Control and Coordination

The governing concept is the stimulus-response pathway in control and coordination. A stimulus must be detected, its information must be transmitted and interpreted, and an effector must then act. If sensing, transmission, processing, or execution fails, the final response may be delayed, absent, or inappropriate. Therefore option A is correct; the other choices make scientifically false absolute claims.

The central conclusion is that living organisms detect changes and coordinate suitable responses. Animals use rapid electrical signals through the nervous system and slower chemical coordination through hormones, while plants respond through chemical messengers and growth-related mechanisms. Option A captures this unified idea. Options B and C deny established biology, and option D incorrectly treats two different signalling systems as identical.

This situation illustrates nervous coordination. Photoreceptors detect the intensity of incoming light, sensory information is processed through the nervous system, and signals reach the muscles controlling the iris. Those muscles alter pupil diameter: contraction limits excess light and dilation permits more light in darkness. Thus option A is correct. Option B concerns endocrine regulation, while C and D describe plant responses unrelated to the pupil.

A reflex arc contains a receptor, sensory pathway, integration centre, motor pathway and effector. Because the receptor is functioning, the stimulus can still be detected and the sensory message can enter the pathway. However, damage to the motor nerve interrupts transmission from the control centre to the muscle or gland, so the response becomes weak or absent. Therefore A is correct; B ignores the working receptor and C and D are unrelated.

Dendrites are branched extensions specialized mainly for receiving signals from receptors or neighbouring neurons and directing them toward the cell body. If they are removed, the neuron loses much of its receiving surface and incoming signals cannot be transmitted normally to the cell body. Consequently, its ability to receive messages decreases greatly, making option A correct. The other options either reverse the effect or describe impossible changes in unrelated structures.

A synapse is the functional junction or tiny gap between two neurons. When an impulse reaches the end of one neuron, chemical neurotransmitters are released into the gap and bind to receptors on the next neuron, allowing the signal to continue. Therefore option A directly describes synaptic transmission. Insulin action is hormonal regulation, whereas root growth and stem growth are plant responses, so B, C and D are distractors.

Feeling touch demonstrates that the touch receptor and at least much of the sensory pathway are functioning. Moving the hand away requires processing followed by a motor command travelling through motor nerves to an effector muscle. If sensation is present but movement fails, the defect may therefore lie in the motor pathway, neuromuscular connection or muscle. Option A is appropriately cautious; B contradicts the evidence and C and D are irrelevant.

The brain shows division of labour because its regions have specialised but coordinated functions. The cerebrum supports conscious thought, memory and voluntary activities; the cerebellum helps coordinate muscular activity and maintain posture and balance; and the medulla regulates important involuntary actions such as breathing and heartbeat. Therefore option A is correct. The other options wrongly assign blood filtration, hormone secretion or memory to these brain regions.

The symptoms point to a region that controls vital involuntary activities rather than conscious thought. The medulla, part of the brainstem, helps regulate breathing, heartbeat and other autonomic functions. Normal thinking suggests that the cerebrum, which supports higher mental activity, may be functioning comparatively normally. Therefore option A is the best conclusion. The tongue is an organ involved in taste and movement, and the pancreas regulates digestive enzymes and hormones, not the primary control of breathing and heartbeat.

The governing concept is the interaction of tropic responses. A root can show hydrotropism towards water and positive geotropism towards gravity, so stimuli acting in different directions may jointly influence its final growth path. Option A states this principle correctly. Roots are not affected only by light, plants do possess chemical regulation, and they do not make decisions through a nervous system like animals.

The governing concept is geotropism, the growth response to gravity. The root grows in the direction of gravitational pull, so it exhibits positive geotropism. The stem grows away from that direction, so it exhibits negative geotropism. Thus option A is correct. Option B reverses the signs, while chemotropism concerns chemicals and a reflex arc is a nervous-system pathway absent from ordinary plant growth.

The governing concept is the difference between nastic movement and tropic growth. Touch-me-not leaves fold rapidly because of changes in cell turgor at the pulvinus; this movement does not require growth and is not direction-dependent. A tendril coils after contact because unequal growth occurs on its two sides, making it a directional thigmotropic response. Therefore option A is correct.

The governing concept is chemotropism, a directional growth response to a chemical stimulus. Chemicals released or present near the ovule guide the pollen tube toward it, allowing the male gametes to reach the female gametophyte for fertilisation. Therefore option A is correct. Sound does not guide the tube, pollen tubes have no spinal-cord connection, and insulin is an animal hormone unrelated to this plant process.

The governing concept is the role of plant hormones in growth. Gibberellin promotes cell elongation and can increase the length of stems, so comparing a treated dwarf plant with an untreated one would provide clear evidence. Therefore option A is correct. Hand withdrawal is a nervous reflex, blood-sugar reduction involves insulin, and pupil contraction is controlled by the nervous system; these effects do not test gibberellin.

The governing concept is receptor-dependent cell signalling. A hormone produces its effect only when it binds effectively to the appropriate receptor. If the receptor changes shape, number, or binding ability, the hormone may bind poorly and the cellular response can weaken even though the hormone concentration is normal. Thus option A is correct. Hormones do not turn into nerve cells, geotropism does not begin in blood, and receptor changes in one target cell cannot stop respiration in all plants.

The governing concept is homeostasis maintained by negative feedback. When blood glucose rises above its normal range, the pancreas releases more insulin; insulin promotes glucose uptake and storage, bringing the level downward. Once the level approaches normal, the stimulus decreases and insulin secretion also falls. This reversal of the original change makes option A correct. Phototropism and thigmotropism are plant movements, while a reflex arc is a nervous pathway, so B, C, and D are unrelated.

The governing concept is insulin-dependent regulation of blood glucose. After a meal, glucose enters the bloodstream. Insulin helps many body cells take up glucose and promotes its storage, especially as glycogen, so the blood level returns toward normal. If insulin secretion is insufficient, uptake and storage are reduced and glucose remains elevated for longer; therefore option A is correct. The other options confuse unrelated hormones, glands, or nervous structures.

The governing concept is the emergency or fight-or-flight response. Fear activates the adrenal glands, and adrenaline helps the liver break down stored glycogen, making more glucose available in the blood. This rapid energy supply supports vigorous muscle activity and immediate action, so option A is correct. Root growth toward water and leaf movement away from light are plant responses, while stopping thyroxine production is not the purpose of adrenaline; hence B, C, and D are distractors.

The governing concept is coordinated fight-or-flight physiology. An increased breathing rate brings oxygen into the lungs more rapidly, while a faster heartbeat delivers oxygen and glucose-rich blood to active muscles more quickly. Together these changes support rapid respiration and ATP production for emergency movement, making option A correct. Root emergence, insulin production by the thyroid, and smell detection by photoreceptors are biologically unrelated, so B, C, and D are incorrect.

The governing concept is hormonal control by the endocrine system. Thyroxine is produced by the thyroid gland, and iodine is an essential raw material needed for its synthesis. Therefore, inadequate iodine may reduce thyroxine production and can contribute to thyroid enlargement, called goitre. Option A correctly connects the nutrient, hormone, gland, and possible consequence. Adrenaline is not the only hormone related to iodine, plants do not produce human thyroxine, and a reflex arc belongs to nervous coordination.