(C) It maintains posture and precision of voluntary actions.

The textbook states the cerebellum "is responsible for precision of voluntary actions and maintaining the posture and balance of the body." Option C matches this exactly. The medulla (hind-brain) controls involuntary actions; the fore-brain handles thinking; the mid-brain relays sensory signals.

Answer: B

When a nerve impulse reaches the end of an axon, chemicals are released that cross the synapse (gap) and start a similar electrical impulse in the dendrite of the next neuron.

Source: Chapter 6, Section 6.1 – Animals – Nervous System

---

• The passage clearly states: "At the end of the axon, the electrical impulse sets off the release of some chemicals. These chemicals cross the gap, or synapse, and start a similar electrical impulse in a dendrite of the next neuron."
• Option A is wrong — electrical impulses do not jump across the gap directly; chemical transmission bridges the synapse.
• Options C and D are completely incorrect and not supported by the text.
• Key term to remember: synapse = the gap between two neurons; transmission across it is chemical, not electrical.

Option C: Adrenaline — Adrenal gland

Adrenaline is secreted by the adrenal glands to prepare the body for 'fight or flight' by increasing heart rate, breathing rate, and diverting blood to muscles.

The passage directly states: "This is done in many animals, including human beings, using a hormone called adrenaline that is secreted from the adrenal glands." Examiners expect you to correctly pair the hormone with its source gland — a common 1-mark trap is confusing adrenal (adrenaline) with pituitary (growth hormone) or pancreas (insulin).

(C) In the spinal cord, because reflex arcs are formed in the spinal cord itself, allowing a rapid response without waiting for signals to travel up to the brain.

The passage explicitly states: "Reflex arcs are formed in this spinal cord itself, although the information input also goes on to reach the brain." The key reason is speed — routing through the brain would take too long. Option C in the given answer choices is Spinal cord, which matches. Examiners expect you to link the location (spinal cord) to the reason (faster response).

(D) Abscisic acid

Abscisic acid inhibits growth, promotes seed dormancy, and causes stomatal closure during water stress conditions in plants.

Abscisic acid (ABA) is often called the "stress hormone" of plants. Examiners expect you to remember that it acts opposite to growth-promoting hormones (auxin, gibberellin, cytokinin). The three key functions — growth inhibition, seed dormancy, and stomatal closure — are the standard points tested in MCQs on plant hormones.

(C) Stimulation of the development of secondary sexual characters in males

This is a function of testosterone, not thyroxin. Thyroxin regulates metabolism, body temperature, heartbeat, and physical/mental development.

The question asks which option is NOT a function of thyroxin. The source passage (Chapter 7) clearly states that testosterone brings about changes in appearance (secondary sexual characters) in boys at puberty — not thyroxin. Thyroxin's functions include regulation of carbohydrate, protein, and fat metabolism, heart rate, body temperature, and childhood development. Examiners expect students to distinguish between hormones and their specific roles.

(B) Cranial nerves and spinal nerves

The peripheral nervous system consists of cranial nerves arising from the brain and spinal nerves arising from the spinal cord, which connect the CNS to the rest of the body.

Source: Chapter 6, Section 6.1.2 Human Brain

---

The passage directly states: "the peripheral nervous system consisting of cranial nerves arising from the brain and spinal nerves arising from the spinal cord." Option A describes the CNS, not PNS. Options C and D are incorrect. For MCQs, briefly state the correct option letter and a one-line justification — examiners award the mark for the correct choice supported by a reason.

Answer: B

Auxin, synthesised at the shoot tip, diffuses to the shady side of the shoot, causing cells there to grow longer, making the shoot bend towards light.

Source: Chapter 6, Section 6.2.2 – Movement Due to Growth

The passage explicitly states: "a hormone called auxin, synthesised at the shoot tip, helps cells grow longer… auxin diffuses towards the shady side… the plant appears to bend towards light." This makes B the only correct option. Remember: auxin = shoot tip, moves to shady side = bending towards light.

(C) Dwarfism — stunted growth and short stature

The question tests knowledge of endocrine gland disorders. A deficiency of growth hormone (secreted by the pituitary gland) during childhood leads to dwarfism (stunted/short stature). Excess growth hormone causes gigantism. Cretinism results from thyroid hormone deficiency; diabetes mellitus involves insulin deficiency — not growth hormone.

Answer: B

Plant cells change shape by changing the amount of water in them, causing swelling or shrinking, which results in movement without any muscle tissue.

Source: Chapter 6, Section 6.2.1 – Immediate Response to Stimulus

---

The passage explicitly states: "plant cells change shape by changing the amount of water in them, resulting in swelling or shrinking." Options A and C describe animal mechanisms (electrical impulses and contractile proteins). Option D is incorrect — auxin promotes cell elongation in growth movements, it does not directly contract the cell wall. Examiners expect you to identify osmotic change (water content) as the key mechanism.

(B) Testosterone — development of male secondary sexual characteristics at puberty

The source passage (Chapter 7, Male Reproductive System) explicitly states that "testosterone brings about changes in appearance seen in boys at the time of puberty," directly supporting option B. The other options incorrectly pair hormones with unrelated processes.

Option C — A centre associated with hunger in a separate part of the fore-brain (the hypothalamus) is responsible for the sensation of hunger.

Source: Control and Coordination, Chapter 6

---

The hypothalamus, located in the fore-brain, contains centres that regulate hunger, thirst, and body temperature. Examiners expect students to identify Option C and, if asked, name the hypothalamus as the specific region. Options A and B relate to the cerebrum; Option D is incorrect as olfactory lobes are not in the mid-brain and are not linked to hunger.

Option C — The brain is protected by a bony box (skull) and a fluid-filled balloon (for shock absorption); the spinal cord is protected by the vertebral column.

The passage (6.1.3) explicitly states: brain sits inside a bony box, further protected by a fluid-filled balloon; the vertebral column protects the spinal cord. Options A and D reverse or incorrectly assign these structures. Examiners expect both protective structures for the brain to be identified for full credit.

Source: Chapter 6, Section 6.1.3 — How are these Tissues protected?

Option A is correct. Both A and R are true, and R is the correct explanation of A. Iodised salt is recommended because iodine is needed for thyroxin synthesis; its deficiency leads to goitre.

The Assertion is a well-known dietary recommendation, and the Reason correctly states the biological purpose of iodine (thyroxin synthesis by the thyroid gland) and the consequence of its deficiency (goitre). Since R directly and completely explains why A is true, option A is the right choice. Note: This topic is from Life Processes (Biology), not the Chemistry chapters provided — but the logic of A-R questions requires checking whether R is the correct explanation of A, which it clearly is here.

Answer: C

Assertion (A) is true — reflex actions occur without conscious thought. Reason (R) is false — in a reflex arc, the nerve signal travels from receptor → spinal cord → effector muscle, bypassing the brain for the response.

The key error in R is the claim that the signal travels to the brain before reaching the effector. The textbook clearly states that reflex arcs are formed in the spinal cord itself, allowing a quick response without waiting for the brain. Information does reach the brain, but only after (or simultaneously with) the reflex response — the brain is not in the reflex arc pathway. That is precisely why reflexes are fast and unconscious (A is true), but R gives the wrong mechanism, making it false.

Option A — Both A and R are true, and R is the correct explanation of A. Hormones are synthesised in one part of the plant and diffuse to another part to bring about the desired effect.

Source: What you have learnt, Chapter 6

The key summary point states: "Hormones produced in one part of an organism move to another part to achieve the desired effect." This directly supports both A (synthesised away from site of action) and confirms R (diffusion/movement) as the correct explanation of A. Choose option A when R logically explains why A is true.

Answer: (D) A is false but R is true.

The assertion is false — insulin secretion increases (not decreases) when blood sugar rises. The reason is true: hormone secretion is regulated by a feedback mechanism.

The feedback mechanism (R) is correct as stated in Chapter 6. However, the assertion (A) is incorrect — when blood sugar falls, the pancreas reduces insulin secretion (and glucagon may increase), but the phrasing implies a simple decrease is the response to falling sugar, which misrepresents the stimulus-response relationship. The key error is in A, while R stands as a true, general principle. Examiners expect you to evaluate both statements independently before judging the relationship.

Option B — Both A and R are true, but R is NOT the correct explanation of A.

Electrical impulses are faster than chemical signals (A is true). Chemical signals (hormones) do reach all cells via blood regardless of nervous connections (R is true), but this wide reach explains the advantage of chemical signals, not why electrical impulses are faster.

Source: Chapter 6, Section 6.3 (Hormones in Animals)

• A is true: The textbook states reflex arcs evolved because "the thinking process of the brain is not fast enough," implying electrical/nerve impulses are the faster mode.
• R is true: The textbook explicitly states that chemical signals "would reach all cells of the body" unlike electrical impulses via nerve cells whose range is limited.
• R does NOT explain A: R describes an advantage of chemical signals (wide reach), not the reason electrical impulses are faster. These are two separate facts about two different systems. Hence option B.

The dendritic tip of a nerve cell first receives information from the environment.

This information sets off a chemical reaction that creates an electrical impulse. The impulse travels from the dendrite → cell body → along the axon to its end. There, it triggers release of chemicals that cross the synapse and start a similar electrical impulse in the dendrite of the next neuron.

Source: Chapter 6, Section 6.1 – Animals – Nervous System

---

• Part 1 (1 mark): Name the part — "dendritic tip." Just "dendrite" is also acceptable but "dendritic tip" is the exact textbook term.
• Part 2 (1 mark): Describe the conversion (chemical → electrical impulse) and the pathway (dendrite → cell body → axon → synapse → next neuron). Examiners look for: the word electrical impulse, the correct sequence, and mention of the synapse. Missing the synapse or the chemical-to-electrical conversion will lose marks.

The cerebellum (part of the hind-brain) is likely damaged.

Justification: The cerebellum is responsible for precision of voluntary actions and maintaining the posture and balance of the body. Since the patient stumbles and cannot maintain balance despite healthy leg muscles, the cerebellum must be malfunctioning.

Source: Human Brain, Chapter 6

---

• Always name the specific part (cerebellum, not just "hind-brain") for full credit.
• The justification must link the function of the cerebellum (posture and balance) directly to the symptom described (stumbling, loss of balance).
• Mentioning that leg muscles are healthy rules out a muscular problem — the fault lies in coordination, which points to the cerebellum. This logical link earns the second mark.

Voluntary actions are those performed with conscious thought and control (e.g., writing).
Involuntary actions occur without conscious control (e.g., heartbeat).

The medulla in the hind-brain controls involuntary actions such as heartbeat and breathing.

Source: Chapter 6, Section 6.1.2 Human Brain

---

• Examiners expect a clear one-line distinction for each type, with one example each — don't elaborate.
• The key fact for the second part is medulla (hind-brain) — writing just "brain" is too vague and may lose the mark.
• The passage explicitly states: "All these involuntary actions including blood pressure, salivation and vomiting are controlled by the medulla in the hind-brain." Heartbeat and breathing follow the same principle.

The movement of touch-me-not leaves is independent of growth — cells change shape by losing water (turgor change), causing rapid folding.

The bending of a seedling towards light is dependent on growth — the hormone auxin causes unequal growth on the two sides of the shoot, making it bend towards light.

Source: Chapter 6, Sections 6.2 and 6.2.2

---

Examiners look for two things: (1) the correct basis of each movement stated clearly, and (2) one reason for each. Touch-me-not = turgor/water change (no growth); seedling bending = differential growth due to auxin. Use the terms "growth-independent" and "growth-dependent" to signal you know the core distinction. Don't confuse the two — mixing them up is the most common error.

Two physiological changes when adrenaline is released:

1. Heart beats faster — This supplies more oxygen to the muscles, preparing the body for immediate physical activity like fighting or running.

1. Breathing rate increases — Contractions of the diaphragm and rib muscles increase breathing rate, ensuring more oxygen intake for greater energy release.

Source: Chapter 6, Section 6.3 — Hormones in Animals

---

The examiner expects two distinct changes with a brief reason for each. Both points are directly from the passage about adrenaline's effect. Common mistakes: writing only one change, or listing changes without explaining why each helps. Note that blood diversion to skeletal muscles (reduced supply to digestive system/skin) is also valid as a third point if asked. Stick to the textbook's exact language — "heart beats faster," "breathing rate increases" — to score full marks.

Insulin is a protein (hormone). If taken orally, it would be digested by proteases in the alimentary canal and become inactive before reaching the bloodstream. Hence, it must be injected directly into the blood.

Role of insulin: It regulates blood glucose levels by promoting the uptake of glucose by body cells, thereby lowering blood sugar.

• The key point examiners look for is that insulin is a protein and would be broken down by digestive enzymes if taken orally — this is the core reason for injection.
• The second part on insulin's role (regulating/lowering blood glucose) must be included for full marks.
• This question is based on general NCERT knowledge from Chapter 7 (Control and Coordination); the source passages do not directly cover it, so standard textbook knowledge applies.
• Keep both parts concise — one line each is sufficient for 2 marks.

Geotropism is the movement of a plant part in response to the force of gravity.

• Positive geotropism: Movement of a plant part towards gravity.

Example: Roots grow downward into the soil — they show positive geotropism.

• Negative geotropism: Movement of a plant part away from gravity.

Example: The stem grows upward against gravity — it shows negative geotropism.

When a seed germinates, the root grows down (positive geotropism) and the shoot grows up (negative geotropism), ensuring the plant is properly anchored and exposed to sunlight.

Source: Chapter 6, Section 6.2 – Coordination in Plants

---

• 1 mark for defining geotropism correctly (response to gravity).
• 1 mark for positive geotropism + example (root grows downward).
• 1 mark for negative geotropism + example (stem grows upward).
• Always mention direction (towards/away from gravity) — that's what distinguishes the two types.
• The passage mentions "the root goes down, the stem comes up" when a seed germinates — use this directly as your example.
• Do not confuse geotropism with phototropism (response to light).

Diagram of a Neuron:

```
Dendrites
|||
_______|||_______
| Cell Body |
| (Nucleus) |
|_________________|
|
| Axon
|
[Myelin Sheath]
|
Nerve Endings
```

(Students must draw a proper labelled diagram showing: dendrites, cell body/nucleus, axon, myelin sheath, and nerve endings.)

Two parts and their functions:

1. Dendrites – They receive information (nerve impulses) from the environment or from other neurons and pass it to the cell body.

1. Axon – It carries the electrical impulse from the cell body to its end, where the impulse is converted into a chemical signal for transmission to the next neuron.

Source: Chapter 6, Section 6.1

---

• The diagram carries marks — label at least 4–5 parts clearly (dendrite, cell body, nucleus, axon, nerve endings/synaptic knob).
• For the two functions, examiners look for specific roles: dendrite = receives impulse; axon = transmits impulse away from cell body.
• Avoid vague answers like "it helps in communication" — use precise terms: electrical impulse, chemical signal, synapse.
• You may choose any two labelled parts, but dendrite and axon are the safest choices as their functions are explicitly stated in the textbook.

At the synapse: When an electrical impulse reaches the end of an axon, it triggers the release of chemicals (neurotransmitters) into the synaptic gap. These chemicals cross the gap and start a similar electrical impulse in the dendrite of the next neuron.

Why chemical signal is important: A chemical signal is necessary because it allows control over whether the next neuron fires or not. It enables signal amplification, integration of multiple inputs, and communication with non-nervous cells like muscle and gland cells. A direct electrical connection would not allow such regulation or flexibility in the body's responses.

Source: Chapter 6, Section 6.1 – Animals: Nervous System

---

• Examiners expect two clear parts: what happens at the synapse (process) and why chemical conversion is important (reason/advantage).
• Key terms to use: synapse, chemicals/neurotransmitters, electrical impulse, dendrite, axon.
• The textbook passage implies the chemical step allows transmission to muscle/gland cells too — mention this for full marks.
• Avoid vague phrases like "it helps the body" — be specific about regulation and flexibility.

Type of movement: Tropic movement (growth-dependent movement), specifically thigmotropism.

Mechanism: The tendril of a pea plant is sensitive to touch. When it comes in contact with a support, the part of the tendril touching the object grows less rapidly than the part away from the object. This unequal growth on the two sides causes the tendril to curve and coil around the support, allowing the plant to cling to it.

Source: Chapter 6, Section 6.2.2 – Movement Due to Growth

---

• Examiners expect you to name the movement type (tropic/thigmotropic) and then explain the differential growth mechanism — slower growth on the contact side vs. faster growth on the free side.
• The key phrase is "unequal/differential growth" — state it clearly.
• Do not confuse this with the sensitive plant's movement, which is growth-independent; tendril coiling is growth-dependent.
• Auxins are involved in causing this differential growth, but the passage focuses on the growth-rate difference — stick to what the source states unless the question specifically asks about auxins.

Nervous System: Uses electrical impulses transmitted through neurons to carry information rapidly to specific target organs (muscles/glands). Actions can be reflex, voluntary, or involuntary.

Endocrine System: Uses hormones secreted into the bloodstream to carry chemical signals to target organs. A feedback mechanism regulates hormone release.

Comparison:
| Feature | Nervous System | Endocrine System |
|---|---|---|
| Medium | Electrical impulses | Chemical (hormones) |
| Speed | Fast | Slow |
| Effect | Short-lived, specific | Long-lasting, widespread |

Advantage of Nervous System: Response is extremely fast, enabling immediate reactions (e.g., reflex actions).

Advantage of Endocrine System: Hormones travel through blood and reach all cells of the body, enabling widespread, long-lasting effects (e.g., adrenaline prepares entire body for fight-or-flight).

Source: Chapter 6, Sections 6.1 and 6.3

---

• Examiners expect a direct comparison of speed, medium, and duration, plus one advantage each — clearly stated.
• The fight-or-flight example (squirrel/adrenaline) from the textbook is the ideal illustration for the endocrine advantage.
• A small table saves words and scores well for comparison questions.
• Do not write long paragraphs — this is 3 marks, so stay crisp.

(i) Production: Auxin is synthesised at the shoot tip.

(ii) Redistribution: When light falls from one side, auxin diffuses towards the shady side of the shoot, making its concentration higher there.

(iii) Cellular mechanism: The higher concentration of auxin causes cells on the shady side to grow longer than cells on the light side. This unequal elongation makes the shoot bend towards the light.

Source: Chapter 6, Section 6.2.2 – Movement Due to Growth

---

The passage gives all three points explicitly. Examiners look for three distinct, labelled steps: (1) site of synthesis = shoot tip, (2) lateral diffusion to shady side, (3) differential cell elongation causing bending. Use the exact textbook terms — "auxin," "shoot tip," "shady side," "grow longer" — to score full marks. Avoid vague phrases like "moves to one side" without specifying which side.

(i) Reflex Arc — Definition, Diagram & Sequence of Events

A reflex arc is the nervous pathway through which a reflex action occurs — connecting the receptor to the effector via the spinal cord, without involving the brain.

Labelled Diagram:

```
Receptor (skin) → Sensory neuron → Spinal cord (relay neuron)
↓
Effector (muscle) ← Motor neuron ←────────
```
(Label: Receptor → Sensory neuron → Spinal cord → Motor neuron → Effector/Muscle)

Sequence of events on stepping on a sharp object:

1. Receptor (pain receptors in foot) detects the stimulus.
2. Electrical impulse travels along sensory neuron to the spinal cord.
3. Relay neuron in spinal cord processes the signal and passes it on.
4. Impulse travels along motor neuron to the leg muscles (effector).
5. Muscle contracts — foot is pulled away instantly.
6. Simultaneously, information is sent to the brain, which registers pain after the action.

(ii) Why Reflex Arcs Evolved

Reflex arcs evolved because the brain's thinking process is too slow for urgent responses — by the time a conscious decision is made, serious harm could occur (e.g., getting burnt). Many simpler animals lack complex neuron networks, so reflex arcs provided efficient survival responses.

Even in animals with highly developed brains, reflex arcs continue to be useful because they allow much quicker responses than brain-directed actions, as the signal need only travel to the spinal cord and back, not all the way to the brain.

Source: Chapter 6, Section 6.1.1 — What happens in Reflex Actions?

---

• Definition must mention: pathway, receptor to effector, spinal cord involvement — examiners check all three.
• The diagram must be labelled with at least: receptor, sensory neuron, spinal cord/relay neuron, motor neuron, effector/muscle. Even a simple block-arrow diagram earns marks if correctly labelled.
• For the sequence, using numbered steps is cleaner and easier to mark. The key detail examiners look for is that the brain is informed after the action, not before.
• For part (ii), directly use the textbook reasoning: brain is slow → reflex arc bypasses it → faster response. Also note: simpler animals with no thinking tissue rely entirely on reflex arcs.
• Avoid writing an essay — tight, numbered points score better in board exams.

(i) Diagram of the Human Brain

(Draw and label: Fore-brain (cerebrum), Mid-brain, Hind-brain — showing Cerebellum and Medulla oblongata)

```
[FORE-BRAIN / Cerebrum]
/
[MID-BRAIN]
\
[HIND-BRAIN]
├── Cerebellum
└── Medulla Oblongata
|
Spinal Cord
```
(A neat diagram with the above three regions and parts labelled is expected.)

---

(ii) Functions:

• Fore-brain: Main thinking part; receives sensory impulses (hearing, smell, sight); interprets and stores information; controls voluntary muscles; contains centres for hunger and other sensations.

• Cerebellum: Part of hind-brain; responsible for precision of voluntary actions and maintaining posture and balance of the body (e.g., walking straight, riding a bicycle).

• Medulla Oblongata: Controls involuntary actions — blood pressure, salivation, vomiting, heartbeat, and breathing.

---

(iii) Medulla oblongata controls vital involuntary functions like heartbeat and breathing. Damage stops these immediately, making it life-threatening. Cerebellum controls balance and coordination of voluntary actions — serious but not immediately fatal, as these functions can partially be managed or compensated over time.

Source: Chapter 6, Section 6.1.2 — Human Brain

---

• For the diagram, even a simple schematic with correct labels of all three major regions (fore-brain, mid-brain, hind-brain) and key parts (cerebellum, medulla) fetches full marks. Neatness matters.
• Examiners look for specific key terms: "precision of voluntary actions," "posture and balance" for cerebellum; "involuntary actions," "blood pressure, salivation, vomiting" for medulla.
• For part (iii), the logic is simple: medulla = vital functions (heart, breathing) → immediate death if damaged; cerebellum = coordination only → serious but survivable. State this contrast clearly.

(i) Two Categories of Plant Movement:

Growth-independent movement: When the leaves of a sensitive plant (Mimosa pudica) are touched, they fold and droop rapidly. No growth is involved. The plant communicates the touch stimulus cell to cell using electrical-chemical signals. Cells then change shape by changing the amount of water in them (swelling or shrinking), causing movement.

Growth-dependent movement: The directional growth of a seedling (e.g., stem growing upward toward light — phototropism) is caused by growth. If growth is prevented, movement does not occur.

(ii) Four Plant Hormones:

| Hormone | Promotes/Inhibits | Specific Function |
|---|---|---|
| Auxin | Promotes | Causes phototropism by promoting cell elongation on the shaded side |
| Gibberellin | Promotes | Promotes stem elongation and seed germination |
| Cytokinin | Promotes | Promotes cell division |
| Abscisic acid | Inhibits | Promotes wilting of leaves; inhibits growth under stress |

Source: Chapter 6, Section 6.2 and 6.2.1

---

• The passage explicitly states the two movement types — examiners expect both named and explained with examples.
• The mechanism for growth-independent movement (water movement in cells) is directly from Section 6.2.1 — quote this closely.
• For hormones, the textbook lists auxin, gibberellin, cytokinin, and abscisic acid. Abscisic acid is the main inhibitor. Memorise one function each — examiners check specificity.
• The table format is acceptable and saves space in a 5-mark answer.

(a) The action described is a reflex action. It is coordinated by the spinal cord (part of the central nervous system).

(b) Pathway of the nerve signal:

Hot iron → Receptor (skin) → Sensory neuron → Spinal cord (relay neuron forms reflex arc) → Motor neuron → Effector (muscle of hand) → Hand is withdrawn

The impulse travels from the dendritic tip of the sensory neuron to the cell body, along the axon, crosses the synapse via chemicals, and reaches the next neuron, finally reaching the muscle.

(c) Although the spinal cord coordinates the reflex action, the sensory information also travels upward to the brain simultaneously. The brain receives and processes this signal a moment later, which is when Rahul consciously feels the pain.

Source: Chapter 6, Section 6.1.1 — Reflex Actions

---

• (a) Always name both — the type of action (reflex) AND the coordinating part (spinal cord). Missing either loses the mark.
• (b) Examiners expect a clear, sequential pathway with all components: receptor → sensory neuron → spinal cord → motor neuron → effector. Mention of synapse/relay neuron adds accuracy.
• (c) The key phrase from the textbook is: "the information input also goes on to reach the brain" — this explains the delayed conscious perception of pain. Make sure to say the signal reaches the brain after the reflex is already complete.

(a) Patient A likely has a deficiency of thyroxin (thyroid hormone). Thyroxin is secreted by the thyroid gland. Deficiency of the dietary mineral iodine prevents the thyroid gland from producing enough thyroxin, causing the gland to swell — a condition called goitre.

(b) Patient B likely has a deficiency of growth hormone, secreted by the pituitary gland. Insufficient growth hormone during childhood leads to stunted growth despite no genetic abnormality.

(c) The body uses a feedback mechanism to regulate hormone levels. When the level of a hormone rises or falls beyond the required amount, a signal is sent to the concerned gland to adjust its secretion accordingly.

Source: Control and Coordination (Chapter 6), What you have learnt

---

• (a) Goitre (swollen neck) is the classic sign of iodine-deficiency leading to low thyroxin. Examiners expect all three: hormone name, gland, and mineral.
• (b) Stunted growth with no genetic cause points to pituitary growth hormone deficiency. Both hormone and gland must be named for full marks.
• (c) "Feedback mechanism" is the exact NCERT term — use it. A brief description of how it works earns the mark. Don't write "negative feedback" specifically unless asked; "feedback mechanism" is sufficient at this level.

(a)
(i) Shoot — positive phototropism (bends towards light).
(ii) Root — negative phototropism (bends away from light).
These responses are opposite in direction.

(b)
The hormone responsible is auxin, produced at the shoot tip.
When light falls from one side, auxin diffuses to the shady side of the shoot. This causes cells on the shady side to elongate more than cells on the light side, making the shoot curve/bend towards the light (the hole in the box).

(c)
Geotropism — roots grow downwards (towards gravity) and shoots grow upwards (away from gravity) in response to the pull of the earth.
(Alternate: Chemotropism — growth of pollen tubes towards ovules.)

Source: Chapter 6, Section 6.2.2 — Movement Due to Growth

---

• (a) Examiners want both the correct term AND the direction qualifier (positive/negative). Stating they are "opposite" is essential for the 1 mark.
• (b) Three points are needed for 2 marks: name of hormone → where produced → mechanism (differential elongation). Don't just name auxin; explain the unequal distribution and resulting unequal cell growth.
• (c) Any valid tropism with a correct example scores the mark. Geotropism is the safest choice as it is explicitly in the textbook; chemotropism (pollen tube → ovule) is also directly mentioned and acceptable.