Control and Coordination

The governing concept is the organization of the nervous system. Neurons join to form connected pathways, and an impulse travels from a receptor through sensory neurons and processing centres to a particular motor neuron and effector. This wiring directs the message to the required body part. Hormones do not block nerves, blood does not create impulses, and muscles do not receive light; therefore option A is correct.

The governing concept is tropic movement, which is a directional growth response to an external stimulus. A root growing away from light shows negative phototropism. Because it grows in the direction of gravitational pull, it shows positive geotropism, also called positive gravitropism. Chemotropism and thigmotropism involve chemicals and touch respectively, so they do not describe both observations; option A is correct.

The governing concept is the distinction between growth-dependent and growth-independent plant movements. A stem bends toward light through unequal growth, so it is a growth movement. A Mimosa leaf folds rapidly because of changes in cell turgor at the pulvinus, not because of new growth, so it is non-growth-based. The pair in option A therefore correctly represents both types.

The governing concept is coordination in living organisms. Animals use the nervous system for rapid responses and hormones for slower, longer-lasting regulation. Plants have no brain or nervous system; they coordinate activities mainly through chemical messengers such as plant hormones and through growth responses. Therefore option A correctly compares both groups. The other choices assign structures that do not perform control in the stated organisms.

A synapse is the small junction or gap through which one neuron communicates with the next neuron. Usually, the first neuron releases neurotransmitters, which cross this gap and trigger a signal in the next neuron. If a medicine blocks these chemical messages, transmission becomes weak or stops. Therefore, option A is correct; the other options describe biologically unrelated or impossible effects.

Adrenaline, also called epinephrine, is released by the adrenal glands during sudden fear, danger, or intense stress. It prepares the body for a rapid emergency response by increasing heart rate, breathing rate, and blood supply to skeletal muscles. Therefore, suddenly seeing danger is the best answer. Peaceful sleep, calm study, and ordinary digestion do not normally cause this strong fight-or-flight response.

A stimulus is a detectable change in the surroundings or inside an organism that can initiate a response. A response is the resulting action or change. In phototropism, light provides the directional stimulus, and the plant stem bends toward the light as its growth response. Therefore, option A correctly identifies both parts; the other pairings confuse organs, substances, and actions.

A receptor is a specialised cell or sensory structure that detects a particular stimulus and converts it into a nerve signal. The retina contains photoreceptors, namely rods and cones, that respond to light entering the eye. Hence, “retina and light” is the correct match. The tongue mainly detects taste, the ear detects sound, and the nose detects chemical odours, so the other pairings are mismatched.

Control and coordination allow different organs and systems to receive information, respond appropriately, and work together in a properly timed manner. The nervous system provides rapid control, while hormones provide chemical regulation that may be slower and longer lasting. This integration maintains effective body functions. Therefore, option A is correct; the other choices are narrow, unrelated, or incomplete descriptions.

A reflex arc follows this route: receptor in the skin detects heat, a sensory neuron carries the impulse to the spinal cord, and a motor neuron carries the response to the muscle, the effector. If the motor nerve is damaged, the spinal command cannot reach the muscle, so withdrawal becomes weak or absent even though the receptor may still detect heat. Thus option B is correct.

The governing concept is synaptic transmission. Neurons at a synapse are separated by a microscopic gap, so the electrical impulse cannot simply pass through continuous cytoplasm. When the impulse reaches the axon terminal, it triggers release of neurotransmitter chemicals; these diffuse across the synaptic cleft and stimulate the next neuron. Thus option A gives the necessary reason. The other choices assign impossible functions to the axon, dendrite or spinal cord.

The governing concept is the interaction of tropic responses. Roots commonly show positive geotropism, growing in the direction of gravity, but they also show positive hydrotropism, growing towards water. In the stated experiment, the root bends towards water even against gravity, so the water stimulus has the stronger observable influence under those conditions. Option A is therefore correct; the other choices confuse light response, deny the response or make a biologically impossible claim.

The governing concept is the difference between nastic movement and tropic growth. Mimosa leaves fold rapidly after touch because changes in water content and turgor pressure occur in specialised pulvinus cells; this does not require directional growth. A stem bends toward light because auxin causes unequal elongation on its two sides, making it a slower growth response. Thus option A is correct; plants have no brain or blood circulation like animals, and both movements involve stimuli.

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.