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Hormones Definition and Examples

Introduction:

  • Hormones are non-nutrient chemicals that act as intracellular messengers and are produced in trace amounts.


History

  • Term Hormone was coined by Starling.
  • First hormone discovered was Secretin.
  • Study of hormones and hormone secretion glands is called Endocrinology.
  • Father of endocrinology - Thomas Addison.


Properties of hormones

  • Non-nutrient compounds
  • Chemical messengers
  • Released directly into blood
  • Required in small quantities (in traces)
  • Non-antigenic
  • Non-species specific
  • Highly specific for target organs.
  • Do not catalyze any reaction.
  • Their excess or deficiency leads to disorders
  • All neurotransmitters are hormones but vice versa is not true
  • Hormones have a specific physiological action (excitatory or inhibitory). These actions coordinate different physical, mental and metabolic activities and maintain homeostasis.

    target-cell
     
  • There are 3 types of glands present in the body:

i. Exocrine Glands:

  • Duct-containing glands. Their secretions are on the body surface or into body cavities.

ii. Endocrine Glands:

  • Ductless glands. Their secretions are secreted directly into the bloodstream or into the lymphatic system.

iii. Heterocrine glands:

  • Contain both exocrine and endocrine glands, the first secreting through ducts and the second directly into the bloodstream.

Topics covered:

  • Types of hormones
  • Hormone vs Enzyme
  • Hormone vs vitamin

Types of hormones
 

Introduction

  • In the body, hormones are secreted in minute quantities and transported through the blood and lymph to targeted organs and cells.
  • Hormones can work in synergy to control processes, e.g. FSH and LH.
  • The action of two hormones working against one another to control a process is called antagonistic action, eg. Insulin and glucagon and calcitonin and parathormone.
  • Hormone receptors are found either exposed on the surface of the cell or within the cell, depending on the type of hormone.
  • In very basic terms, the binding of hormones to receptors triggers a cascade of reactions within the cell that affects function.
  • Hormone secretion is controlled by feedback mechanisms.

Detailed explanation
 

Classification of hormones:
 

I. On the basis of chemical nature

1. Amino acid derivatives: Adrenaline, noradrenaline, thyroid hormones (iodothyronines)
2. Steroid: These hormones are not water-soluble, for example, Testosterone, Progesterone, Oestrogen and Corticosteroids.
3. Glycoprotein hormones: Gonadotropins i.e., FSH, LH and ICSH.
4. Peptide hormones: Calcitonin, all hypothalamic hormones, Glucagon, insulin, Oxytocin, ADH (Anti Diuretic Hormone), MSH (Melanocyte Stimulating Hormone), ACTH, parathormone.

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II. On the basis of source and target cells:

1. Endocrine hormones:

  • Hormones that are secreted from endocrine glands,and can travel in the blood to act on distant target cells are called circulating hormones or endocrine hormones.

    endocrine-hormones
     

2. Local hormones:

  • Local hormones are signalling molecules that do not circulate within the bloodstream.
  • These hormones are produced by certains cells, and they bind to either neighbouring cells or the source cells themselves.
  • These hormones are rapidly activated and deactivated.
  • Examples: prostaglandins, angiotensin, neurotensin, nitric oxide, kinins, histamine, serotonin etc.
  • They can be classified further as below:

a. Autocrine hormones:

  • These hormones affect the same cells that produced them.

b. Paracrine hormones:

  • These hormones act locally by diffusing from their source to target cells in the neighbourhood.

    paracrine-hormones
     

c. Pheromones:

  • Pheromones are chemicals that act as hormones outside the body of the individual.
  • These affect the behaviour of the individual receiving them.
  • Generally, these are secreted as sexual responses but they can also induce other behavioural changes in the animals.
  • Eg., Bombykol (released by female Bombyx mori or silk worm,to attract males)

Hormone Vs Enzyme
 

  Enzyme Hormone
1 Always proteinaceous Maybe proteinaceous, or steroid.
2 High molecular weight Low molecular weight
3 They may act at the site where they are produced or carried to another site by some duct They are produced at one site and are passed by blood to another site for action
4 Acts slowly, but increases the rate of reaction Accelerate or retard a specific reaction. May act slowly or quick
5 Catalyze reversible reaction Hormone controlled reactions are not reversible
6 Are not used up in reactions Are used up in reaction


Hormone Vs Vitamin
 

  Vitamins Hormone
1 Taken along with food from outside. Rarely synthesized in the body Some are synthesized by endocrine glands, while others by neurosecretory cells.
2 Simple organic compounds like amines, esters, organic acids etc. These are steroids or proteinous or amino acid derivatives.
3 These generally act as coenzymes or constituents of coenzymes for enzyme action. These are excitatory or inhibitory in action. They never act as coenzymes
4 Vitamins have catalytic action. Hormones directly influence gene expressions.
5 Their deficiency (avitaminosis) causes specific deficiency diseases. Their deficiency and overproduction cause metabolic disorders
6 Are not used up in reactions Are used up in reaction


Frequently Asked Questions (FAQs):

Q1. What are hormones ?
Ans:
Hormones are non-nutrient chemicals that act as intracellular messengers and are produced in trace amounts.

Q2. What are pheromones ?
Ans: 
Pheromones are chemicals that act as hormones outside the body of the individual. These affect the behaviour of the individual receiving them. Generally, these are secreted as sexual responses but they can also induce other behavioural changes in the animals. Eg., Bombykol (released by female Bombyx mori or silkworm to attract males).

Q3. Which was the first discovered hormone?
Ans:
Secretin

Q4. What are local hormones ?
Ans: 
Local hormones are signalling molecules that do not circulate within the bloodstream. Examples: prostaglandins, angiotensin, neurotensin, nitric oxide, kinins, histamine, serotonin etc. They can be classified into three categories:

1. Autocrine hormones
2. Paracrine hormones
3. Pheromones

Q5. What are the three types of glands on the basis of ducts?
Ans: 
There are 3 types of glands present in the body:
a. Exocrine glands
b. Endocrine glands
c. Heterocrine glands

Q6. Define heterocrine gland with example?
Ans: 
Heterocrine glands contain both exocrine and endocrine glands, the first secreting through ducts and the second directly into the bloodstream. Example - Pancreas. 

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