CBSE Class 11 Biology VBQs Chemical Coordination and Integration

Question. Read the given statements and state them as true or false.
A. Cretinism can be treated by an early administration of thyroid hormones.
B. Conn’s syndrome occurs due to insufficient production of aldosterone.
C. Gigantism affects optic chiasma and vision.
D. Calcitriol deaccelerates bone formation.
E. Male hypogonadism is due to hypofunction of Leydig’s cells.
      A   B    C   D    E
(a) T    F    T    T    F
(b) T    F    T    F    T
(c) T    F    T    T    T
(d) T    F    F    T    T

Answer. B

Question. A tumour in the adrenal zona glomerulosa can cause hypersecretion of hormones produced in that region. Which of the following might you expect to find in a patient with such a tumour?
(a) Increased blood sodium levels
(b) Increased blood glucose levels
(c) Decreased blood calcium levels
(d) Increased dehydration

Answer. A

Question. Polydipsia and ketosis are the symptoms of
(a) hypoglycemia
(b) Cushing’s syndrome
(c) diabetes insipidus
(d) diabetes mellitus.

Answer. D

Question. Estrogen and testosterone are steroid hormones, it most likely binds to
(a) membrane ion channels
(b) enzyme-linked membrane receptors
(c) G-protein coupled membrane receptors
(d) cytoplasmic receptors.

Answer. D

Question. What is the function of calcitonin?
(a) It increases calcium level in blood.
(b) It decreases calcium level in blood.
(c) It stimulates steroid synthesis.
(d) It increases absorption of water in kidney tubules.

Answer. B

Question. Match column I with column II, and choose the correct combination from the given options.

    Column I                   Column II
A. Goitre                      I. Hyperparathyroidism
B. Child birth              II. Hypothyroidism
C. Demineralisation   III. Aldosterone
D. Salt water balance IV. Oxytocin
(a) A-III, B-I, C-IV, D-II
(b) A-II, B-I, C-IV, D-III
(c) A-I, B-II, C-IV, D-III
(d) A-II, B-IV, C-I, D-III

Answer. D

Question. Which one of the following four glands is correctly matched with its description?
(a) Thyroid - hyperactivity in young children causes cretinism
(b) Angiotensin - stimulates production of aldosterone
(c) Parathyroid - secretes parathormone which promotes movement of calcium ions from blood into bones during calcification
(d) Pancreas - delta cells of the islets of Langerhans secrete a hormone which stimulates glycolysis in liver.

Answer. B

Question. Identify the hormone from the given characteristics.
(i) It is secreted by the mucosa of small intestine.
(ii) It stimulates gall bladder to release bile juice.
(a) Enterogastrone
(b) Pancreatic polypeptide
(c) Cholecystokinin-pancreozymin
(d) Vasoactive intestinal peptide

Answer. C

Question. The posterior pituitary gland is not a ‘true’ endocrine gland because
(a) it is provided with a duct
(b) it only stores and releases the hormones secreted by hypothalamus
(c) it is under the regulation of hypothalamus 
(d) it secretes enzymes.

Answer. B

Question. Which of the following cells does not secrete hormone?
(a) Kupffer cell
(b) Leydig cell
(c) Lutein cell
(d) Parafollicular cells of thyroid

Answer. A

Question. The hormone receptors complex induces
(a) release of GDP from G protein
(b) release of G from GDP
(c) conversion of cAMP into ATP
(d) both (a) and (c).

Answer. A

Question. Which of the following option in given table is correct identification of the structures labelled as A, B, C and D and their corresponding function in the above figure?  

Answer. D

Question. Study the given table and identify A, B, C and D.

(a) A-Melanotrophin, B-Hyposecretion, C-Acromegaly, D-Thyroxine
(b) A-Cortisone, B-Hyposecretion, C-Gigantism,  D-Luteinising hormone
(c) A-Oxytocin, B-Hypersecretion, C-Diabetes mellitus, D-Glucagon
(d) A-Thyroid, B-Hypersecretion, C-Diabetes insipidus, D-Parathormone

Answer. D

Question. Choose the incorrect statement.
(a) The secretion of the posterior lobe of pituitary is known as pituitrin.
(b) High blood level of ADH is characterised by dilute blood and high plasma sodium.
(c) Somatostatin is secreted by delta cells of Langerhans.
(d) Thyrocalcitonin is secreted when calcium level in blood is high.

Answer. B

Case II : Read the following passage and answer the questions from 46 to 50 given below.
Hormones act as a intercellular messengers and produce their physiological effects on target tissues by binding to its specific receptors. Receptors present on cell membrane of target cells are membrane bound receptors whereas nuclear receptor present inside the target cell are called intracellular receptors. For e.g.,
steroid hormone being lipid soluble can easily pass through the cell membrane and enters the nucleus with receptor and insulin being water soluble binds with membrane bound receptor.

Question. Identify the incorrect pair.
(a) Peptide hormone - Insulin
(b) Steroid - Oxytocin
(c) Iodothyronines - Thyroine
(d) Amino-acid derivatives - Epinephrine

Answer. B

Question. Number of cAMP molecules required to activate enzymes protein-kinase A is/are
(a) 1
(b) 4
(c) 2
(d) 3

Answer. C

Question. Action of lipid soluble hormones
(a) is slow
(b) is fast
(c) last long
(d) both (a) and (c).

Answer. D

Question. Which of the following is not a second messenger in hormone action ?
(a) cAMP
(b) cGMP
(c) Calcium
(d) ATP

Answer. D

Question. Which set of the hormones are antagonist to each other ?
(a) Estrogen and progesterone
(b) Insulin and glucagon
(c) Thyroxine and parathormone
(d) Both (b) and (c)

Answer. B

Assertion & Reasoning Based MCQs

For question numbers 51-60, two statements are given-one labelled Assertion and the other labelled Reason.
Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is False but reason is true.

Question. Assertion : Insulin is said to be an anabolic hormone.
Reason : Failure of insulin secretion causes diabetes .

Answer. B

Question. Assertion : Prolactin is also known as ‘Mammotropin hormone’.
Reason : Prolactin stimulates the growth of mammary glands during Pregnancy.

Answer. A

Question. Assertion : Excessive amounts of calcitonin hormone in the blood causes softening of bones.
Reason : Parathormone causes mobilisation of calcium from the bones to the blood plasma.

Answer. D

Question. Assertion : Type I diabetes is caused by destruction of b–cells of Islets of Langerhans.
Reason : Insulin can be taken as injections by Type I patients.

Answer. B

Question. Assertion : The parathyroid glands are said to be under the feedback control of blood calcium levels.
Reason : Parathormone of parathyroids mobilises calcium from the bones to the blood plasma.

Answer. B

Very Short Answer Type Questions

Question. Name any two ovarian hormones.
Answer. Estrogen and relaxin

Question. Name any two hormones which produce second messengers to bring physiological effects in the target tissue.
Answer.  Insulin and follicle stimulating hormone.

Question. What do you mean by hypophysis?
Answer. Pituitary gland is also called hypophysis and it lies in the sella turcica of the sphenoid bone.

Question. What is the source of hormone responsible for darkening of skin colour?
Answer. Pars intermedia or intermediate lobe of pituitary gland.

Question. Fill in the blanks : Hormones Target gland
(a) Corticotrophin (ACTH) .....................
(b) Gonadotrophins (LH, FSH) .....................
(c) Melanotrophin (MSH) .....................
Answer. (a) Adrenal cortex (b) Gonads –Testes in male and ovaries in female (c) Skin

Short Answer Type Questions

Question. (a) What are catecholamines? (b) Where they are found?
Answer. (a) Catecholamines are a group of chemicals which acting as neurotransmitters. It is a amine derivative catechol and include adrenaline, noradrenaline and dopamine. (b) Catecholamines are found within the adrenal medulla of adrenal gland.

Question. Which hormone is also called hormone of maternity? What is it’s source gland? Write it’s main physiological effect.
Answer. Prolactin is also called as hormone of maternity. It is secreted by anterior lobe of pituitary gland. It activates growth of breasts during pregnancy and secretion of mammary glands after child birth.

Question. The given figure shows the diagrammatic representation of the mechanism of protein hormone action.

(a) Name the steps labelled as ‘A’, ‘B’ and ‘C’.
(b) What is the role of second messenger in this process?
(c) How is this mechanism different from the mechanism of action of steroid hormone?
Answer. (a) ‘A’ – Generation of second messenger ‘B’ – Biochemical responses ‘C’ – Physiological responses. (b) Second messenger like cAMP activates the enzyme system of the cell and thus accelerates the metabolic reactions in the cell. (c) Protein hormones interact with membrane bound receptors and generate second messengers which in turn regulate cellular metabolism. Steroid hormones enter the cell and interact with intracellular receptors and regulate gene expression or chromosome function by the interaction of hormone – receptor complex with the genome.

Question. Name three zones of adrenal cortex. Also mention the types of hormones secreted and their affect.
Answer. The adrenal cortex is sub-divided into three zones : (i) Zona glomerulosa - secretes mineralocorticoids that affect mineral homeostasis. (ii) Zona fasciculata - secretes glucocorticoids, that affect glucose homeostasis. (iii) Zona reticularis - secretes gonadocorticoids (e.g., androgens having masculinising effect).

Question. Briefly mention the mechanism of action of FSH.
Answer. FSH being glycoprotein is insoluble in lipids, therefore, cannot enter the target cells. It binds to the specific receptor molecules located on the surface of the cell membrane to form hormone – receptor complex. This complex causes the release of an enzyme adenylate cyclase from the receptor site. This enzyme forms cyclic adenosine monophosphate (cAMP) from ATP in the cytoplasm. The cAMP activates the existing enzyme system of the cell. This accelerates the metabolic reactions in the cell. The hormone is called the first messenger and the cAMP is termed the second messenger. The hormone-receptor complex changes the permeability of the cell membrane to facilitate the passage of materials through it. This increases the activities of the cell as it receives the desired materials.

Question. What is the role of second messenger in hormone action?
Answer. Hormones which interact with membrane - bound receptors normally do not enter the target cell, but generate second messenger (e.g., cAMP) which acts as intracellular hormonal mediator, delivering information inside the target cells. This activates appropriate cellular enzyme system by cascade effect which stimulates the cell machinery to perform its specialised function.

Question. Which cells secrete testosterone? What are its various functions?
Answer.  Testosterone is secreted from endocrine cells called interstitial cells or Leydig’s cells, situated in connective tissue present between the seminiferous tubules in testis. Various functions of testosterone are : (i) It stimulates growth and development of male secondary sex organs like the seminal vesicles, prostate and penis. It also helps to maintain their normal functions. (ii) This hormone also stimulates the development of the male secondary sexual characters like beard, moustache and low-pitch male voice in man. (iii) It also stimulates the formation of sperms in the seminiferous tubules of the testes. (iv) This hormone promotes growth of many body tissues such as bones and muscles. It is due to this fact that male has a higher stature than the female.

Long Answer Type Questions

Question. Describe the mechanism of action of steroid hormones.
Answer. Steroid hormones are lipid-soluble and easily pass through the cell membrane of a target cell into the cytoplasm. In the cytoplasm, they bind to specific intracellular receptors (proteins) to form a hormone receptor complex that enters the nucleus. In the nucleus, hormones which interact with intracellular receptors (e.g., steroid hormones, iodothyronines, etc.) mostly regulate gene expression or chromosome function by the interaction of hormone-receptor complex with the genome. Biochemical actions result in physiological and development effect (tissue growth and differentiation, etc.). Infact, the hormone receptor complex binds to a specific regulatory site on the chromosome and activates certain genes (DNA). The activated gene transcribes mRNA which directs the synthesis of proteins and usually enzymes in the cytoplasm. These enzymes promote the metabolic reactions in the cell. The actions of lipid soluble hormones are slower and last longer than the action of water-soluble hormones. The diagrammatic representation of the mechanism of steroid hormone is given below :

Hormones are non-nutrients chemicals which act as intercellular messengers and are produced in trace amount.
According to definition of hormones, some chemicals also considered as hormones which are not produced by endocrine glands.

HUMAN ENDOCRINE SYSTEM:
The endocrine glands and hormone producing diffuse tissue or cells are located in different part of the body.
Pituitary, pineal, thyroid, adrenal, pancreas, parathyroid thymus and gonads are the organised endocrine bodies in human.
Gastrointestinal tract, kidney, liver and heart also produce hormones.

THE HYPOTHALAMUS:
The hypothalamus, the basal part of diencephalon (fore brain) contain several group of neurosecretory cells called nuclei which produce hormones which regulate the synthesis and secretion of pituitary gland.

Nuclei of hypothalamus secretes two types of hormones -:
1) The releasing hormones: they stimulate secretion of pituitary gland. Example- Gonadotrophin releasing hormone (GnRH) stimulates the pituitary synthesis and release of gonadotrophins
2) The inhibiting hormones: they inhibit secretion of pituitary gland. Example somatostatin from the hypothalamus inhibits the release of growth hormone from the pituitary. -
• These hormones originating in the hypothalamic neurons, pass through axons and are released from their nerve endings. These hormones reach the pituitary gland through a portal circulatory system and regulate the functions of the anterior pituitary.
• Anterior pituitary gland is under the control of secretion of nuclei whereas posterior pituitary is under the direct neural regulation of the hypothalamus.

THE PITUTARY GLAND
• The pituitary gland is located in a bony cavity called Sella tursica and is attached to hypothalamus by a stalk.
• The pituitary hormones regulate the growth and development of somatic tissues and activities of peripheral endocrine glands.
• It is divided anatomically into
• 1) adenohypophysis
• 2) neurohypophysis.
• 1) Adenohypophysis consists of two portions, pars distalis and pars intermedia.

PARS DISTALIS-:

• It is also called anterior pituitary secretes six trophic hormones.

• Growth hormone (GH)-: Over-secretion of GH stimulates abnormal growth of the body leading to gigantism and low secretion of GH results in stunted growth resulting in pituitary dwarfism.

• Prolactin -: regulates the growth of the mammary glands and formation of milk in them.

• TSH (thyroid stimulating hormones)-: stimulates the synthesis and secretion of thyroid hormones from the thyroid gland.

• ACTH (adrenocorticotrophic hormone)-: stimulates the synthesis and secretion of steroid hormones called glucocorticoids from the adrenal cortex.
• luteinizing hormone (LH) and
• follicle stimulating hormone (FSH).
• LH and FSH stimulate gonadal activity and hence are called gonadotrophins.

LH-:
LH stimulates the synthesis and secretion of hormones called androgens from testis in males whereas in females, LH induces ovulation of fully mature follicles (Graafian follicles) and maintains the corpus luteum, formed from the remnants of the Graafian follicles after ovulation.

FSH-:
• In males, FSH and androgens regulate spermatogenesis whereas FSH stimulates growth and development of the ovarian follicles in females.
• Pars intermedia secretes only one hormone called melanocyte stimulating hormone (MSH). MSH (MELANOCYTES STIMULATING HORMONES) acts on the melanocytes (melanin containing cells) and regulates pigmentation of the skin.
• In humans, the pars intermedia is almost merged with pars distalis.
• 2)Neurohypophysis (pars nervosa) also known as posterior pituitary, stores and releases two hormones called oxytocin and vasopressin, which are actually synthesised by the hypothalamus and are transported axonally to neurohypophysis.

Oxytocin:
• It acts on the smooth muscles of our body and stimulates their contraction.
• In females, it stimulates a vigorous contraction of uterus at the time of child birth, and milk ejection from the mammary gland.

Vasopressin:
It acts mainly at the kidney and stimulates resorption of water and electrolytes by the distal tubules and thereby reduces loss of water through urine (diuresis). Hence, it is also called as anti-diuretic hormone (ADH).

The Pineal Gland;
• The pineal gland is located on the dorsal side of forebrain.
• Pineal secretes a hormone called melatonin which plays a very important role in the regulation of a 24-hour (diurnal) rhythm of our body. For example, it helps in maintaining the normal rhythms of sleep-wake cycle, body temperature.
• Melatonin also influences metabolism, pigmentation, the menstrual cycle as well as our defence capability.

Thyroid Gland
• The thyroid gland is composed of two lobes which are located on either side of the trachea.
• Both the lobes are interconnected with a thin flap of connective tissue called isthmus.
• The thyroid gland is composed of follicles and stromal tissues.
• Each thyroid follicle is composed of follicular cells, enclosing a cavity. These follicular cells synthesise two hormones-:
• 1) tetraiodothyronine or thyroxine (T4) and 2) triiodothyronine (T3).
• Iodine is essential for the normal rate of hormone synthesis in the thyroid.
• Deficiency of iodine in our diet results in hypothyroidism and enlargement of the thyroid gland, commonly called goitre.
• Hypothyroidism during pregnancy causes defective development and maturation of the growing baby leading to stunted growth (cretinism), mental retardation, low intelligence quotient, abnormal skin, deaf-mutism, etc.
• In adult women, hypothyroidism may cause menstrual cycle to become irregular.
• Due to cancer of the thyroid gland or due to development of nodules of the thyroid glands, the rate of synthesis and secretion of the thyroid hormones is increased to abnormal high levels leading to a condition called hyperthyroidism which adversely affects the body physiology.
• Thyroid hormones play an important role in the regulation of the basal metabolic rate, support the process of red blood cell formation.
• Thyroid hormones control the metabolism of carbohydrates, proteins and fats.
• Maintenance of water and electrolyte balance is also influenced by thyroid hormones.
• Thyroid gland also secretes a protein hormone called thyrocalcitonin (TCT) which regulates the blood calcium levels.

PARATHYROID GLAND
• In humans, four parathyroid glands are present on the back side of the thyroid gland, one pair each in the two lobes of the thyroid gland.
• The parathyroid glands secrete a peptide hormone à parathyroid hormone (PTH).
• The secretion of PTH is regulated by the circulating levels of calcium ions.
• Parathyroid hormone (PTH) increases the Ca2+ levels in the blood.
• PTH acts on bones and stimulates the process of bone resorption (dissolution/ demineralisation), reabsorption of Ca2+ by the renal tubules and increases Ca2+ absorption from the digested food.
• PTH is a hypercalcaemic hormone, i.e., it increases the blood Ca2+ levels.
• Along with TCT, it plays a significant role in calcium balance in the body.

Thymus:
• The thymus gland is a lobular structure located on the dorsal side of the heart and the aorta.
• Thymus plays a major role in the differentiation of T-lymphocytes, which provide the thymus plays a major role in the development of the immune system. It secretes the peptide hormones called thymosin.
• Thymosin play a major role in the differentiation of T- lymphocytes, which provide cell-mediated immunity.
• Thymosin also promote production of antibodies to provide humoral immunity.
• Thymus is degenerated in old individuals resulting in a decreased production of thymosin. As a result, the immune responses of old persons become weak. Adrenal Gland
• One pair of adrenal glands located one at the anterior part of each kidney.
• The gland is composed of two types of tissues. The centrally located tissue is called the adrenal medulla, and outside this lies the adrenal cortex.

ADRENAL MEDULLA:
• The adrenal medulla secretes two hormones called adrenaline or epinephrine and noradrenaline or norepinephrine. These are commonly called as catecholamines.
• Adrenaline and noradrenaline are rapidly secreted in response to stress of any kind and during emergency situations and are called emergency hormones or hormones of Fight or Flight.
• These hormones increase alertness, pupillary dilation, piloerection (raising of hairs), sweating etc.
• Both the hormones increase the heartbeat, the strength of heart contraction and the rate of respiration.
• Catecholamines also stimulate the breakdown of glycogen resulting in an increased concentration of glucose in blood. In addition, they also stimulate the breakdown of lipids and proteins.

ADRENAL CORTEX:
• The adrenal cortex can be divided into three layers called
• zona reticularis (inner layer), Zona fasciculata (middle layer) ,zona glomerulosa (outer layer).
• The adrenal cortex secretes many hormones, commonly called as corticoids.

• Glucocorticoids: à involved in carbohydrate metabolism & cortisol is the main glucocorticoids.
• Glucocorticoids stimulate, gluconeogenesis, lipolysis and proteolysis; and inhibit cellular uptake and utilisation of amino acids.
• Cortisol is also involved in maintaining the cardio-vascular system as well as the kidney functions. Glucocorticoids, particularly cortisol, produces anti-inflammatory reactions and suppresses the immune response. Cortisol stimulates the RBC production.

• Mineralocorticoids: Corticoids, which regulate the balance of water and electrolytes in our body are called mineralocorticoids. Aldosterone is the main mineralocorticoid in our body.
• Aldosterone acts mainly at the renal tubules and stimulates the reabsorption of Na+ and water and excretion of K+ and phosphate ions. Thus, aldosterone helps in the maintenance of electrolytes, body fluid volume, osmotic pressure and blood pressure.

• ANDROGENIC STEROIDS: Small amounts of androgenic steroids are also secreted by the adrenal cortex which play a role in the growth of axial hair, pubic hair and facial hair during puberty.

Pancreas:
• Pancreas is a composite gland which acts as both exocrine and endocrine gland.
• The endocrine pancreas consists of ‘Islets of Langerhans’.
• There are about 1 to 2 million Islets of Langerhans in a normal human pancreas representing only 1 to 2 per cent of the pancreatic tissue.
• The two main types of cells in the Islet of Langerhans are called α-cells and β-cells.

GLUCAGON:
• It is secreted by alpha cells of
• The α-cells secrete glucagon.
• Glucagon is a peptide hormone and plays an important role in maintaining the normal blood glucose levels.
• Glucagon acts mainly on the liver cells (hepatocytes) and stimulates glycogenolysis resulting in an increased blood sugar (hyperglycemia).
• It stimulates the process of gluconeogenesis which also contributes to hyperglycaemia.
• Glucagon reduces the cellular glucose uptake and utilisation. Thus, glucagon is a hyperglycemic hormone.

• INSULIN:
• The β-cells secrete insulin.
• Insulin is a peptide hormone, which plays a major role in the regulation of glucose homeostasis.
• Insulin acts mainly on hepatocytes and adipocytes (cells of adipose tissue) and enhances cellular glucose uptake and utilisation. As a result, there is a rapid movement of glucose from blood to hepatocytes and adipocytes resulting in decreased blood glucose levels (hypoglycaemia).
• Insulin also stimulates conversion of glucose to glycogen (glycogenesis) in the target cells.
• The glucose homeostasis in blood is thus maintained jointly by the two – insulin and glucagons.
• Prolonged hyperglycaemia leads to a complex disorder called diabetes mellitus which is associated with loss of glucose through urine and formation of harmful compounds known as ketone bodies. Diabetic patients are successfully treated with insulin therapy.

Testis:
• A pair of testis is present in the scrotal sac of male individuals.
• Testis has dual functions as a primary sex organ as well as an endocrine gland.
• Testis is composed of seminiferous tubules and stromal or interstitial tissue.
• The Leydig cells or interstitial cells, which are present in the intertubular spaces produce a group of hormones called androgens mainly testosterone.
• Androgens regulate the development, maturation and functions of the male accessory sex organs like epididymis, vas deferens, seminal vesicles, prostate gland, urethra etc.
• These hormones stimulate muscular growth, growth of facial and axillary hair, aggressiveness, low pitch of voice etc.
• Androgens play a major stimulatory role in the process of spermatogenesis.
• Androgens act on the central neural system and influence the male sexual behaviour (libido).
• These hormones produce anabolic (synthetic) effects on protein and carbohydrate metabolism.

Ovary:
• Females have a pair of ovaries located in the abdomen.
• Ovary is the primary female sex organ which produces one ovum during each menstrual cycle.
• ovary produces two groups of steroid hormones called estrogen and progesterone.
• Ovary is composed of ovarian follicles and stromal tissues.

ESTROGEN:
• The estrogen is synthesised and secreted mainly by the growing ovarian follicles.
• Estrogens produce wide ranging actions such as stimulation of growth and activities of female secondary sex organs, development of growing ovarian follicles, appearance of female secondary sex characters (e.g., high pitch of voice, etc.), mammary gland development.
• It also regulates female sexual behaviour.

PROGESTRON:
• It is secreted by corpus luteum.
• Progesterone supports pregnancy.
• Progesterone also acts on the mammary glands and stimulates the formation of alveoli and milk secretion.

HORMONES OF HEART, KIDNEY AND GASTROINTESTINAL TRACT:
• The atrial wall of our heart secretes a very important peptide hormone called atrial natriuretic factor (ANF), which decreases blood pressure. When blood pressure is increased, ANF is secreted which causes dilation of the blood vessels. This reduces the blood pressure.
• The juxtaglomerular cells of kidney produce a peptide hormone called erythropoietin which stimulates erythropoiesis (formation of RBC).
• Endocrine cells present in different parts of the gastro-intestinal tract secrete four major peptide hormones, namely
• Gastrin: Gastrin acts on the gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen.
• Secretin: Secretin acts on the exocrine pancreas and stimulates secretion of water and bicarbonate ions.
• cholecystokinin (CCK): CCK acts on both pancreas and gall bladder and stimulates the secretion of pancreatic enzymes and bile juice, respectively.
• gastric inhibitory peptide (GIP): GIP inhibits gastric secretion and motility.
• Several other non-endocrine tissues secrete hormones called growth factors. These factors are essential for the normal growth of tissues and their repairing/regeneration.

MECHANISM OF HORMONE ACTION
• Hormones produce their effects on target tissues by binding to specific proteins called hormone receptors located in the target tissues only.
• Hormone receptors present on the cell membrane of the target cells are called membrane-bound receptors and Hormones which interact with membrane-bound receptors normally do not enter the target cell, but generate second messengers (e.g., cyclic AMP, IP3, Ca++ etc) which in turn regulate cellular metabolism.
• The hormone receptors present inside the cell (present in the nucleus): Hormones which interact with intracellular receptors (e.g., steroid hormones, iodothyronines, etc.) mostly regulate gene expression or chromosome function by the interaction of hormone-receptor complex with the genome. Cumulative biochemical actions result in physiological and developmental effects.
• Binding of a hormone to its receptor leads to the formation of a hormone-receptor complex.
• Each receptor is specific to one hormone only and hence receptors are specific.
• Hormone-Receptor complex formation leads to certain biochemical changes in the target tissue.
• Target tissue metabolism and hence physiological functions are regulated by hormones.
• On the basis of their chemical nature, hormones can be divided into groups:
• (i) peptide, polypeptide, protein hormones (e.g., insulin, glucagon, pituitary hormones, hypothalamic hormones, etc.)
• (ii) steroids (e.g., cortisol, testosterone, estradiol and progesterone)
• (iii) iodothyronines (thyroid hormones)
• (iv) amino-acid derivatives (e.g., epinephrine).