PLANT PHYSIOLOGY WITH ELEMENTS OF BIOCHEMISTRY - Prytuliak R. M. - 2016

Lecture Notes

TOPIC № 1. INTRODUCTION. PHYSIOLOGY OF THE PLANT CELL

Class="center">Introduction

Outline

1. Subject and objectives of plant physiology. Interdisciplinary connections of plant physiology.

2. Methods in plant physiology.

3. Plant physiology as the theoretical foundation of agronomic sciences.

4. A BRIEF HISTORY OF The Development of plant physiology as a science and The Role of domestic scientists in its advancement.

5. Key trends in modern plant physiology.

1. Subject and objectives of plant physiology.

Modern plant physiology is an integrative discipline that examines the fundamental vital Functions of the plant Organism across various Levels of Organization. It explores the principles of plant life and the methods for regulating them to optimize crop productivity. The subject of plant physiology encompasses the functions of living plant organisms, their Organs, Tissues, Cells, and cellular components, as well as the underlying causes of their physiological manifestations and their dependence on external and internal factors. The methodology of plant physiology is based on METABOLISM/2.html">THE CONCEPT OF the plant organism as a complex, self-regulating system comprising a hierarchy of structural levels—from subcellular (macromolecular) components to the whole plant. Understanding physiological functions is achieved by investigating simpler levels of organization, followed by the integration of data to analyze physiological systems of increasing complexity.

The goal of plant physiology is to understand the principles of plant vital functions, uncover their mechanisms, develop a comprehensive view of the Structural and functional organization of plant systems at various levels, and devise strategies for managing the plant organism.

Living systems, including the plant organism, possess a higher form of integrity, characterized by the inseparable connection and interdependence of their constituent components. One manifestation of this integrity is that, despite The Significance of any individual function, the organism's overall vitality depends on how a specific function relates to others and how these relationships are coordinated with changing environmental conditions.

The exceptional Specificity of chemical composition, morphological Structure, the close relationship between Structure and function, the dependence of metabolic processes on structural states, and the dynamic Nature of the latter are all defining CHARACTERISTICS OF THE subject matter in plant physiology.

The problems and tasks of plant physiology are so broad and complex that their resolution requires the application of a comprehensive set of sophisticated Physicochemical methods and diverse experimental and theoretical approaches. It is precisely for this reason that this discipline is classified among the exact fundamental sciences.

Modern plant physiology maintains close ties with botany, Cytology, biochemistry, biophysics, genetics, immunology, and ecology.

2. Methods in plant physiology.

The primary method for understanding processes and phenomena in physiology is the experiment. Consequently, plant physiology is an experimental science.

To investigate the physicochemical essence of functions and processes, plant physiology widely employs the following methods:

· Laboratory-analytical;

· Vegetation (under controlled conditions);

· Field;

· Production-based;

· Tracer (radioisotope) methods;

· Electron Cell/15.html">Microscopy;

· Ultraviolet and Fluorescence Microscopy;

· Electrophoresis;

· Chromatography.

In addition, phytotrons and controlled-environment laboratories are used to grow plants and conduct experiments under specific air composition, Temperature, and lighting conditions.

Using these methods, physiologists study plants at the molecular, subcellular, cellular, and organismal levels.

3. Plant physiology as a theoretical basis for agronomic sciences.

Plant physiology serves as the theoretical foundation for all agronomic sciences, as managing plant life processes and their Practical Application constitute the core objective of the field.

4. A brief History of plant physiology and the role of domestic scientists in its development.

The Current state of plant physiology is the result of a long developmental journey spanning more than two centuries. It emerged in the 17th and 18th centuries as a branch of botany.

The scientific approach to plant physiology was pioneered by the Dutchman Jan Baptist van Helmont (1579–1644), who conducted the first quantitative vegetation experiment to study plant Nutrition. Based on his findings, he erroneously concluded that Water was the sole nutrient substrate for plants. Only later was The Importance of soil and air as sources of plant nutrition established. It is no exaggeration to say that plant physiology began as the science of plant nutrition.

Marcello Malpighi (1628–1694), one of the founders of plant microscopic anatomy, linked his discoveries regarding specific microstructures to their functions, which proved vital for the future of plant physiology.

In 1727, Stephen Hales (an English botanist and chemist) hypothesized that plants might derive nourishment from the air through their leaves. Hales is undoubtedly considered one of the founders of plant physiology as a science.

Plant physiology as a distinct science took shape nearly a century later, when between 1771 and 1782, Joseph Priestley (England), Jan Ingenhousz (Austria), and Jean Senebier (Switzerland) identified the components responsible for the aerial nutrition of plants.

Nicolas-Théodore de Saussure (1777–1845) proved through precise chemical analysis that plants assimilate carbon from CO₂ in the presence of light. Thus, the Concept of the photosynthetic function of plants gradually took form.

The term "Photosynthesis" was proposed by the German scientist Wilhelm Pfeffer only in 1877. Following the discovery of photosynthesis and the laws of conservation of matter and energy, plant physiology increasingly viewed the atmosphere and sunlight as the primary material and Energy Sources for the existence of green plants.

Plant physiology was finally established as an independent branch of science in 1800, when Jean Senebier (1742–1803) introduced the term "plant physiology." He authored the first textbook on the discipline, in which he defined the subject's essence, methods, and tasks.

In Russia, plant physiology did not separate into an independent discipline for a long time; it was only in 1863, under a new university statute, that departments of plant physiology were created. The first Russian plant physiologist is considered to be S. A. Rachinsky (1833–1902), a professor of plant physiology at Moscow University.

In 1872, the prominent scientist K. A. Timiryazev (1843–1920) began working at the same university. At St. Petersburg University, Prof. A. S. Famintsyn (1835–1918) had been teaching a course in plant physiology since 1861. In its initial stage, plant physiology was primarily a university-based science.

The first Russian textbook on plant physiology was written by Prof. A. S. Famintsyn in 1887, and in 1891, the textbook by V. I. Palladin was published, which was subsequently reprinted nine times.

Significant contributions to various branches of plant physiology in the 19th century were made by: J. Boussingault, A. S. Famintsyn, K. A. Timiryazev, M. S. Tswett (photosynthesis); I. P. Borodin, A. N. Bach, G. Bertrand, V. I. Palladin, L. Pasteur (Respiration); H. Dutrochet, H. de Vries, J. Sachs (water relations); J. Liebig, H. Hellriegel, W. Knop, S. N. Winogradsky, M. Beijerinck, D. N. Pryanishnikov (mineral nutrition); J. V. Baranetsky, H. Vöchting, G. Klebs (Plant GROWTH AND DEVELOPMENT), and others.

At the beginning of the 20th century, the final differentiation of plant physiology into its main branches occurred, with some gaining such significance that they became independent disciplines with their own theoretical foundations and broad Practical Applications: 1902 – virology, 1910 – agricultural chemistry, 1930 – microbiology and biochemistry.

During this period, significant success was achieved in uncovering the Biochemical Mechanisms of respiration (V. I. Palladin, H. Wieland, S. P. Kostychev, O. Warburg, D. Keilin, T. Thunberg, H. Krebs, A. Kornberg), photosynthesis (R. Willstätter, C. van Niel, R. Hill, M. Calvin, D. Arnon), The Study of endogenous growth regulators (N. G. Kholodny, F. Went, F. Kögl, E. Kurosawa, T. Yabuta, F. Skoog), and the study of Plant resistance mechanisms (N. A. Maximov) and mineral nutrition (D. A. Sabinin).

In 1934, the Institute of Plant PHYSIOLOGY OF THE USSR Academy of Sciences was established, which coordinated all research in this field and where A. L. Kursanov, A. A. Nichiporovich, M. Kh. Chailakhyan, P. A. Genkel, I. I. Tumanov, and other scientists worked fruitfully.

Academician A. L. Kursanov became the successor and standard-bearer of plant physiology traditions during the pivotal mid-1950s, a time when the field was undergoing a global transformation. He essentially redefined the trajectory of plant physiology by establishing the renowned Kursanov school (A. T. Mokronosov, O. N. Kulaeva, R. G. Butenko, V. E. Semenenko, D. B. Vakhmistrov, B. B. Vartapetyan, Y. G. Molotkovsky, A. T. Vereshchagin, I. A. Tarchevsky, R. K. Salyaev, G. O. Sanadze, V. V. Polevoy, and others).

New avenues of scientific inquiry emerged: oxygen exchange and transport (B. B. Vartapetyan), the role of Cytokinins in Metabolic Regulation (O. N. Kulaeva), plant donor-acceptor systems (A. T. Mokronosov), the organization of plant membranes (Y. G. Molotkovsky, R. K. Salyaev), ionic Homeostasis in plant cells (D. B. Vakhmistrov), and plant Lipid Metabolism (A. T. Vereshchagin).

Mechanisms of hormonal regulation were studied intensively, including hormone reception and signal Transduction to The Cell's genetic apparatus (O. N. Kulaeva, V. V. Polevoy), as well as phytohormone metabolism and growth regulation (V. I. Kefeli). Research into plant immunochemistry also began (O. D. Volodarsky).

By the end of the 20th century, physiological experimental techniques had evolved, leading to fundamental research that paved the way for entirely new plant biotechnologies. A new stage in plant physiology began, characterized by the assimilation and integration of advancements from other experimental biological sciences.

5. Key directions in the development of modern plant physiology.

As a fundamental biological science and the theoretical foundation of plant cultivation, it addresses two primary, interrelated problems:

1) a comprehensive Study of the metabolic processes of the plant organism to understand The Essence of life—the core theoretical question of biology;

2) the study of plant life patterns during ontogenesis under various ecological conditions to solve practical problems in plant cultivation.

To address these challenges, plant physiology encompasses the following scientific directions.

Biochemical — investigates The Nature and Functional Significance of organic substances within plants, the pathways of their Formation and Transformation during photosynthesis and respiration, and explores aspects of ROOT nutrition and other related issues.

Biophysical — examines energy accumulation pathways in plant cells, The Nature of Biological Membranes, the biophysical processes of photosynthesis and respiration, the physicochemical principles of water relations and root nutrition, Phytochrome regulatory systems, the nature of stimuli, and more.

Synthetic, or cybernetic — studies the energy status and kinetics of interrelated processes such as photosynthesis and respiration, nutrition and Organogenesis, and GENERAL PATTERNS OF plant growth.

Ecological — investigates the Influence of the environment on plant life processes and develops effective methods for managing plants (e.g., Fertilization, optimizing water, light, and air regimes) to ensure resistance to diseases and pests, improve crop yields, etc.

Ontogenetic — studies the age-related patterns of plant development and develops ways to rationally manage these processes (e.g., controlled-environment agriculture, vernalization, hardening, Photoperiodism, etc.).

Evolutionary — studies the Characteristics of Individual species and plant varieties under specific ecological conditions, changes in plant structure depending on environmental factors, and views ontogenesis as a function of the genotype.

PLANT CELL PHYSIOLOGY

Syllabus

1. The Cell as a functional unit of living matter.

2. Structural organization of the cell.

3. The Cell wall: composition, structure, and functions.

4. Cytoplasm: Structure and properties.

5. The Nucleus: STRUCTURE AND FUNCTIONS.

6. Biological membranes.

7. Endoplasmic reticulum.

8. Ribosomes: Chemical Composition and functions.

9. Chloroplasts, their structure and function.

10. Mitochondria, their structure and functions.

1. The cell as a functional unit of living matter.

The cell is the structural and functional unit of the plant organism, just as it is for all other living beings. The term was first proposed in 1665 by the English scientist Robert Hooke, who, while examining cork structure under a microscope, observed formations resembling honeycombs, which he named cells. Later, in 1838–1839, German scientists—histologist T. Schwann and botanist M. Schleiden—proposed the general biological concept of Cell Theory, according to which the cell is the fundamental unit of structure and function for all organisms. This theory was based on two fundamental principles:

1) The cell is the basic unit of all organisms;

2) The growth, development, and differentiation of tissues in Living organisms are driven by The process of new Cell Formation.

This theory was further developed and refined by Rudolf Virchow (1885), who introduced a series of new concepts regarding the role of cellular structures in an organism. The core principle of his theory is: there is no life outside the cell, and every cell originates from a pre-existing cell (*omnis cellula e cellula*). This was confirmed by the subsequent development of biology.

According to modern understanding, the cell is the fundamental structural and functional unit of All living organisms and an elementary living system. Only Viruses represent non-Cellular forms of life.

The plant cell is a functional structural unit of living matter, as it exhibits the following characteristics:

· Metabolism and Energy Exchange;

· Capacity for growth and self-reproduction;

· Storage and transmission of hereditary information.

The stability of cellular structure throughout the organic world is due to the fact that only the cell ensures the optimal storage, reproduction, and transmission of hereditary information. The ability of organisms to store, transfer, and convert energy into work is intrinsically linked to this structure.

2. Structural ORGANIZATION OF THE plant cell.

According to modern concepts, a plant cell consists of three main parts: the cell wall, the protoplast, and the vacuole. The cell wall is relatively rigid, chemically complex, and is a product of the protoplast's activity. The protoplast, which is the living part of the cell, is a colloidal solution containing structural components (nucleus, Plastids, mitochondria, endoplasmic reticulum, Golgi complex, ribosomes). Vacuoles are non-living formations filled with solutions of inorganic salts absorbed by the cell, as well as organic substances—products of the cell's metabolic activity. Cell sizes vary widely (from one to several hundred microns). Morphological differences between cells are determined by the nature of biological processes and the type of metabolism. If a plant cell is grown in isolation, its shape tends toward spherical; if surrounded by other cells, it may take the form of a polyhedron. Cells in the growth zone of a stem or root resemble a box approximately 50 μm long, 20 μm wide, and 10 μm high. Cells of embryonic tissues and the apical meristem are very small.

Young cells have similar longitudinal and transverse dimensions and a thin wall. They are entirely filled with protoplast. Over time, as the cell volume increases, The amount of protoplast gradually grows, and many small vacuoles form within it, which eventually merge into one large vacuole; the wall thickens.

The protoplast is surrounded externally and internally by biological membranes: it is separated from the cell wall by the Plasmalemma and from the vacuole by the tonoplast.

The protoplast and the cell wall are not completely isolated. They are connected via specialized openings called pores. Through these Pores in the wall, the protoplasts of adjacent cells are connected by cytoplasmic strands known as plasmodesmata. Thus, thanks to membrane structures and plasmodesmata, all cells are interconnected. Structurally, plasmodesmata resemble tubes with a diameter of 20 to 100 nm. Within each plasmodesma, There is a channel (desmotubule) through which various substances can pass from one cell to another.

3. The cell wall: composition, structure, and functions.

The cell wall is formed from the products of the protoplast's secretory activity, which are deposited in layers during the cell's development. The main component of the cell wall is Cellulose (fiber - (С₆Н₁₀О₅)n). Cellulose molecules are unbranched chains consisting of D-glucopyranose residues linked by (β1-4)-glycosidic bonds. On average, one cellulose molecule contains up to 8,000 glucose residues.

Between the walls of adjacent cells lies the middle lamella, which is initially formed by gelatinous pectic substances but is later supplemented with cellulose and other Polysaccharides, gaining rigidity. In lignified cells, the wall is impregnated with Lignin.

The primary cell wall consists of cellulose microfibrils embedded in a matrix. Due to the spaces between these fibrils, the cell wall possesses sufficient flexibility.

The ability of the cell wall to stretch depends on the orientation of the cellulose fibrils. When randomly arranged, the wall stretches uniformly in all directions. If the fibrils are arranged in parallel, stretching occurs at a right angle to the axis. During cell formation, the outer wall undergoes significant pressure from the protoplast, stretching while new building material is added—this is how growth and thickening occur. In some cells, such as those of the mesophyll, Cell wall formation concludes as soon as the cell reaches its maximum size. In other tissues, cells that have finished growing develop a secondary cell wall with a rigid structure on their inner side. Such wall thickening reduces the volume of the protoplast. Over time, the entire protoplast dies off, leaving only hollow cylinders of cell walls that perform mechanical or conductive functions.

The secondary cell wall is penetrated by numerous pores. The area of the wall with pores is quite thin, consisting only of the middle lamella and the primary cell wall.

In mature living cells, pores appear as canals connecting the inner part of the secondary cell wall to the outer part of the primary one. Pores can be arranged in groups, forming pit fields, which play an important role in The transport of water and solutions of mineral and plastic substances.

Functions of the cell wall. The cell wall is a secretory product of the protoplast and is likely non-living in a mature cell. However, it contains Proteins, including those with enzymatic activity.

The cell wall provides mechanical strength and support to individual cells and the plant as a whole.

It determines the size, shape, and Stability of the PLANT CELL AND protects the protoplasmic membrane from rupture due to the hydrostatic pressure generated within the cell.

The cell wall serves as an anti-infective barrier and participates in the absorption, transport, and secretion of substances. The system of interconnected cell walls is called the apoplast, which acts as the primary pathway for the Movement of water and dissolved substances throughout the plant. Cytoplasmic strands pass through pores in the cell walls, connecting the contents of individual cells and thus uniting all protoplasts into a single system known as the symplast.

There is evidence that carbohydrate Components of the cell wall interact with phytohormones to induce physiological changes within the cell.

4. Cytoplasm: structure and properties.

The protoplast is a colloidal system. In a living cell, it is in constant motion, which ensures the optimal positioning of Organelles, facilitates biochemical reactions, and Supports the removal of metabolic waste into the vacuole or out of the cell. The cytoplasm forms the foundation of the protoplast, which also includes the nucleus.

The cytoplasm is a semi-fluid, transparent, and viscous homogeneous mass located beneath the cell wall, primarily as a relatively thin peripheral layer.

The chemical composition of the cytoplasm (water 75-85%, PROTEINS AND AMINO acids 10-12%, CARBOHYDRATES 4-6%, fats and LIPIDS 2-3%, other organic substances ~1%, Mineral Substances 2-3%) promotes The formation of a colloidal solution that does not mix with water and the substances contained in vacuoles.

The cytoplasm is characterized by elasticity and relatively high density. For instance, in the parenchyma cells of bean cortex, its viscosity is 24 times higher than that of water.

The viscosity of the cytoplasm is non-uniform. Its peripheral part, adjacent to the cell wall, is more viscous and is separated from it by a surface membrane called the plasmalemma. On the side of the vacuole, the cytoplasm is separated by a second surface membrane, the tonoplast. Between these membranes lies the inner layer of the cytoplasm with lower viscosity, known as the mesoplasm, which serves as the cytoplasmic matrix permeated by The endoplasmic reticulum (the internal membrane system). Metabolic processes occur continuously within the cytoplasmic matrix. It is believed that the cytoplasm contains fibrillar structural elements that contribute to gel formation.

Water plays an exceptionally important role in the structural and functional organization of the cytoplasm. Its properties as a solvent and a substance of great biological significance are determined by the features of its internal molecular structure, primarily the polarity of the molecule. This polarity is caused by the asymmetric arrangement of hydrogen and oxygen electrons in the molecule, resulting in an uneven distribution of positive and negative charges.

5. The nucleus: structure and functions.

The nucleus is the central and most important organelle of the cell, possessing a rather complex structure. Its shape is mostly spherical or oval, and its size varies within a fairly wide range.

Genetic information is concentrated in the nucleus within specific structural units called Chromosomes. Their intertwining forms a cohesive mass known as Chromatin. Furthermore, the nucleus contains one or more nucleoli. The space between nuclear structures is filled with a colorless substance called karyoplasm or nucleoplasm.

The nucleus is enclosed by a porous membrane that contains ribosomes and is connected to the membranes of other cellular components via elements of the endoplasmic reticulum.

The inner membrane occasionally invaginates into the nucleus. A 1 μm area of the nuclear envelope may contain from 10 to 100 pores, each up to 20 nm in diameter. Nuclear pores are dynamic structures that periodically open and close, regulating the exchange of substances between the nucleus and the cytoplasm.

The nucleus not only stores genetic information but also transmits it to the cytoplasm (e.g., via mRNA synthesis) and from Cell to Cell (through nuclear division, Cell Division, reproduction, and inheritance). Nucleoproteins predominate among Nuclear Proteins. The structure of the nucleus depends on the functional state of the cell.

The Internal Structure of the nucleus changes depending on its state. Two periods in the life of a nucleus are distinguished: the metabolic period (interphase) and the division period. During the metabolic period, the nucleus contains one or more nucleoli, which consist of tightly intertwined threads called nucleonema and contain up to 80% protein, 10-15% RNA, and a small amount of DNA.

The nucleus is filled with nucleoplasm and intertwined, coiled chromatin threads. Chromatin consists of DNA, histone and non-histone proteins, and small amounts of RNA and lipids. A characteristic feature of the metabolic period is the process of self-Replication (doubling) of DNA molecules. Only after this does the nucleus proceed to division, i.e., mitosis. In the interphase nucleus, chromosomes appear as somewhat amorphous, swollen masses of chromatin. Before division begins, each chromosome consists of two chromatids, which separate during anaphase. Subsequently, in the new cell, the chromatid replicates to become a chromosome with a full set of genetic information.

The primary function of the nucleus is to control Protein Synthesis AND cellular activity, as well as to preserve and transmit genetic information to daughter cells during cell division. The nucleus is surrounded by a double nuclear membrane and contains chromatin, nucleoli, and nucleoplasm.

Chromosomes, or chromatin, in eukaryotes consist of four types of molecules: 1) DNA (approx. 35%); 2) RNA (approx. 12%); 3) basic low-molecular-weight protein - histone (approx. 40%); 4) acidic non-histone protein, including Enzymes (approx. 10%), as well as small amounts of lipids, polysaccharides, and Metal Ions.

The nucleolus is a dense, rounded structure that lacks a membrane. It consists of material more compact than the rest of the nucleus and contains ribonucleic acid (15%) and proteins (80%). The nucleolus contains A large number of ribosomal subunits. These ribonucleoprotein granules, along with ribonucleoprotein filamentous structures (fibrils), are immersed in the nucleoplasm. Numerous ribosomal proteins are synthesized in the nucleolus, and RNA accumulates there before being exported to the cytoplasm. Nucleolar ribosomal RNA and proteins assemble into ribosomal subunits. Additionally, Other types of RNA (such as tRNA) are assembled in the nucleolus. Therefore, it serves as a site for RNA redistribution.

The primary function of the nucleolus is the synthesis of nuclear proteins. It is likely that a certain amount of specific ribosomal proteins are also synthesized there. Self-assembly of ribosomes may also occur within the nucleolus. If the nucleoli are destroyed, for example by ultraviolet radiation, the nucleus loses its ability to divide.

6. Biological membranes.

The protoplast is bounded externally and internally by specific membranes: the plasmalemma separates it from the cell wall, while the tonoplast separates it from the vacuole. There are also membranes of the nucleus, mitochondria, plastids, Golgi apparatus subunits, and internal Cytoplasmic membranes, such as those of the endoplasmic reticulum, mitochondria, and chloroplasts. Membranes are highly organized cellular structures whose composition depends on their type and function, yet they always contain lipids and proteins.

In 1959, the English scientist J. Robertson proposed a hypothesis regarding the STRUCTURE OF THE "unit" membrane, postulating a structure common to all biological membranes. According to this hypothesis, all membranes appear as a trilaminar structure, in which a central lipid bilayer is situated between two protein layers with a total thickness of 7.5 nm. In 1972, English scientists

S. Singer and G. Nicolson proposed the Fluid Mosaic Model of the membrane, according to which protein molecules in lipids form something resembling a mosaic.

Membrane Lipids consist of Phospholipids, Glycolipids, and sterols. The lipid layer is primarily composed of phospholipids, galactolipids, Fatty acids, and sterols. It is likely that the lipid layer is penetrated by protein molecules, which may be linked via their hydrophobic side chains to the internal hydrophobic PARTS OF THE lipid molecules. The granules in the inner regions of membranes are most often proteins that are hydrophobically anchored within the lipid matrix of the membrane.

Biological membranes divide the cytoplasm into compartments where specific biochemical transformations occur.

The labile structure of membranes enables them to perform A wide variety of functions: barrier, osmotic, transport, electrical, structural, energetic, biosynthetic, secretory, receptor-regulatory, and others.

7. Endoplasmic Reticulum.

The endoplasmic reticulum (ER) is a complex three-dimensional membrane system, the shape and extent of which are determined by the cell type and its stage of differentiation. In three dimensions, it has a lamellar structure consisting of many membrane sacs called cisternae, On the surface of which numerous granules up to 30 nm in diameter can be detected using an Electron microscope—these are ribosomes, where protein synthesis occurs. Due to this structure, it is often called the rough endoplasmic reticulum (RER).

Cells undergoing intensive lipid synthesis possess a different, highly branched smooth tubular endoplasmic reticulum. Both types of ER can exist simultaneously within the same cell, and there is a functional connection between them.

The functions of the rough ER are associated with the transport of proteins synthesized by ribosomes on its surface. At the beginning of protein synthesis, the initial part of the synthesized polypeptide chain consists of a so-called signal sequence, which corresponds in configuration to a specific receptor on the ER membrane. The receptor forms a channel through which the protein enters the ER cisternae for further transport. During the transfer process, the protein undergoes significant modifications, such as phosphorylation, Acetylation, conversion into Glycoproteins, etc.

The endoplasmic reticulum is of great importance in the Biogenesis of Cellular membranes, as it is the primary site for the synthesis of both Membrane Proteins and cellular lipids. The final stages of membrane lipid synthesis, particularly glycolipids and phospholipids, are localized in the ER. The latter ensure the formation of Mitochondrial and Chloroplast membranes. Furthermore, the ER is the site of steroid Biosynthesis and the synthesis of all unsaturated acids; it is here that fatty acids characteristic only of plants—linoleic, linolenic, and arachidonic—are synthesized. The membranes of vacuoles, spherosomes, Microbodies, etc., are derivatives of ER membranes. The reticulum is directly connected to the nuclear envelope. Through the Golgi apparatus membrane system, it participates in the synthesis of plasmalemma components. The transition of membranes into various types of organelles is called membrane flow. There is a concept of the interaction of cellular membrane components known as the endomembrane system, according to which membrane flow explains the functional continuity of membranes and their Participation in the vital activities of all cell organelles.

The endoplasmic reticulum is an extensive membrane system that divides the content of a Eukaryotic Cell into compartments and channels. It forms a single unit with the outer membrane of the nuclear envelope. The rough ER facilitates the synthesis and transport of proteins across the membrane, while the smooth ER is responsible for lipid synthesis and other biosynthetic processes.

8. Ribosomes, their chemical composition and functions.

Ribosomes are small spherical particles with a diameter of 0.2 µm, consisting mainly of high-molecular-weight RNA (up to 60%) and protein. Ribosomal RNA (rRNA) is synthesized in the nucleolus. Through self-assembly, RNA together with protein forms a complex three-dimensional structure. A ribosome consists of a small (40S) and a large (60S) subunit. The small subunit contains one 18S RNA and up to 40 protein molecules, while the large subunit contains one molecule each of 5S RNA, 5.8S RNA, 25S RNA, and up to 45 protein molecules. There are tens of thousands of them in each cell. Ribosomes can be attached to the ER or localized freely in the cytoplasm. They are also present in the nucleus, plastids, and mitochondria. Therefore, Two Types of ribosomes are distinguished: cytoplasmic—with a sedimentation coefficient of 80S (the sedimentation constant is measured in Svedberg units, 1S=10^-13 s) and organelle ribosomes with a coefficient of 70S. Ribosomes quite often form complexes—polyribosomes—which are formed in stages from individual subunits. First, proteins specific to each subunit are assembled with the participation of 28S and 18S rRNA. During this process, the smaller ribosomal subunit interacts with initiator tRNA in the presence of ATP, GTP, and protein initiation factors. This complex joins with mRNA in the presence of magnesium ions, and finally, the large ribosomal unit attaches to the mRNA.

Ribosomes are responsible for Protein synthesis in the cell. The sequence of Amino Acids in protein molecules synthesized on ribosomes is determined by specific Messenger RNA (mRNA) molecules, which carry information from the nuclear genome to the ribosomes. Mitochondria and chloroplasts have their own genome. During Protein Synthesis on Ribosomes, the amino acids that make up the polypeptide chain are added sequentially one after another. It is within the ribosome that amino acid molecules can occupy a strictly defined position relative to one another. Thus, protein synthesis involves mRNA, which carries genetic information; transfer tRNA (tRNA), which delivers the necessary amino acids to the ribosome; and the growing polypeptide chain. Factors responsible for initiation, elongation, and termination of the polypeptide chain are also required. During protein synthesis in an active state, most ribosomes attach to long thread-like mRNA molecules, forming polyribosomes in the presence of magnesium ions. This allows for the simultaneous synthesis of several dozen molecules of the same protein.

9. Chloroplasts, their structure and function.

Plastids are products of the plant cell's vital activity. They are formed from proplastids—small amoeboid bodies (d 0.05-0.5 µm) that originate from initial particles that separate from the nucleus and contain nucleoplasm. Plant cells contain Three types of plastids: leucoplasts (colorless), chloroplasts (green), and chromoplasts (orange). The entire set of plastids is referred to as the "plastidome".

Plastids of all three types can interconvert. For example, leucoplasts turn into chloroplasts when potatoes turn green in the light; in the dark, chloroplasts lose their green color and turn into leucoplasts; and upon chloroplast degeneration and chlorophyll breakdown, chromoplasts can be formed.

Chloroplasts are green due to the presence of chlorophyll. They are capable of self-replication: their high degree of autonomy has been proven, consisting of The biosynthesis of A number of protein and lipid components. The specific DNA of plastids differs from nuclear DNA. Its synthesis directly within the plastids has been experimentally confirmed. A chloroplast contains about 5% of all cellular DNA. Its content varies depending on physiological conditions. The number, size, shape, and Location of DNA localization sites and their Morphology differ among chloroplasts of different plant species.

Light is of great importance in the development of chloroplasts, as it is necessary for the synthesis of chlorophylls. Chlorophyll molecules are localized in internal membranes that form two types of lamellae immersed in a hydrophilic protein matrix or stroma. Some of them extend along the entire plastid—stroma lamellae—while others are shorter and stacked one above another, forming grana. In the absence of light, instead of lamellae, a prolamellar body is formed—an ordered center of vesicles and channels. They were named prolamellar to emphasize that they are precursors to lamellae. After light stimulation, the structures of the prolamellar bodies change their orientation and rapidly transform into a system of lamellar membranes.

Plastids containing prolamellar bodies are called etioplasts. They can be viewed as a specific stage of chloroplast development. Etioplasts are formed in the primary leaves or cotyledons of seedlings before they emerge from the soil into the light. In flowering plants, chloroplasts develop from proplastids only in the light, whereas in some gymnosperms, this transformation occurs even in its absence.

Chloroplasts are found primarily in leaf parenchyma cells, whereas they are absent in the meristem. A cell can contain from one to hundreds of chloroplasts, with a diameter of 5...8 µm and a thickness of up to 1 µm.

Each chloroplast is surrounded by a double membrane with selective permeability. The basic structural unit of the inner chloroplast membrane is the thylakoid, a thin, flat disk surrounded by a single-layer membrane.

Its membrane contains chlorophyll a and b, carotenoids, and proteins that participate in photosynthetic reactions. There are two types of thylakoids: the large stroma thylakoid, which approaches the size of the chloroplast itself in length, and the smaller thylakoid, or granum thylakoid, with a diameter of 30...60 nm. A chloroplast can have 40...60 grana and, as a rule, 5 to 20 thylakoids per granum. Perforations have been discovered in the grana, through which granum membranes connect, and thus their intrathylakoid space is interconnected by narrow tubes called frets.

The thylakoid system is a single compartment separated from the stroma by the thylakoid membrane system. Three important chloroplast compartments are distinguished:

· intermembrane space between the outer and inner membranes surrounding the chloroplast;

· stromal compartment;

· thylakoid lumen.

The outer membrane is permeable to metabolites. This permeability is facilitated by specific membrane proteins known as porins, which form channels in the membrane that allow the free passage of substances with a molar mass of up to 10 kDa. It has been established that the diameter of open pores formed by porins in chloroplasts reaches 3 nm.

The stromal compartment supports essential metabolic cycles of photosynthesis, while the thylakoid lumen is responsible for generating a proton gradient during the light-dependent reactions of photosynthesis.

10. Mitochondria: structure and functions.

Mitochondria are present in the cytoplasm of all Eukaryotic cells. During sexual reproduction, promitochondria are transmitted to offspring via the egg cell. Mitochondria reproduce through binary fission as well as budding. The resulting daughter organelles, or promitochondria, eventually mature into fully functional mitochondria.

Mitochondria are composed of proteins, lipids, Vitamins, ribosomes, RNA, DNA, and an enzymatic complex.

The primary function of mitochondria is to provide the cell with the energy required for vital processes by converting the chemical bond energy derived from The oxidation of respiratory substrates into the high-energy phosphate bonds of adenosine triphosphate (ATP). ATP synthesis occurs through the enzymatic breakdown of carbohydrates, fatty acids, and amino acids during Oxidative Phosphorylation. Furthermore, mitochondria are involved in the biosynthesis of lipids and proteins, which participate in the overall process of ion transport within the cell.

The lifespan of a mitochondrion does not exceed a few days. The constant formation and degradation of these organelles account for the wide variety of their shapes within a single cell. The length of mitochondria typically ranges from 1 to several µm, with a width of approximately 0.5 µm. Occasionally, in highly active cells, mitochondria may be larger. Their number per cell can vary significantly, reaching up to 2,000.

A mitochondrion is bounded by a double membrane—an outer and an inner one. The outer membrane is smooth and porous; it contains enzymes and proteins and is permeable to ions and small molecules. It is highly permeable to Pyruvate, which is produced by the incomplete oxidation of six-carbon compounds in the cytoplasm.

The inner membrane has a complex structure, forming numerous folds of various shapes known as cristae. The surface of this membrane is densely covered with mushroom-shaped (elementary) particles containing factors that facilitate DNA Synthesis. The inner membrane is permeable only to water and small neutral molecules. It houses the Respiratory Chain, which consists of electron carriers and a small number of proteins.

Thanks to the cristae, the internal content of the mitochondrion—the matrix—is divided into compartments. The mitochondrial matrix is a gel-like substance containing up to 50% protein. It houses the enzymes of The Tricarboxylic Acid Cycle (Krebs cycle) as well as those involved in lipid and protein synthesis. The matrix also contains ribosomes and Mitochondrial DNA.

Despite possessing their own genetic and protein-synthesizing systems, mitochondria are not entirely autonomous. A significant portion of the information regarding their structure and function is encoded within the nuclear chromosomes.