Hormones: definition, properties, chemical nature and functions

In this article you will find a detailed explanation of hormones, learning about its and chemical nature of hormones and understanding the differences between hormones and enzymes.


The first hormone was discovered by two English physiologists, William M. Bayliss and Ernest H. Starling in 1902. They discovered secretin hormone which is secreted by intestinal mucosa and stimulating the secretion of pancreatic juice. The term hormone was introduced by Starling in 1905. The word hormone is derived from a Greek word which literally means to stimulate or arouse and was first used in reference to hormone secretin and gastrin.

Hormones are special chemical substances or chemical messengers of the body, which are secreted by endocrine or ductless glands in response to change in the environment inside or outside the body.
According to Thimann and Went, hormones are substances which are produced in one part of an organism and transferred to some other parts where their physiological effects are observed.

Chemically, hormones are polypeptides, steroids and biogenic amines. In simple words we can say hormones are chemical messengers that regulate the biological processes of the organisms.

Properties of hormones

The main properties of hormones are:

  • Hormones have low molecular weight so hormones can easily pass out through the capillaries.
  • Hormones are soluble in water, so hormones can be easily transported with blood stream. Hormones are poured into venous blood.
  • Whenever their function is over, hormones are readily destroyed or inactivated or excreted.
  • Hormones are non antigenic.
  • Hormones are organic catalysts and act as coenzymes of the other enzymes in the body.
  • Hormones always act in very low concentration.
  • Many hormones are produced in an inactive form called prohormone.
  • A hormone in its primary action affects one or a limited number of reaction and does not influence directly all other metabolic activities of the cell.
  • Secretion of the hormone is provoked in response to a given situation and its action is therefore, to fulfill a given task.
  • Hormonal activity is not related to heredity.

Chemical nature of hormones

Chemically hormones may be steroids, proteins, peptides or amino acid derives.

  1. Steroid hormones : The hormones secreted by the testes, adrenal cortex, placenta and ovaries are composed of steroids. E.g. , testosterone, corticosterone, aldosterone, oestrogen and progesterone.
  2. Proteinaceous hormones : Hormones secreted by anterior lobe of the pituitary gland and hormones secreted by the pancreas are composed of proteins.
  3. Peptide hormones : Peptide hormone includes:

    • The hormones secreted by the intermediate lobe of the pituitary gland. Eg. MSH
    • The hormones secreted by posterior lobe of pituitary gland. E.g., vasopressin, ADH and oxytocin.
    • The hormones secreted by the anterior lobe of pituitary gland. E.g., ACTH.
    • The hormones secreted by the parathyroid gland. E.g., parathormone.
    • The hormones secreted by the thyroid gland. E.g., calcitonin.

  4. Amino acid derivative hormones : Amino acid derivative hormones include:

    • The hormones secreted by the thyroid gland. E.g., thyroxine.
    • The hormones secreted by the adrenal medulla. E.g., adrenaline and nonadrenaline.

Differences between hormones and enzymes

The main differences between hormones and enzymes are given below:

  1. Hormones are produced in one part and are transferred to some other parts by blood for action whereas enzymes may act the site of their production or transferred to another site for action.
  2. Hormones may be steroids, proteins, peptides or amino acid derivatives whereas enzymes are simple proteins.
  3. Hormones have low molecular weight whereas enzymes have high molecular weight.
  4. Hormones accelerate or retard the rate of reaction whereas enzymes catalyze the reaction.
  5. Hormonal reactions are irreversible whereas enzymatic reactions are reversible.
  6. Hormones are used up in their reaction whereas enzymes act as catalysts hence not used up in their reactions.

Functions of hormones

The main functions of hormones are as follows:

  1. Metabolism : Some hormones regulate the rate of basal metabolisms. E.g. , thyroxin of thyroid gland.
  2. Homeostasis : Internal environmental factors including temperature regulation, water and ion balance, blood glucose levels, etc are maintained by hormones.
  3. Growth, maturation and regeneration : Hormones control growth by addition of segments. Hormones also control maturation and regeneration in different organisms.
  4. Secondary sexual characters and reproductive activities : Hormones secreted by gonads produce secondary sexual characters and reproductive activities. For example, testosterone in male produces male characters and maturation of sperms. The progesterone controls female secondary sexual characters, maturation of ovum, implantation of fertilized egg, its retention in the uterus and relaxin control the child birth.
  5. Control of other endocrine glands : In some cases hormones secreted by one gland controls the secretary activity of other endocrine glands. For example, trophic hormones of anterior pituitary control the secretion of thyroid, adrenal cortex, gonads, etc.
  6. Adaptations : Adaptation to external factors like visual adaptations to light intensities, control of physiological colour changes are regulated by hormones.

Last words

So, these are detail explanation of hormones. It will be beneficial for the students of every level from high school to college level. If you want some more detail please do not hesitate to ask me.


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