Mechanism of Hormone Action
ஹார்மோன் செயல்படும் இயக்கமுறை
Steroid hormones cross the membrane and bind receptors inside the cell to switch genes on or off; protein and amine hormones stay outside and use second messengers like cyclic AMP. That one split answers every mechanism question.
The idea
Hormones produce their effects on target tissues by binding to specific proteins called hormone receptors located in the target tissues only. Hormone receptors present on the cell membrane of the target cells are called membrane-bound receptors, and receptors present inside the target cell are called intracellular receptors, mostly nuclear receptors (present in the nucleus). Binding of a hormone to its receptor leads to the formation of a hormone-receptor complex. Each receptor is specific to one hormone only, and hence receptors are specific. Hormone-receptor complex formation leads to certain biochemical changes in the target tissue; target tissue metabolism and hence physiological functions are regulated by hormones. On the basis of their chemical nature, hormones can be divided into groups: (i) peptide, polypeptide and protein hormones (e.g. insulin, glucagon, pituitary hormones, hypothalamic hormones); (ii) steroids (e.g. cortisol, testosterone, estradiol and progesterone); (iii) iodothyronines (thyroid hormones); and (iv) amino-acid derivatives (e.g. epinephrine). Hormones which interact with membrane-bound receptors normally do not enter the target cell, but generate second messengers (e.g. cyclic AMP, IP3, Ca++ etc.) which in turn regulate cellular metabolism. Hormones which interact with intracellular receptors (e.g. steroid hormones, iodothyronines) mostly regulate gene expression or chromosome function by the interaction of the hormone-receptor complex with the genome. Cumulative biochemical actions result in physiological and developmental effects.
Why it matters
Water-soluble hormones cannot pass the lipid membrane, so they signal from outside; lipid-soluble steroids pass through and act directly on genes — two routes for two chemistries.
NEET lens
Chemical classes: peptide/protein (insulin, glucagon, pituitary, hypothalamic), steroids (cortisol, testosterone, estradiol, progesterone), iodothyronines (thyroid), amino-acid derivatives (epinephrine). Membrane-bound receptors → second messengers (cAMP, IP3, Ca2+), hormone does not enter. Intracellular (nuclear) receptors → steroid hormones and iodothyronines → hormone-receptor complex regulates gene expression. 'Steroid hormone influences cellular activities by binding to DNA and forming a gene-hormone complex' — the expected answer.
Clinical significance
Some breast cancers grow in response to estrogen through intracellular receptors; drugs that block these receptors are used in treatment. In type 2 diabetes, cells respond poorly to insulin even though its membrane receptors are present.
Introduction
Hormones act only on target cells that carry specific receptors — either on the cell membrane or inside the cell.
Core concept
Hormone receptors: specific proteins in target tissues only; membrane-bound (on cell membrane) or intracellular (mostly nuclear). Hormone + receptor → hormone-receptor complex → biochemical changes. Chemical nature: (i) peptide, polypeptide, protein — insulin, glucagon, pituitary and hypothalamic hormones; (ii) steroids — cortisol, testosterone, estradiol, progesterone; (iii) iodothyronines — thyroid hormones; (iv) amino-acid derivatives — epinephrine. Membrane-bound receptor hormones: do not enter cell; generate second messengers (cyclic AMP, IP3, Ca2+) → regulate cellular metabolism. Intracellular receptor hormones (steroids, iodothyronines): hormone-receptor complex interacts with the genome → regulates gene expression or chromosome function.
Go deeper
Ecdysone (insect moulting hormone) is a steroid; epinephrine is the amino-acid derived hormone among the usual options.
From Class 10
Hormones travel in blood to reach every cell, yet act only on some. This concept explains why — receptors.