NCERT Exemplar Solutions Class 10 Science Chapter 6

Read the prefix, find the sense. Every receptor name in this chapter carries a clue in its first half, so decoding the prefix is the fastest route to the answer.

Concept used. Receptors are stimulus-specific. The tongue holds gustatory (taste) receptors, the nose holds olfactory (smell) receptors, the ear holds auditory (sound) receptors, and the eye holds photo (light) receptors.

1. Gustatory. The root gusta means taste. So these receptors, found in the taste buds of the tongue, detect taste.
2. Olfactory. The root olfact means smell. These receptors line the inside of the nose and detect smell.
3. Test each option. Only (a) keeps both pairs right. (b) wrongly says gustatory detects smell, (c) and (d) swap the senses across organs.
4. So the correct pairing is option (a).

Why this matters. This is also why food tastes flat during a heavy cold: the blocked nose stops the olfactory receptors working, and taste alone feels dull. Question 24 in this same set tests exactly that link.

Option (a): gustatory receptors sense taste, olfactory receptors sense smell.

Follow the shape of the cell. The neuron is built like a funnel that gathers signals at the bushy end and pours them out at the far tip, so the path follows that shape.

Concept used. A neuron has three working zones in series: dendrites that collect, a cell body that processes, and an axon that transmits to its end. The signal flows through them in that order.

1. Dendrites first. The many short branches catch the incoming stimulus, so the impulse begins at the dendrite.
2. Cell body next. The dendrites feed into the cell body, the central part of the neuron.
3. Axon then end. A single long axon carries the impulse away from the cell body to the axonal end.
4. Check the options. Only (c) keeps this real sequence; (a), (b) and (d) jumble the order or run it in reverse.

Why this matters. Fixing this direction in your mind makes the synapse question (Question 3) easy, because the signal must leave one neuron from its axonal end and reach the next neuron at its dendrite.

Option (c): the impulse travels dendrite → cell body → axon → axonal end.

Spot the gap, name its two banks. A synapse has a sending bank and a receiving bank; naming them correctly answers the question.

Concept used. At a synapse the electrical impulse is converted to a chemical signal at the axonal end. The chemical diffuses across the gap and is converted back to an electrical impulse at the dendrite of the next neuron.

1. Sending side. The axonal end of the first neuron holds the chemical messengers, so it is the transmitting bank.
2. The gap. The chemical is released and drifts across the narrow synaptic space.
3. Receiving side. The dendritic end of the second neuron picks up the chemical and fires a new impulse.
4. Eliminate the rest. Options (b) and (c) stay inside one neuron, and (a) reverses the true direction, so all are wrong.

Why this matters. This chemical relay is why messages flow only forward through a nerve circuit, and why drugs and toxins that act on synapses can block or boost nerve signals.

Option (d): axonal end of one neuron to dendritic end of another.

Ask where the chemical is made. A conversion to a chemical signal can only happen where the chemical is stored and released, which narrows the answer at once.

Concept used. The chemical messengers of a synapse are kept at the axonal end. When the electrical impulse arrives there, it makes the end release these chemicals, converting the message from electrical to chemical form.

1. Where the chemicals sit. Only the axonal end stores and releases the chemical messengers.
2. When the swap happens. The arriving electrical impulse triggers their release, so the swap to a chemical signal is at the axonal end.
3. Reject the rest. The dendrite, cell body and axon merely carry the impulse as electricity, so (a), (c) and (d) cannot be the conversion site.

Why this matters. This single fact, that the axonal end is the conversion point, explains the entire one-way design of nerve signalling that the chapter keeps coming back to.

Option (b): at the axonal end, where chemicals are released.

Trace the journey of the signal. Picture touching something hot: follow the message from skin to spinal cord and back to the muscle, and the sequence writes itself.

Concept used. A reflex arc has five parts in series: receptor, sensory (afferent) neuron, spinal cord (central relay), motor (efferent) neuron, and effector muscle. The signal moves through them in that order.

1. Detect. Receptors in the skin sense the heat first.
2. Carry in. The sensory neuron takes the signal towards the spinal cord.
3. Relay. The spinal cord switches the signal to a motor neuron without waiting for the brain.
4. Carry out and act. The motor neuron drives the muscle to pull the hand back. This matches option (d).

Why this matters. Because the spinal cord handles the reflex without the brain, the response is very fast, which is exactly why you pull your hand back before you even feel the full pain.

Option (d): receptor, sensory neuron, spinal cord, motor neuron, muscle.

Judge each line as true or false. For a ``which are true'' question, the safest method is to test every statement on its own and then read off the matching option.

Concept used. Reflex action is automatic and fast; the reflex arc is its pathway. Sensory neurons are incoming (receptor to CNS), motor neurons are outgoing (CNS to effector).

1. Statement (i). A sudden, unplanned response to a stimulus is the textbook meaning of reflex action. True.
2. Statement (ii). It claims sensory neurons go spinal cord to muscles. That is the motor neuron's job. False.
3. Statement (iii). It claims motor neurons go receptors to spinal cord. That is the sensory neuron's job. False.
4. Statement (iv). The full receptor-to-effector path is the reflex arc. True. So only (i) and (iv) hold, giving option (c).

Why this matters. Many board errors come from swapping these two neuron roles. Getting the direction right here is the same skill the reflex-arc labelling in Question 34 will test.

Option (c): only statements (i) and (iv) are true.

Check each claim against the brain map. Hold a simple map in mind, fore-brain for thought and senses, hind-brain for balance and involuntary control, then mark each statement.

Concept used. The fore-brain is the thinking and sensory centre. The hind-brain has the cerebellum (posture, balance) and the medulla (involuntary actions like heartbeat, salivation, vomiting).

1. Statement (i). Thinking is a fore-brain job, not hind-brain. False.
2. Statement (ii). Hearing, smell, memory and sight centres are in the fore-brain. True.
3. Statement (iii). Salivation, vomiting and blood pressure are involuntary and run by the medulla in the hind-brain. True.
4. Statement (iv). It denies the cerebellum's role in balance, but the cerebellum does control posture and balance. False. Hence (ii) and (iii): option (c).

Why this matters. This same fore-brain versus hind-brain split is the backbone of the long-answer Question 46, which asks you to list the brain's parts and their functions.

Option (c): only statements (ii) and (iii) are true.

Name the part by the job. The question gives one job, posture and balance, so we just recall which single part owns it.

Concept used. In the hind-brain, the cerebellum coordinates muscle activity to maintain posture, balance and the precision of movement.

1. The job. Standing upright and balancing needs many muscles tuned together at once.
2. The owner. The cerebellum performs this fine coordination, so it controls posture and balance.
3. Reject the rest. The cerebrum is for thought and willed action, the medulla for involuntary actions, the pons for relaying signals. None of these owns balance.

Why this matters. Activities like cycling, dancing or writing neatly all depend on the cerebellum, which is why a well-practised skill feels automatic and steady.

Option (b): the cerebellum controls posture and balance.

Find where brain ends and cord begins. The spinal cord must start at the lowest part of the brain, so we look for the brain region that sits just above the backbone.

Concept used. The medulla oblongata is the lowest part of the hind-brain. It connects the brain to the spinal cord, so the cord originates from the medulla.

1. Trace downward. Moving down the brain, the last region before the backbone is the medulla.
2. Origin point. The spinal cord continues directly from the medulla and runs down inside the vertebral column.
3. Reject the rest. The cerebrum, pons and cerebellum all lie higher up and do not give rise to the cord.

Why this matters. Because the cord starts at the medulla, an injury where the neck meets the skull is so dangerous: it can cut the link between the brain and the rest of the body.

Option (b): the spinal cord originates from the medulla.

Match the stimulus to the prefix. Each tropism is named after the stimulus that causes it, so identifying the stimulus gives the answer directly.

Concept used. Phototropism is directional growth caused by light. Shoots grow towards light (positive phototropism) because the plant hormone auxin gathers on the shaded side and makes it grow longer, bending the shoot toward the light.

1. Name the stimulus. The shoot is responding to light.
2. Apply the prefix. Light-driven growth is phototropism.
3. Direction. The shoot grows towards the light, so it is positive phototropism.
4. Reject the rest. Gravity (geo), water (hydro) and chemicals (chemo) are not the stimulus here, so option (d) stands.

Why this matters. This is why a potted plant on a windowsill leans toward the glass over a few days, and why Question 19 about the sunflower following the sun is the same idea in action.

Option (d): shoot growth towards light is phototropism.

Find the odd hormone out. Three of the four options describe growth promotion, so the inhibitor hormone must be the one whose job is the opposite.

Concept used. Abscisic acid is the plant's stress and dormancy hormone. It inhibits growth, promotes the closing of stomata, and triggers the dropping of leaves and fruits.

1. List the promoters. Auxin (cell elongation), gibberellin (stem growth) and cytokinin (cell division) all push growth forward.
2. Place abscisic acid. It works against these, slowing or stopping growth, so it is an inhibitor.
3. Match the option. ``Inhibit growth'' is option (c). The other three options describe the promoter hormones, not ABA.

Why this matters. Abscisic acid is why deciduous trees shed leaves before winter and why seeds stay dormant until conditions are right, both protective pauses in growth.

Option (c): the main role of abscisic acid is to inhibit growth.

Sort promoters from inhibitor. Since the chapter has only one inhibitor hormone, the ``not associated with growth'' option must be that one.

Concept used. Auxin lengthens cells, gibberellin promotes stem elongation and cytokinin promotes cell division, so all three drive growth. Abscisic acid is the lone inhibitor, linked to stress and dormancy rather than growth.

1. Group the promoters. Auxin, gibberellin and cytokinin each support a part of plant growth.
2. Spot the inhibitor. Abscisic acid acts against growth, causing wilting, leaf fall and dormancy.
3. Answer. The one not tied to growth is abscisic acid, option (d).

Why this matters. Knowing abscisic acid as the brake, not the accelerator, links directly to Question 20, where it triggers the fall of mature leaves and fruits.

Option (d): abscisic acid is the hormone not associated with growth.

Link the mineral to its hormone. Only one hormone in the list is built from iodine, so the answer is whichever hormone the thyroid makes.

Concept used. The thyroid gland uses dietary iodine to make thyroxin, the hormone that sets the body's metabolic rate. A shortage of iodine means too little thyroxin.

1. Name the gland. Iodine is taken up by the thyroid gland.
2. Name its hormone. The thyroid makes thyroxin, so thyroxin is the iodine-dependent hormone.
3. Reject the rest. Adrenaline (adrenal gland) and insulin (pancreas) need no iodine, and auxin is a plant hormone. So thyroxin, option (b), is correct.

Why this matters. The iodine-thyroxin link is the reason iodised salt is sold across India to prevent goitre, the very point tested in Question 41 later in this set.

Option (b): thyroxin is synthesised using iodine.

Hunt the false line. The question wants the wrong statement, so verify each line and select the one that does not fit insulin.

Concept used. Insulin from the pancreas lowers blood sugar by helping cells take in glucose. Too little insulin raises blood sugar and causes diabetes. It has nothing to do with body growth.

1. Statement (a). Insulin is indeed made in the pancreas. True.
2. Statement (b). Growth and development are controlled by growth hormone, not insulin. False, so this is the answer.
3. Statement (c). Insulin does control blood sugar. True.
4. Statement (d). Too little insulin does cause diabetes. True. So the incorrect line is (b).

Why this matters. Confusing insulin with growth hormone is a classic mix-up. Keeping insulin tied firmly to blood sugar helps you answer the diabetes-injection scenario in Question 29.

Option (b): insulin does not regulate growth and development.

Check each gland-hormone pair. A mis-matched pair question is solved by testing each pairing against the correct source gland.

Concept used. Adrenal gland makes adrenaline, testes make testosterone, ovaries make estrogen, and the thyroid makes thyroxin. A correct pair links the hormone to its true gland.

1. Pair (a). Adrenaline is from the adrenal gland, not the pituitary. Mismatch found.
2. Pair (b). Testosterone is from the testes. Correct.
3. Pair (c). Estrogen is from the ovary. Correct.
4. Pair (d). Thyroxin is from the thyroid gland. Correct. So the only wrong pair is (a).

Why this matters. Building a clean gland-to-hormone table now makes the matching and ``name the gland'' questions later in this chapter, like Questions 43 and 44, quick and safe.

Option (a): the mismatch is adrenaline paired with the pituitary gland.

Ask what swells the cell. A change in cell shape that opens and closes a pore points to water moving in and out, so water is the cause.

Concept used. Guard cells regulate the stomata. Water entering by osmosis makes them turgid and bent, opening the pore; water leaving makes them flaccid and straight, closing the pore.

1. Water in. Turgid, swollen guard cells bow outward and the stoma opens.
2. Water out. Flaccid guard cells lose their curve and the stoma shuts.
3. Pick the cause. The driver of this shape change is the amount of water, option (c).
4. Reject the rest. Protein make-up, temperature and nucleus position do not cause the opening and closing.

Why this matters. This water-driven movement lets a plant control water loss and gas exchange, and it shows that not all plant movements need growth, which is a key idea of this chapter.

Option (c): the change in guard cell shape is due to the amount of water in the cells.

Think which side bends inward. A tendril coils towards the support, so the side that grows more must be the outer side, away from the support.

Concept used. A tendril curls by growing unevenly. The cells on the side not in contact with the support elongate faster, so that side becomes longer and the tendril wraps around the support.

1. Contact happens. The tendril brushes against a support on one side.
2. Uneven growth. Cells on the far side (away from contact) divide and lengthen rapidly.
3. Coiling. The longer far side and shorter near side make the tendril bend around the support.
4. Answer. The cause is rapid growth in cells away from the support, option (c).

Why this matters. This is how climbing plants like peas and gourds hold onto fences and sticks, a touch response that helps weak stems reach sunlight.

Option (c): rapid cell division on the side away from the support coils the tendril.

Name the stimulus that guides the tube. The pollen tube is led to the ovule by chemical signals, so the matching tropism is chemotropism.

Concept used. Chemotropism is growth directed by a chemical. The ovule releases chemical attractants, and the pollen tube grows down the style toward them to carry the male gametes to the egg.

1. The goal. The pollen tube must travel from the stigma down to the ovule.
2. The guide. Chemicals near the ovule attract the tube and set its direction.
3. The name. Growth toward a chemical is chemotropism, so option (b) is correct.
4. Reject the rest. Water (hydro), gravity (geo) and light (photo) do not steer the pollen tube.

Why this matters. This chemical guidance makes fertilisation possible in flowering plants, linking this control-and-coordination idea straight to the reproduction chapter that follows.

Option (b): pollen tube growth towards the ovule is chemotropism.

Match the moving stimulus. The sunflower follows the sun, and the sun is a source of light, so the response is to light, namely phototropism.

Concept used. Phototropism is directional growth caused by light. The growing parts of a young sunflower bend so the flower keeps facing the sun as it moves across the sky.

1. Identify the stimulus. The sunflower responds to the position of the sun, which gives light.
2. Apply the prefix. Light-driven growth is phototropism.
3. Confirm. The head turning to face the sun is a positive phototropic movement, so option (a) is right.
4. Reject the rest. Gravity, chemicals and water are not the stimulus, so (b), (c) and (d) fail.

Why this matters. This sun-tracking helps the sunflower catch the most light for photosynthesis, the same survival logic behind shoots bending toward a window in Question 10.

Option (a): the sunflower tracking the sun is phototropism.

Pick the ageing hormone. Falling of mature leaves and fruits is an ageing event, so the hormone behind it is the one tied to inhibition and senescence, abscisic acid.

Concept used. Abscisic acid inhibits growth and speeds ageing in plant parts. It builds up in old leaves and ripe fruits and causes them to separate and fall.

1. Rule out promoters. Auxin, gibberellin and cytokinin drive growth, not shedding.
2. Identify the role. Falling of old leaves and fruits is an ageing, growth-stopping effect.
3. Match the hormone. Abscisic acid causes this, so option (c) is correct.

Why this matters. This is why trees shed leaves in autumn and why ripe fruit drops from the branch, both controlled pauses that abscisic acid manages for the plant.

Option (c): abscisic acid causes mature leaves and fruits to fall.

Find the one wrong claim. Three statements describe nerve signals correctly; the incorrect one will get the release site wrong.

Concept used. The impulse moves dendrite to axonal end inside a neuron. Chemicals are stored and released only at the axonal end; they cross the synapse and start an electrical impulse in the next dendrite.

1. Statement (a). Impulse from dendrite to axon end is the true direction. Correct.
2. Statement (b). It claims chemicals are released at the dendritic end. They are released at the axonal end. Incorrect, so this is the answer.
3. Statement (c). Chemicals from the axon end crossing the synapse to the next dendrite is correct.
4. Statement (d). A neuron can signal muscles and glands too. Correct. So only (b) is wrong.

Why this matters. Pinpointing the release site as the axonal end is the same fact that explains the one-way synapse, the topic of long Question 53.

Option (b): the statement placing chemical release at the dendritic end is incorrect.

Locate the medulla first. The answer is settled by recalling which brain region the medulla belongs to.

Concept used. The medulla oblongata lies in the hind-brain. It controls involuntary actions like heartbeat, breathing, vomiting and blood pressure.

1. The controller. Involuntary actions are run by the medulla.
2. Its location. The medulla is part of the hind-brain.
3. Combine. So involuntary actions are controlled by the medulla in the hind-brain, option (c).
4. Reject the rest. The medulla is not in the fore-brain, mid-brain or spinal cord, so (a), (b) and (d) are wrong.

Why this matters. Because the medulla keeps the heart and lungs going on their own, we stay alive even while asleep, which is why injury to this region is life-threatening.

Option (c): involuntary actions are controlled by the medulla in the hind brain.

Ask which one you can control. Three actions run on their own; the odd one out is the action you can choose to do or not.

Concept used. Involuntary actions are automatic (vomiting, salivation, heartbeat). Voluntary actions are under conscious control, and chewing is something we deliberately do.

1. Vomiting. Happens automatically, controlled by the medulla. Involuntary.
2. Salivation. Reflex release of saliva, automatic. Involuntary.
3. Heart beat. Continues on its own at all times. Involuntary.
4. Chewing. We start and stop it at will, so it is voluntary, option (d).

Why this matters. Sorting voluntary from involuntary action is the basis for understanding which body processes the brain runs consciously and which the medulla handles automatically.

Option (d): chewing is the voluntary action, so it is not involuntary.

Find the sense a cold blocks. A cold affects the nose, so the sense that fails is smell, controlled by the olfactory receptors.

Concept used. Olfactory receptors in the nose detect smell. A severe cold blocks the nasal passage with mucus, so these receptors cannot work and smells cannot be distinguished.

1. Where the cold acts. It blocks the nose, the home of the smell receptors.
2. Effect. With olfactory receptors covered, the person cannot tell perfume from agarbatti.
3. Reject the rest. Taste (tongue), sight (eye) and touch (skin) receptors are not blocked by a cold, so (a), (c) and (d) are unaffected.
4. So the answer is (b).

Why this matters. This everyday experience proves how sense-specific receptors are, the exact idea introduced in Question 1 about gustatory and olfactory receptors.

Option (b): with a blocked nose, smells like perfume and agarbatti cannot be told apart.

Read the arrows against the rule. The neuron has one fixed signal direction, so compare each option's arrows with the dendrite-to-axon-end rule.

Concept used. A neuron carries the impulse from dendrite, into the cell body, down the axon, to the axonal end. The correct diagram shows arrows following exactly this sequence.

1. Set the rule. The only valid flow is dendrite → cell body → axon → axonal end.
2. Identify the ends. The many short branches are dendrites (input); the single long fibre ends at the axonal end (output).
3. Test the options. Reject any diagram whose arrows leave the axon end and head back to the dendrite, or skip the cell body.
4. Select. Option (c) shows arrows moving from the dendrite through the cell body to the axonal end, the correct flow.

Why this matters. This figure is a visual version of Question 2. Mastering the one-way flow here makes labelling the neuron in Question 38 and explaining the synapse in Question 53 straightforward.

Option (c): the diagram with flow dendrite → cell body → axon → axonal end.

Spot the wrong mineral. The false statement about thyroxin should name the wrong raw material, so check which mineral the thyroid really needs.

Concept used. The thyroid uses iodine to synthesise thyroxin, which then regulates the metabolism of carbohydrates, fats and proteins. Iron is unrelated to thyroxin.

1. Statement (a). It says iron is essential for thyroxin. The correct mineral is iodine, so this is false, the answer.
2. Statement (b). Thyroxin does control metabolism of carbohydrate, protein and fat. True.
3. Statement (c). The thyroid needs iodine. True.
4. Statement (d). Thyroxin is indeed the thyroid hormone. True. So only (a) is untrue.

Why this matters. Keeping iodine (thyroid) separate from iron (blood) prevents one of the most common mineral mix-ups in board exams.

Option (a): the statement that iron is essential for thyroxin is not true.

Link the disorder to the hormone amount. Short height points to a shortage of the hormone that controls growth, so the cause is too little growth hormone.

Concept used. The pituitary's growth hormone controls how tall and large the body becomes. A deficiency during the growing years stunts growth and produces dwarfism.

1. Name the hormone. Body growth is controlled by growth hormone from the pituitary.
2. Direction of the fault. Dwarfism means too little growth, so the cause is too little growth hormone.
3. Match the option. ``Less secretion of growth hormone'' is option (b).
4. Reject the rest. Thyroxin and adrenaline do not set height, and excess growth hormone causes gigantism, not dwarfism.

Why this matters. This shows that the right amount of a hormone matters, a balance idea that returns in the insulin and thyroxin questions of this chapter.

Option (b): dwarfism results from less secretion of growth hormone.

Match each sex hormone to its gland. The correct option must place testosterone in the testes and estrogen in the ovary, so check the pairings.

Concept used. Testosterone is the male sex hormone from the testes; estrogen is the female sex hormone from the ovaries. These hormones drive the body changes seen at puberty.

1. Male side. Testosterone is secreted by the testes.
2. Female side. Estrogen is secreted by the ovaries.
3. Scan the options. Options (a), (b) and (d) put the hormones in wrong glands such as adrenal, pituitary or thyroid.
4. Select. Only (c) gives testosterone-testes and estrogen-ovary, the correct pairing.

Why this matters. These same gland-hormone links underpin Question 30 on male fertility and the mis-matched-pair Question 15, so a firm grasp pays off across the chapter.

Option (c): testosterone from testes and estrogen from ovary.

Match the medicine to the problem. Insulin lowers blood sugar, so it is prescribed for the condition where blood sugar is too high.

Concept used. Insulin from the pancreas reduces blood glucose. In diabetes the body lacks enough working insulin, so blood sugar rises and external insulin is given to control it.

1. Insulin's action. It lowers the sugar level in the blood.
2. When it is needed. It is needed when blood sugar is too high.
3. Match the option. High blood sugar is option (d).
4. Reject the rest. Insulin does not treat low blood pressure, slow heartbeat or goitre, so (a), (b) and (c) are wrong.

Why this matters. This is the everyday treatment for type-1 diabetes and shows why insufficient insulin, mentioned in Question 14, leads straight to high blood sugar.

Option (d): the insulin injection was needed because blood sugar was high.

Pick the male sex hormone. Male fertility is controlled by the male sex hormone, so the answer is testosterone.

Concept used. Testosterone, secreted by the testes, controls male secondary sexual characters and supports sperm formation, thereby increasing male fertility.

1. The need. The question asks for the hormone that raises fertility in males.
2. The hormone. The male sex hormone is testosterone.
3. Confirm. Testosterone supports sperm production, so it increases male fertility, option (b).
4. Reject the rest. Oestrogen is female, insulin is for sugar, growth hormone is for growth, so none fits.

Why this matters. This builds directly on Question 28, where testosterone drives the male body changes at puberty, the same gland and hormone working through life.

Option (b): testosterone increases fertility in males.

Count how many of each gland we have. Three options are glands that come as a pair; the single one is the answer.

Concept used. Paired glands occur twice in the body (adrenals, testes, ovaries). The pituitary is a lone gland at the base of the brain, hence unpaired.

1. Adrenal. One above each kidney, so two. Paired.
2. Testes and ovary. Each occurs as a pair. Paired.
3. Pituitary. A single gland under the brain. Unpaired.
4. So the unpaired gland is the pituitary, option (c).

Why this matters. The pituitary is also the master gland that controls others, so noting it as a single central gland helps you see why it has such wide-reaching control.

Option (c): the pituitary gland is unpaired.

Name the gap by what it joins. The question asks for the junction between two neurons, and that specific gap has one name, the synapse.

Concept used. A synapse is the gap between the axonal end of one neuron and the dendrite of the next. The nerve signal crosses it as a chemical message.

1. Identify the pair. The junction is between two neurons.
2. Name it. That junction is the synapse.
3. Reject the look-alikes. A neuro-muscular junction joins a neuron to a muscle, and ``cell junction'' and ``neural joint'' are not the correct terms.
4. So the answer is synapse, option (d).

Why this matters. The synapse is central to this chapter; getting its name right sets up the synapse questions 3, 21, 42 and 53 that test how the signal crosses it.

Option (d): the junction between two neurons is the synapse.

Recall the two control systems. Coordination in humans rests on exactly two systems, so the right option must name both of them.

Concept used. The nervous system uses electrical impulses for rapid control, while the endocrine system uses hormones for slower, lasting control. Together they regulate all life processes.

1. Fast control. The nervous system gives quick responses through neurons.
2. Slow control. The endocrine system gives gradual control through hormones.
3. Combine. Both together run control and coordination, so the pair is nervous and endocrine.
4. Reject the rest. Reproductive, respiratory and digestive systems do not run overall control, so (a), (b) and (c) are wrong.

Why this matters. This pairing is the heart of the whole chapter and the exact statement justified in long Question 51, where the two systems are shown working as partners.

Option (d): the nervous and endocrine systems control life processes.

Trace the arrows from skin to muscle. The labels fall out once you follow the signal's journey through the reflex arc step by step.

Concept used. A reflex arc routes the signal: receptor in skin, sensory neuron in, spinal cord relay, motor neuron out, muscle responds. The labels (a)–(d) sit along this path in order.

1. (a) Sensory neuron. It connects the skin receptors to the spinal cord and carries the incoming signal.
2. (b) Spinal cord. The central relay that receives the sensory signal and decides the response.
3. (c) Motor neuron. It carries the outgoing command from the spinal cord to the muscle.
4. (d) Muscle. The effector in the arm that contracts to pull the hand away from the flame. Direction of flow: receptor → (a) → (b) → (c) → (d).

Why this matters. Reading this diagram correctly turns the abstract reflex-arc sequence of Question 5 into a real, labelled picture, the form most likely to appear in a board diagram question.

(a) sensory neuron, (b) spinal cord, (c) motor neuron, (d) muscle; signal flows from receptor through (a), (b), (c) to (d).

Match each effect to its hormone. The four effects listed are the signature jobs of the four plant hormones, so pairing them is direct recall.

Concept used. Auxin elongates cells, gibberellin drives stem growth, cytokinin promotes cell division, and abscisic acid causes senescence and the dropping of leaves.

1. (a) Cell elongation. This is the classic role of auxin.
2. (b) Stem growth. Gibberellin promotes elongation of the stem.
3. (c) Cell division. Cytokinin encourages cells to divide.
4. (d) Fall of old leaves. Abscisic acid, the ageing hormone, causes this.

Why this matters. This neat hormone-to-effect table is the answer key for Question 49's longer hormone essay, so memorising it once pays off twice in this chapter.

(a) auxin, (b) gibberellin, (c) cytokinin, (d) abscisic acid.

Read the glands by their position. The labels in this body diagram follow a top-to-bottom order, so naming them in sequence avoids confusion.

Concept used. The pineal and pituitary lie in the brain region, the thyroid sits in the neck around the windpipe, and the thymus is in the upper chest. Each is an endocrine (ductless) gland.

1. (a) Pineal gland. The small gland set deep inside the brain.
2. (b) Pituitary gland. Just below the brain, the master gland that controls others.
3. (c) Thyroid gland. In the neck, wrapped around the windpipe.
4. (d) Thymus gland. Lower still, in the upper chest behind the sternum.

Why this matters. Placing these glands on a real body diagram makes the gland-hormone links of Questions 15, 43 and 44 concrete, since you can now point to where each hormone is made.

(a) pineal gland, (b) pituitary gland, (c) thyroid gland, (d) thymus gland.

Apply the up-shoot, down-root rule. The most accurate figure must obey geotropism, so check which one bends the shoot up and the root down.

Concept used. Shoots are negatively geotropic and also positively phototropic, so they grow upward. Roots are positively geotropic, so they grow downward into the soil, seeking gravity and water.

1. Shoot behaviour. A correctly drawn shoot turns upward, away from gravity and toward light.
2. Root behaviour. A correctly drawn root turns downward, toward gravity.
3. Compare the figures. Only figure (a) shows the shoot bending up and the root bending down together.
4. Conclude. Figure (a) is the most accurate; the others show wrong growth directions.

Why this matters. This is the same geotropism idea that fixes why roots reach water and shoots reach light, the survival logic behind the tropism questions 10, 18 and 19 in this chapter.

Figure (a), since the shoot is negatively geotropic (grows up) and the root is positively geotropic (grows down).

Label along the signal path. Naming the parts in the order the signal travels keeps the four labels in the right places.

Concept used. A neuron is built from dendrites (receive), a cell body (process), an axon (transmit) and a nerve ending (deliver), arranged in the direction the impulse flows.

1. (a) Dendrite. The many short branches that pick up the incoming signal.
2. (b) Cell body. The wide central region holding the nucleus.
3. (c) Axon. The single long fibre carrying the impulse away from the cell body.
4. (d) Nerve ending. The terminal tip that hands the signal to the next neuron or muscle.

Why this matters. This labelled neuron is the picture behind Question 2's signal-direction rule and the impulse-flow figure of Question 25, tying the chapter's neuron questions together.

(a) dendrite, (b) cell body, (c) axon, (d) nerve ending.

Pair the prefix with the organ. Each receptor name reveals its sense, and each sense lives in one organ, so the matches follow at once.

Concept used. Receptors are housed in stimulus-specific organs: nose for smell, skin for temperature, tongue for taste, eye for light.

1. (a) Olfactory → (iii) Nose. Olfact means smell, sensed in the nose.
2. (b) Thermo → (iv) Skin. Thermo means temperature, sensed in the skin.
3. (c) Gustato → (i) Tongue. Gusta means taste, sensed on the tongue.
4. (d) Photo → (ii) Eye. Photo means light, sensed by the eye.

Why this matters. This matching tidies up the receptor ideas introduced in Questions 1 and 24, giving a clean map of which organ senses which stimulus.

(a)–(iii) nose, (b)–(iv) skin, (c)–(i) tongue, (d)–(ii) eye.

Define, then show in action. A clear answer first defines tropism as growth toward or away from a stimulus, then gives one worked example.

Concept used. Tropism is directional growth driven by a stimulus. Differential growth on the two sides of a plant part makes it bend either toward or away from the stimulus.

1. Meaning. A tropic movement is a growth movement of a plant in a direction set by an outside stimulus.
2. Two directions. It is positive if growth is toward the stimulus, negative if away from it.
3. Example. A potted shoot kept near a window bends toward the light, a positive phototropic movement, while its roots bend away from light.

Why this matters. This single definition unlocks every tropism question in the chapter, from phototropism in Question 10 to chemotropism in the pollen-tube Question 18.

Tropic movement is directional growth of a plant part in response to a stimulus; for example, a shoot growing towards light.

Follow the iodine shortage downstream. Trace what happens step by step when the thyroid runs short of its key raw material, iodine.

Concept used. The thyroid uses iodine to make thyroxin, the hormone that sets metabolic rate. A shortage of iodine starves the gland of raw material, so thyroxin falls and the gland enlarges.

1. Raw material drops. Low dietary iodine leaves the thyroid short of what it needs.
2. Hormone falls. The gland makes less thyroxin, so the metabolism of carbohydrate, protein and fat slows.
3. Gland swells. The thyroid grows larger trying to cope, which shows as goitre, a visible swelling in the neck.

Why this matters. This is the real-life reason behind the iodised salt sold across India, and it follows directly from the iodine-thyroxin link of Questions 13 and 26.

Low iodine reduces thyroxin, disturbs metabolism, and can cause goitre.

Describe the relay across the gap. The synapse converts the signal twice, so explain the electrical-to-chemical-to-electrical relay in order.

Concept used. At a synapse the impulse changes from electrical to chemical at the axonal end, diffuses across the gap, and changes back to electrical at the dendrite of the next neuron.

1. Arrival. The electrical impulse reaches the axonal end of the sending neuron.
2. Chemical release. The axonal end releases chemical messengers into the narrow synaptic gap.
3. New impulse. The chemicals cross over and trigger a fresh electrical impulse in the dendrite of the receiving neuron.

Why this matters. This chemical relay is the reason nerve signals travel one way only, the exact point that the long Question 53 asks you to justify.

The electrical signal releases chemicals at the axonal end that cross the synapse and generate a new electrical signal in the next neuron.

Read each clue to its hormone. Every part names an effect or disorder that points clearly to one hormone, so answer them one by one.

Concept used. Oestrogen drives female puberty, growth hormone controls body growth, insulin controls blood sugar, and thyroxin is built from iodine. Each link is fixed and specific.

1. (a) Female puberty. The female sex hormone is oestrogen.
2. (b) Dwarfism. It comes from a lack of growth hormone.
3. (c) High blood sugar. It comes from a lack of insulin.
4. (d) Iodine-built hormone. That hormone is thyroxin.

Why this matters. This part-by-part recall pulls together the hormone facts from Questions 13, 27, 28 and 29 into one quick-fire answer, exactly the kind that appears on board papers.

(a) oestrogen, (b) growth hormone, (c) insulin, (d) thyroxin.

Place each gland by location or role. Each part describes a gland by where it sits or what it does, so identify it from that clue.

Concept used. The pituitary is at the base of the brain, the pancreas is a mixed gland (enzymes and insulin), the adrenals cap the kidneys, and the testes are the male-only sex glands.

1. (a) Linked with the brain. The pituitary gland sits just below the brain.
2. (b) Enzymes plus hormones. The pancreas makes digestive enzymes and the hormone insulin.
3. (c) Linked with the kidneys. The adrenal glands sit on top of the kidneys.
4. (d) Male-only gland. The testes are present in males but not in females.

Why this matters. Knowing each gland's location and dual role, especially the pancreas as a mixed gland, rounds off the endocrine map you built across Questions 15, 31 and 36.

(a) pituitary, (b) pancreas, (c) adrenal, (d) testes.

Draw the path, then explain each stop. Build the neuron as a left-to-right signal path so the diagram and the explanation match exactly.

Concept used. A neuron conducts impulses one way. Its four parts, dendrites, cell body, axon and nerve ending, form a chain that receives, processes, carries and delivers the signal.

1. Start at the dendrites. These branched fibres collect the stimulus and feed it into the cell body.
2. Move to the cell body. It holds the nucleus and processes the signal received from the dendrites.
3. Travel along the axon. The long axon carries the electrical impulse away from the cell body, sometimes over a long distance.
4. End at the nerve ending. Here chemicals are released to pass the signal across the synapse to the next cell, so the impulse can continue into the next neuron, a muscle or a gland.

Why this matters. This structure explains every neuron behaviour in the chapter, including the one-way flow of Question 25 and the chemical hand-off at the synapse in Questions 42 and 53. Because each part has one clear job and the parts are joined in a fixed order, the neuron can carry a message quickly and reliably from one place in the body to another, which is exactly why the nervous system is so fast at control and coordination.

The neuron's four parts, dendrites, cell body, axon and nerve ending, work in series to receive, process, carry and pass on the nerve impulse.

Take the brain region by region. List the three major parts and attach each one's job, working from the front of the brain to the back.

Concept used. The brain divides into fore-brain (thought and senses), mid-brain (a relay for some reflexes), and hind-brain, which holds the cerebellum (balance) and the medulla (involuntary control).

1. Fore-brain. The thinking centre; it handles sensory input (sight, smell, hearing), memory and voluntary actions.
2. Mid-brain. A connecting region that controls some involuntary eye and head reflexes.
3. Cerebellum (hind-brain). It keeps posture and balance steady and makes movements smooth and well-timed.
4. Medulla (hind-brain). It runs involuntary life processes such as heartbeat, breathing and blood pressure.

Why this matters. This complete map answers the brain-statement Question 7 and the posture Question 8 in one go, and it is the standard board long answer on the human brain.

Fore-brain (thinking, senses, memory), mid-brain (reflex relay), hind-brain (cerebellum for balance, medulla for involuntary actions).

Split the system, then armour the centre. First name the two divisions, then explain the bony and fluid protection of the central parts.

Concept used. The CNS (brain and spinal cord) is the control centre; the PNS (nerves) is the wiring. Because the CNS is delicate, it is shielded by bone and a fluid cushion.

1. Central nervous system. It is made of the brain and the spinal cord, which coordinate the whole body.
2. Peripheral nervous system. It is made of the nerves branching from the brain and spinal cord to every part of the body.
3. Brain's protection. The hard skull, or brain box, surrounds it, and cerebrospinal fluid cushions shocks.
4. Spinal cord's protection. The vertebral column (backbone) encloses it, with the same fluid acting as a shock absorber.

Why this matters. This explains why the medulla-to-cord origin of Question 9 is so carefully shielded: damage to the bony armour can injure the irreplaceable central nervous system.

CNS = brain + spinal cord; PNS = nerves. The brain is guarded by the skull and the spinal cord by the vertebral column, both cushioned by fluid.

Give one sharp function each. For a five-part hormone question, state a single precise function per hormone so each part scores fully.

Concept used. Hormones each control one main process: thyroxin sets metabolism, insulin lowers blood sugar, adrenaline readies the body for stress, growth hormone controls growth, and testosterone drives male development.

1. (a) Thyroxin. Controls the rate of metabolism of carbohydrates, fats and proteins.
2. (b) Insulin. Lowers and balances the blood glucose level.
3. (c) Adrenaline. Raises heart rate and redirects blood flow during fear or excitement.
4. (d) Growth hormone. Controls overall body growth; (e) testosterone brings about male features at puberty and aids fertility.

Why this matters. These crisp one-line functions are the building blocks for the chemical-coordination essay of Question 52 and tie back to the disorder questions 27 and 29.

Thyroxin: metabolism; insulin: lowers blood sugar; adrenaline: emergency response; growth hormone: growth; testosterone: male puberty changes.

List four hormones, one effect each. Cover all four plant hormones and pin a single clear physiological effect to each.

Concept used. The four plant hormones share out the control of growth: auxin lengthens cells, gibberellin grows the stem, cytokinin divides cells, and abscisic acid slows growth and ages tissue.

1. Auxin. Promotes cell elongation and directs phototropic bending of shoots toward light.
2. Gibberellin. Promotes stem elongation and helps seeds sprout.
3. Cytokinin. Promotes cell division, especially in young, actively growing parts.
4. Abscisic acid. Inhibits growth, closes stomata under water stress, and triggers leaf and fruit fall.

Why this matters. This complete hormone list expands the quick matches of Question 35 into a full essay, the standard board long answer on plant hormones.

Auxin: cell elongation; gibberellin: stem growth; cytokinin: cell division; abscisic acid: inhibits growth and causes leaf fall.

Define, illustrate, then trace the path. A strong answer defines reflex action, gives two everyday examples, and walks through the reflex arc in order.

Concept used. A reflex action is an automatic, rapid response managed by the spinal cord. The reflex arc is the fixed nerve pathway: receptor, sensory neuron, spinal cord, motor neuron, effector.

1. Meaning. A reflex action is a quick, involuntary reaction to a stimulus, without conscious control.
2. Examples. Withdrawing the hand from a hot object, and blinking when something nears the eye.
3. The arc begins. A receptor detects the stimulus and the sensory neuron carries the signal to the spinal cord.
4. The arc completes. The spinal cord relays the signal to a motor neuron, which drives the muscle to respond at once.

Why this matters. This long answer combines the reflex-arc sequence of Question 5 with the labelled diagram of Question 34, the exact two skills a board reflex question tests.

Reflex action is a fast involuntary response (hand withdrawal, blinking); the reflex arc carries it: receptor → sensory neuron → spinal cord → motor neuron → muscle.

Show why one system is not enough. Justify the statement by proving that each system covers what the other cannot, so both are needed.

Concept used. The nervous system delivers rapid, targeted, brief responses; the hormonal system delivers slow, widespread, lasting ones. Together they give complete control over body processes.

1. Strength of nerves. They act in a fraction of a second, ideal for quick reactions like reflexes.
2. Limit of nerves. Their effect is short and cannot reach every cell, so they cannot manage slow processes like growth.
3. Role of hormones. Carried by blood, hormones reach distant organs and bring lasting changes such as growth, metabolism and puberty.
4. Teamwork. In a stressful moment, nerves react at once while adrenaline sustains the alert state, so both systems together achieve full coordination.

Why this matters. This justification is the heart of the chapter's title and directly extends the two-system answer of Question 33, the most likely framing of a board ``justify'' question here.

Nerves give fast, short control and hormones give slow, lasting control; since neither alone is enough, the two systems together coordinate the body.

Follow the hormone from gland to target. Explain chemical coordination as a delivery chain: gland makes the hormone, blood carries it, target organ responds.

Concept used. The endocrine system coordinates the body chemically. Hormones secreted into the blood travel to specific target tissues, where they cause defined biochemical or physiological responses.

1. Production. Each endocrine gland secretes its own hormone in small, precise amounts.
2. Transport. The hormone enters the blood, which carries it throughout the body.
3. Targeting. The hormone acts only on its specific target organ or tissue, even if that organ is far away.
4. Response. At the target, the hormone triggers a particular change, such as adjusting metabolism or blood sugar, coordinating the body's activity.

Why this matters. This delivery model explains every hormone action listed in Question 48 and shows why chemical coordination, though slower than nerves, can reach and control the whole body.

Endocrine glands release hormones into the blood, which carries them to specific target organs where they trigger particular responses, coordinating the body chemically.

Explain the one-way valve. The synapse behaves like a one-way valve, and the reason lies in where the chemicals are made and where they are received.

Concept used. The axonal end converts the electrical signal to a chemical one by releasing chemicals; the dendritic end converts the chemical back to electrical. Since only the axon end releases chemicals, the flow cannot reverse.

1. Release point. The axonal end alone stores and releases the chemical messengers when an impulse arrives.
2. Receiving point. The dendritic end of the next neuron receives these chemicals and fires a fresh impulse.
3. No backward release. The dendritic end has no chemicals to send back across the gap, so it cannot signal the axon end.
4. Result. The signal can only pass from axonal end to dendritic end, making the synapse strictly one-way.

Why this matters. This one-way design keeps nerve signals flowing in a single, orderly direction through the body, the deeper reason behind the synapse facts of Questions 3, 21 and 42.

Only the axonal end releases the chemical and only the dendrite receives it, so the synapse passes signals one way, from axon end to dendrite, never in reverse.