Учебно-методический комплекс дисциплины " Basis of biochemistry " Для специальности

Hormones Definition

Hormones definition - Hormones are organic chemical substances released by a cell, or organ or a gland or any body part of plants and animals that functions in the regulation of physiological activities and to maintain homeostasis. The chemical discharged from one part affects the cells in other part of the organism. Hormones are released in very minute quantities. Minute quantity of hormones carry out functions evoking responses from the target organs or tissues. The target organs or tissues are adapted to the minute quantities of the hormones. Hormones acts as chemical messengers that transport signal from one cell to the other. 

Hormones are transmitted to their target organs in the blood stream after they are discharged from the glands secreting them. Cells express a specific receptor molecule to the hormone molecules to which they respond. Endocrine secretions is the mode of discharge directly into the bloodstream.

Characteristics of Hormones

General characteristics of hormones are as follows:

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  Hormones are secreted by endocrine cells.
  
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  Hormones are chemical messengers.
  
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  The are chemical signals that circulate in the body fluids.
  
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  The hormones regulate the behavior of the target cells.
  
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  Hormones, unlike enzymes do not catalyze any reaction.
  
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  They are secreted only when needed, they are not stored prior to requirement.
  
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  Hormones may be proteinaceous or non-proteinaceous in nature (amino-acids or steroids).
  
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  The secretion of hormones is regulated by the nervous system through the feed back effect.
  
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  Hormones usually cause long term effects like change in behavior, growth, etc.
  
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  The hormones function is to stimulate or inhibit the target organs.
  

Classification of Hormones

Traditional Classification

Hormones are classified traditionally into three types:

Classical hormones - These hormones are secreted from the endocrine cells into the interstitial fluid. These hormones diffuse into the bloodstream and are distributed to all body parts by the circulatory system.

Neurohormones - These hormones are synthesized by the neuroendocrine cells and are secreted at the nerve terminals. They are transported around the body through the blood vessels, into which they were diffused.

Local hormones - These hormones are secreted into the interstitial fluid and they act locally in two ways. Some hormones act on the neighboring cells and are known as paracrine hormones and some hormones act on the cells from which they were secreted, they are autocrine hormones.

Structural Classification

Hormones are structurally classified into four groups steroids, peptides, amino acids and fatty acids.

Steroid hormones are derived from cholesterol and are soluble in lipids. The steroid hormones include the sex hormones and the hormones produced by the adrenal gland. The sex hormone include androgens, estrogens and progesterone. The adrenal hormones are mineralcorticosteroids and glucocorticosteroids.Steroids hormones are important as they take part in important functions including water balance, sexual development and stress response.

Amino acid derivative hormones - These hormones are derived from amino acids like tyrosine and tyroptophan. Two types of tyrosine derived hormones, they are thyroid hormone and catecholamines.

Thyroid hormone is the most important as it regulates the develpment of organs and metabolism.

Catecholamines - Norepoinephrine and epinephrine are catecholamines. They are stress hormones and ar neurotransmiiters.,

Tryptophan amino acid is the precursor of hormones like serotonin and melatonin. Serotonin regulates the movement of the intestines and is also associated with mood and low levels of this hormones often result in depression.

Peptide hormones - These hormones are derived from peptides. Prohormones are the precursors of for peptide hormones. The prohormones are synthesized by the endoplasmic reticulum. Proper structural configuration is necessary for their functioning. The peptide hormones are stored in the cell vesicle until there is stimuli signals for their release into the blood stream. Examples of peptide hormones are TSH (thyroid stimulating hormone),insulin, prolactin, vassopressin.

Fatty acids derived hormones - Hormones derived from the fatty acids are called eicosanoids, they are derived from arachidonic acid. These hormones are produced by every cell in the body. They have important roles in the body including inflammation, blood pressure and blood clotting.

On the Basis of Mode of Action

Based on the mode of action hormones are classified into quick acting hormones and short acting hormones.

Quick acting hormones - These hormones initiate immediate response from their target cells. These hormones have outer plasma membrane receptors on the target cell, they are large sized. Example: Protein and amine hormones.

Short acting hormones - These hormones initiate a delayed response. These hormones are small in size and they bind to the protein receptors present in the cytosol. Example: steroid hormones of reproductive organs and sdrenal cortex.

Functions of Hormones

Effects of hormones in mammals:

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  They stimulate or inhibit growth.
  
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  Hormones control the wake-up cycle and the circadian rhythms.
  
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  Are responsible for mood swings.
  
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  Induces or suppresses apoptosis.
  
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  Activates or inhibits the immune system.
  
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  Regulates metabolism.
  
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  Prepares body for mating, fleeing, flighting and other activity.
  
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  Also prepares body for new mode of life like puberty, parenting and menopause.
  
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  It controls the activity of the reproductive cycle.
  
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  Controls hunger.
  

Functions of steroid hormones

The male sex hormone - testosterone is produced by the testis and is responsible for male characteristics like deep voice, facial hair during puberty.

Estradiol is the female sex hormone and is responsible for the development of the secondary sexual characteristics in females. It is also participates in control of menstrual cycle.

Progesterone is responsible for preparing the uterus for implantation of the fertilized egg. It also plays a important role as birth control agents.


Hormones of Adrenal Gland

Mineral corticoids are made from different cells of the adrenal cortex. This hormone is concerned with the water-salt balance in the body. It also regulates the NaCl content of blood and causes excretion of potassium in urine.

Glucocorticoids are made by the adrenal cortex, these hormones modify certain metabolic reactions and has and anti-inflammatory effect.

Function of Peptide hormones

Peptide hormones like insulin take part in carbohydrate metabolism. It increases penetrations of cell membranes to facilitate entry of glucose. Hence decreasing the concentration of glucose int eh blood. Insulin is often referred as hypoglycemic factor.

Deficiency of insulin causes diabetes mellitus.

Functions of Amino Acid hormones

Thyroid hormones thyroxin and tri-iodothyroxine, they affect the general metabolism of the body. The thyroid hormones are known as the pace setter of the endocrine system.