Chemical Coordination And Integration Class 11 Biology Chapter 19 Notes

Chemical Coordination and Integration Class 11 Biology Chapter 19 Notes

Endocrine Glands And Hormones

Endocrine glands are a vital component of the human body’s regulatory system.
Unlike exocrine glands, which have ducts to transport their secretions, endocrine glands are ductless.
The secretions of endocrine glands are called hormones.
Hormones are essential chemical messengers that play a crucial role in regulating various physiological processes within the body.
This text explores the fundamental concepts of endocrine glands and hormones, as well as the human endocrine system.

Defining Hormones:

The classical definition of a hormone refers to a chemical produced by endocrine glands.
These chemicals are released directly into the bloodstream, allowing them to travel to specific target organs located some distance away.
A modern scientific definition of hormones is more comprehensive: Hormones are non-nutrient chemicals that act as intercellular messengers.
They are produced in trace amounts and serve to regulate various physiological functions within the body.
This updated definition acknowledges that there are many different types of molecules that function as hormones, extending beyond those produced by the organized endocrine glands.

Variability in Endocrine Systems:

Invertebrates typically have simple endocrine systems that involve only a few hormones.
In contrast, vertebrates, including humans, possess complex endocrine systems where a large number of different chemicals function as hormones.
These hormones work together to provide coordination and maintain homeostasis within the body.

The Human Endocrine System:

The human endocrine system consists of a network of endocrine glands, each producing specific hormones.
These glands include the pituitary gland, thyroid gland, adrenal glands, pancreas, and reproductive glands (testes in males and ovaries in females), among others.
The hormones produced by these glands regulate a wide range of physiological processes, including growth, metabolism, reproduction, and stress response.
The endocrine system plays a critical role in maintaining the body’s internal balance and ensuring that various organs and systems work harmoniously together.

Human Endocrine System

The human endocrine system is a complex network of endocrine glands and hormone-producing tissues/cells dispersed throughout the body.
These components collectively regulate a wide range of physiological processes, ensuring that the body functions smoothly and maintains homeostasis.
The key elements of the endocrine system include organized endocrine glands, such as the pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus, and gonads (testes in males and ovaries in females).
Additionally, other non-glandular organs like the gastrointestinal tract, liver, kidney, and heart also produce hormones.
This text provides an overview of the structure and functions of the major endocrine glands, as well as the role of the hypothalamus, within the human body.

Organized Endocrine Glands:

Pituitary Gland: Often referred to as the “master gland” due to its role in controlling many other endocrine glands. It produces hormones that regulate growth, metabolism, and various other bodily functions.
Pineal Gland: Responsible for the secretion of melatonin, which helps regulate the sleep-wake cycle and influences mood and reproductive function.
Thyroid Gland: Produces hormones, such as thyroxine (T4) and triiodothyronine (T3), which control metabolism and influence the body’s energy levels.
Adrenal Glands: These are located on top of each kidney and produce hormones like cortisol (involved in stress response), adrenaline (fight-or-flight response), and aldosterone (regulates salt and water balance).
Pancreas: Plays a crucial role in regulating blood sugar levels by producing insulin and glucagon. These hormones help maintain glucose homeostasis.
Parathyroid Glands: Regulate calcium and phosphate balance in the body by secreting parathyroid hormone (PTH), which affects bone health and blood calcium levels.
Thymus Gland: It produces hormones involved in the development of the immune system, particularly in childhood.
Gonads (Testes and Ovaries): Testes produce testosterone in males, which influences male sexual characteristics and reproductive functions. Ovaries produce estrogen and progesterone in females, controlling female sexual characteristics and reproductive functions.

Other Hormone-Producing Organs:

Apart from the organized endocrine glands, several non-glandular organs also produce hormones. For example:
- Gastrointestinal Tract: Releases hormones related to digestion, hunger, and satiety.
- Liver: Produces insulin-like growth factor (IGF-1) and angiotensinogen, among others.
- Kidney: Secretes erythropoietin, which stimulates red blood cell production, and renin, which regulates blood pressure.
- Heart: Produces atrial natriuretic peptide (ANP), which influences blood pressure and fluid balance.

The Hypothalamus:

Although not a gland itself, the hypothalamus plays a crucial role in the endocrine system. It acts as a bridge between the nervous and endocrine systems, controlling the release of hormones from the pituitary gland.
The hypothalamus regulates various bodily functions, including body temperature, thirst, hunger, and circadian rhythms.

The Hypothalamus

The hypothalamus is a crucial part of the diencephalon, which is located in the forebrain. It plays a central role in regulating a broad spectrum of bodily functions. Within the hypothalamus, several groups of specialized neurosecretory cells, known as nuclei, are responsible for producing hormones. These hormones have a significant influence on the synthesis and secretion of pituitary gland hormones.

Types of Hypothalamic Hormones:

Releasing Hormones: These hormones stimulate the anterior pituitary gland to synthesize and release its own hormones. For example, Gonadotropin-releasing hormone (GnRH) is a hypothalamic hormone that stimulates the pituitary to produce and release gonadotropins, which regulate the gonads (testes and ovaries).
Inhibiting Hormones: In contrast, inhibiting hormones from the hypothalamus serve to suppress or inhibit the secretion of specific pituitary hormones. For instance, somatostatin, another hypothalamic hormone, inhibits the release of growth hormone from the anterior pituitary.

Hormone Transport from the Hypothalamus to the Pituitary:

The hormones produced by the neurosecretory cells in the hypothalamus travel from their originating neurons through axons. These hormones are then released from the nerve endings into the bloodstream.
These hypothalamic hormones are transported to the pituitary gland via a portal circulatory system, which is a unique blood vessel system connecting the hypothalamus and the anterior pituitary. This system allows the rapid delivery of hypothalamic hormones to their target in the pituitary.

Regulation of the Posterior Pituitary:

Unlike the anterior pituitary, which is regulated by hypothalamic hormones delivered via the portal system, the posterior pituitary is under direct neural regulation by the hypothalamus.
The posterior pituitary stores and releases two hormones, oxytocin and vasopressin (antidiuretic hormone), which are produced in the hypothalamus. When necessary, these hormones are released directly from nerve endings in the posterior pituitary into the bloodstream.

The Pituitary Gland

The pituitary gland, often referred to as the “master gland,” is a critical component of the endocrine system. It is situated within a bony cavity called the sella turcica and is connected to the hypothalamus through a stalk. The pituitary gland is anatomically divided into two distinct parts: the adenohypophysis and the neurohypophysis.

Adenohypophysis (Anterior Pituitary):

Pars Distalis: This region of the anterior pituitary produces several important hormones:
- Growth Hormone (GH): Regulates growth and development.
- Prolactin (PRL): Influences mammary gland growth and milk production.
- Thyroid Stimulating Hormone (TSH): Stimulates the synthesis and release of thyroid hormones from the thyroid gland.
- Adrenocorticotropic Hormone (ACTH): Stimulates the synthesis and secretion of glucocorticoids from the adrenal cortex.
- Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH): Together known as gonadotropins, they regulate gonadal activity, with LH affecting androgen synthesis in males and ovulation and corpus luteum maintenance in females, and FSH stimulating ovarian follicle development in females.
Pars Intermedia: This portion secretes melanocyte-stimulating hormone (MSH). However, in humans, the pars intermedia is often merged with the pars distalis.

Neurohypophysis (Posterior Pituitary):

Also known as the posterior pituitary, it stores and releases two hormones—oxytocin and vasopressin (antidiuretic hormone or ADH).
These hormones are actually synthesized in the hypothalamus and are transported axonally to the neurohypophysis for storage and release.

Functions of Pituitary Hormones:

Over-secretion of GH can lead to gigantism in children and acromegaly (disfiguration, especially of the face) in adults.
Prolactin regulates mammary gland growth and milk production.
TSH stimulates the thyroid gland to produce thyroid hormones.
ACTH stimulates the adrenal cortex to secrete glucocorticoids.
LH and FSH regulate gonadal activity and are involved in reproductive functions.
MSH influences skin pigmentation.
Oxytocin stimulates uterine contractions during childbirth and milk ejection.
Vasopressin, also known as ADH, promotes water and electrolyte resorption in the kidneys, reducing water loss in urine. Impairment of ADH synthesis or release can lead to diabetes insipidus, characterized by excessive urination and dehydration.

The Pineal Gland

The pineal gland is a small, pinecone-shaped gland situated on the dorsal side of the forebrain. It plays a vital role in the endocrine system by secreting a hormone called melatonin. Melatonin is a key regulator of various physiological processes in the body, particularly those related to the diurnal (24-hour) rhythms.

Functions of Melatonin:

Regulation of Sleep-Wake Cycle: Melatonin is crucial in maintaining the normal rhythms of the sleep-wake cycle. It is produced in response to darkness, and its secretion peaks during the night, promoting sleep. In the morning, when light exposure is increased, melatonin levels decrease, signaling wakefulness.
Body Temperature: Melatonin also influences the regulation of body temperature. It helps lower the body’s temperature, which is important for a good night’s sleep.
Metabolism: Melatonin plays a role in regulating metabolism. It can affect metabolic processes, including energy expenditure and the utilization of nutrients.
Pigmentation: While not its primary function, melatonin can have an impact on pigmentation. It can influence skin pigmentation and hair color in some individuals.
Menstrual Cycle: Melatonin has been found to influence the menstrual cycle in women, potentially by interacting with other hormones involved in reproductive processes.
Immune System: There is evidence to suggest that melatonin may influence the body’s defense capability. It can have immunomodulatory effects, helping to regulate immune responses.

Thyroid Gland

The thyroid gland is a butterfly-shaped endocrine gland located in the neck, with two lobes on either side of the trachea. These lobes are connected by a thin strip of connective tissue called the isthmus. The thyroid gland plays a crucial role in the endocrine system and is responsible for producing important hormones.

Structure of the Thyroid Gland:

The thyroid gland is composed of follicles and stromal tissues. Each thyroid follicle is made up of follicular cells that enclose a central cavity.
These follicular cells are responsible for synthesizing two vital hormones: tetraiodothyronine (T4), commonly known as thyroxine, and triiodothyronine (T3). The synthesis of these hormones requires the presence of iodine.

Iodine and Thyroid Hormones:

Iodine is an essential element for the normal synthesis of thyroid hormones in the thyroid gland.
A deficiency of iodine in the diet can lead to hypothyroidism, a condition where the thyroid gland enlarges, causing a condition known as goiter.
Hypothyroidism during pregnancy can result in severe developmental issues in the baby, including stunted growth (cretinism), mental retardation, and other abnormalities.

Hyperthyroidism:

In some cases, the thyroid gland can become overactive, leading to hyperthyroidism. This can occur due to conditions such as thyroid cancer or the development of nodules in the thyroid.
Hyperthyroidism is characterized by the excessive synthesis and secretion of thyroid hormones, resulting in an abnormal increase in hormone levels.
Exopthalmic goiter, a form of hyperthyroidism, is characterized by thyroid gland enlargement, protrusion of the eyeballs, increased basal metabolic rate, weight loss, and other symptoms. This condition is often referred to as Graves’ disease.

Functions of Thyroid Hormones:

Thyroid hormones are essential for regulating the basal metabolic rate, influencing the rate at which the body utilizes energy.
These hormones also support the production of red blood cells and play a role in carbohydrate, protein, and fat metabolism.
Thyroid hormones help maintain water and electrolyte balance within the body.
The thyroid gland also secretes thyrocalcitonin (TCT), a protein hormone that helps regulate blood calcium levels.

Parathyroid Gland

The parathyroid glands are small endocrine glands located on the posterior surface of the thyroid gland. In humans, there are typically four parathyroid glands, with two found in each lobe of the thyroid gland. These glands are essential for maintaining calcium balance in the body and play a critical role in regulating calcium levels.

Parathyroid Hormone (PTH):

The parathyroid glands secrete a peptide hormone called parathyroid hormone (PTH).
The secretion of PTH is tightly regulated by the concentration of calcium ions (Ca2+) in the bloodstream.

Functions of Parathyroid Hormone (PTH):

Increase in Blood Calcium Levels: PTH primarily functions to raise the concentration of calcium ions in the blood, making it a hypercalcemic hormone.
Stimulation of Bone Resorption: PTH acts on bone tissue and stimulates a process known as bone resorption. This involves the dissolution and demineralization of bone tissue, releasing calcium ions into the bloodstream. By doing so, PTH helps elevate the blood calcium levels.
Renal Tubule Reabsorption: PTH also has an impact on the kidneys. It encourages the reabsorption of calcium ions by the renal tubules, preventing excessive loss of calcium in urine.
Enhanced Calcium Absorption: PTH increases the absorption of calcium from the digested food in the intestines. This ensures that more dietary calcium is made available for the body’s use.

Role in Calcium Balance:

Together with the hormone thyrocalcitonin (TCT) produced by the thyroid gland, PTH plays a crucial role in maintaining the delicate balance of calcium within the body.
While PTH increases blood calcium levels, TCT has the opposite effect and lowers them.
This coordinated regulation ensures that calcium concentrations remain within the optimal range, as calcium is essential for various bodily functions, including muscle contractions, nerve signaling, and bone health.

Thymus Gland

The thymus gland is a specialized, lobular structure located in the thoracic cavity, between the lungs and behind the sternum, on the ventral side of the aorta. The thymus is a crucial organ involved in the development of the immune system and plays a significant role in supporting the body’s defense against pathogens.

Thymosins and the Immune System:

The thymus gland secretes a group of peptide hormones known as thymosins. Thymosins are essential for the development and maturation of a specific type of white blood cell called T-lymphocytes or T cells.
T-lymphocytes are integral components of the immune system and are responsible for cell-mediated immunity, which involves the direct destruction of infected or abnormal cells in the body.
In addition to their role in T-cell development, thymosins also promote the production of antibodies, contributing to humoral immunity. Humoral immunity involves the production of antibodies that target pathogens, neutralize toxins, and facilitate their removal from the body.

Aging and the Thymus:

The thymus is particularly active during childhood and adolescence, playing a central role in the development of the immune system during these years.
However, as individuals age, the thymus gradually undergoes a process of degeneration. This results in a decreased production of thymosins and a reduced activity of the thymus.
As a consequence, the immune responses of older individuals become weaker, making them more susceptible to infections and less effective in mounting immune responses.
This age-related decline in thymus function is one of the reasons why the immune system becomes less efficient with age.

Adrenal Gland

The adrenal glands are a pair of endocrine glands located on top of each kidney. They are responsible for producing a variety of hormones that play essential roles in regulating different aspects of the body’s physiology.

Adrenal Gland Structure:

The adrenal gland consists of two main parts: the adrenal medulla and the adrenal cortex.
The adrenal medulla is the central tissue and is responsible for secreting hormones called adrenaline (epinephrine) and noradrenaline (norepinephrine). These hormones are often referred to as catecholamines and are known as “emergency hormones.”
The adrenal cortex, on the outer layer, is further divided into three zones: the zona glomerulosa (outer layer), zona fasciculata (middle layer), and zona reticularis (inner layer). This part of the adrenal gland produces corticoids, including glucocorticoids, mineralocorticoids, and androgenic steroids.

Functions of Adrenal Gland Hormones:

Adrenal Medulla (Catecholamines – Adrenaline and Noradrenaline):

Adrenaline and noradrenaline are rapidly released in response to stress or emergency situations, activating the “fight or flight” response.
These hormones increase alertness, pupil dilation, hair standing on end (piloerection), and sweating.
They enhance heart rate, heart contraction strength, and respiratory rate.
Catecholamines stimulate the breakdown of glycogen, increasing blood glucose levels. They also stimulate the breakdown of lipids and proteins.

Adrenal Cortex (Corticoids):

Glucocorticoids: The primary glucocorticoid is cortisol. These hormones are involved in carbohydrate metabolism. They stimulate gluconeogenesis (the formation of glucose from non-carbohydrate sources), lipolysis (breakdown of fats), and proteolysis (breakdown of proteins). They also inhibit cellular uptake and utilization of amino acids.
Mineralocorticoids: The primary mineralocorticoid is aldosterone. These hormones regulate water and electrolyte balance by promoting the reabsorption of sodium (Na+) and water while excreting potassium (K+) and phosphate ions. This helps maintain electrolyte balance, body fluid volume, osmotic pressure, and blood pressure.
Androgenic Steroids: Small amounts of androgenic steroids are also produced by the adrenal cortex, which contribute to the development of axial hair, pubic hair, and facial hair during puberty.

Role in Inflammation and Immune Response:

Glucocorticoids, particularly cortisol, have anti-inflammatory properties and suppress immune responses. They are commonly used as anti-inflammatory medications in medical treatments.

Pancreas

The pancreas is a multifunctional gland that serves both exocrine and endocrine roles in the body. It plays a critical role in regulating blood glucose levels and is composed of specialized areas known as the “Islets of Langerhans.” These islets are responsible for the secretion of important hormones.

Endocrine Pancreas (Islets of Langerhans):

The endocrine part of the pancreas is comprised of clusters of cells known as the Islets of Langerhans.
There are approximately 1 to 2 million Islets in a normal human pancreas, making up only about 1 to 2 percent of the pancreatic tissue.
The two primary types of cells within the Islets of Langerhans are α-cells and β-cells. These cells are responsible for secreting specific hormones.

Hormones Secreted by Islets of Langerhans:

Glucagon (α-cells):

Glucagon is a peptide hormone secreted by α-cells.
It plays a vital role in maintaining normal blood glucose levels. Glucagon acts predominantly on liver cells (hepatocytes).
Glucagon stimulates glycogenolysis, which is the breakdown of glycogen in the liver, leading to an increase in blood sugar levels (hyperglycemia).
It also promotes gluconeogenesis, the synthesis of glucose from non-carbohydrate sources, contributing to hyperglycemia.
Furthermore, glucagon reduces cellular glucose uptake and utilization, which aids in elevating blood glucose levels. Thus, glucagon is considered a hyperglycemic hormone.

Insulin (β-cells):

Insulin is a peptide hormone secreted by β-cells.
It plays a central role in regulating glucose homeostasis. Insulin acts mainly on hepatocytes (liver cells) and adipocytes (fat cells).
Insulin enhances cellular glucose uptake and utilization, facilitating the rapid movement of glucose from the blood to hepatocytes and adipocytes, resulting in decreased blood glucose levels (hypoglycemia).
Insulin also stimulates the conversion of glucose into glycogen, a process known as glycogenesis.
Together, insulin and glucagon work in a coordinated manner to maintain blood glucose levels within a narrow, healthy range.

Diabetes Mellitus:

Prolonged hyperglycemia, caused by a deficiency of insulin or insensitivity to insulin, leads to a complex disorder known as diabetes mellitus.
Diabetes mellitus is characterized by elevated blood glucose levels, which can result in the loss of glucose through urine and the formation of harmful compounds called ketone bodies.
Diabetic patients can be successfully treated with insulin therapy to help regulate blood glucose levels and manage the condition.

Testis

The testis is a vital organ in the male reproductive system, responsible for both primary sexual functions and endocrine functions. It is located in the scrotal sac outside the abdominal cavity and serves dual roles.

Structure of Testis:

The testis is composed of two main types of tissues: seminiferous tubules and stromal (interstitial) tissue.
The Leydig cells or interstitial cells are found in the intertubular spaces of the testis and are responsible for the production of a group of hormones known as androgens, with testosterone being the primary and most well-known androgen.

Functions of Androgens, Mainly Testosterone

Regulation of Male Accessory Sex Organs: Androgens, particularly testosterone, play a central role in regulating the development, maturation, and functions of the male accessory sex organs. These include the epididymis, vas deferens, seminal vesicles, prostate gland, and urethra.
Secondary Sexual Characteristics: Androgens stimulate the development of secondary sexual characteristics in males. These include the growth of facial and axillary (armpit) hair, an increase in muscle mass, and changes in voice pitch (resulting in a deeper voice). Androgens also play a role in promoting an aggressive attitude in males.
Spermatogenesis: Androgens are crucial for the process of spermatogenesis, which is the formation of spermatozoa (sperm cells). Testosterone, in particular, is necessary for the initiation and maintenance of sperm production in the seminiferous tubules of the testis.
Influence on Male Sexual Behavior: Androgens act on the central nervous system and have a significant impact on male sexual behavior, including libido or sexual desire. They are responsible for driving male sexual characteristics and behaviors.
Metabolic Effects: Androgens have anabolic effects on protein and carbohydrate metabolism. These hormones contribute to muscle growth and repair and can influence energy metabolism and the utilization of nutrients.

Ovary

The ovary is a fundamental organ in the female reproductive system and also functions as an endocrine gland, producing hormones. Women have a pair of ovaries located in the abdominal region.

Structure of Ovary:

The ovary is composed of ovarian follicles and stromal tissues. Ovarian follicles are tiny structures within the ovary, each containing an immature egg (ovum).
The ovary serves two primary functions: the production of ova (eggs) and the synthesis of steroid hormones, primarily estrogen and progesterone.

Hormones Produced by the Ovary:

Estrogen:

Estrogen is one of the main female sex hormones produced by the ovary. It is synthesized and secreted primarily by the growing ovarian follicles.
Estrogen has various functions, including:
- Stimulation of the growth and activity of female secondary sex organs, such as the uterus and vagina.
- Promotion of the development of ovarian follicles.
- Induction of female secondary sexual characteristics, such as changes in voice pitch.
- Mammary gland development.
- Regulation of female sexual behavior.

Progesterone:

Progesterone is another steroid hormone produced by the ovary, but it is primarily secreted by the corpus luteum, a structure that forms after the rupture of an ovarian follicle during ovulation.
Progesterone has several essential roles, such as:
- Supporting pregnancy by preparing the uterus for embryo implantation and maintaining the uterine lining.
- Acting on the mammary glands to stimulate the formation of alveoli, which are sac-like structures responsible for storing milk.
- Promoting milk secretion in preparation for breastfeeding.

Hormones produced by various Non-Endocrine Tissues in the body

Atrial Natriuretic Factor (ANF): Secreted by the atrial wall of the heart, ANF helps decrease blood pressure. When blood pressure is elevated, ANF is released, causing vasodilation, which, in turn, reduces blood pressure.
Erythropoietin: Produced by juxtaglomerular cells in the kidney, erythropoietin is a peptide hormone that stimulates erythropoiesis, the formation of red blood cells (RBCs).
Hormones in the Gastrointestinal Tract:
- Gastrin: Acts on the gastric glands to stimulate the secretion of hydrochloric acid and pepsinogen.
- Secretin: Stimulates the exocrine pancreas, promoting the secretion of water and bicarbonate ions.
- Cholecystokinin (CCK): Acts on both the pancreas and the gallbladder, stimulating the secretion of pancreatic enzymes and bile juice, respectively.
- Gastric Inhibitory Peptide (GIP): Inhibits gastric secretion and motility in the gastrointestinal tract.
Growth Factors: Various non-endocrine tissues secrete growth factors that are essential for normal tissue growth, repair, and regeneration.

These hormones and growth factors play important roles in maintaining homeostasis and regulating various physiological processes in the body.

Mechanism of Hormone Action

Hormones exert their effects on target tissues through a specific mechanism involving hormone receptors. This mechanism allows hormones to regulate various physiological processes in the body. There are different types of hormone receptors, including membrane-bound receptors and intracellular receptors, each playing a distinct role in hormone signaling.

Hormone Receptors:
- Membrane-Bound Receptors: These receptors are located on the cell membrane of target cells. They are involved in the signaling of peptide, polypeptide, and protein hormones. Examples of hormones that interact with membrane-bound receptors include insulin, glucagon, pituitary hormones, and hypothalamic hormones.
- Intracellular Receptors: These receptors are primarily nuclear receptors, located inside the target cell. Steroid hormones (e.g., cortisol, testosterone, estradiol, and progesterone) and iodothyronines (thyroid hormones) primarily interact with intracellular receptors.
Formation of Hormone-Receptor Complex:
- Hormones are specific to their receptors, meaning that each receptor is designed to bind to a particular hormone. When a hormone binds to its receptor, it forms a hormone-receptor complex.
Biochemical Changes in Target Tissue:
- The binding of a hormone to its receptor triggers specific biochemical changes within the target tissue. These changes can vary depending on the type of hormone and the specific receptor involved.
Regulation of Cellular Metabolism:
- Hormone-receptor complex formation regulates target tissue metabolism, ultimately affecting the physiological functions of the tissue. These effects can include changes in enzyme activity, gene expression, and cell signaling pathways.
Groups of Hormones Based on Chemical Nature:
- Hormones can be categorized into different groups based on their chemical nature. These groups include:
  - Peptide, polypeptide, and protein hormones (e.g., insulin, glucagon, and pituitary hormones).
  - Steroids (e.g., cortisol, testosterone, estradiol, and progesterone).
  - Iodothyronines (thyroid hormones).
  - Amino acid derivatives (e.g., epinephrine).
Signaling Pathways:
- Hormones that interact with membrane-bound receptors typically do not enter the target cell. Instead, they activate second messenger systems, such as cyclic AMP (cAMP), inositol trisphosphate (IP3), and calcium ions (Ca++), which regulate cellular metabolism and gene expression.
- Hormones that interact with intracellular receptors, like steroid hormones and thyroid hormones, often regulate gene expression by interacting with the genome.
Physiological and Developmental Effects:
- The cumulative biochemical actions of hormones, whether through membrane-bound or intracellular receptors, result in various physiological and developmental effects in the body. These effects can include changes in growth, metabolism, and overall homeostasis.