Chemical Coordination and Integration: Class 11 Biology Chapter 19 Hinglish mein

Ek gate ke peeche se ek kutta tum par jhapatta hai. Second ke ek chhote se hisse mein tumhare pair tumhe jhatke se peeche kheench lete hain: yeh kaam fixed pathways par daudte nerve impulses ne kiya. Lekin poore ek minute baad bhi tumhara dil zor se dhadak raha hota hai, hatheliyan geeli hoti hain aur tum ekdum alert mehsoos karte ho, jabki kutta kab ka shaant ho chuka hai. Yeh der tak rehne wali state tumhare blood mein bahne wale chemicals ka kaam hai.

Neural control, jo tumne pichhle chapter mein padha, fast aur precise hai lekin thodi der ka hota hai, aur yeh sirf un cells tak pahunchta hai jinhe koi nerve fibre sach mein supply karta hai. Kai kaamon ke liye kuch aur chahiye: saalon tak chalne wali growth, din-raat steady rehne wala blood glucose level, hafton mein time hone wala menstrual cycle. In sab ke liye body hormones (हॉर्मोन) use karti hai, jo blood mein travel karke lagbhag har cell tak pahunchne wale messengers hain. Dono systems partners hain: neural system aur endocrine system milkar body ki physiology ko coordinate aur regulate karte hain (Fig. 19.1).

19.1 Endocrine glands aur hormones

Tumhari body mein do tarah ki glands hoti hain. Salivary gland ya sweat gland apna product ek tube, yaani duct, mein daalti hai, jo use ek jagah jaise mouth ya skin tak pahunchata hai. Yeh exocrine glands hain. Ek endocrine gland (अंतःस्रावी ग्रंथि) mein koi duct hota hi nahi. Iske cells apna product apne aas-paas ke fluid mein chhodte hain, aur wahan se woh paas ki blood capillaries mein chala jaata hai (Fig. 19.2). Isliye endocrine glands ko ductless glands bhi kehte hain, aur unke secretions hormones hote hain.

Purani textbooks mein hormone ka idea kaafi narrow tha: ek endocrine gland ka banaya chemical, jo blood mein daala jaata hai aur door ke target organ tak le jaaya jaata hai. Yeh classic glands par fit baithta tha. Lekin biologists ko aise messengers milte rahe jo is rule ko todte the: heart ki wall, gut lining aur kidney se aane wale hormones, jinmein se koi bhi gland nahi hai, aur aise signals jo apni banne ki jagah ke paas wale cells par hi kaam karte hain. Isliye ab definition yeh batati hai ki hormone kya karta hai, na ki woh kahan se aata hai.

• Non-nutrient: hormone information hai, khaana ya fuel nahi. Blood ka glucose ek nutrient hai; insulin, jo use control karta hai, nutrient nahi hai.
• Cells ke beech messenger: yeh signal ko banane wale cell se respond karne wale cell tak le jaata hai.
• Trace amounts: bahut chhoti concentrations hi kaafi hain, jaisa Example 19.1 dikhata hai.

Simple animals thode se hormones se kaam chala lete hain; jaise ek insect moulting aur metamorphosis ko control karne ke liye bas kuch hi hormones par depend karta hai. Vertebrates apne kai organ systems ko coordinate karne ke liye hormones ki ek badi range use karte hain. Is chapter ka baaki hissa human endocrine system ke baare mein hai.

19.2 Human endocrine system

Tumhare endocrine system ke do parts hain. Ek hai alag-alag, organised glands ka set: pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas aur gonads (males mein testes, females mein ovaries). Doosra hai organs ke andar chhupa hormone banane wala tissue, jin organs ka main kaam kuch aur hai, jaise gastrointestinal tract, liver, kidney aur heart. Hypothalamus, jo brain ka ek part hai, is system ke bade hisse ko direct karta hai. Fig. 19.3 dikhata hai ki har gland kahan hai, aur Table 19.1 unke hormones ki list deta hai.

19.2.1 Hypothalamus

Pichhle chapter mein tum hypothalamus se body temperature, bhookh aur pyaas ke centre ke roop mein mil chuke ho. Yeh forebrain mein diencephalon ke base par hota hai, aur yeh ek hormone factory bhi hai. Iske neurons ke clusters, yaani neurosecretory cells ya nuclei, hormones secrete karte hain. Inmein se lagbhag sabhi ka target ek hi gland hai, pituitary, aur yeh do mein se ek instruction le jaate hain: releasing hormones pituitary ko secrete karne ke liye kehte hain, aur inhibiting hormones use rukne ke liye kehte hain.

Jaise gonadotrophin releasing hormone (GnRH) pituitary se woh hormones banwata aur release karwata hai jo testes aur ovaries ko chalate hain. Somatostatin, jo hypothalamus se hi aata hai, ulta kaam karta hai: yeh pituitary ke growth hormone release ko kam kar deta hai.

Yeh hormones theek neeche wali pituitary tak kaise pahunchte hain? Fig. 19.4 mein do routes follow karo. Pehle route mein releasing aur inhibiting hormones hypothalamic neurons ke axons mein neeche behte hain aur unke nerve endings se nikalte hain. Wahan woh ek chhoti portal circulation mein jaate hain, yaani aisi vessels jo blood ko ek capillary bed se seedhe anterior pituitary ke doosre capillary bed tak le jaati hain, bina pehle poori body ka chakkar lagaye, isliye hormones concentrated form mein pahunchte hain. Posterior pituitary ki wiring alag hai: hypothalamic neurons apne axons seedhe uske andar bhejte hain, isliye yeh direct neural control mein hai.

19.2.2 Pituitary gland

Pituitary (पीयूष ग्रंथि) lagbhag ek matar ke size ki hoti hai. Yeh ek stalk se hypothalamus se latki rehti hai aur skull ke floor ki ek bony jagah mein tiki hoti hai jise sella turcica kehte hain, jo Latin mein 'Turkish saddle' hai aur iske shape ko describe karta hai. Is gland ke do parts hain jo bilkul alag tarah se kaam karte hain:

• adenohypophysis, yaani glandular part, jo pars distalis (anterior pituitary) aur ek patli pars intermedia se bana hai, jo insaanon mein lagbhag pars distalis mein mil chuki hai;
• neurohypophysis, ya pars nervosa (posterior pituitary), jo asal mein hypothalamus ka hi ek outgrowth hai aur apna koi hormone nahi banata.

Pars distalis chhe hormones secrete karta hai, jinhe aam taur par unke abbreviations se jaana jaata hai: TSH (thyroid stimulating hormone), ACTH (adrenocorticotrophic hormone), GH (growth hormone), PRL (prolactin), FSH (follicle stimulating hormone) aur LH (luteinizing hormone). Pars intermedia sirf ek hormone secrete karta hai, MSH (melanocyte stimulating hormone). Neurohypophysis oxytocin aur vasopressin ko store aur release karta hai, jo hypothalamic neurons mein synthesise hote hain aur unke axons se neeche bheje jaate hain. Table 19.2 aur Fig. 19.5 har hormone ko uske target ke saath jodte hain.

Growth hormone dikhata hai ki imbalance ki timing uske effect ko kaise shape karti hai. Puberty se pehle long bones abhi bhi lambi ho sakti hain; adult mein nahi ho sakti, lekin bone aur soft tissue abhi bhi mote ho sakte hain.

Prolactin mammary glands ko develop karta hai aur unmein milk production chalata hai. TSH aur ACTH trophic hormones hain, jo har ek kisi doosri endocrine gland ko on karte hain: TSH thyroid par kaam karta hai, taaki woh apne hormones banaye aur release kare, aur ACTH adrenal cortex par, taaki woh glucocorticoids naam ke steroids banaye aur release kare. Kyunki inka target gonads hain, LH aur FSH ko saath mein gonadotrophins ke group mein rakha jaata hai.

Male mein LH testes ke liye androgens banane ka signal hai, jabki sperm production (spermatogenesis) ke liye FSH ko un androgens ke saath milkar kaam karna padta hai. Female mein FSH ovarian follicles ki growth aur development chalata hai. Phir LH ek poori tarah mature (Graafian) follicle se uska ovum release karwata hai, jo ovulation hai, aur uske baad us corpus luteum ko banaye rakhta hai jismein khaali follicle badal jaata hai.

Skin ka colour MSH ka kaam hai, jo melanocytes, yaani skin ke melanin wale cells, par kaam karta hai. Oxytocin smooth muscle ke liye contract hone ka signal hai, jo sabse zyada childbirth ke time uterus ke strong contractions mein aur baby ke feed karte waqt mammary glands se milk ke ejection mein dikhta hai. Vasopressin mainly kidney par kaam karta hai: distal tubules mein yeh paani aur electrolytes ka reabsorption badhata hai, isliye urine ke roop mein kam paani nikalta hai. Urine ke zyada flow ko diuresis kehte hain, isliye vasopressin ko antidiuretic hormone (ADH) bhi kehte hain.

Jab body kaafi ADH nahi bana ya release kar paati, to insaan chahe kitna bhi kam piye, urine patla aur zyada hi rehta hai, aur lagaatar pyaas aur dehydration hone lagte hain. Is condition ko diabetes insipidus kehte hain.

19.2.3 Pineal gland

Pineal gland forebrain ki dorsal side par ek chhoti si body hai, aur iska hormone melatonin 24-hour, yaani diurnal, cycle ke liye body ki ghadi ki tarah kaam karta hai. Yeh raat ko tumhe neend laata hai aur body temperature ko roz ke schedule par girne aur badhne mein madad karta hai. Iska asar aur aage tak jaata hai: metabolism, skin pigmentation, menstrual cycle aur body ki immune defences sab is par respond karte hain.

Melatonin ka output roshni aur andhere ke hisaab se chalta hai (Fig. 19.6). Din bhar yeh kam rehta hai; shaam dhalne ke baad badhta hai, raat ke pichhle pehar mein peak par pahunchta hai aur subah ki taraf phir girta hai.

19.2.4 Thyroid gland

Adam's apple ke theek neeche apni gardan chhoo kar nigal kar dekho: thyroid wahin trachea ke saamne hota hai. Iske do lobes hain, trachea ke dono taraf ek-ek, jo saamne se connective tissue ki ek patli band se jude hain jise isthmus kehte hain (Fig. 19.7). Andar se yeh gland kai khokhle, gend jaise follicles se bana hai jo stromal (supporting) tissue mein tike hote hain. Har follicle ki wall ek central cavity ke chaaron taraf follicular cells ka ring hai, aur cavity protein-rich colloid se bhari hoti hai.

Follicular cells do hormones banate hain: tetraiodothyronine, jise thyroxine (${\mathrm{T}}_4$) ke naam se zyada jaana jaata hai, aur triiodothyronine (${\mathrm{T}}_3$). Prefixes iodine atoms ginte hain, ek molecule mein chaar aur doosre mein teen. Isi wajah se thyroid apna normal hormone synthesis tab tak nahi chala sakta jab tak diet se kaafi iodine na mile.

Thyroid hormones lagbhag har tissue par kaam karte hain, body ki chemistry ke accelerator ki tarah. Yeh basal metabolic rate, yaani aaram ki halat mein use hone wali energy, set karte hain, aur iske saath yeh bhi ki carbohydrates, proteins aur fats kitni tezi se process hote hain. Nervous system ko develop aur mature hone ke liye inki zaroorat hoti hai; yeh paani aur electrolytes ke balance par bhi asar daalte hain aur red blood cells banne mein madad karte hain. Iske alawa thyroid ek protein hormone, thyrocalcitonin (TCT), bhi secrete karta hai jo blood calcium ko control mein rakhne mein madad karta hai (Section 19.2.5).

19.2.5 Parathyroid glands

Thyroid ko ghuma kar dekho to uski peeth par chaar chhoti parathyroid glands dikhti hain, har lobe par do (Fig. 19.7 ka aakhri step). Inka peptide hormone parathyroid hormone (PTH) hai. Inhe koi doosri gland on nahi karti: parathyroid cells khud paas se behte blood mein calcium ions ($\mathrm{C}\mathrm{a}{\phantom{X}}^{2+}$) ko sense karke apna output adjust karte hain. PTH teen jagah kaam karke blood calcium badhata hai:

• Bone: yeh bone resorption ko stimulate karta hai, yaani bone mineral ka ghulna (demineralisation), jisse $\mathrm{C}\mathrm{a}{\phantom{X}}^{2+}$ blood mein aata hai.
• Kidney: yeh renal tubules se zyada $\mathrm{C}\mathrm{a}{\phantom{X}}^{2+}$ reabsorb karwata hai, isliye urine mein kam nikalta hai.
• Intestine: yeh digest hue khaane se $\mathrm{C}\mathrm{a}{\phantom{X}}^{2+}$ ka absorption badhata hai.

Teenon actions blood calcium ko upar le jaate hain, isliye PTH ek hypercalcemic hormone hai. Thyrocalcitonin ulti disha mein kaam karke ise kam karta hai. Steering wheel par do haathon ki tarah, PTH aur TCT blood calcium ko ek narrow range mein rakhte hain (Fig. 19.8), jo zaroori hai kyunki nerves, muscles (heart bhi) aur blood clotting sab calcium par depend karte hain.

19.2.6 Thymus

Tumhari chhati ki haddi (sternum) ke peeche, dono lungs ke beech aorta ki ventral side par, ek naram, lobed gland hoti hai: thymus. Ise immune system ka training school samjho. Iske release kiye peptide hormones, thymosins, young T-lymphocytes ko guide karte hain jab woh cell-mediated immunity ke zimmedaar cells mein differentiate hote hain, aur yeh antibody production bhi badhate hain, jo humoral immunity ka base hai.

Yeh school umar ke saath chhota hota jaata hai. Buzurg logon mein thymus kaafi had tak degenerate ho chuka hota hai aur bahut kam thymosin banata hai, isliye unki immune defences kamzor hoti hain; yeh ek wajah hai ki jo infection ek teenager aasaani se jhel leta hai woh dada-dadi ke liye khatarnaak ho sakta hai. T-cells aur antibodies se tum Class 12 mein human health and disease wale chapter mein phir miloge.

19.2.7 Adrenal glands

Tumhare paas do adrenal glands (अधिवृक्क ग्रंथि) hain, har ek kidney ke upar topi ki tarah baithi hoti hai. Ek ko kaat kar dekho to do alag tissues dikhte hain: beech mein adrenal medulla jo bahar ke adrenal cortex mein lipta hota hai (Fig. 19.9). Dono bilkul alag hormones banate hain, isliye inhe ek package mein do glands samajhna aasaan hai.

Medulla: emergency hormones. Kisi bhi tarah ke stress ya emergency mein medulla tezi se adrenaline (epinephrine) aur noradrenaline (norepinephrine) chhodta hai, jinhe catecholamines ke group mein rakha jaata hai. Kyunki yeh body ko ladne ya bhaagne ke liye taiyaar karte hain, inhe emergency hormones, ya 'fight or flight' hormones kehte hain. Gate wale kutte ko yaad karo:

• tumhara dil tez dhadakta hai aur har beat zyada zordaar hoti hai, aur saans tez ho jaati hai;
• fuel mobilise hota hai: glycogen toot-ta hai isliye blood glucose badhta hai, aur lipids aur proteins bhi toot-te hain;
• tum alert mehsoos karte ho, pupils chaude ho jaate hain (pupillary dilation), paseena aata hai, aur skin ke baal khade ho jaate hain (piloerection).

Cortex: teen zones, kai corticoids. Bahar se andar, cortex ki layers hain zona glomerulosa, zona fasciculata aur zona reticularis. Yeh milkar kai steroid hormones, yaani corticoids, release karte hain, jo do main families ke hain. Glucocorticoids, jinmein insaanon mein sabse aage cortisol hai, carbohydrate metabolism sambhalte hain. Mineralocorticoids, jinmein sabse aage aldosterone hai, body ke paani aur electrolytes ko manage karte hain.

Doctors severe allergies aur inflammation ke liye synthetic glucocorticoids dete hain, aur iski wajah woh hai jo cortisol naturally karta hai: yeh inflammation ko kam karta hai aur immune response ko dabata hai. Iske metabolic effects kaafi broad hain. Yeh gluconeogenesis (non-carbohydrate sources se naya glucose), lipolysis (fat ka tootna) aur proteolysis (protein ka tootna) chalata hai, aur saath hi cells dwara amino acids ke uptake aur use ko rokta hai. Yeh cardiovascular system aur kidney function ko banaye rakhne mein bhi madad karta hai, aur red blood cell production ko stimulate karta hai.

Aldosterone mainly renal tubules par kaam karta hai. Yeh $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$ aur paani ka reabsorption aur $\mathrm{K}{\phantom{X}}^{+}$ aur phosphate ions ka excretion badhata hai, aur is tarah blood pressure, body fluids ka volume, unka osmotic pressure aur electrolyte balance steady rakhta hai.

Cortex thodi matra mein androgenic steroids bhi release karta hai, jo puberty ke dauraan armpits, pubic region aur chehre par baal ugne mein madad karte hain. Agar cortex bahut kam hormone banata hai, to carbohydrate metabolism bigad jaata hai aur insaan ko tez kamzori aur thakaan hoti hai: yeh Addison's disease hai.

19.2.8 Pancreas

Pancreas (अग्न्याशय) ek composite gland hai, yaani ek saath exocrine bhi aur endocrine bhi. Iska zyada hissa pancreatic juice banata hai jo ek duct ke through duodenum mein jaata hai. Iske andar islands ki tarah bikhre hue lagbhag 1 se 2 million endocrine cells ke chhote clusters hain, islets of Langerhans, phir bhi yeh sab milkar gland ke tissue ka sirf 1–2 % hain. Islet cells ke do main types hain: $\alpha$-cells, jo glucagon secrete karte hain, aur $\beta$-cells, jo insulin secrete karte hain.

Do meals ke beech glucagon hi tumhari blood sugar ko girne se rokta hai. Yeh peptide hormone mainly liver cells (hepatocytes) par kaam karta hai aur unse do tareekon se glucose release karwata hai: stored glycogen ko tod kar (glycogenolysis) aur non-carbohydrate sources se naya glucose bana kar (gluconeogenesis). Saath hi yeh body cells se kam glucose lene aur use karne ko kehta hai. Yeh har effect blood sugar ko upar, hyperglycemia ki taraf le jaata hai, isi liye glucagon ko hyperglycemic hormone kehte hain.

Khaane ke baad yeh kaam insulin ke paas chala jaata hai, jo bhi ek peptide hai. Iske main targets hepatocytes aur adipocytes (fat cells) hain, jinse yeh glucose tezi se andar lene aur use karne ko kehta hai; yeh unse extra glucose ko glycogen ke roop mein store bhi karwata hai (glycogenesis). Glucose blood se bahar nikalta hai aur level girta hai, aur agar yeh normal se neeche chala jaaye to is state ko hypoglycemia kehte hain. Glucagon ise badhata hai aur insulin ghatata hai, isliye blood glucose steady rehta hai: yeh glucose homeostasis hai ((19.1), Fig. 19.10).

Agar insulin nahi hai, ya body ke cells us par respond karna band kar dete hain, to blood glucose din-ba-din high rehta hai. Yeh lambi chalne wali hyperglycemia complex disorder diabetes mellitus (मधुमेह) hai.112023 mein publish hui ek ICMR study ke anusaar India mein lagbhag 101 million log diabetes ke saath jee rahe hain. Kidneys saara extra glucose wapas nahi le paati, isliye kuch urine mein nikal jaata hai; aur jo cells glucose ko theek se use nahi kar paate woh energy ke liye fat ki taraf mudte hain, jisse nuksaandeh ketone bodies bante hain. Insulin therapy is disease ko achhe se control karti hai, aur jin logon ka pancreas insulin nahi banata woh is par depend karte hain.

19.2.9 Testis

Testes (वृषण) abdomen ke bahar scrotal sac mein latke rehte hain, aur double role nibhaate hain: primary male sex organs ke roop mein yeh sperms banate hain, aur endocrine glands ke roop mein hormones. Har testis ka zyada hissa coiled seminiferous tubules hai, jahan sperms bante hain. Hormones banane wale Leydig cells (interstitial cells) hain, jo tubules ke beech ki jagah bharne wale stromal, ya interstitial, tissue mein bikhre hote hain. Yeh androgens secrete karte hain, jinmein testosterone sabse mukhya hai (Fig. 19.12).

• Sperm production: androgens spermatogenesis ke liye ek bada stimulus hain.
• Puberty: yeh muscle banate hain aur chehre aur axillary (armpit) ke baal, bhaari (low-pitched) awaaz aur aggressiveness laate hain.
• Accessory organs: epididymis, vas deferens, seminal vesicles, prostate gland aur urethra ko develop, mature aur kaam karne ke liye androgens chahiye.
• Metabolism: protein aur carbohydrate metabolism par inke anabolic (building-up) effects hote hain aur yeh red blood cells banne ko stimulate karte hain.
• Behaviour: central nervous system par kaam karke yeh male sexual behaviour (libido) ko influence karte hain.

19.2.10 Ovary

Female abdomen ke andar do ovaries (अंडाशय) ke bhi do roles hain. Har menstrual cycle mein ek ovum release hota hai, aur ovaries do groups ke steroid hormones secrete karti hain, estrogen aur progesterone. Kaun sa hormone zyada hoga, yeh is par depend karta hai ki supporting stroma mein bane ovarian follicles kya kar rahe hain (Fig. 19.12 ke aakhri teen steps). Badhta hua follicle mainly estrogen secrete karta hai. Ovum release karne ke baad phata hua follicle corpus luteum ban jaata hai, jiska main product progesterone hai.

Puberty par estrogens female secondary sex characters laate hain, jaise patli (high-pitched) awaaz, aur mammary glands ko develop karte hain. Poori reproductive life mein yeh female secondary sex organs ki growth aur activity ko stimulate karte hain, badhte follicles ko develop hone mein madad karte hain aur female sexual behaviour ko regulate karte hain. Progesterone ka main kaam pregnancy ko support karna hai; mammary glands mein yeh alveoli, yaani milk store karne wale thaili jaise structures, banata hai aur milk secretion ko stimulate karta hai. Class 12 dikhata hai ki yeh hormones LH aur FSH ke saath milkar menstrual cycle kaise chalate hain.

19.3 Heart, kidney aur gastrointestinal tract ke hormones

Har hormone kisi gland se nahi aata. Kai organs, jinka main kaam kuch aur hai, unmein bhi hormone secrete karne wale cells hote hain (Fig. 19.13, Table 19.6).

Heart bhi ek endocrine organ hai. Jab bhi blood pressure badhta hai, iski atrial walls ek peptide release karti hain, atrial natriuretic factor (ANF). ANF blood vessels ko chauda (dilate) karta hai, aur chaudi vessels ka matlab kam pressure, isliye ANF blood pressure par ek built-in brake ki tarah kaam karta hai.22Natriuretic ka matlab hai 'urine mein sodium ke nikalne ko badhana', jo kidney par ANF ka ek aur effect hai.

Kidney mein juxtaglomerular cells erythropoietin release karte hain, ek peptide hormone jo bone marrow ko erythropoiesis, yaani red blood cells banana, badhane ke liye kehta hai. Jin logon ki kidneys fail ho jaati hain woh ise bahut kam banate hain aur aksar anaemic ho jaate hain; unka ilaaj recombinant DNA technology se bane erythropoietin se kiya ja sakta hai.

Gastrointestinal tract mein bikhre endocrine cells chaar major peptide hormones release karte hain, har ek digestion ke ek stage se juda hai. Gastrin gastric glands se hydrochloric acid aur pepsinogen nikalwata hai. Secretin ka target exocrine pancreas hai, jo jawab mein paani aur bicarbonate ions ka flow chhodta hai. Cholecystokinin (CCK) ke do targets hain: pancreas, jo apne enzymes release karta hai, aur gall bladder, jo bile juice release karta hai. Gastric inhibitory peptide (GIP) gastric secretion aur stomach ki churning movements (motility) dono ko rokta hai.

Inke alawa kai non-endocrine tissues growth factors release karte hain. Tissues ko normal growth ke liye aur chot ke baad khud ko repair ya regenerate karne ke liye in signals ki zaroorat hoti hai.

19.4 Hormone action ka mechanism

Glucagon body ke har cell tak pahunchta hai, phir bhi liver cells zor se respond karte hain aur skin cells lagbhag bilkul nahi. Yeh fark hormone receptors (ग्राही) ki wajah se hai, jo aise proteins hain jo ek khaas hormone ko pehchan kar usse bind karte hain aur sirf uske target tissues mein hote hain. Har receptor sirf apne hormone se bind karta hai, isliye receptors specific hote hain. Binding se ek hormone–receptor complex banta hai, aur yeh complex biochemical changes shuru karta hai jo target tissue ke metabolism ko, aur uske through uske physiological functions ko, badal dete hain.

Receptors do mein se ek jagah hote hain. Membrane-bound receptors target cell ki cell membrane mein hote hain. Intracellular receptors cell ke andar hote hain, zyada tar nucleus mein, jahan inhe nuclear receptors kehte hain. Hormone kaun sa type use karega, yeh kaafi had tak uske chemical nature par depend karta hai (Table 19.7).

Peptide hormones aur adrenaline membrane cross nahi kar sakte, isliye yeh membrane-bound receptors se bind karke surface par hi apna message dete hain. Binding receptor ko badal deti hai, jo phir cell ke andar ek second messenger ka production shuru karta hai: jaise cyclic AMP (cAMP), inositol trisphosphate ($\mathrm{I}{\mathrm{P}}_3$) ya calcium ions ($\mathrm{C}\mathrm{a}{\phantom{X}}^{2+}$). Agar hormone first messenger hai, jo signal ko cell surface tak pahunchata hai, to second messenger use andar aage le jaata hai, enzymes ko on ya off karke cellular metabolism ko regulate karta hai. Jab glucagon liver cell se bind karta hai, to cAMP badhta hai aur glycogen todne wale enzymes activate ho jaate hain (Fig. 19.14).

Steroids aur iodothyronines doosra route lete hain. Steroids lipid-soluble hote hain aur seedhe membrane ke paar chale jaate hain, aur yeh hormones apne receptors cell ke andar, zyada tar nucleus mein, paate hain. Wahan hormone–receptor complex khud genome par kaam karta hai, khaas genes ko on ya off karke (gene expression) ya chromosome function par kisi aur tarah asar daal kar, jisse cell proteins ka ek alag set banana shuru kar deta hai. Samay ke saath jud kar yeh biochemical changes physiological aur developmental effects ke roop mein dikhte hain.