological types of cells are distinguished: 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, often tens, hundreds, and sometimes thousands of times. A plant cell consists of a living part—the protoplast, and the products of protoplast activity—ergastic substances, Cell wall, inclusions, and vacuolar cell sap (Fig. 1.1, Table 1.1).

Class="center">Fig. 1.1. Water/32.html">Plant Cell Structure under an Electron microscope (A) and diagram of Membrane Structure ontogeny (B): 1 — Cytoplasm; 2 — Plasmalemma; 3 — smooth Endoplasmic reticulum; 4 — nucleus with nucleolus and Chromatin; 5 — cell wall; 6 — plasmodesmata; 7 — middle lamella; 8 — free Ribosomes scattered in the cytoplasm; 9 — rough endoplasmic reticulum with ribosomes; 10 — Plastids; 11 — Mitochondria; 12 — vacuole with cell sap; 13 — tonoplast; 14 — Golgi vesicles; 15 — Golgi apparatus; 16 — Lysosomes; 17 — microfilaments; 18 — Microbodies and microtubules

Table 1.1. BRIEF CHARACTERISTICS OF cellular structures

Protoplast

The Components of the protoplast include The Nucleus, cytoplasm with membrane structures and Organelles, which encompass: smooth and rough endoplasmic reticulum (ER), facilitating various Chemical Reactions; ribosomes, which synthesize Proteins; the Golgi apparatus, or dictyosomes, involved in the synthesis, accumulation, and export of various substances from cells, as well as The formation of the ER and cell wall; lysosomes, which hydrolyze proteins, Nucleic Acids, and Other Compounds; spherosomes, which synthesize fatty oils; mitochondria, which drive energy release and ATP formation; and plastids, the functions of which will be discussed further.

The plant cell differs from the animal cell by the presence of plastids, a carbohydrate cell wall, plasmodesmata, vacuoles, and crystalline inclusions.

The cytoplasm is a semi-liquid, optically homogeneous, colorless biological colloid with a complex physicochemical structure. In the aqueous dispersion medium of the cytoplasm, dissolved substances do not dissociate into molecules or ions, as occurs in true solutions, but remain as relatively large suspended particles (hundredths of a micron in size) or giant macromolecules. The chemical composition of the cytoplasm is diverse, complex, and variable. Water accounts for 75 — 90 %, with complex proteins (15 — 20 %) and their compounds with other substances (Lipoproteins, Nucleoproteins, Phosphoproteins, Chromoproteins, etc.), phytohormones, and proteinaceous Enzymes predominating. It also contains CARBOHYDRATES (4 — 6 %), fats and fat-like substances (2 — 3 %), Amino Acids (4 — 6 %), nucleic acids (1 — 2 %), Vitamins, Inorganic Compounds, and other substances (2 — 6 %). The reaction of the cytoplasm is close to neutral. It does not mix with the vacuolar contents and exhibits higher viscosity, surface tension, and optical density than the cell sap.

The structure of the cytoplasm is heterogeneous: adjacent to The cell wall is The Plasma Membrane—the plasmalemma, while the vacuoles are separated from the cytoplasm by the vacuolar membrane—the tonoplast. These are three-layered protein-lipid membranes that regulate METABOLISM, selective permeability, cellular interaction with the external environment, and wall formation. Located between the tonoplast and the plasmalemma is the hyaloplasm containing organelles (their characteristics are given in Table 1.1).

The cytoplasm possesses biological properties essential for maintaining life: movement and metabolism, selective permeability regulating the inward and outward Movement of water and solute solutions (which underlies phenomena such as plasmolysis, deplasmolysis, and turgor); irritability—the ability of the cytoplasm to respond to light, Temperature, chemical, mechanical, and other stimuli; and reproduction, growth, and development, ensuring individual life, preservation, and numerical increase of the Organism.

Metabolism and Energy Exchange between the organism and the environment (metabolism) represent the sum of chemical processes that sustain life, self-renewal of cells, and the entire organism. Metabolism consists of two opposing processes—assimilation and dissimilation. Assimilation (anabolism, or plastic metabolism) is The conversion of external substances into the cell's own compounds with the consumption of energy. In green plants, assimilation is based on Photosynthesis. Dissimilation (Catabolism, or Energy Metabolism) is The process of Breakdown and Oxidation of Organic compounds accompanied by energy release.

Cytoplasmic streaming, or cyclosis, promotes the optimal positioning of organelles, enhances biochemical reactions, facilitates the removal of Metabolic waste products, and serves other functions.

The penetration of substances across the membrane occurs via endocytosis, which is based on the cell's ability to actively absorb or ingest nutrients from the environment in the form of small fluid vesicles or solid particles. Passive Transport of substances occurs most readily through membrane pores, which are permeable to specific molecules and act as molecular sieves (selective channels). Passive transport is driven by diffusion along concentration or electrochemical gradients. However, substances more frequently cross membranes against concentration gradients with the aid of specialized transport systems known as carriers. These may be lipoproteins, Antibiotics, or other ionophores capable of temporarily binding to required molecules on one side of the membrane, transporting them, and releasing them on the other side. If the same carrier facilitates transport in one direction and subsequently carries another substance in the opposite direction, the process is termed exchange diffusion. Active Transport of substances across membranes is also widespread. Its characteristic feature is The ability to transport substances against a concentration gradient, which requires Energy Expenditure. Practically all types of membranes contain specialized transport proteins possessing ATPase activity.

The nucleus is the most vital, obligatory component of The Introduction/5.html">Eukaryotic Cell, serving as the control center for all biochemical processes and the carrier of heredity. It participates in Cell wall formation, influences cell growth and plastid division, and regulates photosynthetic processes. Nuclear division (karyokinesis) precedes Cell Division (cytokinesis). Chemically, the nucleus comprises amino acids, nucleoproteins, enzymes, fats, lipoproteins, carbohydrates, mineral salts, and nucleic acids. The Structural components of the nucleus include: karyoplasm, or karyolymph—the nuclear sap, which differs from the cytoplasm by its high DNA content; a double-membraned, porous nuclear envelope with ribosomes on the outer membrane connected to the ER cisternae; non-membranous nucleoli, where RNA is synthesized and preribosomes are formed; and chromatin—a complex of DNA and proteins that makes up Chromosomes, which carry genes with hereditary information.

Plastids are the largest organelles, unique to plant cells. They develop from proplastids in meristematic cells. Like mitochondria, they possess genetic autonomy, as they contain their own DNA, RNA, and ribosomes. Plastids are capable of division, growth, movement, and altering their structure and composition. Unlike other organelles, plastids may contain pigments—chlorophylls, carotenoids, and their derivatives. Depending on structure, coloration, and function, plastids are subdivided into Chloroplasts, chromoplasts, leucoplasts, and chromatophores (Fig. 1.2).

Fig. 1.2. Plastids in cells of Higher Plants and Algae: 1 — chloroplasts in Elodea leaf parenchyma; 2 — chromoplasts in rosehip fruit pulp cells; 3,4 — leucoplasts in Tradescantia leaf epidermis and Elodea meristem; 5 — spiral chromatophore in Cells of the green alga Spirogyra

Chloroplasts are green plastids responsible for photosynthesis, ATP synthesis, and The production of Lipids and proteins. They are typically disc-shaped, with a highly organized, ordered structure (Fig. 1.3). They are bounded by a double, porous, protein-lipid membrane bearing internal invaginations known as lamellae or thylakoids. Embedded within them are photosynthetic chlorophylls (a, b, c, d) and accessory pigments—phycobilins and carotenoids—which regulate radiant Energy Flow. Disc-shaped thylakoids stacked together form grana, On the surface of which the light reactions of photosynthesis take place. The ground substance of the chloroplast (stroma, or matrix) is rich in enzymes, lipids, and sugars, and drives the dark reactions of photosynthesis.

Fig. 1.3. Structure of a chloroplast: 1 — outer membrane; 2 — inner membrane; 3 — stroma; 4 — grana; 5 — thylakoids; 6 — starch grains

In The Human Body, chlorophyll promotes Hemoglobin production, strengthens Blood Vessels, and exhibits bactericidal and antioxidant properties. Chlorophylls and carotenoids are widely used in perfumery and cosmetology, serve as food colorants, and act as active ingredients in pharmaceutical products (such as Chlorophyllipt, Carotolin, and Aecol).

Chromoplasts are plastids pigmented yellow, orange, or red due to the presence of carotenoids—carotene, xanthophyll, and their isomers, including lycopene and rhodoxanthin. They develop from leucoplasts or chloroplasts. Their internal structure is simpler than that of chloroplasts. They exhibit a variety of shapes (triangular, lamellar, thread-like, rod-like, granular, etc.), which are species-specific characteristics. Chromoplasts are commonly found in the Tissues of petals, fruits, and seeds, and less frequently in other Organs, such as ROOT crops. They aid in pollination, reproduction, and the Dispersal of fruits and seeds, while also facilitating secondary metabolite synthesis. Because carotene is a provitamin of vitamin A, it is essential for animal organisms.

Leucoplasts are colorless plastids consisting of a protein-lipid stroma. They typically occur in meristematic cells, storage tissues, and the epidermis. Depending on The Nature of the stored substances, several types of leucoplasts are distinguished: amyloplasts, which synthesize secondary starch; proteoplasts, which accumulate storage proteins; and elaioplasts, which store fatty oils. In epidermal cells, leucoplasts function as optical light filters.

All types of plastids in higher plants are biologically interrelated and can transform into one another under certain conditions: leucoplasts into chloroplasts (as seen when potato tubers turn green upon exposure to light); chromoplasts into chloroplasts (when the illuminated part of a carrot root turns green); and chloroplasts into leucoplasts or chromoplasts (during tomato fruit ripening).

Chromatophores are the plastids of algae. They possess a diverse yet species-specific shape (Fig. 1.2) and contain, In addition to chlorophylls a, b, c, and d, various accessory pigments (such as phycocyanins and phycoerythrins). Furthermore, they contain proteinaceous bodies known as pyrenoids, around which reserve nutrients typically accumulate.