Describe hormone, its effect on target cell, chemical nature of hormone and feedback system


Hormone is specialized chemical messenger that an endocrime gland produces and secretes. Hormones circulate through body fluids and affect the metabolic activity of a target cell in a specific way. By definition a target cell has receptors to which chemical messengers either relatively bind or on which they have an effect. Only rarely does a hormone operate independently one hormone influences, depends on and balances another hormone in a controlled feedback network.

In the target cell, hormones help, control biochemical reactions in three ways:
(1) A hormone can increase the rate of other substances enter or leave the cell.
(2) It can stimulate a target cell to synthesize enzymes, proteins or other substances.
(3) It can promote a target cell to activate or suppress existing cellular enzymes.

Biochemistry of Hormones:
Most hormones are proteins (Polypeptides), derivates of amino acids (amines) or steroids. Few are fatty acid derivates. For example most invertebrate neuro secretary cells produce polypeptides called neuropeptides.

Hormones that vertebrate pancreas secretes are proteins, those that the thyroid gland secretes are amines. The ovaries, testes and cortex of the adrenal glands secrete steroids.

Hormones are effective in extremely small amount. Only few molecules of a hormone may be enough to produce a dramatic response in a target cell. In target cell hormones help control biochemical reactions in three ways. A hormone can increase the rate at which other substances enter or leave the cell. It can stimulate a target cell to synthesize enzymes, proteins or other substances. It can prompt a target cell to activate or suppress existing cellular enzymes. As is the case for enzyme, hormones are not changed by the reaction they regulate.

Feedback mechanism:
You are already familiar with the concept of feedback mechanism in hoemeostasis. Likewise, hormonal secretion is also subjected to feedback mechanism. In case of any change in chemical information of the body, feedback mechanism comes into action and either blocks or promotes further change. The feedback is said to be negative feedback if further secretion of hormone is inhabited. In positive feedback an increase in concentration of a secreted hormone facilitates the process of its further secretion.

The check and balance mechanism in the body is termed as feedback mechanism. As soon as any change in any substance of internal environment takes place, it is detected usually by special organs called receptors and immediately reported to a control centre (e.g. brain in higher animals) which decides about appropriate response and consequently it sends its instructions accordingly to some other organ called effector which could play its role in bringing the target substance back to normal. It has observed that there could be negative as well as positive feedback.

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