Control and Coordination

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.

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.

The governing concept is the reflex arc. Receptors in the skin detect heat, and the sensory nerve carries that impulse to the spinal cord, which coordinates a rapid response through a motor nerve. If the sensory nerve is damaged, the impulse may not reach the spinal cord, so the withdrawal reflex becomes delayed, weak, or absent. The muscle cannot contract faster by itself, and the adrenal gland or plant roots are unrelated.

A synapse is the junction or small gap between two neurons. When an impulse reaches the end of one neuron, chemical neurotransmitters are released and cross the gap to affect the next neuron. If these chemical signals stop, transmission from one neuron to another is interrupted first. The other options concern neither synaptic communication nor the immediate function of a neuron, so they are not the direct result.

A neuron has a functional direction for receiving and transmitting information. Branched dendrites generally receive impulses from receptors or neighbouring neurons and conduct them toward the cell body. The axon carries the nerve impulse away from the cell body toward another neuron, muscle, or gland. Therefore option A correctly distinguishes the two parts. Blood formation, hormone production, digestion, and joining bones are not their functions.

The cerebellum is a brain region that coordinates muscular activity and helps maintain posture and balance. If it is affected, movements may become unsteady, poorly timed, or inaccurate, and walking may be difficult. This explains why option A combines balance and proper movement. Blood-glucose regulation is mainly associated with the pancreas, while thyroid function and complete loss of sensory abilities are not the characteristic combined effects of cerebellar damage.

The medulla oblongata is part of the hindbrain and regulates several involuntary activities essential for survival, including breathing and heartbeat. Damage to it can therefore seriously disturb these automatic functions. The other options describe plant growth responses: phototropism, growth toward water, and pollen-tube guidance. They are controlled by plant tissues and chemical signals, not by the medulla of an animal nervous system.

The governing concept is the protective role of cerebrospinal fluid in the central nervous system. This fluid surrounds the brain and spinal cord, provides buoyancy, and absorbs part of the force produced by sudden movements or impacts. If it is absent, cushioning against shocks would be reduced and the chance of mechanical injury would increase. The other options concern endocrine functions or plant responses, not brain protection.

The governing concept is directional growth, or tropism. “Positive” geotropism means growth in the direction of gravity, so a root generally grows downward into the soil. “Negative” phototropism means growth away from light. Therefore, both descriptions together identify option A. Option B reverses both directions, while options C and D describe chemical or touch responses rather than the two stated stimuli.

The governing concept is that tropisms are named according to the stimulus guiding growth. A root growing toward water shows hydrotropism, because water or moisture is the relevant stimulus. A pollen tube grows toward the ovule in response to chemical attractants released by the female reproductive structures, showing chemotropism. Thus option A is correct; the other choices assign unrelated stimuli to one or both examples.

The governing distinction is between a rapid nastic movement and a slower tropic growth response. Touch causes a quick change in water content and turgor pressure in specialized cells at the Mimosa leaf base, so the leaf folds without directional growth. Phototropism requires unequal growth in response to light and develops more slowly. Therefore option A is correct; the other choices incorrectly invoke seeds, blood, a nervous system, or bones.

The governing concept is coordination without a specialized nervous system. Plants use chemical messengers such as hormones, together with changes in cell growth, water balance, and turgor, to coordinate responses. Tropisms are common examples in which unequal growth changes direction. Thus option A is correct. Plants do not possess a brain, spinal cord, heart, or animal sensory organs as their central control system.

The governing concept is that different plant organs can have different sensitivity and growth responses to the same stimulus. A shoot commonly shows positive phototropism and bends toward light, whereas a root commonly shows negative phototropism and grows away from light; gravity also influences the root strongly. Therefore option A is correct. The other options wrongly attribute the difference to brains, blood, or the origin of light.

Thyroxine is produced by the thyroid gland and regulates the body’s metabolic rate. It influences how cells use nutrients and release energy, and it also supports normal growth and development. Therefore, a low thyroxine level can reduce or disturb metabolism and energy use, making option A correct. Pollen-tube growth, Mimosa movement, and tendril coiling are plant responses controlled by different mechanisms.

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.