Control and Coordination

Homeostasis keeps internal conditions within suitable limits. In negative feedback, a rise in a hormone or its effect is detected by the regulatory system, which reduces stimulation of the relevant endocrine gland and lowers further secretion. Therefore option A describes the usual balancing mechanism. The other options are biologically impossible and do not regulate hormone concentration.

Animals can produce rapid movements because their nervous systems transmit electrical signals and their muscles act as fast effectors. Plants lack neurons and muscles, so they cannot use the same pathway. They still coordinate responses through chemical signals and changes in growth or water balance, such as bending toward light or altering leaf movement. Hence option A is correct; the other statements incorrectly deny plant chemicals, growth or light detection.

The governing concept is tropic movement, in which a plant part grows directionally in response to an external stimulus. Growth towards water is hydrotropism, while a growth response related to light is phototropism; roots commonly show negative phototropism by growing away from light. Therefore option A identifies the relevant stimulus-based growth responses. Thigmotropism and chemotropism are caused by touch and chemicals, so option B does not match the stated stimuli.

A growth-based plant movement results from unequal or directional growth and is generally irreversible. The stem bending towards light is phototropism, root growth towards water is hydrotropism, and pollen-tube growth towards the ovule is chemotropism. All three are plant tropisms, so option A is correct. Mimosa folding is a rapid nastic movement rather than growth; the other options describe animal or physiological processes.

Non-growth-based plant movements are usually rapid and reversible because they result from changes in the water content and turgor pressure of cells, not from the production of new cells. In Mimosa, a stimulus causes water to move out of particular motor cells, making the leaf fold. Thus option A is correct. Options B and C describe growth, while option D is chemotropism, a directional growth response.

Adrenaline is a hormone released during danger, fear, or intense stress. It prepares the body for rapid action by increasing cardiac output and commonly increasing the rate and depth of breathing, so more oxygen and nutrients can reach active muscles. Hence option A is correct. Root gravitropism is a plant response, insulin is produced by pancreatic beta cells rather than the retina, and a Mimosa leaf cannot become a seed.

The nervous and endocrine systems coordinate body functions in complementary ways. The nervous system transmits rapid electrical signals for immediate, targeted responses, such as withdrawing from danger. Endocrine glands release hormones into the blood; these act more slowly but may influence several organs and last longer. Thus option A correctly describes their combined importance. The remaining choices assign unrelated or impossible functions to both systems.

Plants lack an animal-like brain and nervous system, but they are not passive. They detect environmental stimuli and coordinate responses through chemical signals, changes in growth, and rapid changes in cell water content. Phototropism, hydrotropism, gravitropism, thigmotropism, and chemotropism illustrate this ability. Therefore option A is correct; the other options deny plant sensitivity or incorrectly impose animal structures and blood-based control on plants.

The central idea of Control and Coordination is that organisms detect internal or external stimuli and organise suitable responses. Animals mainly use receptors, the nervous system, and hormones, whereas plants rely largely on chemical messengers and changes in growth or cell turgor. Thus option A includes the major mechanisms and applies to living organisms broadly. The other choices restrict coordination to nutrition, bones, or only one group of organisms, so they are incorrect.

A reflex arc normally follows the path receptor → sensory neuron → spinal cord or relay centre → motor neuron → effector. A functioning sensory nerve can carry information from the receptor to the spinal cord, so detection is possible. However, a damaged motor nerve cannot transmit the outgoing command to the muscle or gland. Hence A is correct; B describes sensory damage, and C and D are biologically impossible.

Neurons have functional polarity. Dendrites generally receive stimuli or signals, the cell body integrates them, and the axon carries the resulting nerve impulse away towards another neuron or an effector. This organised direction prevents random transmission and helps a response reach the correct destination. Therefore A is correct. The other choices describe unrelated processes and do not explain the role of dendrites and axons in signalling.

A synapse is the functional junction between two neurons. When an impulse reaches the presynaptic ending, neurotransmitter chemicals are released into the synaptic cleft and bind to receptors on the next cell. This transfers the signal across the gap. If chemical release or reception is blocked, the next neuron may not be excited, so coordination is interrupted. Thus A is correct; B, C and D are unrelated biological claims.

Touching a hot object activates pain receptors in the skin. The sensory impulse reaches the spinal cord, which quickly sends a motor command to withdraw the hand; this is a reflex action that reduces injury. At the same time, the impulse also travels to the brain, where pain is consciously perceived, often slightly later. Therefore A is correct, while B, C and D contradict the reflex pathway.

Tropisms are directional growth responses to external stimuli. Phototropism is a response to light, thigmotropism to touch, and chemotropism to chemicals. When a root bends or grows toward a source of water, the defining stimulus is moisture or water, so the response is hydrotropism. Gravity may also influence roots through geotropism, but the situation specifically highlights growth toward water. Hence B is correct.

In a young shoot, one-sided light causes auxin to redistribute toward the shaded side. In the stem, auxin promotes cell elongation, so cells on the shaded side may elongate more than those on the illuminated side. This unequal growth produces curvature, making the stem bend toward the light. Therefore A is correct. Blood flow and root pulling are irrelevant, and option C does not describe the controlling mechanism.

Mimosa leaf folding is a rapid, non-growth movement caused by changes in turgor or water pressure in specialized cells after touch. A tendril coils around support more slowly because contact causes unequal growth on its two sides; this is a growth movement. Both responses involve a stimulus, but their mechanisms and speed differ. Thus A is correct, while B, C and D incorrectly apply animal processes or deny stimulation.

Nervous coordination uses electrical impulses and synaptic signals, so it can act very quickly and precisely on particular effectors. Endocrine coordination releases hormones into the blood; these may act more slowly, reach target tissues, and produce effects that last longer. Working together allows the body to respond rapidly while also maintaining longer-term regulation such as growth, metabolism and reproduction. Therefore A is correct; B, C and D are false.

The governing concept is hormonal coordination during the emergency or “fight-or-flight” response. When fear occurs, adrenaline increases heart activity and redirects more blood toward skeletal muscles. This supplies additional oxygen and glucose for rapid contraction and action. Therefore option A is correct. The other options describe photosynthesis, an impossible blood-sugar effect, or vision, none of which explains adrenaline’s muscular response.

The governing concept is thyroid hormone production and feedback regulation. Iodine is a necessary raw material for making thyroxine in the thyroid gland. When iodine is deficient, thyroxine production falls; the body may stimulate the thyroid more strongly, causing enlargement of the gland, known as goitre. Therefore option A is correct. The other choices incorrectly connect iodine with insulin transport, reflex arcs, or neuron structure.

The governing concept is control of involuntary, vital activities by the brainstem. The medulla helps regulate breathing, heartbeat, and other automatic functions that must continue without conscious effort. Serious damage can therefore threaten life, unlike many ordinary muscle injuries that mainly impair local movement. Option A is correct. The other options incorrectly attribute skin-colour production, plant-hormone production, or only taste detection to the medulla.