Control and Coordination

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.

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 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 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 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.

A hormone produces an effect only when it reaches a suitable target cell and binds to a functional receptor. Consequently, measuring a normal hormone level does not prove that the target tissue can respond. If the receptors are defective, the hormone may be present in the blood but its message will not be transmitted properly. Thus option A is correct; the other options describe normal plant or reflex situations rather than a failure of hormone action.

A nerve impulse is transmitted along a neuron as an electrical change in the cell membrane. At the synapse, there is a small gap between neighbouring neurons, so neurotransmitter chemicals are released by one neuron and act on the next one. Option A correctly describes this electrical-to-chemical transfer. Food, light, blood, bone, and hormones do not represent the normal mechanism of synaptic transmission.

A synapse is the junctional gap between two neurons. Although an impulse can travel electrically along one neuron, neurotransmitter chemicals are needed to carry the signal across this gap and activate the next neuron. If those chemicals are not formed, transmission to the next neuron will be interrupted. Therefore option B follows directly; insulin production and root growth are unrelated distractors.

Seeing an object requires sensory input, but understanding its meaning, interpreting information, forming memories, and making decisions are higher mental functions. These functions are mainly associated with the forebrain, especially the cerebrum. Therefore option C is correct. The cerebellum coordinates balance and movement, the medulla regulates vital involuntary actions, and the spinal cord conducts impulses and mediates reflexes rather than performing complex interpretation.

Nervous control is rapid and specific because electrical impulses travel along particular neurons and nerve pathways to connected target regions. This pathway-based arrangement limits direct reach to tissues connected with nerves. Hormones, in contrast, are released into the bloodstream and may reach target organs far from the secreting gland, although only cells with suitable receptors respond. Thus option A correctly identifies the limitation; the other statements reverse or exaggerate the properties of the two systems.

The thyroid gland needs iodine to synthesise thyroxine. When iodine intake is insufficient, thyroxine production may fall, and reduced thyroid hormone feedback can stimulate the gland to work harder. This increased stimulation may enlarge the thyroid, producing a swelling called goitre. Thus option A correctly connects iodine deficiency, reduced thyroxine formation, and thyroid enlargement. The remaining choices incorrectly mix insulin, adrenaline, auxin, roots, and blood-glucose regulation.

Mimosa leaf folding is a rapid, non-growth movement caused mainly by changes in water content and turgor pressure in specialized cells. Tendril coiling is a slower growth movement in which touch produces unequal elongation on opposite sides. Thus option A correctly distinguishes the mechanisms. Neither response requires a brain, blood, insulin or a spinal cord because these are not plant nervous-control structures.