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NCERT's section on the mechanism of hormone action is less than a page long, and the NEET syllabus calls it an "elementary idea". It still produces questions, because it lets examiners test the whole chapter from a different side: not what a hormone does, but how it reaches inside the cell to do it. This post sets out the two routes, tells the story behind the second messenger, and works a sorting question that covers every hormone class.
Where is this in the syllabus?
The NEET (UG) 2026 syllabus lists "Mechanism of hormone action (Elementary Idea)" under Chemical coordination and regulation. The text is section 19.4 of NCERT Class 11 Biology Chapter 19, Chemical Coordination and Integration (2026-27 reprint). Check your own year's syllabus.
What do all hormones have in common?
NCERT's starting point: hormones act by binding specific proteins called hormone receptors, found only in target tissues. Binding forms a hormone-receptor complex, and that complex causes biochemical changes in the target tissue. Each receptor is specific to one hormone. That is why a hormone in the blood reaches every organ but changes only a few.
NCERT then sorts hormones into four chemical groups:
Peptide, polypeptide and protein hormones: insulin, glucagon, pituitary and hypothalamic hormones.
Steroids: cortisol, testosterone, estradiol, progesterone.
Iodothyronines: the thyroid hormones.
Amino-acid derivatives: epinephrine.
Route 1: membrane-bound receptors and second messengers
Hormones that bind receptors on the cell membrane "normally do not enter the target cell". Instead the binding generates second messengers, such as cyclic AMP, IP₃ and Ca²⁺, which regulate the cell's metabolism.
The idea came from Earl Sutherland's work on epinephrine and the liver. The Nobel Prize in Physiology or Medicine 1971 went to him "for his discoveries concerning the mechanisms of the action of hormones". The Karolinska press release sets out the chain: epinephrine attaches to a receptor on the cell surface; this stimulates the enzyme adenyl cyclase, which forms cyclic AMP; cyclic AMP activates phosphorylase, which leads to glucose being formed from glycogen. Sutherland called cyclic AMP "the second messenger", the hormone being the first. The release adds that his idea met strong criticism at first, before proof in the late 1960s showed that many polypeptide hormones act through the cell surface in the same way.
The receptors themselves were mapped later. The Nobel Prize in Chemistry 2012 went to Robert Lefkowitz and Brian Kobilka "for studies of G-protein-coupled receptors". The press release describes these as receptors on cell surfaces and says Lefkowitz's team identified the receptor for adrenalin (epinephrine), the β-adrenergic receptor. You will not be asked about G-proteins from NCERT, but it helps to know the membrane route is a real, mapped pathway.
Route 2: intracellular receptors and genes
Hormones that bind intracellular receptors, mostly in the nucleus, include the steroid hormones and the iodothyronines. NCERT says these mostly regulate gene expression or chromosome function through the interaction of the hormone-receptor complex with the genome.
Worked example: sort eight hormones
For each hormone, give its chemical group and its receptor route.
Insulin: protein. Membrane receptor.
Glucagon: peptide. Membrane receptor.
FSH: pituitary hormone, so protein. Membrane receptor.
Epinephrine: amino-acid derivative. Membrane receptor, with cyclic AMP as the second messenger in Sutherland's liver work.
Cortisol: steroid. Intracellular receptor.
Testosterone: steroid. Intracellular receptor.
Progesterone: steroid. Intracellular receptor.
Thyroxine (T₄): iodothyronine. Intracellular receptor, even though it is not a steroid.
Follow-up: a drug (hypothetical) blocks adenyl cyclase in liver cells. Which hormone's effect on glycogen breakdown, from the list above, does it block? Epinephrine, because in Sutherland's chain adenyl cyclase makes the cyclic AMP that activates phosphorylase. Cortisol is not on this route at all: it acts through a receptor inside the cell.
Which mistakes cost the most marks?
Calling thyroid hormones steroids. They are iodothyronines, a separate group, but they share the intracellular route.
Putting epinephrine with the steroids because it is "not a protein". It is an amino-acid derivative and uses a membrane receptor.
Saying the hormone is the second messenger. The hormone is the first messenger; cyclic AMP, IP₃ or Ca²⁺ is the second.
Saying hormones act on every cell they reach. Only cells with the matching receptor respond.
For the glands and disorders behind these hormones, see our guide to chemical coordination and hormones. For how hormone levels are held steady, read hormonal feedback loops.
A practice routine
Sort from memory: list every hormone in Chapter 19 and mark each M (membrane) or I (intracellular). Check against section 19.4 and the gland sections.
Draw both routes as two simple diagrams, like NCERT's Figure 19.5, without looking.
Write two "what if" questions like the adenyl cyclase one above and answer them.
Two days later, repeat step 1 and compare.
If a pathway feels abstract on the page, the Fermi Concept Library covers every concept in biology, each with its own lesson, so you can revise "second messenger" on its own before mixed questions.
Tonight, write the epinephrine chain from receptor to glucose in five boxes, then check it against this post.
