11th Class Biology Chemical Coordination (Endocrine System) Hormones And Their Mechanism

Hormones And Their Mechanism

Category : 11th Class

Hormones are informational molecules secreted by the endocrine cells in one part of the body and carried by blood to another part where they stimulate or inhibit specific physiological process.

Discovery : First hormone discovered was secretin. It was discovered by two English physiologists : William M Bayliss and Ernest H. Starling in 1903.

Term hormone was coined by starling (1905) from Greek word Homone means to excite. It is a mishomer because a number of hormones are known to have inhibitory effect (e.g., Somatostatin).

General function of hormones

(1) Some hormones control Basic Metabolic Rate (BMR) e.g., thyroxine of thyroid gland.

(2) Some hormones control the secretion of other endocrine glands, e.g., Tropic hormones of Anterior pituitary control Thyroid, Adrenal cortex, gonads, etc. 

(3) Some hormone control blood pressure e.g., Aldosterone, Atrial Natriuretic Hormone (ANH) of heart, Vasopressin or ADH’, oxytocin and Renin of kidney.

(4) Increase production of RBC e.g., erythropoietin of kidney.

Properties of hormones

(1) These are secreted by endocrine gland (biogenic in origin).

(2) Their secretions is released directly into blood (except local hormones e.g., gastrin).

(3) These are carried to distantly located specific organs, called target organ.

(4) These have specific physiological action (excitatory or inhibitory). These co-ordinate different physical, mental and metabolic activities and maintain homeostasis.

(5) The hormones have low molecular weight e.g., ADH has a molecular weight of \[6002000\] daltons.

(6) These act in very low concentration e.g., around \[{{10}^{10}}\] molar.

(7) Hormones are non antigenic.

(8) These are mostly short-lived. So have a no cumulative effect.

(9) Some hormones are quick acting e.g., adrenalin, while some acting slowly e.g., oestrogen of ovary.

(10) Some hormones secreted in inactive form called Prohormone e.g., Pro-insulin.

(11) Hormones are specific. They are carriers of specific information to their specific target organ. Only those target cell respond to a particular hormone for which they have receptors.

(12) Hormones after their action destroyed in liver and kidney.

Classification of hormones

(1) On the basis of chemical nature : On the basis of chemical composition hormones are classified into three categories.

(i) Amine hormones : These are derived form tyrosine amino acid and have amino \[(-N{{H}_{2}})\] group e.g., Thyroxine, Epinephrine, Nor-epinephrine.

(ii) Steroids : These are fat soluble and have sterol group. These are derived from cholesterol e.g., hormones of adrenal cortex (cortisol, cortisone, corticosterone, aldosterone) testes (testosterone) and ovaries (estrone, estradiol, progesterone etc.)

(iii) Proteinous and peptide hormones : These are formed of \[3200\] amino acids interlinked by peptide bonds and are water soluble e.g.,

(a) Proteinous hormones like STH, TSH, FSH, LH etc. Out of these FSH and LH are glycoproteins.

(b) Long peptide hormones like insulin and glucagon, ACTH, Paratharmone.

(c) Short peptide hormones like oxytocin, ADH, MSH. These hormones formed of a few amino acids.

(2) On the basis of mode of action

(i) Quick acting hormones : These hormones initiate immediate response from their target cells. There receptor is always located on the outer surface of plasma membrane of target cell because these are large sized. Hormone receptor complex activates a membrane enzyme adenyl cyclase which hydrolyse ATP into cyclic AMP. Which acts as secondary messenger, c-AMP activates an inactive enzyme system by cascade effect. So their mode of action is called second messenger hypothesis. e.g., These includes proteinous, peptide and amine hormones.

(ii) Delayed acting hormones : These hormones initiate response after some time. These are small sized so are diffusable through the plasma membrane of their target cell. These bind their proteinous receptor present in the cytosol. These always operate through de-novo synthesis of m-RNA by activation of certain genes. So their mechanism of action is called m-RNA hypothesis. e.g., These include steroid hormones of testes, ovary and adrenal cortex.

 

Difference between hormone and enzymes

S.No.

Characters

Enzymes

Hormones

1.

Chemistry

Always proteinaceous

May be proteinaceous, or amine or steroids.

2.

Molecular weight

Macromolecules with high molecular weights.

Have low molecular weights.

3.

Diffusibility

Non-diffusible through cell membrane.

Diffusible through cell membrane.

4.

Site of action

Either act intracellularly or carried by some duct to another site.

Generally carried by blood to a target organ.

5.

Mode of action

Always act as biocatalysts and increase the rate of metabolic physiological process.

May be excitatory or inhibitatory in their physiological action.

6.

Reversibility

These catalyze reversible reactions.

Hormone controlled reactions are not reversible.

7.

Effect of concentration

Reaction rate increase with increase in their concentration upto a limit.

Deficiency or excess of hormone causes metabolic disorders and diseases.

8.

Speed

Act quickly

Some are quick acting, while some are slow acting with a lag period.

9.

Consumption

Not used in metabolic functions.

Used up in metabolic functions.

 

Difference between hormone and vitamin

S.No.

Characters

Hormones

Vitamins

1.

Source

Synthesized in the endocrine cells of body.

Taken along with food from outside.

2.

Chemistry

Steroids or proteinous or amino acid derivatives.

Simple organic compounds like amines, esters, organic acids etc.

3.

Action

Either excitatory or inhibatory. Do not act as co-enzymes.

These generally act as co-enzymes for enzyme activity.

4.

Cause of disorders

Both excess as well as deficiency of hormones.

Generally avitaminosis (deficiency of vitamins) leads to deficiency diseases.

 

Difference between Nervous and hormonal control

S.No.

Characters

Nervous control

Hormones control

1.

Speed of action

Always quick acting.

May be quick acting or acting with a long period.

2.

Mode of transmission of informations

As electrochemical nerve impulses.

As chemical messengers.

3.

Path of transmission

Through nerve fibres.

Through blood.

4.

Direction of the informations

Towards a specific direction (effector organ or CNS).

Released in general blood circulation from where taken by specific receptor.

5.

Suitability

For quick reactions like reflexes.

For long-term changes e.g. maintenance of pregnancy.

6.

Durability

Short time effect.

Long lasting.

 

Release of hormones : Hormones are released from endocrine glands by three types of stimuli.

(1) Specific metabolic : The presence of a specific metabolite in the blood elicits the hormone to deal with it. For instance excess of glucose in the blood causes the release of insulin from the pancreas and decrease of glucose in blood causes the release of glucagon from pancreas.

(2) Other hormone : The presence of a specific hormone in the blood induces the release of another hormone. For example TSH stimulate thyroid gland to release thyroxine hormone and ACTH stimulate Adrenal cortex to release their hormones.

(3) Neuronal impulse : Neurons of autonomic system stimulate hormone release from some glands. For example adrenaline and nor-adrenaline are released from adrenal medulla on the arrival of nerve impulses during anxiety, stress and danger.

Mechanism of hormone action : The hormones act in two ways -

(1) On cell surface : The molecules of hormones that are amino acid derivatives, peptides or proteins are large and insoluble in lipid, and can not enter the target cell. Therefore they act at the cell surface. They bind to specific receptor molecules located on the surface of cell membrane. The hormone receptor complex may acts in one of the two ways-

(i) Formation of cAMP : Mechanism of formation of cAMP was discovered by E.W. Sutherland in 1950. The hormone receptor complex causes the release of an enzyme adenylcyclase. From the receptor site. This enzyme hydrolise the ATP into c-AMP. The c-AMP activates the existing enzyme system of the cell. This accelerates the metabolic reactions in cell. The hormone is called first messenger and the c-AMP is termed the second messenger. e.g., Adrenaline causes the secretion of glucose from the liver cell from this mechanism.

 

 

(ii) Change in membrane permeability : The receptor proteins of some hormones are large transmembrane intrinsic protein acting as ion channels for facilitated diffusion of \[N{{a}^{+}},{{K}^{+}},C{{a}^{2+}}\] etc. On binding with specific hormone these receptor proteins undergo conformational changes, so that the membrane permeability for ions is altered, resulting into important changes in metabolism.

For example, insulin promotes the entry of glucose from blood into the muscles cells by increasing the permeability of sarcolemma to glucose.

(2) Within a cell : The steroid hormones act within the cell. Their small, lipid soluble molecules pass through the cell membrane and bind to specific receptor molecules present in the cytoplasm. The receptor molecules carry them into the nucleus. Here, the receptor hormone complex binds to a specific receptor site on the chromosome and activates certain genes that were previously repressed. The activated gene transcribe m-RNA which directs the synthesis of enzyme (protein molecule) in the cytoplasm. The enzyme molecule promote the metabolic reactions in the cell.

 

 

Feedback control of hormone secretion : The secretion of hormones is depends on age, daily routine, health of body. Physiological conditions of body etc. Besides the above factors hormone secretion is also depends on its own amount circulating in the blood. Decrease and increase in the circulating amount of a hormone has a directly inverse effect on the secretion of hormone. This is known as the "pull and push" or "feed-back control" mechanism of hormonal secretion.

 

 

(1) Negative feedback control

Direct feedback control : Thyroid stimulating hormone (T.S.H.) stimulates the thyroid gland to secrete thyroxine hormone. A high amount of thyroxine in the blood exerts an inhibitory effect on pituitary to secrete less T.S.H.. This eventually results a decrease in thyroxine. This is called "Direct feedback control".

(2) Positive feedback control : Oxytocin released by posterior pituitary gland stimulate contraction of uterus during child birth. As the contraction of uterus progresses, more and more of oxytocin is released. This is called positive feed back control.

 

Origin of different endocrine glands

Endocrine glands

Weight

Origin

Pituitary

0.5 gm

Ectoderm

Pineal

5.0 mg

Ectoderm

Thymus (up to 12 yrs.)

20.0 gm

Endoderm

Thyroid

25.0 gm

Endoderm

Parathyroid

20.0 mg

Endoderm

Adrenal cortex

4.0 gm

Mesoderm

Adrenal medulla

1.0 gm

Ectoderm

Testes

-

Mesoderm

Ovary

-

Mesoderm

Pancrease

60.0 gm

Endoderm

 

Number of hormones secreted by different endocrine glands

Endocrine-glands

Number of secreted hormones

Pituitary -Anterior

-

7

 

Hypothalamus

-

2

 

Pineal body

-

2

 

Thymus

-

3

 

Thyroid

-

2

 

Parathyroid

-

1

 

Islets of Langerhans

-

3

 

Adrenal cortex

-

46

 

Adrenal medulla

-

2

 

Testes

-

1

 

Ovary

-

3

 

Placenta

-

2

 

Kidneys

-

2

 

Stomach

-

1

 

Duodenum

-

5

 

Ileum

-

2

 

 

Discovery & Terms

(1) Term 'endocrine' was first used by Bernard.

(2) Thomas Addison is called as father of endocrinology.

(3) Walter canon stated that the hormones maintain homeostasis in the body.

(4) Von Euler coined the term 'prostaglandin'

(5) Kendall for the first time prepared the crystals of thyroxine.

(6) Harrington and Barger studied the molecular structure of thyroxine.

(7) Term 'thyroxine' was coined by Whartson.

(8) Sutherland discovered cAMP.

(9) Parathormone was first isolated by Collip.

(10) Potts discovered the structure of PTH.

(11) Axelord studied the structure of epinephrin and nor-epinephrin.

(12) Endocrine structures of the pancreas were discovered by langerhans.

(13) Structure of insulin was studied by Sanger. He was given Nobel prize in 1958. He was rewarded Nobel prize in 1980 for gene structure.

(14) Human insulin was synthesized by Tsan.

(15) Glucagon was discovered by Kimball and Murlin.

(16) Term ' Secretin' was coined by Beylis and Starling.

(17) Adrenal gland was discovered by Eustachian.

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