Control and Coordination

The thyroid gland synthesizes thyroxine, and iodine is an essential raw material for this hormone. If the diet does not supply enough iodine, the thyroid may not produce adequate thyroxine, which can disturb normal metabolism and growth. Thus option A states the correct causal relationship. Auxin is a plant hormone, insulin is not made using iodine, and reflex actions do not produce thyroxine.

The pituitary gland secretes growth hormone, which helps regulate the growth of bones and other body tissues during childhood. Consequently, an unusually slow or rapid growth rate can indicate a problem involving pituitary regulation, making option A correct. Removing the hand is a nervous reflex, while tendril coiling and pollen-tube movement are plant responses controlled by plant growth mechanisms.

Hormones are transported through the blood, so they may pass near many different cells. However, a hormone produces a response only when a target cell has the appropriate receptor, much like a key fitting a particular lock. Therefore option A is correct. Blood does carry hormones, and hormone action is not restricted to bones or dependent on light.

Nervous control uses electrical impulses and chemical transmission across synapses, so it usually acts rapidly and its effects may end quickly. Hormones travel through the blood and often act more slowly, but their effects can persist longer. Hence option A is correct. Nerves do not normally signal through blood, both systems occur in animals, and hormones act on specific target organs.

Roots can show hydrotropism, a growth response to water, and geotropism, a growth response to gravity. When both stimuli are present, their directions and strengths may jointly influence the final growth pattern. Thus the question tests the combined effect of multiple stimuli, making option A correct. Plants do respond to stimuli, roots are not guided only by sound, and they do not use animal-like nerve cells.

The governing concept is that plant movements may be either growth-independent or growth-dependent. Touch-me-not leaves fold rapidly because changes in cell water content and turgor pressure occur at the pulvinus; this is not caused by growth. A tendril coils after touch because unequal growth develops on its two sides. Thus option A is correct. The other options wrongly invoke blood flow, a spinal cord, or unrelated tropisms.

The governing concept is directional growth, or tropism. A pollen tube grows towards the ovule in response to chemical guidance, which is positive chemotropism. A root grows towards a region containing more water, showing positive hydrotropism. In each case, the direction of growth is related to the direction of the stimulus, so option A is correct. Reflex actions, the medulla and insulin do not explain these plant responses.

Phototropism is a growth response to light, and the sign depends on the direction of growth relative to the light stimulus. Growth towards light is positive phototropism, as commonly seen in shoots; growth away from light is negative phototropism. Therefore, a root growing away from light fits option A. Tendril coiling is thigmotropism, while pollen-tube growth towards chemicals is chemotropism.

The governing rule is that positive geotropism means growth in the direction of gravity, whereas negative geotropism means growth opposite to gravity. Gravity acts downward. The root grows downward and therefore shows positive geotropism; the stem grows upward and therefore shows negative geotropism. Hence option B is correct. The other choices either reverse the signs or name responses to light or chemicals.

Thyroxine is an endocrine hormone that helps regulate the body’s metabolic rate, including how efficiently energy is used and how normal growth is supported. If the thyroid produces too little thyroxine, metabolic activity may be disturbed; therefore option A is correct. Touch detection depends mainly on receptors and the nervous system, while pollen-tube direction and tendril coiling are plant responses controlled by other signals.

Negative feedback is a homeostatic mechanism in which a change triggers a response that opposes or reduces the original change. When blood glucose rises, pancreatic beta cells release insulin, helping cells take up glucose and lowering the blood level. As glucose approaches normal, insulin secretion decreases. Thus option A directly shows self-correction; the other choices describe reflex or plant responses, not feedback regulation.

Adrenaline, also called epinephrine, is released by the adrenal glands during immediate stress. It produces the fight-or-flight response by increasing heart rate, breathing rate and blood flow to skeletal muscles, while making stored energy more available. Therefore option A best explains rapid preparation for a race. Estrogen is a reproductive hormone, whereas cytokinin and abscisic acid are plant hormones.

Adolescent changes such as growth of body hair and maturation of reproductive organs develop gradually because hormones travel through the blood and act comparatively slowly. In contrast, withdrawing the hand from a harmful stimulus is a rapid nervous response, usually organised through a reflex arc involving the spinal cord. Hence A correctly contrasts endocrine coordination with nervous coordination; the other statements reverse or misidentify the processes.

Testosterone is an androgen secreted mainly by the testes. During adolescence it supports male secondary sexual characteristics, including facial and body hair, deepening of the voice and development of muscles and reproductive tissues. If its secretion is low, these changes may be delayed or slower, so option A is correct. Blood glucose is chiefly regulated by insulin and glucagon, while C and D concern plants.

Estrogen is a hormone produced mainly by the ovaries and is important in female reproductive development. During adolescence, it contributes to the development of secondary sexual characteristics and helps regulate maturation of the reproductive system. Therefore option A gives the correct relation. Blood glucose is mainly controlled by insulin and glucagon; bending toward light is a plant response involving auxin, and a reflex arc is a nervous pathway.

Receptor specificity means that a receptor is structurally and functionally suited to detect a particular kind of stimulus. Photoreceptors in the retina respond to light, while olfactory receptors in the nose respond to chemical molecules responsible for smell. Thus option A is correct. Receptors do not detect every stimulus equally, do not merely secrete hormones, and are not restricted to plant roots.

A receptor begins a response by detecting a specific stimulus and sending information through a sensory pathway. If it is activated by an incorrect stimulus, the nervous system may receive misleading information and select an unsuitable response from the effector. Therefore option A is the logical consequence. The other choices do not follow from receptor activity and describe unrelated or impossible changes.

The brain is a coordinated organ with functional division among its regions. The cerebrum is involved in conscious thinking, memory, learning and voluntary actions, whereas the cerebellum helps coordinate muscular activity and maintain posture and balance. Therefore option A correctly expresses division of work. The medulla regulates involuntary activities such as breathing and heartbeat, and the cerebellum is not a hormone-producing gland.

Smell is detected by specialised olfactory receptors in the nasal lining, and the information travels through an olfactory neural pathway to the brain. Since vision and hearing remain normal, the problem is more likely restricted to the smell receptors or their pathway rather than all sensory systems. Thus option A is the best conclusion. Photoreceptor damage would affect vision, while the medulla and pancreas do not specifically explain isolated loss of smell.

The governing concept is receptor-specific hormone action. A hormone produces a response only when it binds appropriately to receptors on or inside its target cells. If the target organ remains present but its receptors are altered, binding or signal transmission may become ineffective, so the response can decrease even though the hormone is circulating. Thus A is correct; B and C are biologically meaningless, while D confuses hormonal action with nerve conduction.