Control and Coordination

Geotropism, also called gravitropism, is directional growth in response to gravity. Roots generally grow downward, in the same direction as gravitational pull, so their response is called positive geotropism. Hence option A is correct. Phototropism concerns light, hydrotropism concerns water, and thigmotropism concerns touch; none of these explains the stated stimulus.

A stem usually grows upward, whereas gravity acts downward. Growth in a direction opposite to the gravitational pull is therefore called negative geotropism, making option A correct. Positive geotropism means growth with gravity, as in roots. Hydrotropism is a response to water, thigmotropism to touch, and chemotropism to chemical substances, so those options do not fit.

Hydrotropism is the directional growth of a plant part in response to water or moisture. When roots grow toward a region containing more water, they display positive hydrotropism; therefore option A is correct. Phototropism is caused by light, geotropism by gravity, and thigmotropism by touch. Although roots also respond to gravity, the stimulus named here is water.

Thigmotropism is directional growth caused by contact or touch. When one side of a tendril touches a support, unequal growth makes it curve and coil around that support, so option A is correct. Phototropism is guided by light, hydrotropism by water, and geotropism by gravity. The essential stimulus in this example is physical contact with the support.

Chemotropism is directional growth in response to chemical substances. During plant reproduction, chemical signals associated with the ovule guide the pollen tube toward it, enabling the male gamete to reach the female reproductive structure. Thus option A is correct. Phototropism responds to light, thigmotropism to touch, and hydrotropism to water, so they are not the governing stimuli here.

A green stem generally shows positive phototropism, meaning it grows toward a source of light. Light causes auxin to redistribute toward the shaded side, where cells elongate more rapidly; this unequal growth bends the stem toward the illuminated side. Hence option A is correct. B describes the opposite response, while C and D refer to directions unrelated to the stated light stimulus.

Touch-me-not leaves fold rapidly after contact because water movement changes the turgor pressure in special motor cells at the leaf base. The response does not depend on the direction from which the touch comes and is not a growth movement, so it is a non-directional touch-induced movement, commonly called thigmonasty or seismonasty. Thus option A is correct; the other choices are directional tropisms.

Tropism is a directional growth response of a plant part to an external stimulus. The direction may be toward the stimulus, called positive tropism, or away from it, called negative tropism. Light, gravity, water, touch, and chemicals can act as stimuli. Therefore option A is correct; digestion, blood flow, and breathing rate are animal or physiological processes, not plant tropic growth.

The governing concept is hormonal or chemical coordination. Endocrine glands produce hormones, which are chemical messengers released into the blood and carried to specific target organs. Therefore, option A is correct. Nerve fibres transmit electrical impulses rather than hormones, bones provide support, and red blood cells mainly transport oxygen and carbon dioxide; none of these performs chemical coordination.

The governing concept is hormonal regulation of blood glucose. Insulin, secreted by the pancreas, promotes the uptake of glucose by body cells and supports its storage as glycogen, thereby lowering an elevated blood-glucose level. Hence option A is correct. Adrenaline can raise blood glucose during stress, while thyroxine regulates metabolism and growth hormone has broader growth-related effects; neither is the principal answer here.

The governing concept is the source and function of endocrine hormones. Insulin is produced by the beta cells of the islets of Langerhans in the pancreas and helps regulate blood glucose. Therefore, option A is correct. The thyroid secretes thyroxine, the pituitary releases several controlling hormones, and the adrenal gland secretes hormones such as adrenaline; these are different from insulin.

The governing concept is the emergency or fight-or-flight response. In sudden danger, adrenaline released by the adrenal glands increases heart rate, breathing readiness, and the supply of glucose and blood to active muscles. Thus, option A is correct. Insulin regulates blood glucose, thyroxine mainly influences metabolism, and auxin is a plant hormone, so they do not provide the characteristic rapid emergency response in humans.

The governing concept is the relationship between an endocrine gland and its hormone. Adrenaline is secreted by the adrenal glands, which are located above the kidneys and help coordinate the body’s rapid response to stress or danger. Therefore, option A is correct. The pancreas produces insulin, the thyroid produces thyroxine, and the ovaries produce reproductive hormones, so these alternatives are not the source of adrenaline.

The governing concept is the mineral requirement for thyroid-hormone synthesis. Iodine is incorporated into thyroxine, which is produced by the thyroid gland and helps regulate metabolism. Therefore, option A is correct. If dietary iodine is insufficient, thyroxine production may be disturbed and the thyroid can enlarge, producing goitre. Iron, calcium, and sodium are important nutrients or ions but are not the specific element asked for.

The governing concept is the source of thyroxine in the endocrine system. Thyroxine is produced and secreted by the thyroid gland in the neck, and it helps regulate the body’s metabolic activities, growth, and development. Hence option A is correct. The pituitary controls several other glands, the pancreas secretes insulin, and the testes produce male reproductive hormones; none of these is the main source of thyroxine.

The governing concept is the link between iodine nutrition and thyroid function. Iodine is required to synthesise thyroxine; when iodine intake is too low, the thyroid may be unable to make enough hormone and can enlarge, causing goitre. Therefore, option A is correct. Diabetes is mainly associated with problems involving insulin, scurvy with vitamin C deficiency, and rickets with vitamin D or mineral deficiency.

The governing concept is endocrine control of growth. Growth hormone, also called somatotropin, is released by the pituitary gland and supports the growth of bones and other body tissues, especially during childhood and adolescence. Therefore option A is correct. Insulin regulates blood glucose, adrenaline prepares the body for emergencies, and auxin is a plant hormone, so these distractors do not control overall human body growth.

The governing concept is the function of endocrine glands. Growth hormone is produced and secreted by the anterior part of the pituitary gland, which influences several other endocrine glands and is therefore often called the master gland. Hence option A is correct. The thyroid mainly secretes thyroxine, the adrenal glands secrete hormones such as adrenaline, and the pancreas secretes insulin and glucagon.

The governing concept is hormonal regulation of puberty. During adolescence, the testes produce increasing amounts of testosterone. This hormone contributes to secondary sexual characteristics in boys, including a deeper voice, facial and body hair, increased muscle development, and maturation of the reproductive system. Thus option A is correct. Estrogen is principally associated with female reproductive development, while insulin and thyroxine have different metabolic roles.

The governing concept is endocrine control of puberty. Estrogen is mainly secreted by the ovaries and helps produce female secondary sexual characteristics during adolescence, such as breast development and changes in body shape. Therefore option A is correct. Testosterone is more strongly associated with male secondary sexual characteristics, adrenaline supports emergency responses, and auxin regulates growth responses in plants rather than humans.

The governing concept is receptor-specific action. Hormones are transported through the bloodstream and may reach many tissues, but they produce a response only where cells contain the matching receptor. The hormone binds to that receptor and initiates a specific cellular change, so option A is correct. Hormones do not destroy all cells, are not restricted to bones, and generally do enter and circulate in the blood; therefore options B, C, and D are incorrect.

The governing concept is the comparison between nervous and hormonal coordination. Nervous control sends electrical impulses along neurons and uses rapid neurotransmitter action at synapses, so it usually produces a response much faster than endocrine control. Hormones must be released into the blood, transported, and detected by target cells. Hence option A is correct. Hormonal responses are often slower but may last longer; the words “always equal” make option C incorrect.

The governing concept is the characteristic time course of endocrine coordination. Hormones are released by endocrine glands into the bloodstream, travel to target cells, and alter cellular activity through specific receptors. This transport and action usually make the response slower to begin than a nerve impulse, while the effect can persist after the initial stimulus. Therefore option A is correct; the other choices incorrectly describe hormonal action as momentary, instantly ending, or ineffective.

The governing concept is sensory coordination by the ear. The outer and middle ear transmit sound vibrations, and the inner ear contains the cochlea for hearing. It also contains the semicircular canals and vestibular apparatus, which detect head movement and help maintain balance. Thus option A is correct. Taste and smell are mainly detected by receptors in the tongue and nasal cavity, while blood sugar is monitored by physiological systems rather than the ear.

The governing concept is reception of chemical stimuli. Taste receptors are sensory cells located mainly in taste buds on the tongue. Dissolved food chemicals stimulate these receptors, which generate nerve signals that travel to the brain for identification of taste. Therefore option A is correct. The eye contains photoreceptors for light, the lungs contain receptors related to breathing, and bones do not serve as the main location of taste receptors.