Control and Coordination

The governing concept is the adrenaline-mediated emergency, or fight-or-flight, response. Adrenaline increases readiness for rapid movement and redirects blood supply toward skeletal muscles. Greater blood flow delivers more oxygen and glucose and helps remove carbon dioxide and other wastes, supporting quick contraction. Therefore option A is correct. Muscle tissue does not produce light, root growth is unrelated, and adrenaline does not simply switch off thyroxine.

The governing concept is thyroid hormone synthesis and feedback regulation. Iodine is an essential raw material for producing thyroxine in the thyroid gland. When iodine is deficient, thyroxine production falls; the pituitary may then stimulate the thyroid more strongly through increased thyroid-stimulating hormone. Persistent stimulation can enlarge the thyroid gland, producing a visible swelling called goitre. Therefore option A is correct; the other options concern insulin, the spinal cord or dendrites and are unrelated.

The pituitary is called the master gland because it secretes tropic hormones that regulate the activity of several other endocrine glands, including the thyroid, adrenal glands and gonads. Thus, its influence is mainly through hormonal control of other glands, not through direct movement or sensation. Therefore option B is correct; the other options describe functions unrelated to the pituitary.

The cerebellum coordinates signals from the brain, muscles and sense organs so that movements are smooth, properly timed and balanced. Walking in a straight line requires continuous adjustment of posture and muscle contraction; damage to the cerebellum can disturb this coordination and cause an unsteady gait. It does not produce insulin or thyroxine, so option A is correct.

The medulla oblongata contains centres that regulate important involuntary activities, such as breathing, heartbeat and aspects of blood-vessel control. These functions continue without conscious effort and are essential for maintaining life. Serious damage may interrupt them and therefore become fatal. Hair colour, plant phototropism and taste are not the basis of this danger, so option A is correct.

Endocrine hormones are carried by the blood to many tissues, but a response occurs only where cells possess the specific receptor or response mechanism for that hormone. Such organs or tissues are called target organs. Other cells may be exposed to the hormone but cannot bind or interpret it effectively. Therefore option A is correct; the blood does not visit only one organ, and hormones do not simply remain in bones.

Positive geotropism means that roots grow in the direction of gravity, generally downward into the soil. This places them where they can anchor the plant and absorb water and mineral salts. Negative geotropism means that the stem grows away from gravity, generally upward, helping leaves reach light for photosynthesis. These complementary responses support nutrition and stability, so option A is correct.

The vestibular parts of the inner ear detect head movement and help the brain judge body position and balance. Their information is compared with signals from the eyes and muscles. If the inner-ear signals become abnormal or inconsistent, the brain may interpret the body as moving when it is not, producing dizziness or loss of balance. The remaining options are biologically unrelated.

The shoot commonly shows positive phototropism by growing towards light, which improves light capture for photosynthesis. The root commonly shows positive gravitropism by growing downward, helping it anchor the plant and absorb water and minerals. These opposite directional responses demonstrate coordination in plants and show that different organs respond to different stimuli according to their functions. Hence A is correct.

Before a race, excitement and anticipation activate the sympathetic stress response. The adrenal glands release adrenaline, which increases heart rate and breathing and directs more blood toward skeletal muscles. These changes prepare the body for rapid action. Insulin regulates blood glucose, while auxin and cytokinin are plant hormones, so they do not explain the runner’s immediate heartbeat increase. Therefore B is correct.

Thyroxine is a hormone secreted by the thyroid gland. It helps regulate the body’s metabolic rate and supports normal growth and development. If its level is too low, cellular energy use and several body processes may slow or become abnormal; prolonged deficiency can also affect development. A photoreceptor, synapse, and tendril are structures, not the function of thyroxine. Hence A is correct.

Plants do not possess a nervous system with a brain, spinal cord, and specialised nerves. Their coordination commonly depends on chemical messengers such as plant hormones, which alter growth or other physiological activities and may act relatively slowly. Animals use electrical impulses in neurons, often producing rapid responses, with hormones also supporting longer-term control. Thus option A gives the valid comparison.

The cerebrum is the major region associated with conscious thought, memory, learning, reasoning, and voluntary actions. The cerebellum does not perform these higher mental functions; it fine-tunes muscular activity and helps maintain posture, balance, and coordinated movement. Thus option A correctly distinguishes their roles. The other options incorrectly describe brain regions as glands, digestive organs, or plant hormones.

The governing concept is stimulus response in plants. Plants lack a brain and an animal-like nervous system, but their cells detect environmental changes and coordinate responses through chemical signals and growth movements. Shoots may bend towards light, roots respond to gravity and water, and tendrils react to touch. Thus option A is correct; B, C, and D wrongly assign animal features to plants.

The governing concept is hormonal balance and regulation of growth. Growth hormone from the pituitary supports the normal growth of bones and other tissues, but its effect depends on an appropriate amount and developmental stage. Too little may cause inadequate growth, whereas too much can cause abnormal excessive growth. Hence option A is correct. Options B and C are biologically false, while D contradicts the hormone’s known role.

The governing concept is the difference between rapid neural control and slower hormonal control. Endocrine glands release hormones into the blood, so their effects usually begin more slowly than a reflex. However, hormones can reach target organs and regulate growth, metabolism, development, and other sustained changes over longer periods. Therefore option A is correct. B reverses the comparison, while C and D contradict how endocrine signals work.

The governing concept is the coordinated stimulus-response pathway. A receptor detects a change, a control centre interprets the information, and an effector produces the response. If any link fails—such as detection, transmission, interpretation, or movement—the final response may be delayed, incorrect, or absent, disturbing body functions. Thus A is correct. B, C, and D make sweeping statements that conflict with normal physiology.

In the usual flow through a neuron, dendrites receive information from a receptor or another neuron. The signal is integrated in the cell body and then travels along the axon to its terminal, where it can influence the next cell across a synapse. Therefore option A correctly describes the direction. Axons do not release messages into blood, nuclei do not send hormones into the spinal cord, and synapses do not always reverse signals.

Blood glucose normally rises after a meal, but insulin released by pancreatic beta cells helps body cells take up and use glucose, bringing the level back toward normal. If glucose remains high for a prolonged period, the body may be producing too little insulin or the cells may not respond effectively to it. Therefore option A is the most suitable conclusion. Auxin and the spinal cord do not regulate blood glucose in this manner.

Negative feedback maintains a physiological variable near its normal level by reducing the original corrective response once the imbalance is corrected. Insulin lowers elevated blood glucose by promoting glucose uptake and storage. When blood sugar returns to the normal range, the stimulus for increased insulin release becomes smaller, so insulin secretion should decrease rather than continue rising. Thus option A is correct; the remaining choices contradict feedback regulation or confuse different organs and hormones.

The thyroid gland synthesizes thyroxine, and iodine is an essential raw material for this hormone. If the diet does not supply enough iodine, the thyroid may not produce adequate thyroxine, which can disturb normal metabolism and growth. Thus option A states the correct causal relationship. Auxin is a plant hormone, insulin is not made using iodine, and reflex actions do not produce thyroxine.

The pituitary gland secretes growth hormone, which helps regulate the growth of bones and other body tissues during childhood. Consequently, an unusually slow or rapid growth rate can indicate a problem involving pituitary regulation, making option A correct. Removing the hand is a nervous reflex, while tendril coiling and pollen-tube movement are plant responses controlled by plant growth mechanisms.

Hormones are transported through the blood, so they may pass near many different cells. However, a hormone produces a response only when a target cell has the appropriate receptor, much like a key fitting a particular lock. Therefore option A is correct. Blood does carry hormones, and hormone action is not restricted to bones or dependent on light.

Nervous control uses electrical impulses and chemical transmission across synapses, so it usually acts rapidly and its effects may end quickly. Hormones travel through the blood and often act more slowly, but their effects can persist longer. Hence option A is correct. Nerves do not normally signal through blood, both systems occur in animals, and hormones act on specific target organs.

The governing concept is that plant movements may be either growth-independent or growth-dependent. Touch-me-not leaves fold rapidly because changes in cell water content and turgor pressure occur at the pulvinus; this is not caused by growth. A tendril coils after touch because unequal growth develops on its two sides. Thus option A is correct. The other options wrongly invoke blood flow, a spinal cord, or unrelated tropisms.