BIOCHEMISTRY FOR TEACHERS — F.F. BOYECHKO — 1985

MAJOR CHEMICAL COMPONENTS OF CELLS

HORMONES AS REGULATORS OF METABOLIC PROCESSES

Ensuring the survival of the Organism as a single, complex, ordered system capable of self-reproduction, growth, and development requires a high degree of integration and coordination of all vital Functions. To survive, the organism must rapidly respond to changing environmental conditions and constantly maintain a dynamic equilibrium across all metabolic processes. Consequently, evolution gradually shaped a centralized control apparatus whose primary function is intercellular regulation and the programming of various Organs and Tissues. In most animals and humans, this role is performed by the nervous and endocrine systems.

The Endocrine System emerged at early stages in the Evolution of the animal kingdom, following the Differentiation of the Nervous system.

Higher animals and humans possess Two Types of glands: exocrine (from the Greek ekso — outside, krinein — to secrete) and endocrine (from the Greek endo — within). Exocrine glands (salivary, sweat, and gastrointestinal glands) release their secretions directly into the Blood.

Vertebrates have approximately 10 Endocrine glands that produce biologically active secretions, forming the endocrine system. The most important among them include the Hypothalamus, Pituitary gland (hypophysis), Pineal Gland (epiphysis), Thyroid Gland, Thymus, Pancreas, Adrenal Glands, and Gonads. Furthermore, certain organs such as the Kidneys, Liver, and Spleen are believed to contain cellular elements that can be classified as endocrine tissues. These organs secrete substances that play a crucial role in regulating vascular tone and various metabolic processes.

The gastrointestinal tract also produces substances that influence specific metabolic pathways. In addition to these glands, which are found in most vertebrate classes, mention should be made of a specialized endocrine gland of mammalian Pregnancy—the Placenta—which produces a large quantity of substances affecting reproductive functions and supporting pregnancy.

Endocrine glands are absent in most lower invertebrates and occur only in their highly evolved representatives, such as cephalopods, crustaceans, and insects. These glands control the most critical aspects of invertebrate ontogeny, including growth, molting, metamorphosis, reproduction, and adaptation. The secretions produced by ductless glands are called HORMONES (from the Greek hormanin — to set in motion, to excite). This group of substances was named in 1902 by the British scientists W. Bayliss and E. Starling following their research on secretin, a substance that stimulates pancreatic juice secretion. It was later revealed that hormones are specific products of endocrine glands that exert a distant regulatory effect.

The range of hormonal action on metabolic processes is quite diverse, spanning from Ion transport across Plasma Membranes to genome METABOLISM/31.html">Transcription. Hormones are capable of altering the structures and functions of Cells, tissues, and organs, as well as their physiology and Morphology. Their undeniable influence extends to the synthesis of energy-rich compounds (macroergic bonds), DNA, and Proteins, The formation of the cellular phenotype, body GROWTH AND DEVELOPMENT, sex differentiation, adaptation, and the Maintenance of Homeostasis.

THYROID HORMONES

The Thyroid Gland is one of the most thoroughly studied endocrine glands. Located in the region of the Larynx and Trachea, it consists of two lobe-like parts connected by a thin isthmus. The lobes are 5–8 cm long and 3–4 cm wide. Weighing 25–30 g, it is one of the largest endocrine glands. The thyroid tissue is composed of numerous glandular structures (follicles) filled with a viscous yellowish fluid (colloid) produced by the glandular cells. Each follicle is entwined with a dense capillary network, ensuring an intensive Blood supply to the gland's tissues. The volume of blood passing through the gland in 1 minute is 3–5 times greater than the mass of the gland itself. Within an hour, nearly all the blood circulating in the human Vascular System passes through this endocrine organ.

The colloid contained within the thyroid follicles incorporates a complex glycoprotein called thyroglobulin, which contains 0.5–1% iodine and 8–10% CARBOHYDRATES. It serves as the essential precursor from which thyroid hormones are synthesized:

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Triiodothyronine exhibits the highest biological activity; however, since it is produced in small quantities, the specific biological action of the hormone is primarily manifested through thyroxine.

Thyroid hormones are active regulators of metabolic processes within the organism, ensuring the normal functioning of many Organ Systems. Consequently, thyroid dysfunction leads to a range of pathological changes whose nature and severity depend on numerous factors.

The primary pathologies associated with thyroid dysfunction include Graves' disease (Basedow's disease), Myxedema, endemic goiter, and cretinism.

Graves' Disease. This condition is caused by thyroid hyperfunction—an excessive secretion of the hormones synthesized within the gland. Because many of these hormones, particularly thyroxine, exert toxic effects on The Human Body, the disorder is also known as thyrotoxicosis.

Hyperfunction of the thyroid gland was first described by the German physician C. Basedow in 1840. A characteristic feature of the disease is an elevated basal metabolic rate and accelerated breakdown of tissue proteins, carbohydrates, and fats due to increased activity of the Enzymes that control these processes (hexokinase, succinate dehydrogenase, thiol, and Proteolytic Enzymes). The enhanced degradation of various compounds is accompanied by the accumulation of large amounts of intermediary metabolites, whose oxidation requires vast amounts of oxygen. This leads to oxygen starvation, which negatively impacts the functioning of several body systems, particularly the Central Nervous System.

Thyrotoxicosis is one of the most common endocrine disorders, occurring predominantly in women aged 20–40. It is frequently triggered by psychological trauma. As S.P. Botkin noted, grief, anger, and fear can precipitate severe thyrotoxicosis, which sometimes develops very rapidly—within a matter of hours. The underlying cause is intense excitation of the Cerebral Cortex, which is transmitted to the thyroid gland either directly via Neural Pathways or through the pituitary gland, prompting the release of large quantities of hormone.

A characteristic outward sign of thyrotoxicosis is exophthalmos (protruding eyes), which gives the face an angry or somewhat startled expression (an expression of frozen terror). Patients experience rapid weight loss, dropping 12–16 kg within a few months.

At the end of the 19th century, in the absence of reliable treatments, the majority of patients with severe thyrotoxicosis died within 2–3 years. Today, therapies that suppress thyroid activity (radioactive iodine, thiouracil and imidazole derivatives) are widely used alongside surgical interventions.

Myxedema arises from thyroid hypofunction and is the clinical opposite of Graves' disease. Specifically, core metabolic processes are suppressed, and blood pressure and body Temperature drop. Patients become lethargic and apathetic, experiencing generalized weakness, fatigue, and drowsiness. They feel perpetually cold; their Skin is dry and peeling, particularly at the elbows and shins. Memory deteriorates, and body temperature falls to 35–36°C, with even infectious diseases failing to trigger a significant fever. Mucinous Swelling and facial puffiness are also observed.

If thyroid hypofunction occurs during childhood or prenatal development and is accompanied by iodine deficiency, a condition known as cretinism develops. Affected children experience profound delays in growth and development. They begin walking late—at 2 to 3 years of age—and tooth eruption is delayed. Their faces appear puffy with a dull, vacant expression, the Mouth remains half-open, and the voice is coarse and hoarse. Such children are frequently born deaf-mute.

In severe cases of the disease, only the basic sensations of hunger and pain remain, accompanied by profound psychiatric disturbances.

Besides these conditions, another type of pathology known as endemic goiter occurs quite frequently. The cause of this disease is the body's diminished capacity to assimilate iodine or a deficiency of iodine in food and Water. This condition is particularly prevalent in mountainous and marshy regions.

Pancreatic Hormones

The pancreas is an unpaired organ of mixed secretion located in the Abdominal cavity to the left of The Stomach. Its Structure is quite complex. The exocrine function of the pancreas is performed by adipose (zymogenic) tissue, which secretes pancreatic juice directly into the duodenum.

The intrasecretory (endocrine) function is provided by the islets of Langerhans, which are clusters of cells of various types: α, β, and D. The total mass of islet cells in humans accounts for 1/30 to 1/100 of the entire pancreatic mass.

Pancreatic islet tissue is characterized by a constant cellular composition, containing 80% α-cells, 20% β-cells, and 2—8% D-cells. Each Cell type produces specific hormones characteristic of it, which regulate corresponding Metabolic Pathways in the bodies of humans and animals.

β-Cells contain zinc ions and produce the hormone Insulin (from insula, meaning island), which is a vital regulator of carbohydrate, lipid, and Protein metabolism. About 150 mg of insulin is contained in 1 kg of pancreatic tissue. Pancreatic β-cells can be selectively damaged by alloxan, magnesium salts, and dehydroascorbic acid.

α-Cells also contain zinc ions and produce the hormone Glucagon (hyperglycemic-glycogenolytic factor, HGF). Both hormones form chelate complexes with zinc. D-cells (definitive cells) presumably produce three hormones: Somatostatin, pancreagastrin, and secretin. The pancreas also contains hormone-like substances such as lipocaic, centrophenine, and vagotonin.

Insulin is a hormone produced by β-Cells of the pancreas. It was discovered as a factor that reduces hyperglycemia in pancreatectomized animals.

By chemical nature, insulin is a globular protein with a relative Molecular Weight of 6,000. The insulin molecule consists of two polypeptide chains, A and B, containing 21 and 30 amino acid residues, respectively, linked by two disulfide bridges formed between Cysteine residues.

Insulin is synthesized in the ribosomal apparatus of the β-cells of the islets of Langerhans as an inactive precursor, proinsulin, with a relative molecular weight of 10,000. This precursor contains 78—84 amino acid residues, depending on the animal species. In secretory granules, under The Influence of proteolytic enzymes, proinsulin is cleaved into a C-peptide and the active form of insulin. The Introduction/19.html">Primary Structure of insulin has minor variations across animal species, primarily concerning amino acid residues at positions 8, 9, and 10 (see Protein Structure). This hormone can exist as a monomer with a relative molecular weight of 6,000, as well as in di-, tri-, tetra-, hexa-, and octameric forms. The formation of insulin oligomeric forms occurs due to the interaction of zinc ions with the imidazole rings of Histidine residues in the polypeptide chain B.

The transport of insulin to target cells occurs with the participation of α- and β-globulins, which bind to the hormone upon its exit from secretory granules. Only the free form of insulin, released upon The breakdown of its protein complex, possesses biological activity. This process is particularly intensive in adipose tissue.

The primary target cells for insulin are the cells of the liver, Muscles, and adipose tissue. Additionally, insulin affects A number of other Cells and Tissues (fibroblasts, leukocytes, mammary gland tissue).

The MOLECULAR MECHANISMS OF the biological Action of Insulin, like other protein hormones, are based on its interaction with receptor proteins localized On the surface of the plasma membranes of target cells. It is believed that the Insulin Receptor is of a glycoprotein nature; in particular, neuraminic acid derivatives—sialic acids—play a crucial role in the reception process.

In the Human and Animal body, insulin plays an extremely vital role. Insufficient secretion of insulin by the pancreatic β-cells leads to disruptions in many metabolic pathways, primarily accompanied by hyperglycemia (elevated blood sugar) and glucosuria (the presence of sugar in the urine). These conditions arise due to impaired Glucose Transport into cells and its utilization by body tissues. It is believed that upon binding to The Cell surface, insulin modifies the cells in a way that facilitates glucose penetration into the cell, where its utilization takes place. This is the so-called primary effect of insulin. All Other effects of insulin are evidently merely consequences dependent on this primary effect. Secondary effects of insulin include a decrease in blood sugar levels, an increase in Glycogen stores in muscles, enhanced synthesis of fats and Peptides, etc.

By stimulating glucose entry into cells, insulin thereby causes a decrease in its blood levels, facilitates the phosphorylation of glucose under the influence of glucokinase, and promotes its subsequent utilization by cells. The process of glucose phosphorylation is of considerable importance, as subsequent conversion of glucose into end products is only possible in its phosphorylated form.

Impairment of the endocrine function of the pancreas (its hypofunction) results in a disease known as diabetes. Its main symptoms include hyperglycemia, glucosuria, polydipsia (excessive thirst), and polyphagia (increased appetite). The loss of significant amounts of sugar through urine leads to physical exhaustion, weakness, decreased working capacity, and visual impairment. Furthermore, the functions of several Internal Organs—the liver, kidneys, cardiovascular, and nervous systems—are disrupted.

The causes of this disease vary. Sometimes the illness develops following psychological trauma, infectious diseases, overeating, or due to tumors and inflammatory processes in the pancreas. Hereditary factors also play a certain role; in more than 40% of diabetic patients, parents or close relatives are similarly affected.

In diabetes, due to impaired glucose utilization and cellular energy deprivation, the demand for alternative Energy Sources increases, thereby intensifying protein and Lipid Metabolism. An elevation in protein-derived Gluconeogenesis and increased urinary nitrogen excretion are observed.

The mobilization of Lipids from fat depots leads to lipemia, and fatty degeneration of the liver may occur. The breakdown of fats is accompanied by the appearance of large quantities of incompletely oxidized products of lipid metabolism, namely Ketone Bodies (β-hydroxybutyric and acetoacetic acids). At a concentration of 20 mmol/L, acidosis occurs, altering blood pH. Acidosis impairs the delivery of glucose to central nervous system cells and reduces oxygen utilization by Brain tissue. As a result of all these changes, a condition known as diabetic coma develops.

Excessive insulin secretion (hyperinsulinism) caused by an adenoma or other pancreatic disorders can lead to hypoglycemia. This condition is the exact opposite of Diabetes Mellitus. Its symptoms include weakness, sweating, hunger, tachycardia, and loss of consciousness. Such a state can also occur as a result of administering excessively high doses of insulin during the Treatment of diabetes mellitus.

Glucagon. In addition to insulin, another hormone—glucagon—is synthesized in the pancreas. Glucagon was obtained in crystalline form in 1953. Subsequently, its chemical nature was investigated, revealing that Glucagon is a low-molecular-weight peptide consisting of 29 amino acid residues and their amides.

Considering its specific action, glucagon is referred to as the hyperglycemic-glycogenolytic factor (HGF). Similar to insulin and other polypeptide hormones, glucagon binds in specific target cells to glucagon-specific receptor systems, leading to the activation of membrane adenylate cyclase and an increase in intracellular cAMP. This subsequently activates protein Kinases and accelerates the phosphorylation of specific proteins. Additionally, glucagon increases the Rate of protein proteolysis in the liver.

In the liver, glucagon inhibits the synthesis of Fatty acids and Cholesterol from acetate, stimulates ketogenesis, activates lipases, and increases the levels of acetyl-CoA and acyl-CoA. In the kidneys, the hormone enhances Glomerular Filtration, accelerates blood flow, and increases The excretion of sodium, chloride, and uric acid ions.

Glucagon causes an increase in blood glucose by stimulating Glycogenolysis exclusively in the liver, unlike adrenaline, which stimulates glycogen breakdown in both The Liver and muscles. This is because glucagon promotes The conversion of inactive liver phosphorylase into its active form in the presence of a specific enzyme that is absent in Skeletal Muscle.

Adrenal hormones

The adrenal glands are situated directly above the kidneys, lying tightly against them. These paired organs weigh 7–12 g. The hormones produced by these glands play a vital role in numerous metabolic processes, ensuring the normal functioning of a wide range of organs and body systems. They are essential for life, and experimental animals typically die within 2–3 days following their removal.

Each Adrenal gland consists of two distinct structures: the medulla and the cortex, with the latter accounting for ²/₃ of the gland's mass. Their embryonic origins, structure, and secreted hormones differ, yet they function in close physiological coordination within the body.

Hormones of The adrenal medulla. The adrenal medulla produces two catecholamines possessing hormonal activity: adrenaline and noradrenaline.

In addition to adrenaline and noradrenaline, the adrenal medulla synthesizes a third biologically active catecholamine as an intermediate product of their Biosynthesis: 3,4-dihydroxyphenylethylamine (dopamine).

Adrenaline and noradrenaline influence several metabolic pathways, particularly those associated with Energy Metabolism. Both catecholamines share the same qualitative biological effects, but noradrenaline acts as a more potent mediator than adrenaline in certain contexts.

An important function of catecholamines is their ability to exert vasopressor effects. Specifically, adrenaline constricts Blood Vessels in the skin and internal organs while dilating coronary Vessels of the Heart; it increases heart rate and myocardial contractility, and relaxes the smooth musculature of the Lungs, Bronchi, and other visceral organs. Noradrenaline exhibits a somewhat weaker effect and, notably, does not promote bronchial muscle relaxation.

Hypofunction of the adrenal medulla is relatively rare. Hyperfunction has been identified in association with chromaffin tissue adenomas, leading to Hypertension, glucosuria, and an elevated basal metabolic rate.

The Hormones of the adrenal cortex form a group known as corticosteroids, which regulate a vast array of metabolic processes in the organism.

THE SPECTRUM OF action of these hormones is exceptionally broad. It is difficult to conceive of any physiological aspect or type of metabolism that is not directly or indirectly influenced by them to some degree. Investigations by the Soviet scientist O. O. Bogomolets and his coworkers were pivotal in studying this group of hormones. They explored the chemical nature and structural characteristics of corticosteroids and successfully isolated the first steroid hormone preparations. Over 60 years ago, Academician O. O. Bogomolets demonstrated that the secretion of the adrenal cortex—a lipid-like substance contained within the glandular cells—possesses hormonal activity. Thus, it was proven for the first time that Adrenocortical Hormones are lipid in nature. Today, the structural and chemical identity of a large family of Steroid Hormones has been elucidated. In total, over 40 Steroid compounds have been isolated from the adrenal cortex. However, only a fraction of these have been studied in detail: corticosterone, 11-dehydrocorticosterone, 17-hydroxycorticosterone, cortisone, hydrocortisone, and aldosterone. The most critical of these compounds, which are continuously secreted into the bloodstream and exert hormonal activity, are hydrocortisone, corticosterone, and aldosterone. These substances are indispensable for life; administering them can prolong the survival of adrenalectomized animals. Other adrenocortical substances have little impact on metabolic processes, and some serve merely as intermediates in corticosteroid synthesis.

Chemically, corticosteroids are structurally related to cholesterol, Bile acids, and vitamin D. They are derivatives of the cyclic hydrocarbon sterane. Based on their chemical structure, adrenocortical hormones are subdivided into several groups. The first group contains 21 carbon atoms and represents derivatives of the hydrocarbon pregnane. This group comprises the classic adrenal hormones.

In addition, Steroids containing 19 carbon atoms, derived from androstane, are also synthesized here.

Histologically, the adrenal cortex is divided into three zones. The outer zone (zona glomerulosa) produces mineralocorticoids—such as aldosterone and 11-deoxycorticosterone—which regulate Water and Electrolyte balance. The middle zone (zona fasciculata) produces glucocorticoids—such as hydrocortisone and corticosterone—which regulate Carbohydrate Metabolism. The inner zone (zona reticularis) produces androgens and estrogens, structurally and functionally akin to Sex Hormones. It should be noted that this functional division of corticosteroids is somewhat relative. For instance, hydrocortisone, which primarily regulates carbohydrate metabolism, may also exhibit Other types of activity to a certain extent; however, its impact on Mineral Metabolism is negligible. Aldosterone predominantly affects mineral metabolism, while its influence on other metabolic pathways amounts to about 1/3 of The activity of hydrocortisone. Corticosterone is less active than both hydrocortisone and aldosterone. In other words, hydrocortisone and aldosterone occupy the opposing extremes in terms of the physiological responses they elicit, whereas corticosterone occupies an intermediate position.

The human body synthesizes 10–30 mg of hydrocortisone (cortisol), 2–4 mg of corticosterone, and 300–400 mcg of aldosterone daily. Consequently, hydrocortisone accounts for 80% of the total corticosteroid pool, serving as the principal hormone of the adrenal cortex.

Even from an incomplete list of the BIOLOGICAL FUNCTIONS OF corticosteroids, it is evident that they profoundly influence Various metabolic pathways—specifically regulating protein, carbohydrate, lipid, water, and electrolyte metabolism—while positively affecting hematopoiesis and enhancing the body's resistance to adverse environmental factors.

Pituitary Hormones

The pituitary gland is one of the most vital endocrine glands, influencing various metabolic processes as well as the Development of Body functions and structure. Given its functional significance, the pituitary is widely regarded as the master center of the endocrine system.

The pituitary gland is ellipsoidal in shape, with a mass of 0.5–0.7 g. Structurally, it comprises three functionally distinct zones: the anterior lobe (adenohypophysis), the intermediate zone (pars intermedia), and the posterior lobe (neurohypophysis). Each of these zones secretes specific hormones that regulate particular aspects of the body's metabolic processes.

Studies of pituitary function revealed that its removal in animals leads to the dysfunction (and eventual atrophy) of several other endocrine glands—such as the thyroid, adrenals, and gonads—along with impaired growth and numerous metabolic disturbances. Conversely, transplanting pituitary tissue into hypophysectomized animals restores these impaired functions. Thus, it was established that the pituitary secretes substances that regulate the functional activity of multiple endocrine glands and influence systemic metabolism.

It was later discovered that pituitary hormones cannot compensate for the systemic alterations that occur following the removal of a specific peripheral endocrine gland whose functional activity depends on the pituitary.

Therefore, the Influence of the pituitary on various metabolic pathways is not direct, but rather mediated through its regulatory control over other endocrine glands. For this reason, pituitary hormones are referred to as tropic hormones (from the Greek tropos, meaning "turn" or "direction").

Chemically, pituitary hormones are Polypeptides or low-molecular-weight proteins. The anterior pituitary synthesizes several key hormones, among which the most important are Growth Hormone (somatotropin, STH), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), gonadotropins (luteinizing hormone, follicle-stimulating hormone, and prolactin), as well as lipotropins.

Somatotropin (growth hormone) was isolated from adenohypophyseal extracts by researchers H. Evans and J. Long in 1921. The hormone is proteinaceous in nature and possesses an oligomeric structure, with a relative molecular mass of 23,000–46,000. Growth hormone consists of two polypeptide chains containing a total of 191 amino acid residues.

A characteristic feature of the growth hormone is its high species Specificity. Although the hormones of certain animal species exhibit biological activity when administered to other animals, only the primate growth hormone is biologically active in humans.

Unlike the action of other adenohypophyseal hormones, the diverse effects of the somatotropic hormone on various metabolic processes in humans and animals are not mediated by its influence on other endocrine glands. Impaired secretion of the somatotropic hormone leads to disruptions in several metabolic processes, notably impairing anabolic processes and growth. Excessive hormone secretion (gland hyperfunction) during childhood, prior to Puberty and the completion of

ossification processes, results in accelerated growth of body organs and tissues, leading to gigantism. In such cases, an individual's height may reach 210–240 cm. Excessive secretion of growth hormone in adulthood leads to a condition known as acromegaly, which arises from pituitary hyperplasia and adenomas. The clinical signs of the disease include disproportionate overgrowth of certain body parts—excessive growth of the BONES OF THE extremities, HEAD, and face, as well as soft tissue enlargement of the Nose, Lips, and chin. Additional manifestations include abnormal body Hair growth, enlargement of internal organs (splanchnomegaly), and hypertrophy of several endocrine glands.

When growth hormone secretion is insufficient (hyposecretion), growth slows down, leading to Pituitary dwarfism. In this condition, height typically does not exceed 90–120 cm, and secondary sexual characteristics are absent. Unlike thyroid-origin dwarfs, pituitary dwarfs show no signs of skeletal deformation or disproportion of body parts, although their head is somewhat enlarged. Their intellectual development is normal and does not differ from that of healthy individuals.

The specific influence of the somatotropic hormone on growth processes indicates that it plays a vital role in supporting anabolic processes, promotes the synthesis of proteins and Nucleic Acids, increases blood glucose levels (diabetogenic effect), enhances the transport of compounds into cells and their assimilation, and stimulates reticulocytosis, chondrogenesis, and osteogenesis.

Growth hormone affects body tissues both directly, by activating membrane-bound adenylate cyclase, and indirectly, via other agents such as somatomedins—Secondary Growth-promoting factors isolated from various organs, including the liver, muscles, kidneys, and blood.

Thyroid-stimulating hormone (TSH) ensures the normal functioning of the thyroid gland. Following hypophysectomy in animals, the thyroid gland undergoes involution with symptoms characteristic of hypothyroidism. Administration of a pituitary extract to such animals promotes reparative processes and the normalization of thyroid function.

Chemically, thyroid-stimulating hormone is a complex protein—a glycoprotein with a relative molecular mass of 28,000. It has an oligomeric structure consisting of two subunits, α and β, linked by non-covalent bonds. The protein moiety contains a high number of Sulfur-Containing Amino Acids that form intrachain Disulfide Bonds, five of which stabilize The structure of the α-subunit and six the β-subunit of the hormone molecule. The sequence of amino acid residues in the α-subunit is identical across all animal species, whereas the primary STRUCTURE OF THE β-subunits varies among species. Therefore, it is believed that the β-subunits are responsible for the specific hormonal activity of TSH and the Regulation of Thyroid functions. Thyroid-stimulating hormone affects metabolic processes within the thyroid tissue. Under the influence of TSH, processes such as the uptake of iodides from the blood, the incorporation of iodine into thyroid hormones, and their release from the gland are stimulated through the activation of follicular proteolytic enzymes that cleave the precursor protein. Furthermore, TSH stimulates the formation of cAMP in thyroid tissue, the synthesis of RNA and proteins, phosphoglycerides, and Sphingolipids, and regulates glucose Transport and Metabolism. The action of TSH on thyroid cells and other target cells begins with the binding of the hormone to specific glycoprotein-natured receptors in the plasma membranes. Sialic acids, which are components of Glycoproteins, are thought to play a crucial role in receptor function. Subsequent processes triggered by hormone-receptor binding are typical of Peptide and Protein hormone action.

Adrenocorticotropic hormone (ACTH) stimulates the growth and functional activity of the adrenal cortex and regulates the Synthesis and Secretion of corticosteroid hormones. Chemically, ACTH is a polypeptide with a relative molecular mass of 20,000, composed of 39 residues of 15 different amino acids.

ACTH is synthesized in the adenohypophysis as a precursor with a significantly higher relative molecular mass, which is converted into the active hormone form through the hydrolytic Cleavage of a portion of the polypeptide chain. It has been found that the first 24 amino acid residues located near the N-terminus of the molecule are identical across all animal species and presumably account for its biological activity. The remaining amino acid residues (25–39) vary among species and determine the species-specific immunological CHARACTERISTICS OF THE hormone. Chemical synthesis has produced a polypeptide containing 24 amino acid residues that exhibits the same biological activity as the natural hormone. Residues of Diaminomonocarboxylic Acids, namely Lysine and Arginine, are of key importance in ensuring biological activity.

The primary role of ACTH is to accelerate the synthesis of corticosteroids from cholesterol in the adrenal cortex. It can act both directly on the tissues of various organs and indirectly via the adrenal cortex.

Insufficient secretion of ACTH manifests all the characteristic symptoms of adrenal cortex hypofunction.

Hypersecretion of ACTH leads to a condition known as pituitary basophilism (Cushing's Disease). This disorder is characterized by hyperglycemia, bone demineralization, facial hirsutism, and inflammation of the Sebaceous Glands.

The Mechanism of ACTH action on adrenal cortex cells is based on a series of interrelated processes that are stimulated within the cells following the binding of ACTH to specific target cell receptors and the activation of membrane-bound adenylate cyclase.

The mammalian neurohypophysis secretes Peptide Hormones—oxytocin and vasopressin. Both hormones are synthesized in the hypothalamus from a single precursor protein (neurophysin) and stored in the neurohypophysis. In most lower vertebrates, a hormone called vasotocin is found, which exhibits The properties of both hormones.

Chemically, oxytocin and vasopressin are nonapeptides with relative molecular masses of 1,082 and 1,007, respectively. They are composed of six amino acid residues and three amides:

As shown in the given formulas, oxytocin and vasopressin have very similar structures. The primary structure of the hormones differs only in minor details: vasopressin lacks isoleucine and leucine, whereas oxytocin lacks phenylalanine and arginine. Recently, A large number of oxytocin and vasopressin analogues have been synthesized, and their biological activity has been studied. The function of neurohypophyseal hormones is crucially important for the organism.

Vasopressin regulates the activity of one of the body's most vital systems—the water balance regulation system.

The neurohypophysis responds quite sensitively to changes in body water content by increasing or decreasing vasopressin secretion. Vasopressin stimulates water reabsorption in the distal renal tubules, hence its name, antidiuretic hormone.

An important function of the hormone is believed to be its activation of the enzyme hyaluronidase, which breaks down the mucopolysaccharide hyaluronic acid, thereby increasing cell membrane permeability and, consequently, enhancing water reabsorption in the renal tubules. Vasopressin does not affect glomerular filtration.

Damage or Atrophy of the neurohypophysis leads to reduced hormone release, resulting in a condition known as diabetes insipidus. Patients experience constant thirst accompanied by a dramatic increase in urine output. In severe cases, a person may drink up to 20 liters of water per day and excrete an equivalent volume of urine (polyuria).

Oxytocin selectively increases the tone of the smooth Muscles of the Uterus and Mammary Glands, promoting stronger contractions. The secretion and activity of oxytocin depend on the functional state of the body; for instance, they decrease significantly during pregnancy and increase markedly during labor. In gynecological practice, the hormone is widely used to stimulate labor contractions.

Oxytocin also promotes the contraction of intestinal and bladder muscles and stimulates milk secretion by activating the adenohypophyseal hormone prolactin. The secretion of neurohypophyseal hormones is influenced by a number of specific factors, primarily the central nervous system and pharmacological agents.

Hypothalamic hormones

The hypothalamus is the primary organ of hormonal regulation and the site of direct interaction between the nervous and endocrine systems. It is located in the subthalamic region of the Diencephalon, which functions simultaneously as a neural center and an endocrine organ. In specific areas of the hypothalamus, neural impulses are transformed into an endocrine process. The neuroendocrine centers of the hypothalamus are directly connected to various brain regions through Direct and Indirect pathways mediated primarily by the Reticular Formation and the Gray matter of the Midbrain. At the same time, the hypothalamus maintains direct connections with another vital

endocrine organ, the pituitary gland, which controls the activity of numerous endocrine glands. Together, the hypothalamus and the pituitary gland form a unified Structural and functional complex that plays a crucial role in regulating many metabolic processes in the body.

The histochemical structure of the hypothalamus is heterogeneous. The magnocellular nuclei of the anterior hypothalamus synthesize the neurohypophyseal hormones oxytocin and vasopressin, which are transported to the neurohypophysis, where they are stored and utilized as needed.

The parvocellular nuclear zone produces a group of adenohypophyseal releasing factors (from English "to release"). These factors play a leading role in regulating the functions of both the adenohypophysis and the neurohypophysis. The hypothalamic nuclei synthesize seven stimulators (releasing hormones or liberins) and three inhibitors (statins) of pituitary hormone secretion. Some of these factors have been studied in detail, their structures established, and their synthesis achieved.

Thyrotropin-releasing hormone (the stimulator of thyrotropin secretion) is a tripeptide composed of pyroglutamic acid, histidine, and Proline residues:

The releasing factor for the lactogenic hormone (prolactin-releasing factor), which is an oligopeptide, and the releasing factor for the somatotropic hormone (growth hormone-releasing hormone), which has a polypeptide nature, have both been studied.

Several statins have also been isolated and studied, including somatostatin (an inhibitor of growth hormone release), prolaktostatin (an inhibitor of prolactin secretion), and others. A characteristic feature of releasing factors is their high specificity in stimulating or inhibiting the release of a specific hormone into the blood.

Plant Hormones

Just as in human and animal organisms, plant organisms exhibit precise Regulation of Metabolic processes at both the intracellular and intercellular levels. Great attention is paid to studying The Nature of the factors that determine Intercellular Communication in plants, because creating an effective system for controlling Plant Growth and Development is one of the most important tasks facing researchers in the field of crop science.

Extensive research into the modes of interaction among plant cells, tissues, and organs, as well as their growth and development processes, has made it possible to establish the Chemical Nature of the signals that drive these processes. These endogenous factors regulating the growth and development of plant organisms primarily include phytohormones—substances synthesized within plant tissues.

In plant organisms, phytohormones perform diverse functions. They regulate growth, development, and the differentiation of organs and tissues, the transition from one developmental phase to another, flower Sex Determination, the processes of flowering and fruiting, and the induction or breaking of dormancy. Hormones regulate all phases of a plant's life cycle. Communication between individual organs and tissues of a plant is carried out through its Circulatory system, which ensures the transport of various substances upward from The ROOT System via the xylem and downward from the leaves via the phloem. In addition, Active Transport occurs across cell membranes against a concentration gradient. Thus, phytohormones synthesized in certain organs and tissues are transported to others, where they direct and stimulate specific metabolic processes, ensuring the functional integrity of the plant organism and coordinating the activity of all its systems. The following groups of substances are classified as phytohormones: Auxins, Gibberellins, Cytokinins, Abscisic acid, and Ethylene.

Auxins (growth stimulators). The discovery of auxins is associated with The Study of phototropism in oat coleoptiles. It was established that emerging plant seedlings always bend toward a light source, bringing their photosynthetic organs closer to it. Furthermore, the time elapsed from the onset of the stimulus to a visible effect is 25–60 minutes. By shading various PARTS OF THE coleoptile, researchers discovered that light sensitivity resides exclusively in its apex (~250 µm). Located just below is the bending zone, where uneven cell growth is observed.

Consequently, the coleoptile tip contains some form of information carrier regarding the direction of light, and the displacement of this carrier determines the phototropic curvature of the coleoptile.

The Role of auxins in phototropism was elucidated in the hormonal theory put forward by the American researcher F. Went. Auxin produced at the coleoptile tip is transported downward, and its concentration is significantly higher on the shaded side of the plant than on the illuminated side. This ensures more intensive growth on the shaded side, causing the plant to turn toward the light source. Phototropic phenomena are particularly striking in plants such as the sunflower.

Auxins are found in the stem tips not only of higher plants but also in Algae, Fungi, and Bacteria. Plants often acquire auxins from their parasites or symbionts. For instance, root nodule formation in legumes occurs under the influence of auxins produced by rhizobia bacteria. Similarly, the formation of various galls and tumors on plants is caused by infection with pathogenic microorganisms that secrete auxins.

Auxins are distributed throughout the plant from their site of synthesis—the stem apex—down to the root growth zone. They act to stimulate stem growth while inhibiting the growth of lateral roots.

When investigating the chemical nature of auxins, researchers discovered analogous substances in urine, feces, and Yeast. It turned out that the active constituent of auxins is indole-3-acetic acid, which is formed through a series of transformations from The amino acid Tryptophan:

Indole-3-acetic acid regulates both Cell Division and cell growth, influencing many processes occurring within the cell and the extracellular environment. In particular, auxins participate in regulating flowering, Fertilization, and SHOOT growth, and they control the abscission of leaves, Ovaries, and fruits. Abscission is associated with the formation of a specialized (abscission) layer in the petioles or pedicels, which secretes enzymes that break down the bonds between individual cells, while the vascular bundles are plugged with polysaccharide plugs. These processes are linked to a decrease in auxin concentration in specific parts of the plant. Introducing exogenous auxins prevents this outcome.

The action of auxins depends on their concentration. Threshold concentrations stimulate growth processes, whereas concentrations above the threshold inhibit them. Therefore, plants possess a regulatory system that controls auxin content and maintains its concentration below the level that exerts an inhibitory effect. This process is accomplished by degrading the auxin or binding it to other molecules to form inactive complexes. In this form, the phytohormone can circulate within the plant without affecting the tissues it remains in constant contact with, effectively creating a storage depot for regulatory substances.

The molecular mechanisms of the biological action of auxins are not yet fully understood, though it is hypothesized that they (similarly to Other Hormones) stimulate transcription processes.

Gibberellins. The discovery of gibberellins is tied to investigations by the Japanese scientist E. Kurosawa into the causes of a rice disease characterized by abnormal, excessive elongation of infected plants. It was discovered that the disease was caused by the pathogenic fungus Gibberella, which secretes BIOLOGICALLY ACTIVE SUBSTANCES into the plant tissues, later named gibberellins. Shortly thereafter, a substance that stimulated plant growth was isolated in crystalline form from this phytopathogenic fungus. Later, gibberellins were extracted from the tissues of many other plants, proving to be normal products of their metabolism.

Chemically, gibberellins are tetracyclic carboxylic acids belonging to the class of Diterpenes—compounds that are quite widespread in nature and also form The basis of camphor, turpentine, and various resins.

Today, more than 50 gibberellins have been isolated from plant tissues. The most common is gibberellin A₃ (gibberellic acid). Like auxins, gibberellins influence plant growth and development processes. Growth stimulation is associated with the acceleration of cell division, although The Effect of cell elongation is also significant. Stem growth, both in herbaceous and woody plants, is particularly intense under the influence of gibberellins. In addition, the action of gibberellins increases the size of leaves, flowers, inflorescences, and fruits. However, most importantly, they ensure flowering and germination processes, meaning the transition of plants from one developmental phase to another.

As early as 1880, the German botanist Julius Sachs, supporting Charles Darwin's hypothesis about growth-stimulating substances, suggested the existence of morphogenetic substances. However, for a long time, this remained unconfirmed. It was only in the 1930s that Soviet physiologists developed the Hormonal theory of flowering. In the 1960s, studying the effect of gibberellins on flowering processes, researchers established that these substances are synthesized in plants under prolonged illumination and lowered temperatures, thereby inducing flowering. Gibberellins are produced in seeds, leaves, apical buds, and roots. Their content in plants ranges from 1·10^-8 to 1·10^-6 % of the dry matter mass. The seeds of growing fruits secrete gibberellins into the fruit body; if the seeds are removed, fruit growth ceases.

Gibberellins act as Chemical factors that stimulate the dormancy break in plants. The mechanism of gibberellins' influence on biochemical processes in plant organisms is not yet fully understood. It is believed that they exert their effects through enzyme systems involved in the hydrolytic breakdown of Reserve Polysaccharides (starch) into Monosaccharides—the primary energy source for cells. At the initial stage of germination, the embryo secretes gibberellins, which induce the synthesis of α-amylase and the hydrolytic breakdown of polysaccharides into monosaccharides necessary for embryonic growth. A single gibberellin molecule induces the formation of 2,000 amylase molecules.

Cytokinins. In the 1930s, a substance that stimulated cell division was isolated from an alcoholic extract of beans. Subsequently, substances with similar properties were extracted from the fruits and seeds of many plants. They were found in particularly high concentrations in corn seeds during the milk-wax ripeness phase. From 70 kg of such seeds, 1 mg of a substance was isolated, which was named zeatin. Given the specific impact of these substances on Cell Growth and Division processes, they were named cytokinins (from cytos—cell, and kinesis—division). The primary function of cytokinins is to ensure mutual influence and connection between the root system and the aerial part of the plant. In 1949, Soviet physiologist D. A. Sabinin expressed the view that plant roots synthesize a phytohormone that regulates the growth of stems and leaves. This hypothesis was based on the coordination between the growth and Development of the above-ground parts of plants and the state of their root system, as well as its ability to supply the aerial parts with necessary substances—a vital condition for normal functioning and survival in adverse environments. Consequently, the roots must possess a regulatory system for shoot growth independent of the plant's nutritional conditions. It turned out that the root system regulates not only shoot growth but also The ratio of actively functioning to dying leaves, as maintaining the proper proportion between them is a key factor in the harmonious Development of Plants. Furthermore, by bridging the root and shoot systems, cytokinins not only regulate nutrient uptake and stimulate Protein Synthesis but also enhance the primary function of green plant parts: Photosynthesis.

With the involvement of cytokinins, Plant resistance to adverse factors—such as dehydration, chemical agents, radiation, infectious diseases, and high temperatures—is enhanced.

The effect of cytokinins on plant organisms is quite diverse. They are particularly active in stimulating cell division in the meristematic zones of stems and in the embryos of germinating seeds. Cytokinins have The ability to overcome apical dominance. Applying cytokinins to the apical bud can transfer primary shoot dominance to one of the lateral shoots or induce the simultaneous growth of several lateral branches. The growth point that receives additional cytokinins begins to grow vigorously and attracts nutrients. The ability of cytokinins to direct nutrient flow to various parts of the plant is exploited by various plant parasites and symbiotic organisms. By synthesizing cytokinins and introducing them into the plant, these organisms ensure an influx of nutrients to their Location.

The most common cytokinins are zeatin, kinetin, and N,N'-diphenylurea.

The mechanism of cytokinins' action on target cells is not yet fully understood. It is believed that they affect cell membranes by altering permeability to various ions, and also enhance synthetic processes within cells as a result of increased nutrient influx. Increased RNA and Protein synthesis elevates the number of Ribosomes, mRNA, and tRNA—thus expanding the synthetic apparatus and its activity, possibly through the activation of the RNA polymerase enzyme.

Ethylene. The Biological Role of ethylene has been studied through research on fruit ripening and leaf abscission. These processes result from physiological changes leading to the Aging of organs and the organism as a whole. They are accompanied by a decrease in cell division and a weakening of metabolic and photosynthetic processes. It has been established that the transition into senescence is a hormonal process beginning in leaves after they reach their maximum size, and in plants as a whole during the formation of reproductive organs. Senescing tissues are particularly sensitive to ethylene. The movement of ethylene from leaves into the petiole serves as a signal for leaf fall. Damage to leaf blades induces active ethylene production and subsequent abscission.

Chemically, ethylene is an unsaturated gaseous hydrocarbon. In its state of aggregation, it differs from other phytohormones. Its synthesis in plants occurs with the participation of the amino acid Methionine:

Aminocyclopropanecarboxylic acid easily cleaves off ethylene and is evidently its transport form within the plant organism. Oxygen is required to convert aminocyclopropanecarboxylic acid into ethylene; therefore, creating anaerobic conditions can delay fruit ripening processes.

Large amounts of ethylene are found in senescing plant organs, pre-abscission leaves, withered flowers, and ripening fruits. The hormonal functions of ethylene are quite diverse. In addition to stimulating senescence, it regulates fruit ripening and color development. Fruit ripening is invariably accompanied by an increase in ethylene concentration. Ethylene can be used to stimulate flowering in certain plants, alter the ratio of staminate and pistillate flowers, and induce seed germination by breaking dormancy.

Abscisic acid. In the 1950s, a substance that inhibited growth, induced leaf fall, and stimulated dormancy was isolated from plant tissue extracts. Given the nature of its action, it was named abscisin (derived from English to mean shedding). Later, the most active of these substances was found to have acidic properties and was designated as abscisic acid (ABA).

The Physiological Role of abscisic acid is multifaceted: it inhibits the sprouting of root crops and various seeds, accelerates leaf abscission and aging, and suppresses photosynthesis. ABA inhibits not only growth but also the Cellular energy supply—specifically, it reduces membrane permeability and the cellular uptake of inorganic phosphate, which hampers the synthesis of high-energy compounds. Abscisic acid also regulates the state of plant Stomata, through which Respiration and Transpiration occur. If a plant loses 10% of its water, the ABA concentration in the leaves increases hundreds of times within a short period, leading to stomatal closure. The inhibition of vital processes by ABA does not negatively impact plants, because its physiological role depends on its concentration in plant tissues. When the concentration drops, its inhibitory effect is lifted. Furthermore, for normal life processes to proceed in plant organisms, a regulatory effect on various metabolic pathways must be ensured. This requires the complex action of different phytohormones, some of which stimulate while others inhibit specific links of metabolism. Thus, during the formation, growth, and ripening of fruits, both simultaneous and sequential actions of various hormones are observed. All known groups of phytohormones participate in the Formation of the ripe fruit. Growth hormones are involved in Ovary formation and fruit body growth, while the action of ethylene and abscisic acid is crucial during fruit ripening and abscission. Other vital processes in plant organisms are similarly regulated by the combined action of various hormones.

The Use of Hormones in Medicine and Agriculture

Nowadays, hormonal preparations are widely used in medicine to normalize metabolic processes in the body. They can be divided into three groups. The first group includes extractive preparations of natural hormones obtained directly from fresh animal endocrine glands. Insulin, adrenaline, estrone, and other hormones are extracted in a pure crystalline state from animal organs. Hormones that cannot be isolated in pure form are used as extracts, which represent mixtures of hormonal substances along with ballast impurities. These include cortin, parathyroidin, pituitrin, as well as the hormonal preparation made from dried thyroid glands—thyroidin.

The second group of hormonal preparations consists of synthetic analogs of steroid and sex hormones, Adrenal Cortex Hormones, etc. With the advent of chemical synthesis for hormonal preparations, the possibilities of hormone therapy have expanded significantly. The Importance of this becomes clear when considering that The amount of hormones in biological material—the animal endocrine glands from which they are isolated—is extremely small. Establishing industrial production of hormonal preparations through chemical synthesis opens up great opportunities for treating many dangerous diseases. Moreover, synthetic hormone analogs often possess much higher biological activity, allowing smaller doses to be used for treatment and Prevention, thereby avoiding unwanted side effects on the body.

The third group of hormonal preparations comprises hormone substitutes that differ in certain structural chemical details but exhibit analogous hormonal action. These include groups of estrogenic, androgenic, corticoid, gonadotropic, and other preparations.

Hormonal preparations are widely utilized to treat endocrine disorders as well as various metabolic disturbances. Hormone therapy is based on exploiting the pharmacodynamic properties of hormonal preparations to influence pathological processes by altering body reactivity, regulating metabolism, and eliminating disease symptoms. Depending on the pathology, compensatory, stimulating, and blocking hormone therapy are applied in treating endocrine diseases.

Compensatory therapy is used in cases of endocrine gland deficiency or partial hypofunction. Diabetes mellitus is treated with insulin, Addison's disease with adrenal cortex extract, and tetany with parathyroid hormone. The administration of hormones in these cases eliminates the disturbances but does not address their root cause—namely, the function of the respective gland—and is therefore temporary in nature.

Stimulating hormone therapy is used to correct gland hypofunction using tropic hormone preparations from the adenohypophysis. Blocking therapy is used in certain endocrine disorders associated with the hyperfunction of specific glands.

Hormonal preparations are used not only to treat endocrine disorders. For instance, insulin is used to treat various toxic conditions, mental illnesses, and internal organ diseases.

Hormonal preparations are employed not just in medicine, but also in livestock farming for the treatment or biostimulation of specific metabolic pathways to enhance animal productivity. Using hormones as feed additives is not always rational, as many hormones lose their biological activity in the process. Mostly, they are administered via injections or through the subcutaneous implantation of endocrine glands. Insulin and growth hormone injections yield a 10–18% weight gain in 5–6-month-old piglets. The administration of thyroid hormone increases wool yield from lambs.

To enhance livestock productivity, synthetic antithyroid drugs (goitrogens) such as thiouracil derivatives, specifically methylthiouracils, are also employed. Applying them in poultry farming at a dose of 0.03 g per 100 g of feed for 14–16 days increases poultry productivity by more than 30 %.

In crop production, phytohormones are widely used—these are substances that regulate the complex physiological processes of plant organisms. The agricultural Application of phytohormones became feasible only after detailed studies established their chemical structure and enabled the synthesis of compounds with analogous effects on plant metabolic processes, thereby making phytohormones broadly accessible for practical use in crop farming.

Among such substances, the most common are stimulants (auxins, gibberellins), growth retardants, herbicides, defoliants, and others. Currently, phenoxyacetic acid derivatives are widely utilized in crop production practices:

Most of these compounds are either structural analogues of phytohormones in terms of their effects on plants or otherwise influence the plant hormonal balance by regulating specific metabolic pathways. Depending on the dosage, these substances can either stimulate or inhibit plant growth processes. In the former case, they act as auxins or gibberellins; in the latter, as herbicides.

Synthetic regulators are applied across various agricultural sectors. Their use in managing plant dormancy is particularly promising. For instance, treating potatoes with solutions of gibberellins and thiourea stimulates tuber sprouting within a few days. The growth regulator ethephon is used for this purpose, as well as for controlling weeds whose seeds can remain dormant in the soil for a long time without germinating, making them insensitive to conventional herbicides. Applying 0.01–1 kg/ha of ethephon to the soil stimulates the germination of weed seeds, which can then be easily eradicated with herbicides.

Plant growth regulators are also employed to increase the yield of various crops. For example, treating grapes with gibberellins (at a dose of 30 g/ha) leads to cluster loosening and increased berry size, resulting in a yield increase of 50–100 %.

One of the most critical challenges in crop production is enhancing the resistance of high-yielding crop varieties to adverse environmental conditions—such as high and low temperatures, or moisture deficits and excesses. Moreover, breeding Methods aimed at increasing productivity typically compromise these tolerance traits. Consequently, modern agriculture makes extensive use of substances with antigibberellin activity known as retardants, which block the gibberellin-mediated system of stem growth and cell elongation. These compounds cause shortening and thickening of plant stems, broadening of leaf blades, intensification of green coloration, and stimulation of the root system without adversely affecting fruiting organs. The most prevalent retardants include nicotine derivatives, quaternary ammonium salts, and succinic acid derivatives. Some of these are used to prevent lodging in cereal crops such as winter wheat and rye, as well as in greenhouse vegetables to prevent excessive elongation under low-light conditions.

Succinic acid dimethylhydrazide is the active ingredient in the foreign retardant "Alar" and the domestic equivalent "Nora". They are used in horticulture to stimulate flower bud formation. Treating apple trees with a 0.2–0.5 % solution of Alar two weeks after blooming accelerates the onset of fruiting in young orchards.

Growth regulators play a vital role in enhancing photosynthetic efficiency. Under normal conditions, only 1 % of the light energy striking a leaf is converted into chemical energy. In field conditions, the efficiency of this process is limited by the plants' capacity to fix carbon dioxide (CO). For example, 1 ha of corn during its active growth period must assimilate 22,000 kg of CO₂ and process 100,000 t of air. To improve this process, it is necessary to modify the canopy architecture of the plants to enhance air Circulation within the crop stand, as well as to regulate the aperture of stomata through which CO₂ enters the leaves.

Defoliants—substances that promote leaf abscission—are widely used in crop production. The most common among them are synthetic diphenylurea derivatives in which phenyl rings are replaced by heterocyclic groups. These are so-called hormonal herbicides, which exhibit regulatory activity at low doses and toxic effects at high doses. For instance, the defoliant thidiazuron is extensively applied to induce leaf fall in cotton plantations. Cotton defoliation is an essential pre-harvest operation to prevent contamination of the valuable raw fiber with foliage.

Crop production also utilizes substances that regulate flowering and fruiting processes. These compounds ensure simultaneous, early fruit ripening, which facilitates mechanical harvesting and prevents crop losses. Ethylene producers are used for this purpose.

Auxins are likewise employed to regulate fruiting. Under unfavorable conditions during the flowering period, tomatoes may drop their blossoms and form small fruits, a phenomenon associated with a deficiency of endogenous auxins. Treating them with auxins, specifically 4-chlorophenoxyacetic acid, promotes better fruit set. For year-round citrus cultivation, fruit is treated with a 0.0008 % solution of 2,4-dichlorophenoxyacetic acid prior to harvest, allowing the fruit to remain green and fresh on the trees for an extended period.

Growth regulators are additionally utilized for storing agricultural produce. In particular, to prevent sprouting in potato storage facilities, the methyl ester of $\alpha$-naphthylacetic acid is applied at a dose of 3 kg per 1 t of potatoes. Maleic hydrazide (MH) also prevents sprouting. To apply this, the above-ground part of the potato plants is treated with the regulator at a dose of 2.5 kg per 1 ha 15 days before harvesting. The regulator penetrates the tubers, accumulates within them, and inhibits subsequent sprouting.

It should be noted that most growth regulators exhibit low toxicity toward humans, animals, and the plants themselves. Thus, modern crop farming possesses a vast array of substances that enable the targeted manipulation of plant metabolic processes in a direction beneficial to humanity.