Hormone Action : Mechanism And Receptors

We know the nervous system and endocrine system are the two controlling systems of the body. The endocrine system controls the body with the help of hormones they release.The endocrine glands produce hormones. These endocrine glands release hormones that bind with specific receptors on target cells which further proceed to trigger intracellular signals either by altering membrane permeability, acting through second messenger system or activating specific genes to form new proteins. To better understand the mechanism of hormone action, it is crucial to comprehend various definitions.

Hormone Action

A Hormones is a tiny chemical messenger, traveling along the bloodstream. It exists to maintain internal balance or in other words, keep the human body in a state of homeostasis. This is an extremely rudimentary definition of hormones, as in reality there are numerous hormones that perform an immense number of functions for various purposes.

 Starting with receptors, they are specific for every different hormone. Their sensitivity and responsiveness are dependent on both, the number of receptors and its affinity.  Also, the site of the receptor might be:

  • Present in or on the surface of the cell membrane. g. protein or peptide hormones and catecholamines.
  • In the cytoplasm. E.g. steroid hormones; Or
  • In the cell nucleus. E.g. thyroxine.

Furthermore, hormones can be divided into groups on the basis of their chemical nature:

  • Peptide, polypeptide, protein hormones.
  • Steroids
  • Iodothyronines (thyroid hormone).
  • Amino acid derivatives.

The hormones that interact with membrane-bound receptors generally don’t enter target cells, rather generate second messengers that proceed to regulate cellular metabolism. On the other hand, hormones which interact with intracellular receptors mostly regulate gene expression or chromosome function by the interaction of hormone receptor complex with the genome. The sum total of all the biochemical actions results in physiological and developmental effects.

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Practise This Question

The given diagrams I and II represent the mechanism of action for two categories of hormones. Which of the following options correctly identifies the options A to H?