Chapter 7 - Control and Coordination (CBSE Champion)

Topic Analysis and Overview

• CBSE Board Examination Weightage Analysis (2020–2011):
  • 7.1 Animals - Nervous System: Includes Very Short Answer (VSA), Short Answer I (SA I), Short Answer II (SA II), and Long Answer (LA) questions.
  • 7.2 Coordination in Plants: Highest weightage for Long Answer (LA) type questions.
  • 7.3 Hormones in Animals: Highest overall weightage in the chapter, featuring the maximum number of Very Short Answer (VSA) and Short Answer II (SA II) type questions.
• Fundamental Biological Principles:
  • All movements in living organisms occur as a direct response to changes in environmental factors (stimuli).
  • Environmental stimuli include light, heat, cold, sound, smell, taste, pressure, and touch.
  • Responding to stimuli requires integrated coordination among multiple organs within the body.

Evolutionary Patterns of Nervous Coordination in Animals

• Unicellular Organisms:
  • Respond to environmental stimuli by physical movement toward or away from the stimulus, termed taxis.
  • Positive Taxis: Direct movement toward the source of stimulus.
  • Negative Taxis: Direct movement away from the source of stimulus.
• Lower Multicellular Organisms:
  • Coelenterates: Possess a diffused nervous system composed of an epidermal and gastrodermal nerve net made of nerve cells.
  • Flatworms: Possess a ladder-type nervous system consisting of a ganglionated nerve ring and longitudinal nerve cords.
  • Annelids: Possess a Central Nervous System (CNS) comprising a circumpharyngeal nerve ring and a ventral nerve cord.
  • Insects: Possess a Central Nervous System (CNS) consisting of a circumoesophageal nerve ring and a ventral nerve cord.
• Higher Animals and Humans:
  • Control and coordination are fully integrated through two distinct, co-operating systems: the Nervous System and the Endocrine System.

Human Nervous System

• System Overview:
  • The human nervous system is the most complex structural network in the body, comprising the brain, spinal cord, and nerves.
  • Sense Organs: Act as sensory input centers (eyes, ears, nose, tongue, skin) containing specialized receptor cells.
• Receptor Classification and Functions:
  • Photoreceptors: Sensitive to light stimuli; located in the eyes.
  • Phonoreceptors: Sensitive to sound stimuli; located in the internal ears.
  • Olfactory Receptors: Sensitive to smell stimuli; located in the nasal epithelium.
  • Gustatory Receptors: Sensitive to taste stimuli; located in the taste buds of the tongue.
  • Thermoreceptors: Sensitive to temperature (heat or cold) stimuli; located in the skin.

Structural and Functional Unit of the Nervous System: Neurons

• General Characteristics:
  • Neurons are the fundamental structural and functional units of the nervous system.
  • The neuron is the longest cell in the human body.
• Structural Components of a Neuron:
  • Cell Body (Cyton):
    • Broad, rounded, pyriform, or stellate central part of the neuron.
    • Contains abundant cytoplasm termed neuroplasm, distinct Nissl's granules, and a relatively large, spherical nucleus.
    • Primarily responsible for metabolic maintenance and cellular growth.
    • Receives electrical impulses from dendrites and conducts them toward the axon.
  • Dendrites:
    • Multiple short, tapering, highly branched protoplasmic processes extending outward from the cyton.
    • Act as information-acquisition sites where sensory stimuli are converted into electrical impulses that travel toward the cyton.
  • Axon:
    • A single, extremely long, cylindrical nerve fibre of uniform diameter originating from the cyton.
    • Highly branched at its terminal ends, ending in enlarged knob-like terminals.
    • Axon terminals form synapses with the dendrites of adjacent neurons.
    • Conducts nerve impulses away from the cyton toward target cells or other neurons.
• Functional Types of Neurons:
  • Sensory (Receptor) Neurons: Transmit nerve impulses from sensory receptor cells to the Central Nervous System (CNS).
  • Motor (Effector) Neurons: Transmit nerve impulses from the CNS to effector organs (muscle cells or glands).
  • Relay (Connector / Interneurons): Located within the CNS; serve as connecting links between sensory neurons and motor neurons.

Structural Organization of the Human Nervous System

• Human Nervous System Branches:
  • Central Nervous System (CNS):
    • Brain
    • Spinal Cord
  • Peripheral Nervous System (PNS):
    • Voluntary Nervous System: Under conscious voluntary control; connects CNS directly to skeletal muscles and external body structures.
    • Autonomic Nervous System (ANS): Involuntary control system originating from visceral nerves; regulates internal visceral organs (heart, lungs, kidneys, bladder, blood vessels, glands). Divided into:
      • Sympathetic Nervous System
      • Parasympathetic Nervous System

Central Nervous System (CNS)

• The Brain:
  • Highest coordinating center of the body; situated inside the cranial cavity of the skull (cranium).
  • Protected by three structural membranes called meninges.
  • The space between the meninges is filled with cerebrospinal fluid (CSF), which absorbs mechanical shocks and protects brain tissue.
  • Gives rise to $12\text{ pairs}$ of cranial nerves.
  • Subdivisions of the Brain:
    • Forebrain (Cerebrum):
      • Main thinking, cognitive, and integrative region of the brain.
      • Site of learning, reasoning, intelligence, personality, memory, sensations, thoughts, and voluntary movements.
    • Midbrain:
      • Lacks complex sub-divisions; contains two main longitudinal fibre tracts called crura cerebri connecting the hindbrain to the forebrain.
      • Controls reflex movements of the head, neck, and trunk in response to visual and auditory stimuli.
      • Controls reflex movements of eye muscles, changes in pupil size, and alterations of lens shape.
    • Hindbrain:
      • Pons (Pons varolii): Directly involved in the regulation of respiration.
      • Cerebellum: Coordinates voluntary muscular movements and maintains bodily posture, static balance, and equilibrium.
      • Medulla Oblongata: Controls crucial involuntary actions including heartbeat (blood circulation), breathing rate, blood pressure, and peristaltic movements of the alimentary canal. Serves as the reflex center for swallowing, coughing, sneezing, salivation, and vomiting.
• The Spinal Cord:
  • Cylindrical neural structure beginning as a downward continuation of the medulla oblongata.
  • Extends through the vertebral column down to the early lumbar region, continuing to the end of the spine as a fibrous thread termed the filum terminale.
  • Enclosed and protected within the bony vertebral column and encased by meninges.
  • Gives rise to $31\text{ pairs}$ of spinal nerves.
  • Functions:
    • Conducts sensory impulses toward the brain and motor impulses away from the brain.
    • Serves as the primary integration center for spinal reflex actions, bypassing direct brain processing to ensure immediate responses.

Peripheral Nervous System (PNS)

• Peripheral Nerves:
  • Cranial Nerves: $12\text{ pairs}$ originating directly from the brain, extending throughout the head and neck.
    • Sensory Cranial Nerves: Nerves $I$, $II$, and $VIII$
    • Motor Cranial Nerves: Nerves $III$, $IV$, $VI$, $XI$, and $XII$
    • Mixed Cranial Nerves: Nerves $V$, $VII$, $IX$, and $X$
  • Spinal Nerves: $31\text{ pairs}$ originating along the length of the spinal cord, spreading across all body regions except the head. All spinal nerves are mixed nerves carrying both sensory and motor nerve fibres.
  • Visceral Nerves: Arise primarily from the spinal cord (and selected brain areas) to innervate internal visceral organs (heart, kidneys, lungs, bladder, vessels), forming the Autonomic Nervous System.

Reflex Action and Reflex Arc

• Reflex Action:
  • An immediate, rapid, involuntary, and automatic motor response to a specific sensory stimulus, executed without conscious brain intervention.
  • Designed for protective functions (e.g., knee jerk, diaphragm movement during breathing, coughing to clear the windpipe, yawning, eye blinking, sneezing, and pupil constriction in intense light to prevent retinal damage).
• Reflex Arc Pathway:
  • The precise anatomical pathway traversed by electrical nerve impulses during a reflex action.
  • Sequence: Stimulus received by Receptor (Sensory Neuron) $\rightarrow$ Electrical Impulse along Sensory Neuron $\rightarrow$ Spinal Cord / Relay Neuron $\rightarrow$ Electrical Impulse along Motor Neuron $\rightarrow$ Effector Organ (Muscle or Gland) $\rightarrow$ Biological Response.

Human Endocrine System

• Endocrine System Features:

  • Composed of ductless endocrine glands that secrete chemical regulators called hormones directly into the bloodstream.
  • Gland Types:
    • Exocrine Glands: Possess ducts to transport secretions directly to target regions (e.g., salivary glands).
    • Endocrine Glands: Ductless glands that pour secretions directly into surrounding capillaries for transport via the circulatory system.

• General Characteristics of Hormones:

  • Secreted in trace amounts by endocrine glands.
  • Poured directly into blood and distributed throughout the organism by blood circulation.
  • Function at target sites anatomically distinct from their site of synthesis (referred to as chemical messengers).
  • Exert specific biological effects on designated target tissues or organs.
  • Regulate metabolic processes, tissue growth, development, and physiological coordination.

• Comprehensive Table of Human Endocrine Glands, Hormones, and Functions:

  • Hypothalamus:
    • Releasing Hormones (RH): Stimulate the anterior and intermediate lobes of the pituitary gland to release hormones.
    • Inhibiting Hormones (IH): Inhibit hormone secretion from the anterior and intermediate lobes of the pituitary gland.
  • Pituitary Gland (Attached to the ventral surface of the brain; divided into three lobes):
    • Anterior Lobe:
      • Growth Hormone (GH) / Somatotropic Hormone (STH): Regulates total body development, muscle, bone, and soft tissue growth. Hyposecretion causes dwarfism; hypersecretion causes gigantism.
      • Thyroid Stimulating Hormone (TSH): Controls growth and secretory function of the thyroid gland; stimulates thyroxine production.
      • Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to synthesize and secrete glucocorticoids (cortisol).
      • Follicle Stimulating Hormone (FSH): In males, stimulates spermatogenesis. In females, stimulates ovarian follicle maturation into eggs.
      • Luteinising Hormone (LH): In males, stimulates interstitial cells to produce testosterone. In females, stimulates ovulation and secretion of oestrogen and progesterone.
      • Prolactin Hormone (PRL): Stimulates mammary gland development during pregnancy and milk production post-partum.
    • Intermediate Lobe:
      • Melanocyte Stimulating Hormone (MSH): Stimulates melanin synthesis within skin cells.
    • Posterior Lobe:
      • Oxytocin: Stimulates smooth muscle contraction during labor; facilitates milk ejection during lactation.
      • Vasopressin / Antidiuretic Hormone (ADH): Maintains body water retention and electrolyte osmotic balance.
  • Pineal Gland (Located between the cerebral hemispheres):
    • Melatonin: Regulates sleep-wake cycles and circadian biological rhythms.
  • Thyroid Gland (Located ventrally in the neck, featuring two lateral lobes straddling the trachea):
    • Thyroxine ($T_4$) & Triiodothyronine ($T_3$): Stimulate basal metabolic rate ($BMR$), cellular oxidation, and carbohydrate, lipid, and protein metabolism.
    • Calcitonin: Lowers elevated blood calcium levels by inhibiting calcium liberation from bone tissue.
  • Parathyroid Glands (Four small oval bodies embedded within the posterior surface of thyroid lobes):
    • Parathyroid Hormone (PTH) / Parathormone: Raises blood calcium levels by mobilizing calcium ions from bone into circulation when blood levels drop (antagonistic to calcitonin).
  • Thymus Gland (Located in the upper chest region anterior to the heart; undergoes progressive atrophy in adults):
    • Thymosin: Promotes differentiation, maturation, and functional development of $T$-lymphocytes.
  • Adrenal Glands (Suprarenals) (Paired glands situated on top of each kidney):
    • Adrenal Cortex (Outer zone):
      • Glucocorticoids: Regulate carbohydrate, lipid, and protein metabolism and maintain blood glucose levels.
      • Mineralocorticoids (Aldosterone): Regulate electrolyte balance (sodium/potassium) and body water content.
      • Sexcorticoids: Stimulate development of secondary sexual characters in males and females.
    • Adrenal Medulla (Inner zone):
      • Adrenaline (Epinephrine) & Noradrenaline (Norepinephrine): Mobilize bodily emergency responses ("fight-or-flight"); increase heart rate, blood pressure, respiration rate, glucose release, and smooth muscle relaxation.
  • Pancreas (Heterocrine/compound gland located posterior to the stomach; endocrine tissue consists of Islets of Langerhans):
    • Insulin: Converts excess blood glucose into glycogen in the liver and muscles, lowering blood sugar levels.
    • Glucagon: Stimulates glycogen breakdown into glucose, raising blood sugar levels.
  • Ovaries (Paired female gonads in the lower abdominal cavity):
    • Oestrogen: Promotes ovulation and controls female secondary sexual characteristics (breast development, female hair distribution, voice pitch).
    • Progesterone: Prepares the uterine lining for implantation of the fertilized ovum and maintains pregnancy.
  • Testes (Paired male gonads located extra-abdominally within the scrotum; Leydig/interstitial cells):
    • Testosterone: Stimulates spermatogenesis, regulates male accessory sex organ development, and controls male secondary sexual traits (facial/pubic hair, larynx enlargement/deepening voice, scrotal/penile enlargement).

• Feedback Mechanism of Hormonal Control:

  • Hormone concentration and secretion timing are strictly controlled by internal feedback systems.
  • Example: Elevated blood glucose is detected by pancreatic Islets of Langerhans $\rightarrow$ Pancreas secretes insulin into blood $\rightarrow$ Glucose levels drop to normal set-point $\rightarrow$ Insulin secretion automatically declines.

Control and Coordination in Plants

• Plant Hormones (Phytohormones):
  • Naturally occurring chemical compounds in plants that regulate physiological processes at extremely low concentrations.
  • Coordinate plant responses by influencing growth stages: cell division, cell enlargement, and cell differentiation (specialization).
• Phytohormone Classification and Functional Roles:
  • Auxins:
    • Promote cell elongation and cell differentiation.
    • Promote stem elongation and fruit enlargement.
    • Regulate growth tropisms (e.g., phototropism).
    • Induce parthenocarpy (development of seedless fruits without fertilization).
    • Site of Synthesis: Meristematic tissues at shoot and root tips.
  • Gibberellins:
    • Promote cell enlargement and cell differentiation in synergistic presence of auxins.
    • Promote stem elongation and fruit growth.
    • Induce bolting (rapid internodal elongation) and flowering in rosette plants.
    • Induce parthenocarpy in various crops.
    • Site of Synthesis: Young leaves, developing roots, and shoot apices.
  • Cytokinins:
    • Promote active cell division (cytokinesis).
    • Play critical roles in plant morphogenesis.
    • Break seed and bud dormancy.
    • Delay leaf senescence (aging).
    • Promote stomatal opening and fruit development.
    • Site of Synthesis: Synthesized primarily in root tips; transported upward via xylem.
  • Ethylene:
    • Promotes fruit growth and accelerates fruit ripening.
    • Breaks bud and seed dormancy.
    • Stimulates formation of the abscission layer in leaves, flowers, and fruits.
    • Promotes leaf yellowing and senescence.
    • Site of Synthesis: Formed throughout plant tissues including leaves, roots, flowers, seeds, and ripening fruits.
  • Abscisic Acid (ABA):
    • Acts primarily as a growth inhibitor.
    • Enforces seed and bud dormancy.
    • Stimulates stomatal closure, mitigating water loss during drought.
    • Promotes leaf abscission (falling) and tissue senescence.
    • Site of Synthesis: Produced across various tissues, most abundantly inside chloroplasts of green leafy cells.

Types of Plant Movements

• General Movement Principles:
  • Plants do not exhibit whole-body locomotion.
  • Movements occur in individual plant organs (shoots, roots, leaves) via differential growth rates or turgor pressure changes in response to external stimuli.
• Tropic Movements (Tropisms):
  • Directional, growth-dependent movements where the direction of response is determined by the direction of stimulus.
  • Phototropism: Growth response to directional light stimuli.
    • Positive Phototropism: Growth movement toward light (e.g., growing plant shoots/stems).
    • Negative Phototropism: Growth movement away from light (e.g., plant roots).
  • Geotropism: Growth response to gravitational force.
    • Positive Geotropism: Downward growth toward gravity (e.g., primary roots).
    • Negative Geotropism: Upward growth against gravity (e.g., growing shoots/stems).
  • Chemotropism: Growth response to specific chemical stimuli.
    • Positive Chemotropism: Movement toward a chemical gradient (e.g., growth of a pollen tube down the style toward the ovule during fertilization).
  • Hydrotropism: Growth response to moisture/water gradients.
    • Positive Hydrotropism: Bending/growth of roots toward water sources.
  • Thigmotropism: Directional growth response to tactile physical contact.
    • Positive Thigmotropism: Coiling and growth of climbing tendrils around physical supports (e.g., bitter gourd, bottle gourd, grape vine).
• Nastic Movements:
  • Non-directional, growth-independent movements where the response direction is independent of stimulus orientation.
  • Seismonastic Movements: Rapid non-directional responses triggered by mechanical touch, shock, or vibration.
    • Example: Folding of leaves in Mimosa pudica (sensitive plant / 'Chhui-mui' / 'Lajwanti') driven by turgor loss in pulvini cells.
  • Nyctinastic Movements (Sleep Movements):
    • Diurnal positional variations of leaves and flowers responding to day-night transitions.
    • Photonastic Movements: Non-directional movements regulated by light intensity variations (e.g., opening/closing of dandelion flowers).
    • Thermonastic Movements: Non-directional movements regulated by environmental temperature fluctuations.

CBSE Board Examination Questions and Detailed Solutions

• Question 1 (VSA 1 Mark, Board Term I, 2013):
  • Question: Which is the largest and most prominent part of the brain?
  • Solution: The cerebrum is the largest and most prominent part of the human brain.
• Question 2 (SA I 2 Marks, 2018):
  • Question:
    • (a) Name one gustatory receptor and one olfactory receptor in human beings.
    • (b) Write $a$ and $b$ in the given flow chart of neuron through which information travels as an electrical impulse: $\text{Dendrite} \rightarrow a \rightarrow b \rightarrow \text{End point of Neuron}$.
  • Solution:
    • (a) Gustatory receptors are taste-detecting cells located within taste buds on the tongue. Olfactory receptors are smell-detecting cells located within the nasal mucous membrane.
    • (b) In the flowchart, $a$ corresponds to the cyton (cell body) and $b$ corresponds to the axon.
• Question 3 (SA II 3 Marks, Board Term I, 2017):
  • Question: Write the main functions of the following: (a) sensory neuron, (b) cranium, (c) vertebral column, (d) motor neuron.
  • Solution:
    • (a) Sensory neuron: Located in sense organs; receives environmental stimuli via dendrites and transmits electrical impulses toward the central nervous system (brain and spinal cord).
    • (b) Cranium: Bony protective case enclosing the brain, shielding it from mechanical shock and external injury.
    • (c) Vertebral column: Bony structure that surrounds and protects the delicate spinal cord while supporting upper body mass.
    • (d) Motor neuron: Transmits regulatory motor impulses from the central nervous system out to effector organs (muscles or glands) to elicit physiological responses.
• Question 4 (SA II 3 Marks, AI 2019):
  • Question: Why does the flow of signals in a synapse from axonal end of one neuron to dendritic end of another neuron take place but not in the reverse direction? Explain.
  • Solution: At the synapse, the enlarged axon terminal forms a pre-synaptic knob containing neurotransmitter-filled synaptic vesicles, separated from the post-synaptic dendritic membrane by a fluid-filled synaptic cleft. When an electrical impulse reaches the pre-synaptic knob, neurotransmitters are released exclusively into the synaptic cleft. These molecules diffuse across the cleft to bind receptors on the post-synaptic dendritic membrane, generating a new electrical signal. Because chemical neurotransmitters are synthesized and stored strictly on the pre-synaptic axonal side, synapses act as one-way valves, preventing reverse impulse transmission.
• Question 5 (SA II 3 Marks, Board Term I, 2017):
  • Question: "Reflex arcs continue to be more efficient for quick responses". Justify this statement giving reason.
  • Solution: Reflex arcs evolved as rapid emergency survival mechanisms because complex conscious processing by brain neural networks requires significant time. By routing sensory signals directly through the spinal cord to effectors, the reflex arc enables near-instantaneous responses to dangerous stimuli, minimizing physical injury and preventing brain fatigue or cognitive overloading.
• Question 6 (SA II 3 Marks, Board Term I, 2016):
  • Question:
    • (a) Define reflex arc.
    • (b) Trace the sequence of events which occur in our body when a bright light is focussed on your eyes.
  • Solution:
    • (a) A reflex arc is the physical neural pathway traveled by nerve impulses from a sensory receptor organ to the spinal cord and back to an effector organ during a reflex action.
    • (b) Sequence of events during bright light illumination:             $\text{Photoreceptors in eye} \rightarrow \text{Sensory neuron} \rightarrow \text{Brain / CNS} \rightarrow \text{Motor neuron} \rightarrow \text{Eye sphincter muscle} \rightarrow \text{Constriction of pupil}$.
• Question 7 (SA II 3 Marks, Board Term I, 2015):
  • Question:
    • (a) Draw a neat diagram of a neuron and label (i) dendrite and (ii) axon.
    • (b) Which part of the human brain is: (i) the main thinking part of the brain? (ii) responsible for maintaining the posture and balance of the body?
  • Solution:
    • (a) Structural neuronal components include the central cell body (cyton) with a prominent nucleus, branching dendrites extending from the cyton, a long cylindrical axon, and terminal axon endings.
    • (b) (i) The forebrain (specifically the cerebrum) is the main thinking part of the brain. (ii) The cerebellum (part of the hindbrain) maintains body posture, muscular coordination, and physical equilibrium.
• Question 8 (SA II 3 Marks, Board Term I, 2014):
  • Question: Mention three major regions of brain. Write one function of each.
  • Solution:
    • (i) Forebrain: Contains the cerebrum; functions in conscious thinking, learning, memory, and processing sensory inputs.
    • (ii) Midbrain: Integrates visual and auditory reflexes, regulating head/neck movements and pupil diameter.
    • (iii) Hindbrain: Contains the pons (respiratory regulation), cerebellum (posture/balance), and medulla oblongata (involuntary control of heartbeat, breathing, and blood pressure).
• Question 9 (VSA 1 Mark, Board Term I, 2015):
  • Question: State one example of chemotropism.
  • Solution: The growth of a pollen tube through the style toward the ovule inside the ovary during flower fertilization.
• Question 10 (VSA 1 Mark, Board Term I, 2013):
  • Question: What is meant by tropic movements?
  • Solution: Directional growth movements of plant parts in direct response to directional external stimuli.
• Question 11 (SA I 2 Marks, Board Term I, 2016):
  • Question: State the two types of movements seen in plants. Give one example of each type.
  • Solution:
    • (i) Nastic movements: Growth-independent, non-directional movements (e.g., rapid closing of leaves in Mimosa pudica upon contact).
    • (ii) Tropic movements: Growth-dependent, directional movements (e.g., positive phototropic bending of stems toward light).
• Question 12 (SA II 3 Marks, 2020):
  • Question: Define geotropism. Draw a labelled diagram of a plant showing geotropic movement of its parts.
  • Solution: Geotropism is the directional growth orientation of plant organs in response to gravity. Shoots exhibit negative geotropism by growing upward against gravity, while roots exhibit positive geotropism by growing downward toward gravity.
• Question 13 (SA II 3 Marks, Delhi 2019):
  • Question: What are plant hormones? Name the plant hormones responsible for the following: (i) Growth of stem, (ii) Promotion of cell division, (iii) Inhibition of growth, (iv) Elongation of cells.
  • Solution: Plant hormones (phytohormones) are naturally synthesized chemical growth substances that regulate physiological actions at low concentrations.
    • (i) Stem growth: Gibberellins
    • (ii) Cell division promotion: Cytokinins
    • (iii) Growth inhibition: Abscisic Acid (ABA)
    • (iv) Cell elongation: Auxins (and Gibberellins)
• Question 14 (SA II 3 Marks, Board Term I, 2016):
  • Question: Define phototropism. Name the plant hormone which is responsible for phototropism.
  • Solution: Phototropism is the directional growth of plant organs responding to light stimuli (stems grow toward light; roots grow away). The hormone responsible is Auxin.
• Question 15 (LA 5 Marks, Board Term I, 2017):
  • Question:
    • (a) What are phytohormones? List four types of phytohormones. Where are these hormones synthesised?
    • (b) What happens when a growing plant detects light? Explain in brief.
  • Solution:
    • (a) Phytohormones are physiological plant signaling molecules synthesized in trace quantities:
      • Auxins: Synthesized in shoot and root apical meristems.
      • Gibberellins: Synthesized in expanding young leaves, growing stems, and roots.
      • Cytokinins: Synthesized in root tips and transported upward through xylem.
      • Ethylene: Synthesized in ripening fruits, senescing leaves, and nodal tissue.
      • Abscisic Acid: Synthesized throughout organs, heavily concentrated in leaf chloroplasts.
    • (b) When light hits one side of a shoot, auxin synthesized at the tip migrates horizontally away from light toward the shaded side. Higher auxin concentration on the shaded side causes those cells to elongate faster than cells on the illuminated side, bending the stem toward the light source.
• Question 16 (LA 5 Marks, Board Term I, 2016):
  • Question: List the sequences of events that occur when a plant is exposed to unidirectional light, leading to bending of a growing shoot. Also name the hormone synthesised and the type of movement that takes place.
  • Solution: Unidirectional light exposure causes shoot tip cells to synthesize Auxin. Auxin diffuses downward, accumulating primarily along the shaded side. Increased local auxin concentrations induce differential cell elongation on the shaded side relative to the lit side, causing the shoot tip to curvature-bend toward light. Hormone: Auxin. Movement type: Positive Phototropism (tropic movement).
• Question 17 (LA 5 Marks, Board Term I, 2014):
  • Question:
    • (a) Define reflex arc. Draw a flow chart showing the sequence of events which occur during sneezing.
    • (b) List four plant hormones. Write one function of each.
  • Solution:
    • (a) A reflex arc is the dedicated pathway traversed by nerve impulses executing a reflex action. Sneezing flow chart:             \text{Foreign particle in nasal cavity} \rightarrow \text{Olfactory sensory receptors stimulated} \rightarrow \text{Sensory nerve impulse} \rightarrow \text{Spinal cord / CNS processing} \rightarrow \text{Motor nerve impulse} \rightarrow \text{Effector diaphragm & intercostal rib muscles} \rightarrow \text{Forced expiration (Sneeze)}.
    • (b) Four plant hormones:
      • Auxin: Promotes cell elongation and tropic stem movements.
      • Gibberellin: Promotes internodal stem elongation and breaks seed dormancy.
      • Cytokinin: Stimulates active cell division and delays leaf aging.
      • Ethylene: Accelerates fruit ripening and stimulates organ abscission.
• Questions 18–21 (VSA Data-based 1 Mark each, 2020):
  • Passage Summary: The bilobed thyroid gland in the neck produces thyroxine, requiring dietary iodine. Thyroxine controls carbohydrate, fat, and protein metabolism. Excess causes hyperthyroidism; deficiency/underactivity causes hypothyroidism and goitre, manageable via iodized salt.
  • Question 18: Where is thyroid gland situated?
    • Solution: Situated in the neck region, ventral to the trachea.
  • Question 19: State the function of thyroxine in human body.
    • Solution: Regulates carbohydrate, lipid, and protein metabolic rates and promotes tissue growth.
  • Question 20: What is hyperthyroidism?
    • Solution: A metabolic clinical condition resulting from excessive secretion of thyroxine hormone by an overactive thyroid gland.
  • Question 21: How can we control hypothyroidism?
    • Solution: By consuming iodized salt in daily diets to ensure adequate iodine for thyroxine synthesis.
• Questions 22–25 (VSA Reference Data Analysis, 2020):
  • Data Table: TSH Levels in Women:
    • $18 - 29\text{ years}$: Normal $0.4 - 2.34\,mU/L$; Low $< 0.4\,mU/L$
    • $30 - 49\text{ years}$: Normal $0.4 - 4.0\,mU/L$; Low $< 0.4\,mU/L$
    • $50 - 79\text{ years}$: Normal $0.46 - 4.68\,mU/L$; Low $< 0.46\,mU/L$
    • $5\%$ of US women have thyroid disorders vs $3\%$ of men. High TSH in older women with thyroid nodules correlates with elevated thyroid cancer risk.
  • Question 22: A $35\text{-year-old}$ woman has TSH level $6.03\,mU/L$. What change should she bring in her diet to control this level?
    • Solution: Her TSH exceeds the normal upper limit ($4.0\,mU/L$) for her age group, indicating hypothyroidism. She should incorporate iodized salt into her diet.
  • Question 23: When do women face a greater risk of abnormal TSH level?
    • Solution: During menstruation, pregnancy/childbirth, and post-menopause.
  • Question 24: State the consequence of low TSH level.
    • Solution: Indicates excess circulating thyroxine (hyperthyroidism), causing the pituitary gland to suppress TSH output.
  • Question 25: Name the mineral that is responsible for synthesis of hormone secreted by thyroid gland.
    • Solution: Iodine.
• Questions 26–29 (VSA Pregnancy Data Analysis, 2020):
  • Data Table: TSH Levels During Pregnancy:
    • First trimester: Normal $0.2 - 2.5\,mU/L$; Low $< 0.2\,mU/L$; High $2.5 - 10\,mU/L$
    • Second trimester: Normal $0.3 - 3.0\,mU/L$; Low $< 0.3\,mU/L$; High $3.01 - 4.5\,mU/L$
    • Third trimester: Normal $0.8 - 5.2\,mU/L$; Low $< 0.8\,mU/L$; High $> 5.3\,mU/L$
  • Question 26: Give the full form of TSH.
    • Solution: Thyroid Stimulating Hormone.
  • Question 27: State the main function of TSH.
    • Solution: Stimulates the thyroid gland to produce and release thyroxine ($T_4$ and $T_3$).
  • Question 28: Why do TSH levels in pregnant women need to be monitored?
    • Solution: Abnormally high TSH levels indicating hypothyroidism increase the risk of maternal miscarriages and fetal developmental issues.
  • Question 29: A pregnant woman has TSH level of $8.95\,mU/L$. What care is needed for her?
    • Solution: This indicates elevated TSH (hypothyroidism); she requires medical evaluation and hormone regulation therapy under a doctor's care.
• Question 30 (SA II 3 Marks, 2020):
  • Question: A squirrel is in a scary situation. Its body has to prepare for either fighting or running away. State the immediate changes that take place in its body so that the squirrel is able to either fight or run.
  • Solution: The sympathetic nervous system triggers the adrenal medulla to secrete adrenaline into the bloodstream. Adrenaline increases heart rate, breathing rate, and blood pressure, dilates respiratory passages, redirects blood flow from skin/digestive organs toward skeletal muscles, and stimulates liver glycogen breakdown into glucose, supplying rapid energy for flight or fight.
• Question 31 (SA II 3 Marks, 2020):
  • Question: Why is chemical communication better than electrical impulses as a means of communication between cells in a multicellular organisms?
  • Solution: Electrical nerve impulses can only reach cells directly wired to nervous tissue. Furthermore, after firing an electrical impulse, a neuron requires time to reset its membrane potential before generating another, preventing continuous signaling. Chemical signals (hormones) diffuse through blood to reach every cell regardless of neural connections, generating longer-lasting metabolic actions.
• Question 32 (SA II 3 Marks, 2020):
  • Question: A cheetah, on seeing a prey moves towards him at a very high speed. What causes the movement of his muscles? How does the chemistry of cellular components of muscles change during this event?
  • Solution: Visual recognition of prey transmits sensory electrical impulses to the CNS, which sends motor signals to leg muscles alongside adrenal release of adrenaline. Adrenaline elevates muscle blood flow and glucose metabolism. At the cellular level, muscle proteins (actin and myosin) alter their structural shape and spatial arrangement in response to nervous and hormonal stimulation, causing rapid muscle fiber contraction.
• Question 33 (SA II 3 Marks, 2018):
  • Question: Name the hormones secreted by the following endocrine glands and specify one function of each: (a) Thyroid, (b) Pituitary, (c) Pancreas.
  • Solution:
    • (a) Thyroid: Secretes Thyroxine ($T_4$), which regulates basal metabolism of carbohydrates, fats, and proteins.
    • (b) Pituitary: Secretes Growth Hormone (GH), controlling general somatic development, bone, and muscle growth.
    • (c) Pancreas: Secretes Insulin, lowering blood sugar levels by promoting glucose storage as glycogen.
• Question 34 (SA II 3 Marks, Board Term I, 2017):
  • Question:
    • (a) How does chemical coordination take place in animals?
    • (b) It is advised to use iodised salt. Give reason.
  • Solution:
    • (a) Chemical coordination occurs via ductless endocrine glands secreting hormones into blood. Hormones circulate to reach distant target tissues, binding specific receptors to regulate growth, metabolism, and organ functioning.
    • (b) Iodine is a mandatory structural component required by the thyroid gland to synthesize thyroxine. Iodine deficiency causes thyroxine underproduction, leading to thyroid enlargement (goitre).
• Question 35 (SA II 3 Marks, Board Term I, 2016):
  • Question:
    • (a) An old man is advised by his doctor to take less sugar in his diet. Name the disease from which the man is suffering. Mention the hormone due to imbalance of which he is suffering from this disease. Which endocrine gland secretes this hormone?
    • (b) Name the endocrine gland which secretes growth hormone. What will be the effect of the following on a person: (i) deficiency of growth hormone, (ii) excess secretion of growth hormone?
  • Solution:
    • (a) Disease: Diabetes mellitus. Imbalanced hormone: Insulin. Secretory gland: Endocrine part of the Pancreas (Islets of Langerhans).
    • (b) Endocrine gland: Pituitary gland.
      • (i) GH deficiency during growth years causes dwarfism.
      • (ii) GH hypersecretion during growth years causes gigantism.
• Question 36 (SA II 3 Marks, Board Term I, 2015):
  • Question: Name the hormone required for the following. Also mention the name of endocrine gland from which that hormone is secreted: (a) Lowering of blood glucose, (b) Development of moustache and beard in human males, (c) Metabolism of carbohydrates, fats and proteins.
  • Solution:
    • (a) Hormone: Insulin. Gland: Pancreas.
    • (b) Hormone: Testosterone. Gland: Testes.
    • (c) Hormone: Thyroxine. Gland: Thyroid gland.
• Question 37 (SA II 3 Marks, Board Term I, 2015):
  • Question:
    • (a) Complete the following table:
      • (i) Hormone: Thyroxine | Gland: Thyroid | Function: [?]
      • (ii) Hormone: Growth Hormone | Gland: [?] | Function: Regulates growth and development of the body
      • (iii) Hormone: Insulin | Gland: Pancreas | Function: [?]
    • (b) List three characteristics of animal hormones.
  • Solution:
    • (a) Completed Table Entries:
      • (i) Function of Thyroxine: Regulates carbohydrate, lipid, and protein metabolism and growth.
      • (ii) Gland secreting Growth Hormone: Pituitary gland.
      • (iii) Function of Insulin: Regulates blood glucose levels by promoting glucose-to-glycogen conversion.
    • (b) Three Characteristics of Animal Hormones:
      1. Synthesized by specialized ductless endocrine glands and secreted directly into blood.
      2. Act on specific distant target cells/organs relative to their site of origin.
      3. Highly potent organic signaling molecules (peptides, proteins, amines, or steroids) required in trace amounts.
• Question 38 (SA II 3 Marks, Board Term I, 2013):
  • Question: List in tabular form three differences between nervous control and chemical control.
  • Solution:
    • Nervous Control:
      1. Signals travel ultra-rapidly as electrical action potentials along axons.
      2. Messages routed directly along nerve fibres to specific localized effectors.
      3. Functional effects are immediate and short-lived.
    • Chemical Control:
      1. Signals travel slowly as endocrine chemical messengers transported in blood.
      2. Hormones circulate systemically throughout the body, targeting widespread tissues containing specific receptors.             3