Control and Coordination

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.

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 governing concept is the stimulus-response pathway in control and coordination. A stimulus must be detected, its information must be transmitted and interpreted, and an effector must then act. If sensing, transmission, processing, or execution fails, the final response may be delayed, absent, or inappropriate. Therefore option A is correct; the other choices make scientifically false absolute claims.

The central conclusion is that living organisms detect changes and coordinate suitable responses. Animals use rapid electrical signals through the nervous system and slower chemical coordination through hormones, while plants respond through chemical messengers and growth-related mechanisms. Option A captures this unified idea. Options B and C deny established biology, and option D incorrectly treats two different signalling systems as identical.

This situation illustrates nervous coordination. Photoreceptors detect the intensity of incoming light, sensory information is processed through the nervous system, and signals reach the muscles controlling the iris. Those muscles alter pupil diameter: contraction limits excess light and dilation permits more light in darkness. Thus option A is correct. Option B concerns endocrine regulation, while C and D describe plant responses unrelated to the pupil.

Dendrites are branched extensions specialized mainly for receiving signals from receptors or neighbouring neurons and directing them toward the cell body. If they are removed, the neuron loses much of its receiving surface and incoming signals cannot be transmitted normally to the cell body. Consequently, its ability to receive messages decreases greatly, making option A correct. The other options either reverse the effect or describe impossible changes in unrelated structures.

A synapse is the functional junction or tiny gap between two neurons. When an impulse reaches the end of one neuron, chemical neurotransmitters are released into the gap and bind to receptors on the next neuron, allowing the signal to continue. Therefore option A directly describes synaptic transmission. Insulin action is hormonal regulation, whereas root growth and stem growth are plant responses, so B, C and D are distractors.

The brain shows division of labour because its regions have specialised but coordinated functions. The cerebrum supports conscious thought, memory and voluntary activities; the cerebellum helps coordinate muscular activity and maintain posture and balance; and the medulla regulates important involuntary actions such as breathing and heartbeat. Therefore option A is correct. The other options wrongly assign blood filtration, hormone secretion or memory to these brain regions.

The governing concept is geotropism, the growth response to gravity. The root grows in the direction of gravitational pull, so it exhibits positive geotropism. The stem grows away from that direction, so it exhibits negative geotropism. Thus option A is correct. Option B reverses the signs, while chemotropism concerns chemicals and a reflex arc is a nervous-system pathway absent from ordinary plant growth.

The governing concept is chemotropism, a directional growth response to a chemical stimulus. Chemicals released or present near the ovule guide the pollen tube toward it, allowing the male gametes to reach the female gametophyte for fertilisation. Therefore option A is correct. Sound does not guide the tube, pollen tubes have no spinal-cord connection, and insulin is an animal hormone unrelated to this plant process.

The governing concept is the role of plant hormones in growth. Gibberellin promotes cell elongation and can increase the length of stems, so comparing a treated dwarf plant with an untreated one would provide clear evidence. Therefore option A is correct. Hand withdrawal is a nervous reflex, blood-sugar reduction involves insulin, and pupil contraction is controlled by the nervous system; these effects do not test gibberellin.

The governing concept is homeostasis maintained by negative feedback. When blood glucose rises above its normal range, the pancreas releases more insulin; insulin promotes glucose uptake and storage, bringing the level downward. Once the level approaches normal, the stimulus decreases and insulin secretion also falls. This reversal of the original change makes option A correct. Phototropism and thigmotropism are plant movements, while a reflex arc is a nervous pathway, so B, C, and D are unrelated.

The governing concept is insulin-dependent regulation of blood glucose. After a meal, glucose enters the bloodstream. Insulin helps many body cells take up glucose and promotes its storage, especially as glycogen, so the blood level returns toward normal. If insulin secretion is insufficient, uptake and storage are reduced and glucose remains elevated for longer; therefore option A is correct. The other options confuse unrelated hormones, glands, or nervous structures.

The governing concept is the emergency or fight-or-flight response. Fear activates the adrenal glands, and adrenaline helps the liver break down stored glycogen, making more glucose available in the blood. This rapid energy supply supports vigorous muscle activity and immediate action, so option A is correct. Root growth toward water and leaf movement away from light are plant responses, while stopping thyroxine production is not the purpose of adrenaline; hence B, C, and D are distractors.

The governing concept is coordinated fight-or-flight physiology. An increased breathing rate brings oxygen into the lungs more rapidly, while a faster heartbeat delivers oxygen and glucose-rich blood to active muscles more quickly. Together these changes support rapid respiration and ATP production for emergency movement, making option A correct. Root emergence, insulin production by the thyroid, and smell detection by photoreceptors are biologically unrelated, so B, C, and D are incorrect.

The governing concept is hormonal control by the endocrine system. Thyroxine is produced by the thyroid gland, and iodine is an essential raw material needed for its synthesis. Therefore, inadequate iodine may reduce thyroxine production and can contribute to thyroid enlargement, called goitre. Option A correctly connects the nutrient, hormone, gland, and possible consequence. Adrenaline is not the only hormone related to iodine, plants do not produce human thyroxine, and a reflex arc belongs to nervous coordination.

Thyroxine is an endocrine hormone that helps regulate metabolism and supports normal growth and development. Thus an unusual metabolic rate or abnormal growth is the situation in which its role should be considered first, making option A correct. Pulling the hand away from a hot object is a rapid nervous reflex. Tendril coiling is a plant growth response, and pollen-tube movement toward chemicals is chemotropism; neither primarily indicates thyroxine action.

Growth hormone from the pituitary helps regulate normal growth of the body, especially during childhood and adolescence. Too little growth hormone can lead to unusually limited growth, whereas excessive secretion can produce abnormal excessive growth; the effects depend on age and developmental stage. Therefore option A correctly states why both conditions matter. The other choices describe no recognized function of growth hormone and incorrectly mix plant responses, imaginary blood changes, or nervous pathways with endocrine control.

A reflex action is a rapid, automatic response carried through a nervous pathway, often involving the spinal cord. Adolescent changes such as development of body shape, reproductive maturity, and secondary sexual characters arise from hormones released by endocrine glands and develop progressively over months or years. Hence option A is correct. Option B describes the opposite of adolescent hormonal change, while options C and D are false because glands and hormones are involved and the changes occur in humans.

Testosterone is an endocrine hormone produced mainly by the testes and is involved in male reproductive development and secondary sexual characters, such as changes occurring during adolescence. If its level is insufficient, the appearance or progression of these characters may be delayed or reduced; therefore option A is the logical result. Low testosterone does not automatically cause an immediate fall in blood sugar, prevent phototropism in plant stems, or stop every chemical signal at synapses.

Estrogen, produced mainly by the ovaries, is transported in the blood and acts on suitable target tissues. It contributes to reproductive development and several changes associated with puberty in girls. This demonstrates that hormones can produce coordinated effects that develop over a relatively long period, so option A is correct. Hormones do not create only reflex actions, are not restricted to plant roots, and must act on target cells or organs through specific receptors.