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Every NCERT book that matters for UPSC — open a book, then a chapter. Learn each chapter's concepts, then how UPSC tests them — with diagrams, flowcharts and the past questions to attempt.
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← Science — all chaptersControl and Coordination
Start here: why this chapter matters
In simple wordsA city works because traffic lights, phone calls and public notices tell everyone what to do at the right time. Your body has two such systems: nerves are the phone line — fast, wired and person-to-person; hormones are the public notice — slower, but they reach everyone. Plants have no nerves, so they rely mostly on chemical notices and on growing in the right direction.
This is the sixth chapter of NCERT's Class 10 Science textbook (pages 100–112 of the 2026-27 reprint; the contents page and the PDF agree). It has three parts — animals: the nervous system (6.1: reflex actions, the human brain, protection, how nerves cause action), coordination in plants (6.2: immediate response, movement due to growth) and hormones in animals (6.3) — with 4 activities, Figures 6.1–6.7, Table 6.1 (to be completed by the reader), one ‘Do You Know?’ box, three in-text ‘Questions’ boxes (14 questions), a summary and 12 exercise questions.
- Neighbouring chapters: Life Processes (Chapter 5) described the maintenance jobs — digestion, breathing, circulation, excretion — that these two systems run; insulin, adrenaline's effect on the heart and the medulla's control of blood pressure tie back to it. How do Organisms Reproduce? (Chapter 7, next) uses the sex hormones and the pollen tube's chemotropism. In Class 12, Sexual Reproduction in Flowering Plants is already live.
- Prelims — science: UPSC has asked about insulin and the pancreas (1980, 1981), goitre and iodine (1981), iodine in thyroxine (1995), nerve impulses (1983), endocrine glands (1983), the pituitary (1983), gland–hormone matches (1992, 2000), blood sugar control (1994), diabetes (1996, 2000) and the medulla (2007).
- GS2 — health: iodine deficiency and salt iodisation, diabetes as a non-communicable disease. GS3 — science and agriculture: plant hormones as growth regulators in farming and horticulture.
| Word | What it means here |
|---|---|
| Stimulus / response | A change in the surroundings / the movement or change made because of it |
| Receptor | Specialised nerve tip that detects a stimulus (taste, smell, heat…) |
| Neuron | Nerve cell: dendrite, cell body, axon |
| Synapse | The gap between two neurons (or a neuron and a muscle or gland) crossed by chemicals |
| Reflex action / reflex arc | A quick, automatic response / the nerve pathway that makes it |
| CNS / PNS | Brain + spinal cord / cranial and spinal nerves linking the CNS to the body |
| Voluntary / involuntary | Actions we decide on (writing) / actions we cannot easily control (heartbeat) |
| Tropism | Directional growth towards or away from a stimulus (light, gravity, water, chemicals) |
| Hormone | Chemical messenger made in one place that acts somewhere else |
| Endocrine gland | Gland that releases hormones directly into the blood |
| Feedback mechanism | The level of a substance itself regulates how much hormone is released |
Reading NCERT's chapter opening (page 100)A small drawing of a cut human brain beside the title, and a QR code (its label, 1064CH07, is a leftover of the book's older chapter numbering). The text asks why we link movement with life: some movements are growth (a seedling pushing up through soil), others are not (a cat running, children on swings, buffaloes chewing cud). Movement is a response to change, used to the organism's advantage — and it is controlled: we whisper rather than shout in class. The point: organisms need systems of control and coordination, built from specialised tissues.
Common confusions
- Coordination = making many parts work together in the right order; control = deciding which response, when and how much.
- Plants have no nervous system and no muscles — yet they do respond and move.
- Hormones are not only an animal feature: plants have hormones too.
At a glance — the six ideas to carry away
- Why control? Movement is a response to change, and each change needs the right response. Animals use nervous and muscular tissue plus hormones; plants use chemicals (hormones) and water-driven changes in cell shape.
- Neuron: dendrite (information acquired; chemical reaction → electrical impulse) → cell body → axon → nerve ending, where the impulse releases chemicals that cross the synapse to the next neuron, a muscle or a gland. Receptors: gustatory (taste), olfactory (smell) and others in sense organs.
- Reflex arc: receptor → sensory neuron → relay in the spinal cord → motor neuron → effector (muscle), fast and without thinking; the message also reaches the brain. CNS = brain + spinal cord; PNS = cranial and spinal nerves.
- Brain: fore-brain (thinking, sensory, association and motor areas, hunger centre), mid-brain, hind-brain — medulla (blood pressure, salivation, vomiting), cerebellum (precision, posture, balance), pons. Protected by the skull and a fluid cushion; the spinal cord by the backbone.
- Plants: Mimosa folds by water loss (no growth); tendrils coil and shoots bend by growth — phototropism, geotropism, hydrotropism, chemotropism (pollen tube → ovule). Auxin collects on the shady side → that side grows longer → bend to light. Gibberellins (stem growth), cytokinins (cell division), abscisic acid (inhibits growth).
- Hormones in animals: adrenaline (adrenal; heart and breathing faster, blood to skeletal muscles), thyroxin (thyroid; needs iodine — lack causes goitre), growth hormone (pituitary; lack in childhood → dwarfism), testosterone/oestrogen (puberty), insulin (pancreas; blood sugar — diabetes), releasing hormones (hypothalamus). Feedback sets how much is released.
1. Nerve cells and the nerve impulse
In simple wordsA neuron is a relay runner with a baton. It picks up news at its fingertips (dendrites), runs it as an electric signal along its long arm (axon), and at the end it cannot jump across to the next runner — so it throws a chemical baton across the small gap (synapse), and the next runner starts its own electric signal.
All information from the surroundings is detected by the specialised tips of some nerve cells — receptors, usually in sense organs such as the inner ear, nose and tongue: gustatory receptors detect taste, olfactory receptors smell. The information picked up at the end of a dendrite sets off a chemical reaction that creates an electrical impulse, which travels from the dendrite to the cell body and along the axon to its end. There the impulse sets off the release of chemicals that cross the gap, the synapse, and start a similar impulse in a dendrite of the next neuron. A similar synapse delivers impulses from neurons to muscle cells or glands (the neuromuscular junction). Nervous tissue is thus an organised network of neurons specialised for conducting information by electrical impulses.
| Part of the neuron | What happens there (NCERT's three questions, page 101) |
|---|---|
| Dendrite (dendritic tip) | information is acquired; a chemical reaction starts an electrical impulse |
| Cell body | the impulse passes through it; holds the nucleus |
| Axon | carries the electrical impulse to its end |
| Nerve ending (end of the axon) | impulse sets off release of chemicals → chemical signal across the synapse |
⤵ How a message travels through nerves
Receptor at a dendrite tip detects a changee.g. heat on the skin, a smell in the nose
Chemical reaction → electrical impulsestarts in the dendrite
Dendrite → cell body → axonthe impulse runs to the end of the axon
Chemicals released at the nerve endingthey cross the synapse
Next cell respondsa new impulse in the next neuron — or a muscle contracts, or a gland secretes
Reading NCERT's Figure 6.1 (page 101)(a) Structure of a neuron: a star-shaped cell body with its nucleus, branched dendrites, a long axon in segments and a branched nerve ending. (b) Neuromuscular junction: an axon's branches ending on a muscle fibre beside a capillary, with an enlarged box showing the axon end full of small sacs against the folded muscle surface and a mitochondrion. The point: the nerve signal is handed to the muscle chemically, at a junction — redrawn above in our own style.
Reading NCERT's Activity 6.1 (page 101)Taste sugar, then taste it again with the nose pinched; then eat lunch with the nose blocked; compare with having a cold. No figure. The point: much of what we call ‘taste’ is smell — gustatory and olfactory receptors work together, so blocking the nose dulls the flavour of food.
Common confusions
- The impulse is electrical within a neuron but chemical across a synapse — both, not either.
- Order: dendrite → cell body → axon → nerve ending. Information is acquired at the dendrite, travels along the axon, and is converted to chemicals at the axon's end.
- Gustatory = taste; olfactory = smell.
- The brain's messages to the body travel as nervous impulses (UPSC 1983).
Beyond NCERT
- The chemicals released at a synapse are called neurotransmitters (for example acetylcholine at the neuromuscular junction); a synapse passes signals one way only, because only the axon end releases them.
- The insulating segments on many axons are the myelin sheath, which speeds the impulse.
Why this matters for PrelimsThe form of nerve messages (1983) and the parts of a neuron are standard; drugs, toxins and diseases that act at synapses (snake venom, nerve agents) appear in science-and-technology questions.
2. Reflex actions and the reflex arc
In simple wordsIf every hot touch had to wait for the brain to think ‘that is hot, that will burn, I should move’, you would already be burnt. So the body keeps a short-cut wire in the spinal cord: the heat signal comes in and the ‘pull back!’ order goes straight out. The brain is told afterwards.
‘Reflex’ means doing something without thinking or feeling in control — jumping out of a bus's way, pulling the hand from a flame, the mouth watering when we are hungry. Thinking is a complex interaction of many impulses in the dense neuron networks at the front of the skull, and would take too long in an emergency. Instead, the nerves that detect heat are connected more simply to the nerves that move muscles: a reflex arc (Figure 6.2). The best place to join the input and output nerves is where they first meet — nerves from all over the body meet in a bundle in the spinal cord on their way to the brain, so reflex arcs are formed in the spinal cord, although the information also goes on to the brain. Reflex arcs probably evolved as efficient ways of working in animals with little or no complex neuron network for thinking, and remain more efficient for quick responses even where such networks exist.
⤵ Touching a flame: the reflex arc step by step
Receptorheat/pain receptors in the skin detect the flame
Sensory neuroncarries the impulse to the spinal cord
Relay neuron in the spinal cordpasses it straight to a motor neuron — no thinking step
Motor neuroncarries the order to the arm
Effector: arm musclecontracts and pulls the hand away
Meanwhilethe message also travels up to the brain — we feel the pain a moment later
Reading NCERT's Figure 6.2 (page 103)A hand above a hot pan; labels for receptors (heat/pain receptors in the skin), a red sensory neuron running up the arm to a cross-section of the spinal cord (CNS), a relay neuron inside it, a motor neuron running back to the effector (muscle in the arm), and an arrow ‘message to brain’. Below, the book asks you to trace the sequence when bright light falls on the eyes. The point: input → spinal cord → output, with the brain informed but not consulted.
Common confusions
- Sequence of a reflex arc: receptor → sensory neuron → relay neuron → motor neuron → effector.
- In a spinal reflex the brain is informed but does not decide (in-text Q5).
- NCERT simplification: the book says reflex arcs ‘are formed in the spinal cord’, then asks about bright light on the eyes. Reflexes of the head — the pupil narrowing in bright light, blinking, sneezing — run through the brain stem (mid-brain and hind-brain), not the spinal cord (Beyond NCERT). Write: ‘spinal reflexes are in the spinal cord; the pupil reflex is controlled in the brain’.
- Reflex ≠ involuntary action in NCERT's scheme: reflexes are sudden responses to a stimulus (knee jerk, pupil); involuntary actions are continuing body activities (heartbeat, breathing, digestion) run by the mid- and hind-brain.
Beyond NCERT
- The knee-jerk reflex has no relay neuron — the sensory neuron synapses directly on the motor neuron, the simplest (monosynaptic) arc.
- Doctors test reflexes to check whether the spinal cord and nerves are intact after an injury.
Why this matters for PrelimsThe reflex-arc sequence and the place of the spinal cord are classic statement material; spinal-cord injury (exercise 7) links to disability and road-safety discussions.
3. The human brain and voluntary and involuntary actions
In simple wordsThe brain is a head office with departments. The fore-brain is management — it sees, hears, thinks and decides. The cerebellum is the coaching department that keeps every movement smooth and balanced. The medulla is the maintenance crew that keeps the heart, blood pressure and breathing running without anyone asking.
The spinal cord is made of nerves that supply information to think about; thinking itself is concentrated in the brain, the main coordinating centre. Brain + spinal cord = central nervous system (CNS), which receives information from all parts of the body and integrates it. Voluntary actions — writing, talking, moving a chair, clapping — are based on deciding what to do next, so the brain also sends messages to muscles (the second way the nervous system talks to muscles). Communication between the CNS and the rest of the body runs through the peripheral nervous system: cranial nerves from the brain and spinal nerves from the spinal cord.
The brain has three regions. The fore-brain is the main thinking part: it has areas that receive sensory impulses (separate ones for hearing, smell, sight and so on), association areas that interpret sensory information together with other inputs and stored information, and motor areas that control voluntary muscles such as those of the legs. Feeling full after eating comes from a centre associated with hunger in a separate part of the fore-brain. Between simple reflexes (the pupil's size) and thought-out actions (moving a chair) lie involuntary actions we cannot control by thinking — the mouth watering, the heartbeat, breathing, digestion. Many are controlled by the mid-brain and hind-brain; blood pressure, salivation and vomiting are controlled by the medulla in the hind-brain. Walking in a straight line, riding a bicycle or picking up a pencil depend on the cerebellum, also in the hind-brain, which gives precision to voluntary actions and keeps posture and balance.
| Region | Parts named by NCERT | What it does (NCERT) |
|---|---|---|
| Fore-brain | Cerebrum; hypothalamus (Fig. 6.3) | Thinking; sensory areas (hearing, smell, sight); association areas; motor areas for voluntary muscles; hunger/fullness centre |
| Mid-brain | — | Many involuntary actions (with the hind-brain) |
| Hind-brain | Pons, medulla, cerebellum | Medulla: blood pressure, salivation, vomiting · Cerebellum: precision of voluntary actions, posture, balance |
| Reflex action | Voluntary action | Involuntary action | |
|---|---|---|---|
| Example | Pulling the hand from a flame; pupil narrowing in bright light | Writing, talking, moving a chair, walking | Heartbeat, breathing, salivation, digestion |
| Thinking involved? | No — automatic and sudden | Yes — decided | No — and cannot easily be controlled |
| Controlled by | Spinal cord (spinal reflexes) | Fore-brain (motor areas) with cerebellum for precision | Mid-brain and hind-brain (medulla) |
Reading NCERT's Figure 6.3 (page 104)A cut-open side view of the brain inside the cranium (skull): the folded cerebrum bracketed as the fore-brain, the mid-brain, the hypothalamus with the pituitary gland hanging below it, and the hind-brain's pons, medulla and cerebellum, continuing into the spinal cord. The text asks you to study it and find each part's function. The point: three regions, each with its own jobs — our page maps them as regions rather than tracing the drawing.
Common confusions
- Cerebellum (hind-brain) = balance, posture, precision of voluntary movement (in-text Q3). Cerebrum (fore-brain) = thinking. One letter apart, very different jobs.
- Medulla controls blood pressure, salivation and vomiting (UPSC 2007: swallowing and vomiting — medulla oblongata).
- The hunger/fullness centre is in the fore-brain (Beyond NCERT: in the hypothalamus).
- Exercise 3: the brain is responsible for thinking, regulating the heartbeat and balancing — answer (d) all of the above.
- CNS = brain + spinal cord; the nerves leaving them are the PNS.
Beyond NCERT
- There are 12 pairs of cranial nerves and 31 pairs of spinal nerves (43 pairs in all).
- The full name of the medulla is the medulla oblongata; mid-brain, pons and medulla together form the brain stem.
- The hypothalamus also regulates body temperature, thirst and hunger (UPSC 1987, 1991 asked about the body's thermoregulatory centre — tagged to Class 11 physiology).
Why this matters for PrelimsMatching brain parts to functions (medulla, cerebellum, hypothalamus) is a recurring Prelims format (2007).
4. Protecting the nervous system, and how nerves cause action
In simple wordsThe brain travels like a phone in a hard case filled with gel: a bony skull outside, a fluid cushion inside. The spinal cord runs through a stack of bony rings — your backbone. And when a nerve finally says ‘move’, it is the muscle cell that does the work, by shortening itself.
A delicate organ like the brain sits inside a bony box (the skull), and within it is held in a fluid-filled balloon that absorbs shocks. The hard, bumpy vertebral column (backbone) down the middle of the back protects the spinal cord. When an action is to be performed, muscle tissue does the final job. Muscle cells move by changing shape so that they shorten: they contain special proteins that change both their shape and their arrangement in the cell in response to nervous electrical impulses, giving the cell a shorter form. Class IX distinguished voluntary muscles (moved at will) from involuntary muscles.
Reading NCERT's ‘Questions’ box (page 105)Five questions: reflex action vs walking; what happens at a synapse; which part of the brain keeps posture and balance; how we smell an agarbatti; and the brain's role in a reflex action. Outlines in the in-text section.
Common confusions
- The skull protects the brain; the vertebral column protects the spinal cord; the fluid absorbs shocks.
- Muscle cells shorten (contract) because special proteins change shape and arrangement; plant cells change shape by water instead (Section 5).
- Voluntary muscles are under our will (legs); involuntary ones are not (Beyond NCERT examples: gut, blood vessels; heart muscle is a third, cardiac type).
Beyond NCERT
- The fluid is cerebrospinal fluid, held within three membranes, the meninges; their infection is meningitis.
- The contractile proteins in muscle are actin and myosin, which slide past each other.
Why this matters for PrelimsShort, factual: which structure protects what, and the protein basis of muscle movement — easy marks in statement questions on the human body.
5. Coordination in plants: fast movements and tropisms
In simple wordsA plant cannot walk, but it can do two things. It can let air out of its tyres — touch-me-not cells quickly lose water, so the leaves flop. And it can grow lopsided on purpose — grow faster on one side so that a shoot curves to the light or a tendril curls round a stick. The first is quick; the second takes days.
Plants have neither a nervous system nor muscles, yet they respond. Touch the leaves of chhui-mui (the sensitive or touch-me-not plant, Mimosa) and they fold and droop — fast, with no growth. A germinating seed sends its root down and its stem up — a movement caused by growth that stops if growth is stopped. So plants show two kinds of movement: independent of growth and dependent on growth. Immediate response (6.2.1): the leaf moves at a point different from where it is touched, so the information is passed on — by electrical-chemical means from cell to cell, but with no specialised conducting tissue; then cells change shape by changing the amount of water in them, swelling or shrinking, instead of using muscle proteins.
Movement due to growth (6.2.2): tendrils of plants such as the pea are sensitive to touch; where a tendril touches a support, that side grows more slowly than the side away from it, so the tendril circles the support. More commonly plants grow in a particular direction in response to a stimulus — tropic movements, towards or away from the stimulus. In phototropism, shoots bend towards light and roots away from it. In geotropism, roots grow down and shoots up, away from the earth. Hydrotropism (water) and chemotropism (chemicals) also exist — the growth of pollen tubes towards ovules is chemotropism. Speeds differ: the sensitive plant is very quick, sunflowers' movement over day and night is slow, and growth movements slower still.
| Tropism | Stimulus | Shoot | Root | Example from the book |
|---|---|---|---|---|
| Phototropism | Light | Towards (positive) | Away (negative) | Bean seedlings in a box facing a window (Activity 6.2) |
| Geotropism | Gravity | Away — upwards (negative) | Towards — downwards (positive) | Plant in a pot laid on its side (Fig. 6.6) |
| Hydrotropism | Water | — | Towards water (Beyond NCERT detail) | In-text Q5: design an experiment |
| Chemotropism | Chemicals | — | — | Pollen tube growing towards the ovule |
⤵ Activity 6.2: light changes the direction of growth
Set upgerminated bean seeds on wire mesh over a water-filled flask, in a box open on one side facing a window
After 2–3 daysshoots bend towards the light; roots away from it
Turn the flask roundshoots now face away from the light, roots towards it
Old partskeep their old bend — growth already done cannot be undone
New growthturns again: shoots towards light, roots away
Conclusiondirectional growth follows the stimulus: phototropism
| Sensitive plant (touch) | Our legs (walking) | |
|---|---|---|
| Signal | Electrical-chemical, cell to cell, no special tissue | Nerve impulses along neurons, chemicals at synapses |
| What changes shape | Plant cells lose or gain water | Muscle cells: special proteins change shape and arrangement |
| Growth involved? | No | No |
| Speed | Quick | Quick |
Reading NCERT's Figure 6.4 (page 106)Two photographs of a sensitive-plant branch against a dark background: before touch, the feathery leaflets spread wide; after touch, the leaflets folded together and the leaf stalks drooping. The point: a fast movement with no growth — water leaving cells, not muscle.
Reading NCERT's Activity 6.2 with Figures 6.5–6.6 (page 107)Figure 6.5: a conical flask with a seedling standing in a box whose open side faces a window; light streams in and the shoot bends towards it while the roots in the flask point away. Figure 6.6: a pot tipped on its side; the shoot curves upward (negatively geotropic) and the root curves downward (positively geotropic). The point: light and gravity steer growth.
Common confusions
- Touch-me-not = no growth (water changes in cells); tendril coiling = growth (unequal growth on two sides).
- In a tendril the side touching the support grows slower; in a shoot facing light the side away from light grows faster — in both, the far side outgrows the near side.
- Shoots: positively phototropic, negatively geotropic; roots: negatively phototropic, positively geotropic.
- Pollen tube → ovule is chemotropism, not phototropism.
- NCERT's ‘Mimosa family’ is loose: the touch-me-not is the species Mimosa pudica, of the legume family (Fabaceae — Beyond NCERT). Write Mimosa pudica.
Beyond NCERT
- Touch-driven growth is thigmotropism; the Mimosa response, which does not depend on the direction of the stimulus, is a nastic movement (seismonasty).
- Flowering in response to day length is photoperiodism; low-temperature treatment that hastens flowering is vernalisation (UPSC 1983, 1992 — tagged to plant-physiology chapters).
Why this matters for PrelimsTypes of tropism and the difference between growth and non-growth movements are standard; they also underpin questions on plant hormones in agriculture.
6. Plant hormones
In simple wordsElectric wires are fast but reach only the houses they are connected to, and need a pause between messages. A chemical message is like a smell spreading through a house — slower, but it reaches every room and can keep coming steadily. Plants run almost entirely on such chemical messages: some say ‘grow’, one says ‘stop’.
Electrical impulses have two limits: they reach only cells connected by nervous tissue, and a cell needs time to reset before it can fire again. So most multicellular organisms also use chemical communication: stimulated cells release a compound that diffuses around; cells with special molecules on their surface detect it. It is slower, but it can reach all cells, regardless of nervous connections, steadily and persistently. These compounds are hormones. Plant hormones coordinate growth, development and responses to the environment; they are made away from where they act and diffuse there.
Auxin, made at the shoot tip, helps cells grow longer. When light comes from one side, auxin diffuses to the shady side; the cells there grow longer, so the shoot bends towards light (the mechanism behind Activity 6.2). Gibberellins, like auxins, help the growth of the stem. Cytokinins promote cell division and so are found in higher concentration where cells divide rapidly, such as fruits and seeds. Plants also need signals to stop growing: abscisic acid inhibits growth; its effects include wilting of leaves (NCERT).
| Hormone | Promotes or inhibits growth | What NCERT says it does |
|---|---|---|
| Auxin | promotes | made at the shoot tip; makes cells grow longer; gathers on the shady side → shoot bends towards light |
| Gibberellins | promotes | help the growth of the stem |
| Cytokinins | promotes | promote cell division; plentiful where cells divide fast, e.g. fruits and seeds |
| Abscisic acid | inhibits | inhibits growth; NCERT: its effects include wilting of leaves |
⤵ Why a shoot bends towards a window
Light falls on one side of the shootthe tip senses its direction
Auxin, made at the shoot tip, moves to the shady sideit diffuses there
Shady-side cells grow longerauxin makes cells elongate
Unequal growththe far side outgrows the lit side
The shoot curves towards the lightpositive phototropism
Reading NCERT's ‘Questions’ box (page 108)Five questions: what plant hormones are; how the sensitive plant's movement differs from a shoot bending to light; a growth-promoting plant hormone; how auxins help a tendril grow round a support; and an experiment to show hydrotropism. Outlines in the in-text section.
Common confusions
- Auxin moves to the shady side, and the shady side grows more — that is why the shoot bends towards light.
- Cytokinins = cell division; auxins and gibberellins = cell and stem growth (elongation); abscisic acid = growth inhibitor.
- Of insulin, thyroxin, oestrogen and cytokinin, only cytokinin is a plant hormone (exercise 1: (d)).
- NCERT gap: in-text Q4 asks how auxins help a tendril coil, but the text explains coiling only by touch and unequal growth. Answer with the unequal growth and add that a growth hormone such as auxin is thought to mediate it (Beyond NCERT).
- NCERT wording: ‘effects of abscisic acid include wilting’. Wilting itself is water loss; standard texts describe abscisic acid as closing stomata under water stress and promoting dormancy and leaf fall (Beyond NCERT). For Class 10, ‘inhibits growth’ is the safe core.
Beyond NCERT — plant hormones in farming (GS3)
- Ethylene is a gaseous plant hormone that ripens fruit — used, under food-safety rules, to ripen fruit evenly; calcium carbide, which releases acetylene, is banned for the purpose in India.
- Synthetic auxins are used to root cuttings and as selective weedkillers (2,4-D); gibberellins enlarge grapes and help malting; spraying hormones on flowers can give seedless fruit (UPSC 1996 — tagged to the flowering-plants chapter).
- Abscisic acid is called the stress hormone; it also keeps seeds and buds dormant.
Why this matters for Prelims and GS3Plant growth regulators come up in agriculture questions (seedless fruit, ripening, rooting); the four hormones and their single-word roles are the base.
7. Hormones in animals: the endocrine system
In simple wordsWhen a squirrel sees a cat it must be ready to fight or run — and it does not yet know which. Rather than phone each organ one by one, the body broadcasts: a single hormone, adrenaline, flows through the blood and every organ gets ready at once. Other hormones work more slowly, over months and years, guiding growth and puberty.
In a scary situation an animal must prepare to fight or run away — different activities that both need a lot of energy and many tissues working together. Nerves alone would reach only a limited range of tissues; a chemical signal reaches all cells. Many animals, including humans, use adrenaline from the adrenal glands, released directly into the blood. Its targets include the heart, which beats faster and supplies more oxygen to muscles; blood to the digestive system and skin is reduced by contraction of muscles around small arteries, diverting blood to skeletal muscles; the breathing rate rises as the diaphragm and rib muscles contract harder. These hormones form the endocrine system, a second way of control and coordination.
⤵ Adrenaline in an emergency (page 109)
Danger sensedthe body must get ready to fight or run
Adrenal glands release adrenalinestraight into the blood
Heartbeats faster → more oxygen to muscles
Small arteries to gut and skinnarrow → blood diverted to skeletal muscles
Diaphragm and rib musclesbreathing rate rises
Body readyfor either fighting or running
Hormones also guide controlled growth — we do not grow fingers on our faces. Iodised salt: iodine is needed by the thyroid gland to make thyroxin, which regulates carbohydrate, protein and fat metabolism for the best balance for growth; lack of iodine may cause goitre, whose symptom is a swollen neck (the thyroid sits in the neck). Growth hormone from the pituitary regulates growth and development; its deficiency in childhood causes dwarfism. The dramatic changes around 10–12 years of age — puberty — come from testosterone in males and oestrogen in females. Insulin, made by the pancreas, regulates blood sugar; if too little is secreted, blood sugar rises with many harmful effects — diabetes, which some patients treat with insulin injections.
| No. | Hormone | Endocrine gland | Functions |
|---|---|---|---|
| 1 | Growth hormone | Pituitary gland | Stimulates growth in all organs |
| 2 | Thyroxin (filled) | Thyroid gland | Regulates metabolism for body growth |
| 3 | Insulin | Pancreas (filled) | Regulates blood sugar level |
| 4 | Testosterone | Testes | Development of male sex organs; changes of puberty in males (filled) |
| 5 | Oestrogen (filled) | Ovaries | Development of female sex organs, regulates menstrual cycle, etc. |
| 6 | Adrenaline | Adrenal gland | Prepares the body for emergency: faster heartbeat and breathing, blood diverted to skeletal muscles (filled) |
| 7 | Releasing hormones | Hypothalamus (filled) | Stimulates pituitary gland to release hormones |
Reading NCERT's Figure 6.7 and Activity 6.3 (pages 109–110)Two body outlines, (a) male and (b) female, with the glands marked: hypothalamus, pineal gland and pituitary gland in the head; thyroid gland and parathyroid glands in the neck; thymus in the chest; adrenal glands (on the kidneys) and pancreas in the abdomen; testis in the male, and uterus and ovary in the female. Activity 6.3 asks you to identify these glands and find out about the ones not covered in Table 6.1 from library books and teachers. The point: glands are scattered through the body but all pour into the same blood — mapped above.
Reading NCERT's Activity 6.4 and Table 6.1 (page 111)Table 6.1, ‘Some important hormones and their functions’, has seven rows — growth hormone, (thyroid), insulin, testosterone, (ovaries), adrenaline, releasing hormones — with six cells left blank for the reader to fill from the text (counted in the source). Our completed version is above; filled cells are marked. The point: hormone ↔ gland ↔ function is a three-way match that exams love.
Common confusions
- Insulin comes from the pancreas (UPSC 1980, 1981); the pancreas is both a digestive gland (Chapter 5) and an endocrine gland.
- Iodine → thyroid → thyroxin; lack → goitre (UPSC 1981). Thyroxin is the hormone that contains iodine (UPSC 1995).
- Growth hormone is from the pituitary, not the thyroid; childhood deficiency → dwarfism.
- Adrenaline reduces blood flow to the gut and skin and increases it to skeletal muscles.
- Endocrine glands release hormones into the blood; salivary glands pour into ducts (exocrine).
- NCERT spells thyroxin and oestrogen; other books write thyroxine and estrogen — same hormones.
Beyond NCERT — GS2 health links
- India's National Goitre Control Programme (1962) was renamed the National Iodine Deficiency Disorders Control Programme in 1992; its main tool is universal salt iodisation. Iodine deficiency in pregnancy can also impair a child's brain development.
- Diabetes: in type 1 the pancreas makes little or no insulin; in type 2 the body responds poorly to insulin. An ICMR–INDIAB study (2023) estimated about 101 million people with diabetes in India.
- Excess growth hormone in childhood causes gigantism — the ‘giants’ NCERT mentions.
- The parathyroid hormone regulates blood calcium (UPSC 2007); the pineal gland makes melatonin.
Why this matters for Prelims and GS2Gland–hormone matches (1992, 2000), insulin (1980, 1981, 1996), goitre and iodine (1981, 1995) and the pituitary (1983) are among the most-asked biology items; iodine deficiency and diabetes are GS2 public-health topics.
8. Keeping hormones in balance: feedback, and nerves vs hormones
In simple wordsA room heater with a thermostat switches off when the room is warm and on when it cools. Hormone glands work the same way: the level itself is the signal. High blood sugar switches insulin up; when sugar falls, insulin is turned down.
Hormones must be secreted in precise quantities, so the timing and amount are regulated by feedback mechanisms. If blood sugar rises, cells of the pancreas detect it and produce more insulin; as blood sugar falls, insulin secretion is reduced. The hypothalamus plays an important role in releasing many hormones: when the growth-hormone level is low, it releases a growth hormone releasing factor that stimulates the pituitary to release growth hormone.
⤵ Insulin feedback (page 111)
Blood sugar risese.g. after a meal
Pancreas cells detect itthey secrete more insulin
Blood sugar fallsinsulin helps bring it down
Insulin secretion is reducedthe falling level itself switches it down — feedback
| Nervous control | Hormonal control | |
|---|---|---|
| Signal | Electrical impulse in neurons; chemicals at synapses | Hormones carried in the blood (chemical) |
| Reach | Only cells connected by nerves | All cells with the matching receptors |
| Speed | Very fast | Slower |
| Duration | Short; a cell must reset before firing again | Steady and persistent |
| Examples | Reflex, walking, talking | Fight-or-flight (adrenaline), growth, puberty, blood sugar |
| In plants? | No nervous system (cell-to-cell electrical-chemical signals only) | Yes — auxin, gibberellins, cytokinins, abscisic acid |
Reading NCERT's ‘Do You Know?’ (page 110), ‘Questions’ box (page 111) and ‘What you have learnt’ (page 111)The box: the hypothalamus releases a growth hormone releasing factor when growth hormone is low, which makes the pituitary release it. The Questions box asks how chemical coordination works in animals, why iodised salt is advisable, how the body responds to adrenaline and why some diabetes patients need insulin injections (outlines in the in-text section). The summary: control and coordination come from the nervous system and hormones; nervous responses are reflex, voluntary or involuntary; nerves use electrical impulses, get information from sense organs and act through muscles; chemical coordination occurs in plants and animals; hormones act away from where they are made; and feedback regulates them. The point: two systems, one aim — the right response in the right amount.
Common confusions
- Feedback here is negative: a rise in sugar leads to the response that brings it down (Beyond-NCERT term).
- The hypothalamus controls the pituitary through releasing factors — so the pituitary is not the top of the chain (Table 6.1, row 7).
- Blood sugar stays fairly steady even after a sugar-rich meal because of pancreatic hormones (UPSC 1994).
- Diabetes mellitus = high blood sugar with low insulin (UPSC 1996); it is a hormonal disorder (UPSC 2000).
Beyond NCERT
- The pancreas has an opposing hormone, glucagon, which raises blood sugar when it falls too low; insulin and glucagon together hold it in range.
- Insulin is a protein (peptide) hormone, so it is digested if swallowed — which is why it is injected (UPSC 2004).
Why this matters for Prelims and GS2Blood-sugar control (1994), diabetes (1996, 2000) and the nervous–hormonal comparison are frequent; the feedback idea also explains drug-dose and lifestyle-disease questions.
NCERT's in-text questions and activities — short answers
Short outlines in page order for the three ‘Questions’ boxes (14 questions), the questions in the running text, and the four activities.
Page 101: In a neuron, where is information (i) acquired, (ii) carried as an electrical impulse, (iii) converted to a chemical signal?
(i) At the dendritic tip; (ii) along the axon (via the cell body); (iii) at the end of the axon (nerve ending), where chemicals are released into the synapse.
Page 103: Trace the sequence of events when a bright light is focussed on your eyes.
Light receptors in the eye detect it → impulse along a sensory nerve → reflex centre (in the brain — Beyond NCERT: mid-brain) → motor nerve → muscles of the iris contract → the pupil narrows, letting in less light; it happens automatically, without thinking.
Page 105, Q1: What is the difference between a reflex action and walking?
A reflex is sudden and automatic, made through a reflex arc in the spinal cord without thinking. Walking is a voluntary action decided by the fore-brain and made smooth and balanced by the cerebellum.
Page 105, Q2: What happens at the synapse between two neurons?
The electrical impulse reaching the axon end releases chemicals; they cross the gap and start a similar electrical impulse in a dendrite of the next neuron.
Page 105, Q3: Which part of the brain maintains posture and equilibrium of the body?
The cerebellum in the hind-brain.
Page 105, Q4: How do we detect the smell of an agarbatti (incense stick)?
Its molecules reach the olfactory receptors in the nose; they start nerve impulses that travel to the smell area of the fore-brain, which interprets them (with memory) as the smell of incense.
Page 105, Q5: What is the role of the brain in reflex action?
The response is made by the reflex arc in the spinal cord; the brain is informed at the same time (so we feel and remember it) but does not decide the response.
Page 108, Q1: What are plant hormones?
Chemicals made in one part of the plant that diffuse to another part and coordinate growth, development and responses to the environment — auxin, gibberellins, cytokinins, abscisic acid.
Page 108, Q2: How is the movement of leaves of the sensitive plant different from the movement of a shoot towards light?
Sensitive plant: fast, no growth, cells lose water and change shape; not directed by the stimulus. Shoot: slow, caused by unequal growth (auxin on the shady side), directed towards the light (phototropism).
Page 108, Q3: Give an example of a plant hormone that promotes growth.
Auxin (also gibberellins; cytokinins promote cell division).
Page 108, Q4: How do auxins promote the growth of a tendril around a support?
When the tendril touches a support, the side in contact grows more slowly and the far side faster, so the tendril coils round it. The book does not name auxin here; the unequal growth is thought to come from auxin acting more on the far side (Beyond NCERT).
Page 108, Q5: Design an experiment to demonstrate hydrotropism.
Grow seedlings in dry soil (or sawdust) in a box with a porous clay pot of water buried at one side. After some days, dig carefully: the roots have bent towards the water source rather than growing straight down — hydrotropism. A control box with water spread evenly shows straight roots.
Page 111, Q1: How does chemical coordination take place in animals?
Endocrine glands release hormones into the blood; the blood carries them to target organs, which respond (adrenaline → heart, muscles); the amounts are regulated by feedback.
Page 111, Q2: Why is the use of iodised salt advisable?
The thyroid needs iodine to make thyroxin, which regulates carbohydrate, protein and fat metabolism for growth. Lack of iodine can cause goitre (swollen neck). Iodised salt is a cheap, everyday way to supply iodine.
Page 111, Q3: How does our body respond when adrenaline is secreted into the blood?
The heart beats faster (more oxygen to muscles), blood is diverted from the digestive system and skin to skeletal muscles, and the breathing rate rises — the body is ready to fight or run.
Page 111, Q4: Why are some patients of diabetes treated by giving injections of insulin?
Their pancreas does not secrete enough insulin, so blood sugar stays high and harms the body; injected insulin lowers the blood sugar level.
Activity 6.1 (page 101): Does sugar or food taste different with the nose blocked? Why?
Yes — food tastes flat. Flavour combines taste (gustatory receptors) with smell (olfactory receptors); blocking the nose, or a cold, cuts out the smell part.
Activity 6.2 (page 107): After turning the flask, do old parts change direction? What about new growth? Conclusion?
Old parts stay bent; new growth turns — shoots again towards light, roots away. Conclusion: light controls the direction of growth (phototropism) through growth itself.
Activity 6.3 (page 109): Identify the endocrine glands in Figure 6.7.
Hypothalamus, pineal, pituitary, thyroid, parathyroids, thymus, adrenals, pancreas, testes (male), ovaries (female) — mapped in Section 7.
Activity 6.4 (page 111): Complete Table 6.1.
Row 2 thyroxin; row 3 pancreas; row 4 development of male sex organs and changes at puberty; row 5 oestrogen; row 6 prepares the body for emergencies (faster heartbeat and breathing, blood to skeletal muscles); row 7 hypothalamus. Full table in Section 7.
NCERT exercise — answer outlines
Reading NCERT's Exercises (page 112)Twelve questions — three multiple-choice, then explanations and comparisons. No figures. NCERT's answers page (page 218) gives 1 (d), 2 (b), 3 (d).
1. Which of the following is a plant hormone? (a) Insulin (b) Thyroxin (c) Oestrogen (d) Cytokinin.
(d) Cytokinin.
2. The gap between two neurons is called a (a) dendrite (b) synapse (c) axon (d) impulse.
(b) synapse.
3. The brain is responsible for (a) thinking (b) regulating the heart beat (c) balancing the body (d) all of the above.
(d) all of the above — fore-brain (thinking), medulla (involuntary actions), cerebellum (balance).
4. What is the function of receptors in our body? Think of situations where receptors do not work properly. What problems are likely to arise?
Receptors detect changes (light, sound, smell, taste, touch, heat) and start nerve impulses. If they fail, the body cannot sense danger or respond: no pain from a burn (injury), no smell of gas or spoiled food, poor taste with a cold, difficulty in balance or hearing.
5. Draw the structure of a neuron and explain its function.
Draw dendrites, cell body with nucleus, a long axon and branched nerve endings (diagram in Section 1). Function: dendrites acquire information and start an electrical impulse, which passes through the cell body and along the axon; at its end the impulse releases chemicals that cross the synapse to the next neuron, muscle or gland.
6. How does phototropism occur in plants?
Light from one side makes auxin, made at the shoot tip, diffuse to the shady side; cells there grow longer, so the shoot bends towards the light. Roots bend away from light (diagram and flow in Section 6).
7. Which signals will get disrupted in case of a spinal cord injury?
Signals between the body below the injury and the brain — sensory messages going up (touch, pain) and motor orders coming down (voluntary movement) — and the spinal reflexes at that level. The result can be loss of sensation and paralysis below the injury.
8. How does chemical coordination occur in plants?
Through plant hormones made at one place and diffusing to where they act: auxin (cell elongation, phototropism), gibberellins (stem growth), cytokinins (cell division), abscisic acid (inhibits growth).
9. What is the need for a system of control and coordination in an organism?
Organisms must detect changes and make the right response in the right order and amount; in a many-celled body the organs must work together (e.g. fight or flight), and growth and internal levels (blood sugar) must be kept in balance. Without it, responses would be random and the body would fail.
10. How are involuntary actions and reflex actions different from each other?
Reflex: a sudden, automatic response to a stimulus, through a reflex arc, mostly in the spinal cord (pulling a hand from a flame). Involuntary: continuing body activities not under conscious control, run by the mid-brain and hind-brain (heartbeat, breathing, salivation).
11. Compare and contrast nervous and hormonal mechanisms for control and coordination in animals.
Nervous: electrical impulses in neurons; very fast; short-lived; reach only connected cells. Hormonal: chemicals in the blood; slower; steady and persistent; reach all cells with receptors. Both coordinate responses and often work together (adrenaline with nerves in emergencies). Table in Section 8.
12. What is the difference between the manner in which movement takes place in a sensitive plant and the movement in our legs?
Sensitive plant: information passes cell to cell by electrical-chemical means without special tissue, and cells change shape by losing water. Legs: nerve impulses reach muscles, whose special proteins change shape and arrangement so the cells shorten. Neither involves growth (table in Section 5).
What to remember
Numbers
Brain regions3: fore-brain, mid-brain, hind-brain · hind-brain: pons, medulla, cerebellum
Nerves (Beyond NCERT)12 pairs cranial + 31 pairs spinal = 43 pairs
Plant hormones in NCERT4: auxin, gibberellins, cytokinins (promote) · abscisic acid (inhibits)
Table 6.17 rows · 6 cells left blank for the reader
Pubertychanges around 10–12 years of age (testosterone, oestrogen)
Activity 6.2shoots bend towards light after 2–3 days
Chapter sizepages 100–112 · 4 activities · Figures 6.1–6.7 · Table 6.1 · 14 in-text + 12 exercise questions
One-liners
- Receptors at dendrite tips; impulse: dendrite → cell body → axon → nerve ending.
- Electrical within a neuron, chemical across a synapse.
- Reflex arc: receptor → sensory neuron → relay (spinal cord) → motor neuron → effector.
- CNS = brain + spinal cord; PNS = cranial + spinal nerves.
- Fore-brain: thinking, senses, voluntary movement, hunger centre.
- Medulla: blood pressure, salivation, vomiting; cerebellum: precision, posture, balance.
- Skull + fluid protect the brain; backbone protects the spinal cord.
- Muscles shorten as special proteins change shape and arrangement.
- Mimosa: water loss, no growth; tendrils and tropisms: growth.
- Auxin gathers on the shady side → shoot bends to light.
- Cytokinins: cell division; gibberellins: stem growth; abscisic acid: inhibits growth.
- Adrenaline: faster heart and breathing, blood to skeletal muscles.
- Iodine → thyroxin; lack → goitre. Growth hormone lack in childhood → dwarfism.
- Insulin from the pancreas lowers blood sugar; feedback sets its amount.
- Hypothalamus releasing factors → pituitary.
Traps to avoid
- Cerebellum (balance) ≠ cerebrum (thinking).
- Reflex (sudden, spinal) ≠ involuntary action (continuous, mid- and hind-brain).
- Shoot: towards light, away from gravity; root: away from light, towards gravity.
- Auxin moves to the SHADY side, which grows MORE.
- Cytokinin is the plant hormone among insulin, thyroxin, oestrogen, cytokinin.
- Growth hormone is from the pituitary, thyroxin from the thyroid, insulin from the pancreas.
- Touch-me-not movement involves no growth; tendril coiling does.
- NCERT slips: ‘reflex arcs are formed in the spinal cord’ (the pupil reflex is in the brain); in-text Q4 links auxin to tendrils without explaining it; ‘Mimosa family’; abscisic acid ‘wilting’.
How to strategise
Prelims — what UPSC asks
- Hormones and glands: insulin and the pancreas (1980, 1981), goitre and iodine (1981), iodine in thyroxine (1995), the pituitary and body development (1983), endocrine = ductless (1983), gland–hormone matches (1992, 2000), steady blood sugar (1994), diabetes (1996) and diabetes as a hormonal disorder (2000).
- Nervous system: messages as nervous impulses (1983) and the medulla's control of swallowing and vomiting (2007).
- Likely next: brain-part functions (cerebellum, hypothalamus), reflex arcs, plant growth regulators in farming (ethylene ripening, gibberellins, synthetic auxins), iodine deficiency and salt iodisation, type 1 vs type 2 diabetes, insulin as a protein.
- Answer-key note (1983, ‘hormones necessary for the development of the human body’): the stored key (b) pituitary stands — growth hormone ‘stimulates growth in all organs’ — but thyroxin also ‘regulates metabolism for body growth’ (Table 6.1), so (a) is not absurd. Treat it as a slightly flawed question; the pituitary is the expected answer.
Mains — where it appears and how to answer
- No GS Mains question in the bank so far is built on this chapter's core content. It supplies the biology for GS2 answers on iodine deficiency and salt iodisation, diabetes and other non-communicable diseases, and for GS3 answers on plant growth regulators in agriculture and horticulture.
- Answer frame: the mechanism in one line (iodine → thyroxin; insulin and feedback) → the health or farm consequence → the programme or rule (iodine deficiency programme and universal salt iodisation; diabetes screening; ripening rules) → positions on open questions, attributed, no verdict.
- A small sketch — the reflex arc or the insulin feedback loop — fits in a 150-word answer.
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. In a neuron, the electrical impulse is converted into a chemical signal at the:
2. The correct sequence in a reflex arc is:
3. Which part of the brain controls blood pressure, salivation and vomiting?
4. When light falls on a shoot from one side, auxin:
5. Which plant hormone inhibits growth?
6. The folding of touch-me-not leaves is due to:
7. Deficiency of iodine in the diet may cause:
8. When blood sugar rises, the pancreas responds by:
UPSC questions from this chapter
Open any question to attempt it in the Question bank — your attempt is saved to your progress.