Ans: A synapse is the junction (gap) between two neurons — specifically between the axon terminal of one neuron and the dendrite of the next. Transmission across a synapse: (1) A nerve impulse (electrical signal) travels along the axon of the first neuron to its axon terminal. (2) The impulse triggers release of chemical messengers called neurotransmitters (e.g., acetylcholine) from vesicles in the axon terminal. (3) Neurotransmitters diffuse across the synaptic cleft (gap). (4) They bind to receptors on the dendrite of the next neuron, generating a new electrical impulse. (5) Neurotransmitters are then broken down or reabsorbed. This chemical relay ensures one-way transmission of signals.

Ans: When light falls on a plant shoot from one side, auxin (a plant hormone produced at the shoot tip) moves away from the light and accumulates on the shaded side. The cells on the shaded side have more auxin and therefore elongate more than cells on the lit side. This unequal growth causes the shoot to bend toward the light source. This is positive phototropism. Roots show negative phototropism (bend away from light). This response helps plants optimise light exposure for photosynthesis. Without auxin (e.g., if the shoot tip is removed), the plant does not bend.

Ans: The cerebellum is located at the back and bottom of the brain (hindbrain). Its main functions are: (1) Coordination of voluntary muscle movements — ensures movements are smooth and precise. (2) Maintaining balance and posture — receives information from muscles and joints, adjusting body position. (3) Learning motor skills — helps in activities like walking, riding a bicycle, playing sports. Damage to the cerebellum causes lack of coordination (ataxia) — the person cannot walk in a straight line, their movements become jerky and uncontrolled.

Ans: Insulin (from β-cells of pancreatic islets of Langerhans): lowers blood glucose level by promoting uptake of glucose by cells and its conversion to glycogen in the liver. Deficiency causes diabetes mellitus. Adrenaline (from adrenal glands): "fight-or-flight" hormone released during stress/fear. Increases heart rate, dilates air passages and blood vessels to muscles, raises blood glucose — prepares body for emergency action. Thyroxine (from thyroid gland): regulates basal metabolic rate (how fast cells use energy). Promotes normal growth and development, especially of the brain in children. Requires iodine for synthesis; deficiency causes goitre (swelling of neck).

Ans: Testosterone, secreted by the testes.

Ans: Adrenaline, secreted by the adrenal glands.

Ans: This is thigmotropism. The part of the tendril in contact with the support grows more slowly, while the part away from it grows faster because of auxin. This unequal growth makes the tendril coil around the support.

Ans: Voluntary actions are under the control of the will and are controlled by the fore-brain, e.g. writing or walking. Involuntary actions occur without conscious control and are controlled mainly by the mid-brain and hind-brain (medulla), e.g. heartbeat, breathing, vomiting.

Ans: The impulse triggers the release of chemical messengers (neurotransmitters) from the axon end. They diffuse across the synaptic gap and bind to the dendrite of the next neuron, where they start a new electrical impulse.

Ans: In a reflex action the impulse is processed at the spinal cord and is not sent all the way to the brain for decision. Since the pathway is shorter, the response is quick, which protects the body from harm, e.g. withdrawing a hand from a hot object.

Ans: Phototropism: shoots bend towards light. Geotropism: roots grow downwards towards gravity. Hydrotropism: roots grow towards water in soil. In phototropism, auxin synthesised at the shoot tip diffuses to the shaded side and makes the cells there grow longer, so the shoot bends towards the light. Similar unequal distribution of auxin explains other tropic bending.

Ans: A feedback mechanism is a way by which the level of a hormone in the blood is regulated by its own effects. When blood sugar rises, the pancreas detects it and secretes more insulin, which lowers the sugar level by letting cells and liver take up glucose. When the sugar level falls, the pancreas secretes less insulin. This keeps the sugar level in balance.

Ans: Growth hormone: pituitary gland; regulates growth and development of the body. Thyroxine: thyroid gland; regulates metabolism of carbohydrates, proteins and fats for balanced growth. Insulin: pancreas; regulates blood sugar level.

Ans: Heat is detected by thermoreceptors in the skin (receptor). The sensory neuron carries the impulse to the spinal cord. In the spinal cord, the relay (connecting) neuron passes the signal to a motor neuron, which carries the impulse to the muscle of the arm (effector). The muscle contracts and the hand is pulled away. Reflex arcs are necessary because the quick response protects the body from harm before the brain can decide, and the brain is also informed afterwards. They are used where fast response is needed since thinking would take longer.

Ans: Nervous: uses electrical impulses and chemical neurotransmitters; hormonal: uses chemical hormones. Nervous: response is very fast; hormonal: slower. Nervous: effect is short-lived; hormonal: effect is long-lasting. Nervous: reaches specific cells or organs through nerves; hormonal: carried by blood to target tissues, so wider effect. Nervous: involves neurons, brain and spinal cord; hormonal: involves endocrine glands. Example: reflex action vs growth by growth hormone.