Control and Coordination

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.

Nerve messages are electrical or electrochemical signals conducted rapidly along specific neurons and pathways, producing relatively immediate responses. Hormones are chemical messengers released by endocrine glands and transported through the bloodstream to target cells; their effects are usually slower and may last longer. Therefore option A correctly compares both systems. B reverses their usual timing, while C and D are biologically false.

Adrenaline is released during fear, stress, or danger and prepares the body for a rapid emergency response. It increases heart rate, breathing efficiency, blood flow to muscles, and the availability of glucose for immediate energy. This coordinated fight-or-flight response explains why option A is correct. Adrenaline is not a plant-only substance, does not induce sleep, and does not cause phototropism, so B, C, and D are incorrect.

Fear activates the body’s emergency response. Adrenaline helps the liver convert stored glycogen into glucose and release more glucose into the blood, supplying quick energy to muscles and other organs. Hence option B is correct. Insulin generally promotes glucose uptake and storage, estrogen mainly regulates reproductive functions, and cytokinin is a plant hormone involved in cell division, so none of those fits the situation.

The coiling of a tendril is a directional growth response called thigmotropism. When one side of the tendril touches a support, auxin-related growth effects produce unequal elongation on the two sides. The untreated or less-contacted side grows relatively faster, bending the tendril around the support. Therefore A is correct; the other options describe no valid mechanism for plant tendril coiling.

The governing idea is tropic movement: plant parts show directional growth in response to external stimuli. A root growing toward water demonstrates positive hydrotropism, while a stem growing toward light demonstrates positive phototropism. These observations support option A, showing that different organs can respond directionally to different stimuli. B denies plant coordination, and C and D contradict the examples given.

Hormonal regulation commonly produces gradual effects that develop over a long period and may influence many body systems. During adolescence, hormones cause progressive changes such as growth, maturation of reproductive organs, and development of secondary sexual characteristics over months or years. Thus option A best illustrates a long-lasting hormonal effect. The other choices are rapid protective reflexes or responses, mainly associated with nervous coordination.