Control and Coordination

The governing concept is reflex action, a rapid, automatic protective response. Heat stimulates pain receptors in the skin; sensory neurons carry the impulse to the spinal cord, where an interneuron connects it to a motor neuron. The motor neuron activates arm muscles before the brain has fully analysed the sensation, so withdrawal is quick. The brain later receives information and becomes aware of pain. The other options deny normal nerve or muscle function.

The governing concept is directional transmission in a neuron. Dendrites are short, branched extensions that receive stimuli or nerve impulses from receptors or nearby neurons. The impulse then passes through the cell body and travels along the axon toward its terminal, where it can affect another cell. Therefore, option B correctly states both functions; the other options assign unrelated functions such as food-making, blood formation, breathing, or water storage.

A synapse is the tiny junction or gap between two neurons. When an electrical impulse reaches the end of the first neuron, it causes chemical messenger substances to be released. These chemicals cross the synaptic gap and stimulate the next neuron, which then carries the impulse onward. Thus option C is correct. Bones, blood cells, and digestive juice do not transmit messages across a synapse.

The governing concept is endocrine coordination by chemical messengers. Hormones are released by endocrine glands into the blood and may remain in circulation or continue affecting target cells after the initial release. Their action usually begins more slowly than a nerve impulse but can persist longer and influence several organs. Thus option A is correct; the remaining options describe false or unrelated processes.

The governing concept is the emergency or fight-or-flight response. Adrenaline released by the adrenal glands prepares the body for rapid action by increasing heart rate, breathing rate, and blood supply to active muscles. These changes help deliver oxygen and energy quickly. Therefore option A is correct. Sleep, absence of blood changes, and complete muscle stoppage are opposite to the normal action of adrenaline.

The governing concept is the role of iodine in thyroid hormone synthesis. The thyroid gland requires iodine to produce thyroxine, a hormone involved in regulating metabolism and development. When iodine intake is insufficient, thyroxine production may fall, and prolonged deficiency can contribute to enlargement of the thyroid gland called goitre. Therefore option A is correct; the other options describe no valid physiological relationship.

The governing concept is the division of functions among brain regions. The cerebellum coordinates the activity of muscles and helps maintain posture, balance and precise voluntary movement. Therefore, damage to it most directly causes difficulty in coordinated movement and balance. Blood-sugar regulation is mainly hormonal, thyroxine is produced by the thyroid gland, and skin colour is not the cerebellum’s principal function.

The governing concept is phototropism caused by unequal growth. When light comes from one side, auxin becomes distributed unevenly in the shoot. In a typical shoot, the shaded side receives more auxin and its cells elongate more than cells on the lighted side. This difference in elongation produces curvature, so the stem bends toward the light. Blood, roots pulling light and stopping photosynthesis do not explain it.

The governing concept is geotropism, a directional growth response to gravity. The word positive means growth in the same direction as the stimulus. Since gravity acts downward, roots generally grow downward along the direction of gravitational pull; therefore they show positive geotropism. Growth toward light describes phototropism, upward growth is opposite to gravity, and roots do respond to environmental stimuli.

The governing concept is the direction of growth relative to gravity. Gravity pulls downward, whereas the stem generally elongates upward toward the surface and light. Growth opposite to the direction of gravity is called negative geotropism, so option A is correct. Downward growth would be positive geotropism, movement relative to water concerns hydrotropism, and a stem does grow rather than remaining inactive.

The governing concept is thigmotropism, a directional growth response to touch. When a tendril touches a support, auxin distribution and cell elongation become unequal on its two sides. The untouched side generally grows more, so the tendril bends toward and coils around the support. Thus option A is correct; the other choices do not explain plant growth movement or involve blood, food production, or complete loss of light.

The governing concept is nastic movement caused by changes in cell turgor, not permanent growth. Mimosa leaves fold within seconds after touch because water shifts in specialised motor cells at the leaf base, changing their pressure. No lasting increase in cell length is required. Therefore option A is correct; the movement is not restricted to roots, and it is unrelated to darkness or seed formation.

The governing concept is coordination through two complementary control systems. The nervous system transmits electrical impulses rapidly for immediate, local responses, while the endocrine system releases hormones into blood for slower effects that may last longer and reach several target tissues. Therefore option A is correct. The other options deny their established roles or incorrectly restrict them to plants.

The governing concept is specificity in endocrine coordination. Hormones may travel widely through the bloodstream, but they affect only cells or organs containing the appropriate receptors. Such responsive organs are called target organs. Hence option A is correct. The distractors are biologically meaningless: hormones do not act on stones, merely colour skin, or become defined by being lost in air.

The governing concept is sensory transduction in vision. Retinal photoreceptors, mainly rods and cones, absorb light and convert its energy into electrical changes that begin nerve signals. These signals are processed in the retina and carried toward the brain through the optic nerve. Therefore option A is correct; filtering blood, joining bones, and producing taste are functions of other body structures.

Seeing attractive food stimulates the eyes, which send sensory information to the brain. The brain then activates the salivary glands through nerve signals, causing saliva secretion. This demonstrates coordination between a sense organ, the nervous system, and a gland, so option A is correct. Bone growth, plant growth, and blood clotting do not explain this immediate response to food.

Phototropism is the directional growth response of a plant to light, while geotropism, or gravitropism, is the directional growth response to gravity. Although the stimuli differ, both involve growth in a particular direction and are regulated by plant growth mechanisms such as unequal distribution of auxin. Thus option A states the correct similarity.

Thyroxine is a hormone secreted by the thyroid gland. Its major general effect is to regulate the body's metabolic rate, meaning how quickly cells use energy, and it also supports normal growth and development. Iodine is needed for its production. Thus option A is correct. Taste, hearing, and sweating are not the primary general functions controlled by thyroxine, so options B, C, and D are unsuitable.

During running, skeletal muscles contract more frequently and perform greater work than they do at rest. Their cells therefore need increased oxygen and glucose for aerobic respiration, which releases ATP for muscle contraction. Blood delivers these materials and removes carbon dioxide and other wastes. Hence option A is correct. The other options do not explain the increased circulation to active muscles.

Control and coordination enable an organism to detect internal or external stimuli, process the information, and produce an appropriate response. A suitable response helps maintain safety, balance, and normal activity; an unsuitable response may cause injury or waste energy. For example, withdrawing from excessive heat protects tissues. Therefore option A expresses the governing purpose, while the other options contradict basic facts about responses and coordination.

The chapter shows that organisms detect stimuli and coordinate responses through different control systems. Animals use electrical nerve impulses and chemical hormones, while plants rely mainly on chemical messengers and regulated growth movements. These mechanisms help organisms respond, survive, and maintain internal functioning. Therefore option A gives the most complete conclusion. Options B, C, and D deny established responses or reduce them incorrectly to digestion alone.

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.