Chemical Nature of Hormones
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Direct answer
Chemistry predicts how a hormone behaves: NCERT classifies human hormones into four chemical classes, and each class carries its own mechanism. Peptide, polypeptide and protein hormones — insulin, glucagon, the hypothalamic and pituitary hormones — are water-soluble and bind receptors on the cell surface, generating second messengers inside. Steroid hormones — cortisol, aldosterone, testosterone, estradiol, progesterone — derive from cholesterol, slip through cell membranes and act on intracellular receptors, largely regulating gene expression. The iodothyronines, thyroxine (T4) and triiodothyronine (T3), are iodinated amino-acid derivatives that also act through intracellular receptors, and the catecholamines adrenaline and noradrenaline, derived from the amino acid tyrosine, act at surface receptors like the peptide class.
What you must remember
- Four classes, verbatim: (i) peptide, polypeptide, protein hormones; (ii) steroids; (iii) iodothyronines; (iv) amino-acid-derived catecholamines — NCERT's own grouping, so learn it as printed.
- Class membership map: insulin, glucagon, pituitary and hypothalamic hormones = peptides; cortisol, aldosterone, testosterone, estradiol, progesterone = steroids from cholesterol; T3 and T4 = iodothyronines; adrenaline and noradrenaline = catecholamines from tyrosine.
- Mechanism divide: peptide hormones act through membrane receptors and second messengers such as cyclic AMP; steroid hormones bind intracellular (mostly nuclear) receptors and modulate transcription — the pair of statements NEET builds its assertion-reason items on.
- Adrenaline's double life: a catecholamine from the adrenal medulla and the emergency "fight or flight" hormone, increasing heart rate and mobilising glucose — never a steroid, whatever the option implies.
- Pituitary split: anterior lobe secretes FSH, LH, ACTH, TSH, prolactin and growth hormone; posterior lobe releases only oxytocin and vasopressin (ADH), which are peptide hormones made in the hypothalamus.
- Non-endocrine sources: the heart's atria release ANF, the kidney makes erythropoietin, and the GI tract secretes hormones — endocrine signalling outside classic glands.
- Pineal timing: melatonin, from the pineal gland, regulates the 24-hour circadian rhythm and influences pigmentation in lower vertebrates — the biological-clock answer.
Reason from structure to mechanism with two hormones
Take insulin, a small protein. It cannot cross the lipid bilayer, so it docks on a surface receptor; the engaged receptor relays the signal through second messengers that drive GLUT transporters onto the membrane, and glucose entry rises within minutes. Now take cortisol, a cholesterol-derived steroid: lipid-soluble, it diffuses straight through the membrane and finds its receptor in the cytoplasm or nucleus, and the complex switches specific genes on or off — new proteins appear over hours, not minutes. Surface-receptor speed against nuclear-receptor stamina is why stress responses are layered, and why the same mechanism split decides drug delivery: insulin must be injected because digestion would destroy it, a clinical footnote Indian vivas commonly probe.
Thyroxine completes the triangle: synthesised from tyrosine with iodine atoms attached — the reason dietary iodine deficiency devastates the gland, and the reason India iodises salt — it behaves like the steroid class at the receptor level despite its amino-acid ancestry. Adrenaline is the counter-example: also a tyrosine derivative, yet water-soluble and surface-acting, so ancestry alone does not settle the mechanism; the four-way class table does.
Where students lose marks
The adrenaline-steroid confusion is the single most planted error: it is a catecholamine, and questions expecting "steroid hormone of the adrenal medulla" lose the mark to anyone who checks. Second, mechanism wording: steroids regulate gene expression through intracellular receptors; peptides generate second messengers through surface receptors — swap either half and the statement is false. Third, thyroxine's classification: iodothyronine, a distinct NCERT class, not simply "amino acid derivative" when the four-way key is offered. And remember the posterior pituitary secretes peptides, not steroids, since oxytocin and ADH are hypothalamic peptide hormones merely stored below.
Frequently asked questions
Into which chemical classes does NCERT group hormones?
Peptide/protein hormones, steroid hormones, iodothyronines, and amino-acid-derived catecholamines.
How do peptide and steroid hormones differ in action?
Peptide hormones bind cell-surface receptors and act through second messengers, while steroids cross membranes and regulate gene expression via intracellular receptors.
What is the chemical nature of adrenaline and thyroxine?
Adrenaline is a catecholamine derived from tyrosine; thyroxine is an iodinated derivative of tyrosine placed in the separate iodothyronine class.
From what precursor are steroid hormones synthesised?
Cholesterol, in the adrenal cortex and the gonads.
Which hormones does the posterior pituitary release, and of what chemical type?
Oxytocin and vasopressin (ADH) — peptide hormones synthesised in hypothalamic neurons and only stored and released by the posterior pituitary.