Question Bank

Reflex Action: A rapid, automatic, involuntary response to a stimulus that does not require the brain's decision-making.

Components of Reflex Arc (in order):

1. Receptor — detects stimulus (e.g., skin pain receptor)
2. Sensory (Afferent) Neuron — carries impulse to spinal cord
3. Relay Neuron — processes signal in spinal cord (CNS)
4. Motor (Efferent) Neuron — carries response signal to effector
5. Effector — muscle or gland produces response

Example: Touching a hot plate → hand withdraws instantly.

Parts and Functions:

• Dendrites — receive electrical impulses from other neurons; carry them toward the cell body
• Cell Body (Cyton) — contains nucleus and organelles; metabolic and integration centre
• Axon — carries impulses away from cell body to next neuron or effector
• Myelin Sheath — fatty insulation by Schwann cells; speeds up conduction (saltatory)
• Nodes of Ranvier — unmyelinated gaps; allow rapid saltatory (jumping) conduction
• Axon Terminals (Synaptic Knobs) — release neurotransmitters across synapse

Draw: elongated cell body → branching dendrites (left) → long axon with myelin → nodes → bulb-like terminals (right).

The brain is protected by a three-tier system:

1. Cranium (skull) — hard bony case that encloses the brain
2. Meninges — three layers of membranes (Dura mater, Arachnoid mater, Pia mater) lining the skull
3. Cerebrospinal Fluid (CSF) — cushioning fluid between meninges that acts as a shock absorber

Master Gland: The pituitary gland secretes tropic hormones that regulate the secretion of all other endocrine glands (thyroid, adrenal, gonads). This control over all other glands earns it the name "master gland." It is itself controlled by the hypothalamus.

Two hormones:

• Growth Hormone (GH) — regulates overall body growth and cell reproduction
• TSH (Thyroid Stimulating Hormone) — stimulates thyroid gland to produce thyroxine

Phototropism: The directional growth movement of a plant part in response to light. Shoots show positive phototropism (grow toward light); roots show negative phototropism (grow away).

Mechanism:

1. Auxin (IAA) is synthesised at the shoot tip.
2. When light falls from one side, auxin migrates laterally to the shaded side.
3. Higher auxin concentration → greater cell elongation on shaded side.
4. More elongation on one side → shoot curves/bends toward the light source.

First demonstrated by Charles Darwin (1880). Auxin named by Fritz Went (1926).

Feature	Endocrine	Exocrine
Duct	Ductless (hormones into blood)	Has ducts (secretion to surface)
Secretion	Hormones	Enzymes, sweat, saliva etc.
Examples	Thyroid, Adrenal, Pituitary	Salivary, Sweat, Liver (bile)

Negative Feedback: A self-regulating mechanism where the product of a process inhibits further production, maintaining homeostasis.

Example — Blood Glucose Regulation:

1. Blood glucose ↑ (after meal)
2. Pancreas β cells detect rise → secrete insulin
3. Insulin promotes glucose uptake by cells and glycogen storage → blood glucose ↓
4. Low blood glucose → insulin secretion stops
5. If glucose falls too low → glucagon released → glycogen broken down → glucose ↑

Tropism	Stimulus	Example
Phototropism	Light	Shoot bends toward light
Geotropism	Gravity	Roots grow downward
Hydrotropism	Water	Roots grow toward moisture
Thigmotropism	Touch/contact	Tendrils coil around support

Parameter	Nervous System	Endocrine System
Mode of signal	Electrical impulse via neurons	Chemical hormones via blood
Speed	Very fast (milliseconds)	Slow (seconds to hours)
Duration of effect	Short-lived	Long-lasting / sustained
Scope	Specific (targeted organ)	Widespread (via bloodstream)

The hypothalamus is a forebrain structure with multiple critical functions:

• Thermoregulation — maintains body temperature (37°C)
• Osmoregulation — controls water balance via ADH secretion
• Hunger and thirst — appetite regulation centres
• Pituitary control — secretes releasing/inhibiting hormones that regulate pituitary → links nervous and endocrine systems
• Sleep-wake cycle — circadian rhythm regulation
• Emotional responses — fear, aggression, pleasure

Growth Promoters:

• Auxin (IAA) — promotes cell elongation; responsible for phototropism, apical dominance
• Gibberellin — promotes stem elongation, seed germination, fruit development
• (Cytokinin also promotes cell division — acceptable as third)

Growth Inhibitor:

• Abscisic Acid (ABA) — the only plant growth inhibitor; causes stomatal closure, dormancy, leaf fall

Note: Only ABA is a true inhibitor among the four main phytohormones.

Division	Functions
Cerebellum	Coordinates voluntary movements, maintains balance and posture, fine-tunes motor activity
Medulla Oblongata	Controls ALL involuntary vital functions: heartbeat, breathing, blood pressure, swallowing, sneezing
Pons	Acts as bridge between cerebrum and cerebellum; role in sleep, respiration, bladder control

Important: Medulla damage → death (controls vital functions). Cerebellum damage → loss of balance, no death.

Diagram: Draw a lateral view showing: Cerebrum (largest, convoluted), Cerebellum (at back, folded), Brainstem (Pons + Medulla at bottom), Thalamus and Hypothalamus (inner, below cerebrum).

Functions:

• Cerebrum (Forebrain) — voluntary actions, conscious thought, memory, reasoning, speech, personality, sensory perception. Two hemispheres joined by corpus callosum. Contains sensory and motor cortex areas.
• Thalamus (Forebrain) — relay station: routes all incoming sensory information to appropriate cerebral areas. Acts as gatekeeper.
• Hypothalamus (Forebrain) — thermoregulation, hunger, thirst, emotion, sleep. Controls pituitary → links nervous+endocrine.
• Cerebellum (Hindbrain) — balance, posture, coordination. Receives input from muscles and cerebrum. Damage → ataxia (uncoordinated movement).
• Pons (Hindbrain) — bridge connecting cerebellum to cerebrum; roles in sleep, respiration.
• Medulla Oblongata (Hindbrain) — controls involuntary vital functions (heartbeat, BP, breathing, swallowing). Damage = fatal.

Protection: Cranium → Meninges (3 layers) → CSF cushion.

Endocrine Glands: Ductless glands that secrete hormones directly into the bloodstream, which then act on distant target organs. Form the endocrine system.

Gland	Hormone	Function	Disorder
Pituitary	GH, TSH, ADH…	Growth, controls other glands	Gigantism / Dwarfism
Thyroid	Thyroxine	Metabolism, growth	Goitre (iodine deficiency)
Adrenal	Adrenaline	Emergency fight-or-flight	Addison's disease
Pancreas	Insulin, Glucagon	Blood glucose regulation	Diabetes mellitus
Testes/Ovaries	Testosterone/Oestrogen	Secondary sex characters	Infertility, menstrual disorders

Auxin and Phototropism:

1. Auxin (Indole Acetic Acid, IAA) is produced at the shoot apex (tip).
2. When light falls from one side, photoreceptors at the tip detect directionality.
3. Auxin is transported laterally — it moves to the shaded (darker) side of the shoot.
4. On the shaded side: higher auxin → greater cell elongation.
5. On the lit side: lower auxin → less elongation.
6. Differential growth → shoot curves/bends toward the light (positive phototropism).

Shoot vs Root response to same auxin concentration:

• Shoots — promote growth at moderate-high auxin concentration; shoot is positively phototropic.
• Roots — inhibited by the same higher concentration; roots are negatively phototropic. (Roots are much more sensitive to auxin — even small amounts inhibit elongation.)

Geotropism comparison: In response to gravity, auxin accumulates on the lower side → root (sensitive) shows inhibition → bends down (+ve geotropism); shoot (insensitive) shows promotion → bends up (−ve geotropism).

Historical note: Darwin (1880) showed coleoptile tip required for phototropic response. Went (1926) isolated auxin from oat coleoptile tip.

Diagram: Draw: Receptor (skin) → Sensory neuron → Spinal cord (relay neuron in grey matter) → Motor neuron → Effector (muscle). Show brain connected by a separate pathway (awareness signal).

Spinal vs Cranial Reflex:

Feature	Spinal Reflex	Cranial Reflex
Processing centre	Spinal cord	Brain (brainstem)
Example	Knee-jerk, hot object withdrawal	Pupil dilation in bright light, blinking
Nerves involved	Spinal nerves	Cranial nerves

Importance of Reflexes:

• Survival — protects body from harm before brain can react
• Speed — short reflex arc avoids time delay of brain processing
• Frees the brain — routine protective actions handled automatically; brain can focus on complex tasks
• Maintains homeostasis — cranial reflexes regulate pupil size, swallowing, blood pressure

Conditioned reflex (Pavlov): Learned reflex — association of neutral stimulus with response through repeated pairing.

Plant Coordination: Plants lack a nervous system but coordinate through chemical messengers (phytohormones) that are synthesised in one part and transported to another to cause a response. They also respond via electrical signals (turgor changes) for rapid movements like in Mimosa.

Four Phytohormones:

1. Auxin (IAA) — Growth promoter. Synthesised at shoot tip.
   • Causes cell elongation → phototropism, geotropism
   • Causes apical dominance (suppresses lateral bud growth)
   • Used commercially as rooting powder for cuttings
2. Gibberellin — Growth promoter. Produced in roots and young leaves.
   • Promotes stem elongation, breaking seed dormancy, fruit growth without fertilisation (parthenocarpy)
   • Used in brewing industry (malting), seedless grapes
3. Cytokinin — Growth promoter. Produced in actively dividing cells.
   • Promotes cell division (cytokinesis), delays leaf senescence (aging)
   • Keeps cut flowers fresh; used in tissue culture
   • Works synergistically with auxin: high auxin/cytokinin ratio → roots; low ratio → shoots
4. Abscisic Acid (ABA) — Only growth INHIBITOR.
   • Causes stomatal closure during drought (water stress response)
   • Promotes seed/bud dormancy during winter
   • Causes leaf, flower, fruit abscission (fall)
   • Antagonises gibberellin during germination

Key distinction: Tropic movements = growth-based, directional, permanent. Nastic movements (Mimosa, Venus flytrap) = turgor-based, non-directional, reversible.