Previous Year Questions: Control and Coordination - Class 10 PDF Download

Ans: (d) Auxins
Role of Auxins: Auxins are a class of plant hormones responsible for cell elongation in shoots, particularly in response to light (phototropism). When a plant shoot is exposed to unidirectional light, auxins accumulate on the shaded side due to their movement away from light. This higher concentration promotes cell elongation on the shaded side, causing the shoot to bend toward the light source, a phenomenon known as positive phototropism.

Other Options:

• Cytokinins: These hormones promote cell division, primarily in meristematic tissues (e.g., shoot and root tips), and are not directly involved in cell elongation for phototropism.

• Gibberellins: These stimulate overall stem elongation and seed germination but do not specifically mediate light-directed growth.

• Adrenaline: This is an animal hormone (produced in adrenal glands) involved in stress responses, not present in plants.

Mechanism: The uneven distribution of auxins is triggered by light perception in the shoot tip, leading to differential growth. For example, in a sunflower shoot, auxin accumulation on the darker side causes those cells to elongate more, bending the shoot toward sunlight.
Significance: This ensures plants maximize photosynthesis by orienting toward light, critical for survival.

Ans: (c) Change in amount of water in cells
Chhui-mui Plant Response: The chhui-mui or touch-me-not plant (Mimosa pudica) exhibits a rapid, non-growth-dependent movement called thigmonasty when its leaves are touched. This is not a tropic movement (directional, growth-dependent) but a nastic movement triggered by touch.
Mechanism of Folding: The folding occurs due to changes in turgor pressure in specialized cells called pulvini at the base of leaflets and petioles. When touched, a mechanical stimulus triggers an electrical signal (action potential) that causes ion movement (e.g., potassium ions) out of pulvinar cells. This leads to water loss from these cells via osmosis, reducing turgor pressure and causing the leaves to fold and droop.

Other Options:

• Hormonal effect: Hormones like auxins or abscisic acid are not directly responsible; the response is due to turgor changes, not hormone-mediated growth.
• Thermal effect: Temperature changes are not involved in this rapid response.
• Electromagnetic effect: No electromagnetic forces are at play; the response is biomechanical and biochemical.

Significance: This rapid folding may protect the plant from herbivores or environmental stress by reducing exposed surface area.
Example: If you touch a Mimosa leaf, within seconds, the leaflets fold inward, and the entire leaf may droop, demonstrating a turgor-driven response.

Ans: (c) Cerebellum
Cerebellum’s Role: The cerebellum, located in the hind-brain, is responsible for coordinating voluntary movements, maintaining posture, and ensuring balance. It integrates sensory input from the eyes, ears (vestibular system), and muscles to fine-tune motor actions, ensuring smooth and precise movements.
Mechanism: The cerebellum receives information about body position from proprioceptors (sensors in muscles and joints) and the vestibular system (inner ear). It processes this data to adjust muscle contractions, maintaining equilibrium and posture. For example, when walking on uneven ground, the cerebellum coordinates leg muscles to prevent falling.

Other Options:

• Pons: Acts as a relay station between the cerebrum and cerebellum, also regulates breathing, but does not directly control posture or balance.
• Cerebrum: Responsible for higher functions like thinking, memory, and voluntary actions, not posture.
• Medulla: Controls involuntary actions like heart rate and breathing, not balance.

Significance: Damage to the cerebellum (e.g., due to injury) can lead to ataxia, causing unsteady gait or difficulty maintaining posture, as seen in drunken walking.

Ans: (b) Cytokinins
Cytokinins’ Role: Cytokinins are plant hormones that stimulate cell division (cytokinesis) in meristematic tissues, such as shoot tips, root tips, and developing seeds. They are found in high concentrations in areas of active cell division, promoting growth in these regions.
Mechanism: Cytokinins work with auxins to regulate cell division and differentiation. For example, in tissue culture, cytokinins promote shoot formation by enhancing cell division in callus tissue.
Other Options:

• Auxin: Promotes cell elongation, especially in shoots, and is involved in tropisms, not primarily cell division.
• Gibberellins: Stimulate stem elongation, seed germination, and flowering but are less specific to cell division.
• Abscisic acid: Inhibits growth, promotes dormancy, and triggers leaf fall, not cell division.

Significance: Cytokinins ensure rapid growth in meristems, essential for plant development, such as in seed germination or shoot branching.

Ans: (b) Both A and R are true, but R is not the correct explanation of A.

• Assertion (a): True. Writing and talking involve voluntary actions controlled by the nervous system. The brain (cerebrum) sends signals via motor neurons in the peripheral nervous system (PNS) to skeletal muscles, enabling movements like hand motion for writing or tongue movement for speech.
• Reason (R): True. The peripheral nervous system comprises cranial nerves (e.g., controlling facial muscles) and spinal nerves (e.g., controlling limb muscles), which transmit signals between the central nervous system (CNS) and the body.
• Why R is not the explanation: R describes the components of the PNS but does not specifically explain how the nervous system communicates with muscles for writing or talking. The actual mechanism involves motor neurons in the PNS, initiated by the cerebrum, not just the existence of cranial/spinal nerves.

Ans: (d) Chemotropism

• Chemotropism: This is the directional growth of a plant part (e.g., pollen tube) in response to a chemical stimulus. In flowering plants, pollen tubes grow toward ovules due to chemical attractants (e.g., sugars or proteins) secreted by the ovule or stigma, guiding fertilization.
• Mechanism: After pollination, chemicals from the ovule create a gradient that directs pollen tube growth through the style to the ovule, ensuring successful fertilization.

Other Options:

• Phototropism: Growth toward light (e.g., shoots bending toward sunlight).
• Hydrotropism: Growth toward water (e.g., roots growing toward moisture).
• Geotropism: Growth in response to gravity (e.g., roots growing downward).

Significance: Chemotropism ensures precise pollen tube growth, critical for plant reproduction.

Ans: (d) Medulla

• Medulla Oblongata’s Role: Located in the hind-brain, the medulla controls involuntary actions essential for survival, including salivation (via salivary gland stimulation), vomiting (via the vomiting center), heart rate, and breathing.
• Mechanism: The medulla contains nuclei that regulate autonomic functions. For example, it triggers salivation in response to food stimuli and coordinates vomiting to expel harmful substances.

Other Options:

• Cerebellum: Coordinates voluntary movements and balance, not involuntary actions like salivation.
• Cerebrum: Handles conscious activities like thinking and voluntary movements.
• Pons: Relays signals between brain regions and assists in breathing but does not directly control salivation or vomiting.

Significance: The medulla’s control ensures automatic responses without conscious effort, vital for homeostasis.

Ans: (b) Hunger

• Medulla Oblongata Functions: The medulla, in the hind-brain, regulates involuntary actions like salivation (via salivary glands), vomiting (via the vomiting center), and blood pressure (via the vasomotor center).
• Hunger Regulation: Hunger is controlled by the hypothalamus, located in the forebrain, which monitors blood glucose and hormone levels to regulate appetite.
• Mechanism: The medulla responds to stimuli (e.g., food for salivation, toxins for vomiting) via autonomic pathways, while hunger involves complex signals processed by the hypothalamus, not the medulla.
• Significance: This distinction highlights specialized brain regions for different functions, with the medulla handling autonomic survival actions.

Ans: (b) Cytokinins and Abscisic acid\

• Cytokinins: These hormones promote rapid cell division (cytokinesis) in seeds, shoot tips, and roots, enhancing growth in meristematic regions. For example, during seed germination, cytokinins stimulate cell division in the embryo.
• Abscisic Acid (ABA): ABA induces wilting and leaf fall by promoting senescence and closing stomata during water stress, reducing turgor pressure and causing leaves to droop.

Other Options:

• Auxins: Promote cell elongation, not division, and are involved in tropisms.
• Gibberellins: Enhance stem elongation and seed germination but are not specific to cell division or wilting.

Significance: Cytokinins drive growth, while ABA protects plants under stress, balancing development and survival.

Ans: (b) Dendrite → Cell body → Axon → Nerve ending

• Neuron Structure and Signal Flow: A neuron consists of dendrites (receive signals), cell body (processes signals), axon (transmits signals), and nerve endings (release signals to the next cell).
• Mechanism: An electrical impulse (action potential) begins when dendrites receive a stimulus (e.g., neurotransmitter binding). The signal travels to the cell body, where it is integrated. If strong enough, it propagates along the axon as an action potential and reaches the nerve endings, releasing neurotransmitters to the next neuron or effector.

​Other Options:

• A: Incorrect, as signals do not start at nerve endings.
• C: Incorrect, as dendrites, not the cell body, receive the initial stimulus.
• D: Incorrect, as the cell body is not the final destination.

Significance: This unidirectional flow ensures efficient communication in the nervous system, critical for coordinated responses.

Ans:

• Parts of Hind-Brain: Cerebellum, Pons, Medulla Oblongata.
• Part Controlling Involuntary Actions: Medulla Oblongata.

Hind-Brain Components:

• Cerebellum: Coordinates voluntary movements, posture, and balance.
• Pons: Acts as a bridge, relaying signals between the cerebrum and cerebellum, and assists in breathing regulation.
• Medulla Oblongata: Controls involuntary actions, including blood pressure (via the vasomotor center), salivation (via salivary gland stimulation), vomiting, and heart rate.

Mechanism: The medulla contains autonomic nuclei that regulate these functions without conscious control. For example, it adjusts blood pressure by controlling blood vessel constriction and stimulates salivary glands in response to food.
Significance: The medulla ensures survival by maintaining essential autonomic functions, freeing the conscious brain for higher tasks.

Ans:
(a) Root: Geotropism (positive); Stimulus: Gravity.
(b) Shoot: Phototropism (positive); Stimulus: Light.

• Root (Geotropism): Roots exhibit positive geotropism, growing downward in response to gravity. This is mediated by auxins, which accumulate on the lower side of the root, inhibiting elongation there and causing downward growth.
• Shoot (Phototropism): Shoots show positive phototropism, growing toward light. Auxins accumulate on the shaded side, promoting cell elongation and causing the shoot to bend toward light.

Mechanism:

• In roots, gravity causes auxin redistribution, directing growth downward to anchor the plant and access water.
• In shoots, light triggers auxin movement to the shaded side, enhancing growth toward light for photosynthesis.

Significance: These tropic movements optimize plant survival by ensuring roots access resources and shoots maximize light capture.

Ans:
The fore-brain, particularly the cerebrum, is critical for planning and executing the task of making a shopping list.
Cerebrum’s Role: The cerebrum, the largest part of the fore-brain, handles higher cognitive functions like memory, decision-making, and voluntary actions.
Mechanism in List-Making:

• Frontal Lobe: Involved in planning and decision-making, it helps decide what items are needed based on requirements (e.g., groceries for the week).
• Temporal Lobe: Recalls memories of previous purchases or household needs, aiding in list creation.
• Parietal Lobe: Processes sensory information (e.g., visualizing the kitchen to check stock).
• Motor Cortex: Coordinates hand movements for writing the list, sending signals via motor neurons to hand muscles.

Example: When listing “milk,” the cerebrum recalls low milk stock (memory), decides to include it (planning), and directs the hand to write (motor control).
Significance: The cerebrum integrates cognitive and motor functions, ensuring the task is performed efficiently and accurately.

Ans:
(a) Brain Protection: The brain is protected by the skull, meninges (dura mater, arachnoid, pia mater), and cerebrospinal fluid (CSF).
(b) Region and Part of Brain: Hind-brain, Cerebellum.

Brain Protection:

• Skull: The bony cranium provides a hard protective barrier against physical injury.
• Meninges: Three layers (dura mater, arachnoid, pia mater) cushion the brain and prevent infections.
• Cerebrospinal Fluid (CSF): Surrounds the brain, absorbing shocks and maintaining pressure stability.

Cerebellum’s Role in Posture and Balance:

• The cerebellum, in the hind-brain, integrates sensory inputs from proprioceptors (muscles/joints), the vestibular system (inner ear), and visual cues to coordinate muscle movements for posture and balance.
• Mechanism: It fine-tunes motor signals to ensure smooth movements. For example, when standing, it adjusts leg muscle contractions to maintain upright posture.
• Symptoms of Dysfunction: A patient with cerebellar damage may exhibit ataxia, unsteady gait, or difficulty balancing, as seen in neurological disorders.

• Significance: These protective mechanisms and the cerebellum’s role ensure the brain’s safety and the body’s ability to maintain stability during movement.

Ans:
(a) Communication of Information: The stimulus of touch is communicated via electrical-chemical signals. When leaves are touched, mechanoreceptors in the pulvini (swollen leaf bases) detect the stimulus, generating an action potential (electrical impulse) that spreads through cells, triggering ion movement and subsequent response.
(b) Enabling Observable Response: Changes in turgor pressure in pulvinar cells cause the response. The action potential leads to potassium and chloride ion movement out of cells, reducing water content via osmosis, causing cells to lose turgor and the leaves to fold.
(c) Differentiation from Pea Tendril Movement:

• Chhui-mui Movement: Thigmonasty, non-directional, rapid, reversible, due to turgor changes, not growth.
• Pea Tendril Movement: Thigmotropism, directional (toward contact), growth-dependent, permanent, mediated by auxins promoting cell elongation on the opposite side of contact.

Communication: Unlike animals, plants lack a nervous system but use electrical signals (action potentials) and chemical changes (ion fluxes) to transmit stimuli. In Mimosa pudica, touch triggers rapid signal propagation to pulvini.
Turgor Mechanism: The loss of water in pulvinar cells collapses them, folding leaves. This is reversible as cells regain turgor.
Comparison: Thigmonasty in chhui-mui is a protective, non-growth response, while thigmotropism in pea tendrils involves auxin-driven growth to coil around supports, aiding climbing.

Ans:
(a) Glands:
(i) Adrenaline: Adrenal medulla (part of adrenal glands, located above kidneys).
(ii) Thyroxin: Thyroid gland (located in the neck).
(b) Regulation of Hormone Release (Example: Adrenaline): The release of adrenaline is regulated by a feedback mechanism. During stress (e.g., a threat), the hypothalamus signals the adrenal medulla via the sympathetic nervous system to release adrenaline. This increases heart rate and energy availability. Once the stress subsides, feedback to the hypothalamus reduces adrenaline secretion, maintaining homeostasis.

Adrenaline: Secreted in response to acute stress, it prepares the body for fight-or-flight by increasing heart rate, dilating pupils, and mobilizing glucose.
Thyroxin: Regulates metabolism, growth, and development, controlled by the pituitary gland’s thyroid-stimulating hormone (TSH).
Feedback Mechanism: The hypothalamus and pituitary monitor hormone levels. For example, high thyroxin levels inhibit TSH release, reducing thyroid activity, while low levels stimulate it. This negative feedback ensures precise hormone levels, preventing over- or under-secretion.
Significance: Proper regulation prevents disorders like hyperthyroidism (excess thyroxin) or adrenal insufficiency.

Ans:
(a) Hormone 'X' and Gland: Hormone: Adrenaline; Gland: Adrenal medulla.
(b) Role in Scary Situations: Adrenaline triggers the fight-or-flight response by increasing heart rate, dilating airways, and mobilizing glucose for energy, enhancing alertness and physical readiness to confront or escape danger.

Identification: Adrenaline (epinephrine) is secreted by the adrenal medulla during emergencies like encountering a threat.
Mechanism:

• Heart Rate: Increases cardiac output, supplying more oxygen to muscles.
• Airways: Dilates bronchioles, improving oxygen intake.
• Energy Mobilization: Breaks down glycogen to glucose, providing quick energy.
• Other Effects: Dilates pupils, enhances blood flow to muscles, and heightens alertness.

Example: If chased by a dog, adrenaline surges, enabling faster running or quick reactions.
Significance: This rapid response enhances survival in acute stress scenarios.

Ans:
(a) Hormone Definition: A hormone is a chemical messenger secreted by endocrine glands into the bloodstream, regulating specific physiological processes in target organs or tissues.
(b) Feedback Mechanism (Example: Thyroxin): Thyroxin, secreted by the thyroid gland, regulates metabolism. The hypothalamus and pituitary gland monitor thyroxin levels. If levels are high, the pituitary reduces thyroid-stimulating hormone (TSH) secretion, decreasing thyroxin production. If low, TSH increases, stimulating the thyroid. This negative feedback ensures precise thyroxin levels, preventing metabolic disorders.

• Hormone Characteristics: Hormones (e.g., insulin, adrenaline) act at low concentrations, target specific cells, and are transported via blood.
• Feedback Mechanism: Negative feedback loops maintain homeostasis. For thyroxin, high levels signal the hypothalamus to reduce TRH (thyrotropin-releasing hormone), lowering TSH and thyroxin output. Low levels trigger the opposite, ensuring balance.
• Significance: Dysregulation (e.g., excess thyroxin causing hyperthyroidism) can disrupt metabolism, highlighting the need for precise control.

Ans:
(a) Reflex Arc Definition and Location: A reflex arc is the neural pathway for a reflex action, involving a sensory neuron, relay neuron (in spinal cord), and motor neuron, bypassing the brain. It is formed in the spinal cord for spinal reflexes.
(b) Purpose of Reflex Arcs: Reflex arcs evolved to enable rapid, automatic responses to harmful stimuli, protecting the body from injury without requiring conscious brain processing.

Reflex Arc Mechanism:

• Components: Receptor (detects stimulus), sensory neuron (transmits to spinal cord), relay neuron (in spinal cord, connects sensory and motor neurons), motor neuron (signals effector), and effector (muscle/gland).
• Example: In the knee-jerk reflex, tapping the knee stretches the tendon, activating sensory neurons that signal the spinal cord, which via a motor neuron causes quadriceps contraction.
• Location: Spinal cord for spinal reflexes (e.g., withdrawing hand from heat); cranial reflexes (e.g., blinking) involve brainstem nuclei.

Evolutionary Purpose: Reflex arcs provide quick responses (e.g., pulling hand from a hot surface), reducing reaction time compared to brain-mediated responses, thus enhancing survival.
Significance: They protect against immediate dangers, like burns or falls, by acting faster than conscious decisions.

Ans:
Limitations of Electrical Impulses:

• Limited Reach: Electrical impulses require physical connections (neurons or gap junctions) and cannot travel to distant or non-adjacent cells without neural pathways.
• Speed and Specificity: They are fast but limited to cells connected by synapses, lacking the ability to target widespread or specific cell types without direct contact.
  Why Chemical Communication is Better: Chemical communication (via hormones) allows signals to reach distant cells through the bloodstream, targets specific cells with receptors, and enables prolonged effects, unlike the short-lived electrical impulses.

Electrical Impulse Limitations:

• Reach: Neurons must physically connect to target cells, limiting communication to nearby or wired cells. For example, nerve signals cannot directly influence distant organs like the liver without extensive neural networks.
• Specificity: Electrical signals are rapid but less versatile for targeting diverse cell types across the body.

Advantages of Chemical Communication:

• Long-Distance Signaling: Hormones travel via blood, reaching all body parts (e.g., insulin from pancreas affects muscle cells).
• Specificity: Hormones bind to specific receptors, ensuring targeted effects (e.g., thyroxin affects metabolism in specific tissues).
• Duration: Hormonal effects last longer (minutes to hours), suitable for sustained responses like growth, unlike brief neural signals.

Significance: Chemical communication complements electrical signals, enabling coordinated, widespread physiological regulation in complex multicellular organisms.

Ans: (d) Plumule
Plumule Definition: The plumule is the embryonic shoot that grows upward during seed germination, developing into the shoot system (stem and leaves).
Mechanism: During germination, the plumule responds to light (phototropism) via auxins, growing upward, while the radicle (embryonic root) grows downward (geotropism).

Other Options:

• Radicle: The embryonic root, growing downward.
• Stem: The mature structure the plumule develops into, not the embryonic shoot.
• Cotyledon: Seed leaves that store or supply nutrients, not the growing shoot.

Significance: The plumule’s upward growth ensures the plant reaches light for photosynthesis, critical for seedling establishment.

Ans: (a)
(a) Situations and Reasons:
(i) Iodine Deficiency: Causes goitre, a swollen neck due to thyroid gland enlargement. Iodine is essential for thyroxin synthesis, which regulates metabolism. Deficiency reduces thyroxin, causing the pituitary to release more TSH, overstimulating the thyroid, leading to swelling.

(ii) Short Heights (Dwarfs): Results from growth hormone (GH) deficiency, secreted by the pituitary gland. Insufficient GH during childhood impairs bone and tissue growth, causing dwarfism.

(iii) Thick Facial Hairs in Boys: Due to testosterone secretion by testes during puberty (10–12 years). Testosterone, a male sex hormone, triggers secondary sexual characteristics like facial hair growth.
(b) Need for Chemical Communication:

Long-Distance Signaling: Hormones travel via blood to distant organs (e.g., insulin from pancreas regulates glucose in muscles), unlike electrical impulses limited to neural pathways.

Specific and Sustained Effects: Hormones target specific cells with receptors and produce prolonged effects (e.g., thyroxin regulates metabolism over hours), suitable for coordinating complex processes like growth or stress response.

Ans: (a) Hormone Definition: A hormone is a chemical substance produced by endocrine glands, released into the bloodstream, and acting on specific target cells to regulate physiological processes like growth, metabolism, or stress response.
(b) Feedback Mechanism (Example: Insulin): Insulin, secreted by the pancreas, regulates blood glucose levels. When glucose rises (e.g., after eating), the pancreas releases insulin, promoting glucose uptake by cells, lowering blood sugar. High insulin levels signal the pancreas to reduce secretion via negative feedback. If glucose drops too low, glucagon (another pancreatic hormone) is released to raise it, maintaining balance.

• Hormone Characteristics: Hormones act in small quantities, are specific (bind to receptors), and regulate diverse functions (e.g., adrenaline for stress, insulin for glucose).
• Feedback Example: The pancreas monitors blood glucose via receptors. High glucose triggers insulin release; as glucose normalizes, feedback inhibits further insulin secretion, preventing hypoglycemia.
• Significance: This feedback prevents hormonal imbalances, like diabetes from insulin dysregulation, ensuring homeostasis.

Ans:
(a) (i) Hormone: Adrenaline
OR
(a) (ii) Source Gland: Adrenal medulla
(b) Responses:

• Increased heart rate to pump more blood to muscles.
• Dilated pupils to enhance vision in low light.

(c) Differences Between Hormones and Electrical Impulses:

• Speed: Hormones (chemical signals) act slower (seconds to minutes) via bloodstream; electrical impulses (via neurons) are faster (milliseconds).
• Duration: Hormonal effects are prolonged (minutes to hours); neural impulses are brief.
• Reach: Hormones reach distant cells with specific receptors; electrical impulses are limited to neural pathways.

Explanation:

• Adrenaline’s Role: Secreted by the adrenal medulla during fear (e.g., climbing a risky hill), adrenaline triggers fight-or-flight responses, preparing the body for action.
• Responses: Increased heart rate enhances oxygen delivery; pupil dilation improves visual acuity. Other responses include increased breathing rate and glucose release.
• Hormone vs. Neural Signals: Hormones like adrenaline act systemically, affecting multiple organs (e.g., heart, lungs), while neural signals are precise and rapid but limited to connected cells.
• Significance: The combination of rapid neural and sustained hormonal responses ensures effective handling of emergencies.

Ans:
(i) Action and Definition: Reflex action; a rapid, involuntary response to a stimulus, bypassing conscious brain processing, to protect the body from harm.
(ii) Roles:

(a) Motor Neuron: Transmits signals from the spinal cord or brain to effectors (muscles/glands), causing action (e.g., muscle contraction to withdraw hand).
(b) Relay Neuron: Connects sensory and motor neurons in the spinal cord, processing and relaying the signal for a reflex response.
(iii) (a) Types of Nervous System and Components:
Central Nervous System (CNS): Brain and spinal cord.

Peripheral Nervous System (PNS): Cranial nerves (except II), spinal nerves, and their branches.
OR
(iii) (b) Brain Parts:

(a) Thinking: Cerebrum (frontal lobe).
(b) Picking up a pencil: Cerebrum (motor cortex).
(c) Controlling blood pressure: Medulla oblongata.
(d) Controlling hunger: Hypothalamus.

• Reflex Action: Touching a flame triggers a reflex arc: receptors detect heat, sensory neurons signal the spinal cord, relay neurons connect to motor neurons, and muscles withdraw the hand instantly.
• Neuron Roles: Motor neurons execute the response; relay neurons integrate signals in the spinal cord for rapid reflexes.
• Nervous System: CNS processes and coordinates; PNS connects CNS to the body.
• Brain Functions: The cerebrum handles conscious tasks, medulla regulates autonomic functions, and hypothalamus controls homeostatic drives like hunger.

Ans:
(I) Movements:

• X (Downwards): Geotropism (positive, e.g., roots).
• Y (Upwards): Phototropism (positive, e.g., shoots).

(II) Hormones:

• (i) Falling of Leaves: Abscisic acid.
• (ii) Rapid Cell Division: Cytokinins.
  

(III) (a) Sensitive Plant Movement:

• Communication: Touch triggers an action potential in pulvinar cells, causing ion (e.g., K⁺) movement, leading to water loss via osmosis.
• Movement: Loss of turgor in pulvini causes leaves to fold and droop rapidly (thigmonasty).

OR

(III) (b) Hormone and Phototropism:

• Hormone: Auxin, synthesized at the shoot tip.
• Mechanism: Auxin accumulates on the shaded side of the shoot, promoting cell elongation there, causing the shoot to bend toward light.

Finally;

• Tropisms: Geotropism directs roots downward (gravity response); phototropism directs shoots toward light (light response).
• Hormones: Abscisic acid promotes leaf senescence; cytokinins drive cell division in growing tissues.
• Thigmonasty: Mimosa pudica’s rapid leaf folding is a turgor-driven, non-growth response to touch, unlike growth-dependent tropisms.
• Phototropism: Auxin redistribution ensures shoots optimize light capture for photosynthesis.