Control and Coordination

The governing concept is the functional division of the brain. The cerebrum is responsible for conscious thought, intelligence, learning, memory, and interpretation, so “cerebrum and intellectual functions” is correctly paired in option D. The cerebellum chiefly maintains balance and coordinates movement, whereas the medulla controls involuntary activities such as breathing and heartbeat. Therefore options A, B, and C mismatch each brain part with its principal function.

The governing concept is receptor specificity. A hormone may circulate through the blood and reach many tissues, but it produces a response only where cells possess receptors with the appropriate shape and chemical compatibility. Therefore option B is correct. Hormones do not equally stop every organ, they are not restricted to the brain, and they are not immediately destroyed on entering the blood. Receptors determine the target-cell response.

The governing concept is the role of iodine in thyroid hormone production. The thyroid gland uses iodine to synthesize thyroxine, which helps regulate metabolism, growth, and development. Long-term dietary deficiency can reduce thyroxine production and may lead to thyroid enlargement or goitre. Therefore option B is correct. The pancreas, adrenal glands, and testes have important endocrine functions but do not require iodine as the defining raw material for their principal hormones.

The governing concept is coordination by endocrine hormones. The pituitary secretes hormones such as thyroid-stimulating hormone, adrenocorticotropic hormone and gonadotropins, which regulate the activity of several other endocrine glands. Growth hormone and other pituitary hormones also act directly on tissues. Thus it is traditionally called the master gland. It does not make digestive juice, join bones or filter blood, so option C is correct.

The governing concept is phototropism caused by unequal auxin distribution. When light comes from one side, auxin moves and accumulates more on the shaded side of a young shoot. In shoots, this higher auxin concentration promotes greater cell elongation there. The shaded side therefore grows faster, pushing the stem toward the light. The light side does not elongate more, and the other choices do not explain the directional bend; hence B is correct.

The governing concept is organ-specific tropic response. Light is the same stimulus, but the stem generally shows positive phototropism by growing toward it, whereas the root commonly shows negative phototropism by growing away. Different organs have different structures, hormone distribution and growth responses, so their reactions need not be identical. This disproves the other choices; therefore option A is correct.

The governing concept is the difference between tropic growth and nastic movement. Tropism is a relatively directed growth response whose direction depends on the direction of the stimulus. Touch-me-not leaves fold rapidly after touch because changes in water pressure in specialised pulvinus cells cause movement; this is not directional growth. Since a stimulus is present but growth is not responsible, option A is correct.

Phototropism is the directional growth of a plant shoot in response to light. The shoot tip produces auxin, a plant hormone that promotes cell elongation. When light comes from one side, auxin distribution helps the shaded side elongate more, causing the shoot to bend toward light. Cutting off the tip removes an important source of auxin, so this unequal growth and bending may become weak. Blood, spinal cord, and insulin are unrelated to plant phototropism.

The thyroid gland secretes thyroxine, a hormone that helps regulate the body’s metabolism, so option C is correctly matched. The pancreas produces insulin, not adrenaline; adrenaline is secreted by the adrenal glands. The pituitary gland produces several animal hormones, whereas auxin is a plant hormone produced mainly in growing regions. Because each of the other pairs places the wrong hormone with the stated gland, thyroid gland and thyroxine is the only correct pair.

Growth hormone is secreted by the pituitary gland and supports the normal growth of bones and other body tissues during childhood. If too little is produced, the child may show reduced or abnormally slow growth, often resulting in shorter stature. Low growth hormone does not directly cause an immediate fall in blood sugar, which is related mainly to insulin regulation. Goitre is associated with thyroid problems, while rapid hand withdrawal is a reflex action.

Testosterone, secreted mainly by the testes, is the principal male sex hormone responsible for many changes during male adolescence. It supports development of secondary sexual characteristics, including growth of the larynx and thickening or lengthening of the vocal cords, which makes the voice deeper. Estrogen is mainly associated with female reproductive changes, insulin regulates blood glucose, and auxin is a plant hormone. Therefore option B is correct.

Estrogen is the principal hormone associated with the development of several female secondary sexual characteristics during adolescence, including breast development. It is produced mainly by the ovaries and also helps regulate the reproductive cycle and maturation of the female body. Adrenaline prepares the body for emergency responses, thyroxine regulates metabolism, and insulin helps control blood glucose. Since none of these performs the described primary role, option B is correct.

Seeing tasty food provides a visual stimulus. Receptors in the eyes send nerve impulses to the brain, and the brain coordinates a response through autonomic nerves that stimulate the salivary glands. The resulting secretion of saliva is therefore a nervous-control response, even though it is involuntary. Blood sugar regulation depends mainly on hormones such as insulin, while plant-hormone control and root growth are unrelated. Hence option B is correct.

The governing concept is the stimulus-response pathway. A receptor is a specialised cell or sense organ that detects a stimulus such as heat, light, pressure, or chemicals and converts it into a nerve signal. The signal is carried to a coordinating centre, which sends instructions to an effector. Muscles or glands act as effectors and produce the response. Therefore option B is correct; option A reverses the functions, while C and D describe neither structure.

An effector is the organ that carries out the response after receiving a command from the nervous system. When a person touches a hot object, heat receptors detect the stimulus and the message is processed through a reflex pathway. Motor neurons then cause the arm muscles to contract, pulling the hand away. Thus option C shows muscles acting as effectors. The eye and nose in A and B mainly contain receptors, while D is a physiological change rather than a muscle response.

A reflex arc depends on a complete pathway: receptor, sensory neuron, spinal cord, motor neuron, and effector. The sensory nerve carries information from the receptor towards the spinal cord. If it is damaged, the stimulus message may be delayed, weakened, or blocked before reaching the coordinating centre. Consequently the reflex response may become slow or absent. Option A states the direct effect; B, C, and D are unrelated or scientifically unsupported consequences.

The governing concept is the direction of impulse conduction. A motor nerve, or motor neuron pathway, carries commands from the brain or spinal cord to an effector such as a muscle. If this nerve is damaged, the receptor may still detect the stimulus and the central nervous system may still form a command, but the command cannot reach the muscle effectively. Therefore option C is correct. A concerns sensory reception, while B and D have no logical connection with motor-nerve damage.

Coordination means detecting changes and producing suitable responses. Animals coordinate rapidly through nerve cells and also use hormones for slower, longer-lasting regulation. Plants do not possess a nervous system or a brain; instead, chemical messengers such as plant hormones influence growth and movements, often through changes in cell growth. Thus option A correctly states the difference. B and D deny well-established animal features, and C incorrectly applies the animal blood-transport system to plants.

Receptors are specialised because their structures respond best to particular forms of stimulation. Photoreceptors in the eye respond to light, whereas smell receptors in the nose respond to chemical molecules that create odours. Comparing these two examples shows that the nervous system uses different receptors for different stimuli. Therefore option B is correct. Option A contradicts receptor specialisation, and C and D wrongly restrict receptors to blood or plants.

Thyroxine is a hormone secreted by the thyroid gland. Its major school-level role is to regulate the rate of metabolism, meaning the chemical reactions through which cells release and use energy; it also supports normal growth and development. Hence option B is correct. Insulin, not thyroxine, primarily regulates blood glucose, so A is incorrect. Reflex action is nervous coordination, and D belongs to plant reproduction.

Insulin is a hormone released by the pancreas when blood glucose rises, especially after a meal. It promotes the uptake of glucose by body cells, where it can be used to release energy, and supports conversion of excess glucose into glycogen for storage, mainly in the liver and muscles. This lowers blood glucose towards a normal level, so A is correct. B is impossible, C is unrelated, and D confuses an endocrine hormone with a nervous structure.

Chemical coordination occurs when a chemical messenger changes the activity of cells and produces a response. Auxin is a plant hormone; when light falls unevenly, its distribution causes unequal cell elongation on the two sides of a stem. The resulting differential growth bends the stem towards light, called phototropism. Therefore A is correct. B and C mainly involve nervous and muscular coordination, while D only describes sensory detection without identifying a chemical messenger.

A response pathway includes detection by a receptor, transmission through sensory and motor neurons, processing by a coordinating centre, and action by an effector. The condition in the question says that the nerve message has already reached the effector, so the receptor and sensory pathway have successfully performed their roles. If no response follows, the effector itself, such as a muscle or gland, may be damaged or unable to act. Therefore C is correct; A, B, and D do not fit the stated evidence.

The governing concept is that voluntary actions are consciously controlled, whereas involuntary actions occur automatically without deliberate choice. Writing, walking, or raising a hand can normally be started or stopped by our will, so they are voluntary. Heartbeat and digestion continue automatically, so they are involuntary. Therefore option A is correct; B wrongly limits voluntary actions to plants, C reverses the idea, and D denies a real physiological difference.

The cerebrum is the major part of the brain responsible for conscious thought, learning, memory, intelligence, and voluntary actions. Memorising a poem requires learning and storing information, so option B is the best answer. Heartbeat is mainly regulated by involuntary centres, a knee jerk is a spinal reflex, and leaf wilting is a plant response rather than a human cerebrum function. Thus B is clearly correct.