Control and Coordination

The governing concept is the reflex arc, in which the spinal cord can coordinate a rapid response before the brain fully processes the information. If a hand touches a hot object, muscles can withdraw it immediately, reducing tissue damage. Therefore option A is correct. The other options are unrelated: reflex action does not stop hormone formation, does not keep muscles inactive, and has no direct role in making plants grow faster.

The governing concept is the role of the medulla oblongata in controlling vital involuntary activities. Breathing is regulated through respiratory centres in this region, so damage can quickly disturb this life-supporting function. Thus option C is correct. Memory and drawing depend mainly on higher brain regions and learned coordination, while taste detection begins with sensory receptors and pathways; these are not the most immediate medulla-controlled functions among the choices.

The governing concept is the structural polarity of a neuron. Dendrites generally receive signals from receptors or neighbouring neurons; the cell body integrates them, and the axon carries the impulse toward its terminal endings. At a synapse, transmission normally proceeds to the next cell rather than backward into the same neuron. Therefore option B is correct, while the other choices reverse the pathway or describe processes that do not occur.

The governing concept is synaptic transmission. When a nerve impulse reaches the terminal of one neuron, it causes neurotransmitter chemicals to be released into the tiny synaptic cleft. These chemicals diffuse across the gap and bind to receptors on the next neuron, initiating a new impulse. Therefore option C is correct. Blood pressure, bone movement, and digestion do not transmit a nerve message across a synapse.

The governing concept is the functional division of the brain. The cerebrum is responsible for conscious thought, intelligence, learning, memory, and interpretation, so “cerebrum and intellectual functions” is correctly paired in option D. The cerebellum chiefly maintains balance and coordinates movement, whereas the medulla controls involuntary activities such as breathing and heartbeat. Therefore options A, B, and C mismatch each brain part with its principal function.

The governing concept is receptor specificity. A hormone may circulate through the blood and reach many tissues, but it produces a response only where cells possess receptors with the appropriate shape and chemical compatibility. Therefore option B is correct. Hormones do not equally stop every organ, they are not restricted to the brain, and they are not immediately destroyed on entering the blood. Receptors determine the target-cell response.

The governing concept is the role of iodine in thyroid hormone production. The thyroid gland uses iodine to synthesize thyroxine, which helps regulate metabolism, growth, and development. Long-term dietary deficiency can reduce thyroxine production and may lead to thyroid enlargement or goitre. Therefore option B is correct. The pancreas, adrenal glands, and testes have important endocrine functions but do not require iodine as the defining raw material for their principal hormones.

The governing concept is hormonal control of blood glucose. Insulin, secreted by the pancreas, helps body cells take up and use glucose and promotes its storage as glycogen. If insulin is deficient, glucose remains in the bloodstream, so blood sugar rises and diabetes may develop. Adrenaline can temporarily raise glucose during stress, while thyroxine and estrogen have different primary functions; therefore option A is correct.

The governing concept is the emergency or fight-or-flight response. In a frightening situation, the adrenal glands release adrenaline. This hormone increases heart rate, breathing rate and blood flow to muscles so that the body can respond rapidly and obtain more oxygen and energy. Thyroxine regulates general metabolism, insulin regulates blood glucose, and growth hormone supports growth; none gives this immediate fear response. Hence option C is correct.

The governing concept is coordination by endocrine hormones. The pituitary secretes hormones such as thyroid-stimulating hormone, adrenocorticotropic hormone and gonadotropins, which regulate the activity of several other endocrine glands. Growth hormone and other pituitary hormones also act directly on tissues. Thus it is traditionally called the master gland. It does not make digestive juice, join bones or filter blood, so option C is correct.

The governing concept is phototropism caused by unequal auxin distribution. When light comes from one side, auxin moves and accumulates more on the shaded side of a young shoot. In shoots, this higher auxin concentration promotes greater cell elongation there. The shaded side therefore grows faster, pushing the stem toward the light. The light side does not elongate more, and the other choices do not explain the directional bend; hence B is correct.

The governing concept is tropism, a directional growth response to an external stimulus. Water provides hydrotropic stimulation and gravity provides geotropic stimulation. Roots grow toward water and generally downward in response to gravity, showing that plant growth can be coordinated without a nervous system. Photosynthesis, nerves and blood circulation do not explain this directional growth. Therefore option A correctly identifies the feature.

The governing concept is organ-specific tropic response. Light is the same stimulus, but the stem generally shows positive phototropism by growing toward it, whereas the root commonly shows negative phototropism by growing away. Different organs have different structures, hormone distribution and growth responses, so their reactions need not be identical. This disproves the other choices; therefore option A is correct.

The governing concept is the difference between tropic growth and nastic movement. Tropism is a relatively directed growth response whose direction depends on the direction of the stimulus. Touch-me-not leaves fold rapidly after touch because changes in water pressure in specialised pulvinus cells cause movement; this is not directional growth. Since a stimulus is present but growth is not responsible, option A is correct.

The governing concept is geotropism, also called gravitropism. Positive tropism means growth in the direction of the stimulus. Gravity acts downward, and roots usually grow downward in response to it; therefore downward root growth is positive geotropism. Upward stem growth is generally negative geotropism, tendril coiling is related to touch, and pollen-tube growth toward an ovule is chemotropism. Hence option B is correct.

The governing concept is negative geotropism, or negative gravitropism. Negative tropism means growth away from the direction of the stimulus. Since gravity acts downward, the stem or shoot growing upward is growing opposite to gravity and therefore shows negative geotropism. Root growth toward water is hydrotropism, tendril bending is a touch response, and pollen-tube growth toward a chemical signal is chemotropism. Thus option B is correct.

Phototropism is the directional growth of a plant shoot in response to light. The shoot tip produces auxin, a plant hormone that promotes cell elongation. When light comes from one side, auxin distribution helps the shaded side elongate more, causing the shoot to bend toward light. Cutting off the tip removes an important source of auxin, so this unequal growth and bending may become weak. Blood, spinal cord, and insulin are unrelated to plant phototropism.

The thyroid gland secretes thyroxine, a hormone that helps regulate the body’s metabolism, so option C is correctly matched. The pancreas produces insulin, not adrenaline; adrenaline is secreted by the adrenal glands. The pituitary gland produces several animal hormones, whereas auxin is a plant hormone produced mainly in growing regions. Because each of the other pairs places the wrong hormone with the stated gland, thyroid gland and thyroxine is the only correct pair.

Growth hormone is secreted by the pituitary gland and supports the normal growth of bones and other body tissues during childhood. If too little is produced, the child may show reduced or abnormally slow growth, often resulting in shorter stature. Low growth hormone does not directly cause an immediate fall in blood sugar, which is related mainly to insulin regulation. Goitre is associated with thyroid problems, while rapid hand withdrawal is a reflex action.

Testosterone, secreted mainly by the testes, is the principal male sex hormone responsible for many changes during male adolescence. It supports development of secondary sexual characteristics, including growth of the larynx and thickening or lengthening of the vocal cords, which makes the voice deeper. Estrogen is mainly associated with female reproductive changes, insulin regulates blood glucose, and auxin is a plant hormone. Therefore option B is correct.