Control and Coordination

The governing concept is receptor-specific hormonal action. Blood can carry a hormone throughout the body, but only cells or organs with the appropriate receptors can recognize and respond to that chemical messenger. Such responsive tissues are called target organs. Therefore option A is correct. Blood does not flow only to those organs, hormones do not turn into stones, and every organ does not destroy every hormone, so options B, C, and D are incorrect.

The governing concept is phototropism, in which unequal auxin distribution causes unequal cell elongation on the two sides of a stem. The side with greater elongation becomes longer and the stem bends toward the side with less growth. If auxin remains equally distributed, elongation on both sides is nearly equal, so the growth difference and resulting bending decrease. Therefore option A is correct; the other options describe biologically impossible changes unrelated to auxin-controlled growth.

The governing concept is thigmotropism, a directional growth response to touch. When a tendril contacts a support, growth on its two sides becomes unequal. The side away from the support generally elongates more, while the contacted side grows less, causing the tendril to curve and coil around the support. Thus option A correctly explains the evidence. A spinal cord, blood pumping, and thyroxine production are animal functions and cannot account for this plant movement.

The governing concept is classification of receptors according to the kind of stimulus they detect. Olfactory receptors in the nose respond to chemicals present in air, while taste receptors in the tongue respond to dissolved chemicals in food and saliva. Because both systems detect chemical stimuli rather than light, pressure, or gravity, they are called chemoreceptors. Therefore option A is correct; the remaining choices describe unrelated or impossible functions.

The governing concept is coordination of balance by the vestibular apparatus of the inner ear. The semicircular canals and related structures detect head movement and changes in body orientation, then send signals to the brain. The brain combines these signals with information from the eyes and muscles to maintain posture and equilibrium. If inner-ear signals are faulty, balance can be disturbed despite normal vision. Therefore option A is correct; the other options are unrelated endocrine or plant functions.

The governing concept is nervous coordination followed by glandular response. Seeing attractive food provides a visual stimulus detected by the eyes. Nerve impulses travel to the brain, where the information is processed; the brain then activates autonomic pathways to the salivary glands, causing secretion of saliva. Thus option A correctly identifies coordination among a sense organ, the brain, and an effector gland. The other options combine unrelated plant and animal structures or invent nonexistent connections.

The governing concept is the receptor–coordinator–effector pathway. A receptor detects the stimulus, the nervous system processes the information and transmits a command, and the effector produces the final response. If the muscle receives the signal but cannot contract, detection and transmission may have occurred, while the response-producing effector link has failed. Therefore option A is correct. The other options refer to earlier stages or to hormonal secretion, which is not required for every muscle response.

The governing concept is coordinated directional growth, or tropic response. Roots commonly show positive hydrotropism and grow towards water, while stems commonly show positive phototropism and grow towards light. These opposite responses in different organs demonstrate that plant parts detect and respond to different environmental stimuli according to their functions and needs. Therefore option A is correct. Options B, C, and D wrongly deny coordination or reverse the usual tropic responses.

The governing concept is hormonal coordination during the emergency or stress response. Anticipation of a race can produce excitement or stress, causing the adrenal glands to release adrenaline. This hormone increases heart rate and helps direct the body toward rapid muscular activity. Phototropism concerns plant growth, insulin regulates blood glucose rather than taste, and roots do not grow through spinal control. Thus, option A is correct.

The governing concept is endocrine regulation of metabolism. Thyroxine, secreted by the thyroid gland, helps regulate the rate at which cells use nutrients and release energy. If its level is deficient, metabolic activity and several normal body functions may become slower or disturbed, subject to the cause and severity of deficiency. A synapse, retina, and tendril have different roles, so they do not explain this effect. Option A is correct.

The governing concept is the stimulus-response pathway. A receptor detects the stimulus, the control centre receives and interprets the information, and the effector carries out the instructed action. These parts form a linked chain, so failure of any one part can interrupt or distort the response even if the other parts work normally. The other options wrongly deny the role of one component. Therefore, option A is correct.

The governing concept is the difference between plant and animal coordination. Plants lack a nervous system and usually respond through chemical messengers, changes in cell growth, and movements such as tropisms; these responses are often relatively slow. Animals possess nervous tissue that can transmit rapid electrical signals, although they also use hormones. Plants do not have a human-like brain, and they are not always faster. Hence, option A is correct.

The governing concept is coordination of blood distribution during physical activity. Running sharply increases the muscles’ demand for oxygen, glucose, and removal of carbon dioxide and other wastes. The body therefore prioritises active muscles, while digestive activity becomes relatively less urgent. Adrenaline contributes to this fight-or-flight adjustment, although blood flow is controlled by several mechanisms. The other choices describe unrelated processes, so option A is correct.

The governing concept is the difference between nervous and hormonal coordination. Nerve impulses are excellent for rapid, precise, short-lived actions, whereas hormones travel through blood and may act more gradually but persist for longer, depending on the hormone and target tissue. Sustained regulation of growth and metabolism therefore often benefits from endocrine control. Hormones do reach target organs and are not confined to synapses, so option A is correct.

The governing concept is information processing in the control centre. A receptor only detects a stimulus and sends coded information; it does not decide the final action. The control centre must interpret that information and select an appropriate command for the effector. If its interpretation is wrong, the command may be unsuitable even though detection was accurate, producing an incorrect response. Therefore, option A is correct; the other statements confuse receptor, effector, and spinal-cord roles.

The governing concept is tropism, a directional growth response of a plant part to an external stimulus. Phototropism is growth in response to light, hydrotropism to water, and geotropism or gravitropism to gravity. Since the direction of stimulus influences the direction of growth in each case, option A is correct. They are not human reflexes, do not depend on blood circulation, and are not produced by muscles.

At a synapse, the electrical impulse reaching the end of one neuron causes chemicals called neurotransmitters to be released. These chemicals diffuse across the synaptic gap and bind to receptors on the next neuron, initiating a new electrical signal. If their release or action is blocked, the gap cannot be crossed and the next neuron is not stimulated. Hence option A is correct; the other statements describe impossible or irrelevant events.

Sensitivity means the ability to detect and respond to a stimulus; it does not require a brain or a nervous system like that of humans. Plants respond through chemical coordination, growth movements, and changes in cell water balance. Examples include bending toward light, growing toward water, responding to gravity, and reacting to touch. Therefore option A is correct, whereas the other choices wrongly attribute animal structures or locomotion to plants.

Growth hormone, secreted by the pituitary gland, regulates growth of bones and other body tissues, especially during childhood. Biological control depends on an appropriate concentration: too little may result in unusually slow or insufficient growth, while too much during development may cause excessive growth. Thus option A is correct. The hormone is neither food nor a stoma, and it clearly has important effects on the body.

The retina contains photoreceptors that convert light into electrical signals and pass them through the optic nerve. These signals alone are not the complete experience of seeing. The visual areas of the brain analyse and interpret patterns, colour, shape, and movement. If that area is damaged, retinal signals may still be produced but cannot be properly understood as vision. Therefore option A is correct; the other choices contradict eye anatomy or physiology.