Control and Coordination

The governing concept is division of control functions between the nervous and endocrine systems. Nerve impulses travel rapidly and are suited to immediate, localized responses such as withdrawal from danger. Hormones travel through blood more slowly and often produce broader, longer-lasting effects such as growth and puberty changes. Hence A is correct; B and C are too narrow, and D is false because the systems differ in speed.

The governing concept is plant coordination without an animal-like nervous system. Plants detect stimuli and respond through chemical messengers, electrical or chemical signalling, and differential growth; these responses are generally slower and less centralized than animal nervous responses. Option A correctly states this limitation. B falsely gives plants a brain and spinal cord, C incorrectly substitutes blood for plant transport tissues, and D denies well-known stimulus responses.

The governing concept is directional growth in response to gravity. In a horizontal plant, gravity provides the relevant stimulus. A root growing downward grows in the direction of gravity and is therefore positively geotropic. A stem bending upward grows opposite to gravity and is negatively geotropic. Thus A combines both observations correctly. Phototropism requires light direction, chemotropism requires chemicals, and plants do not use an animal-like nervous system.

The governing concept is stimulus-specific tropic growth. Phototropism is growth in response to light, so normal phototropism shows that the light-response pathway is functioning. A pollen tube is guided toward the ovule by chemical signals released in the reproductive tissues; this is chemotropism. Therefore A is correct. Geotropism responds to gravity and hydrotropism to water, so neither directly explains the stated defect.

The governing concept is the stimulus-response pathway of coordination. First, a receptor detects a change in the internal or external environment. The message is then transmitted to an integrating or control centre, which processes it and sends instructions to an effector such as a muscle or gland. The effector produces the response, so A is correct. B, C and D describe isolated or irrelevant events, not the complete pathway.

The governing concept is the reflex arc. Receptors in the skin detect heat, and the sensory nerve carries that impulse to the spinal cord, which coordinates a rapid response through a motor nerve. If the sensory nerve is damaged, the impulse may not reach the spinal cord, so the withdrawal reflex becomes delayed, weak, or absent. The muscle cannot contract faster by itself, and the adrenal gland or plant roots are unrelated.

A synapse is the junction or small gap between two neurons. When an impulse reaches the end of one neuron, chemical neurotransmitters are released and cross the gap to affect the next neuron. If these chemical signals stop, transmission from one neuron to another is interrupted first. The other options concern neither synaptic communication nor the immediate function of a neuron, so they are not the direct result.

The key idea is division of work in a reflex arc. The spinal cord provides the rapid coordinating centre for the immediate withdrawal response, allowing the body to react before conscious analysis is complete. However, nerve impulses can also travel to the brain, where the person becomes aware of pain and the event. Thus the brain is not the first centre for the quick response, but it is not completely excluded from the information pathway.

A neuron has a functional direction for receiving and transmitting information. Branched dendrites generally receive impulses from receptors or neighbouring neurons and conduct them toward the cell body. The axon carries the nerve impulse away from the cell body toward another neuron, muscle, or gland. Therefore option A correctly distinguishes the two parts. Blood formation, hormone production, digestion, and joining bones are not their functions.

The cerebellum is a brain region that coordinates muscular activity and helps maintain posture and balance. If it is affected, movements may become unsteady, poorly timed, or inaccurate, and walking may be difficult. This explains why option A combines balance and proper movement. Blood-glucose regulation is mainly associated with the pancreas, while thyroid function and complete loss of sensory abilities are not the characteristic combined effects of cerebellar damage.

The medulla oblongata is part of the hindbrain and regulates several involuntary activities essential for survival, including breathing and heartbeat. Damage to it can therefore seriously disturb these automatic functions. The other options describe plant growth responses: phototropism, growth toward water, and pollen-tube guidance. They are controlled by plant tissues and chemical signals, not by the medulla of an animal nervous system.

The central nervous system needs strong skeletal protection. The brain is enclosed within the skull, while the spinal cord passes through and is protected by the vertebral column, or backbone. Thus option A correctly pairs each nervous-system organ with its protective structure. Ribs mainly protect organs in the chest, and the thigh bone, skin, pancreas, thyroid, and ovary do not form the protective coverings of the brain and spinal cord.

The governing concept is the protective role of cerebrospinal fluid in the central nervous system. This fluid surrounds the brain and spinal cord, provides buoyancy, and absorbs part of the force produced by sudden movements or impacts. If it is absent, cushioning against shocks would be reduced and the chance of mechanical injury would increase. The other options concern endocrine functions or plant responses, not brain protection.

The governing concept is directional growth, or tropism. “Positive” geotropism means growth in the direction of gravity, so a root generally grows downward into the soil. “Negative” phototropism means growth away from light. Therefore, both descriptions together identify option A. Option B reverses both directions, while options C and D describe chemical or touch responses rather than the two stated stimuli.

The governing concept is that tropisms are named according to the stimulus guiding growth. A root growing toward water shows hydrotropism, because water or moisture is the relevant stimulus. A pollen tube grows toward the ovule in response to chemical attractants released by the female reproductive structures, showing chemotropism. Thus option A is correct; the other choices assign unrelated stimuli to one or both examples.

The governing distinction is between a rapid nastic movement and a slower tropic growth response. Touch causes a quick change in water content and turgor pressure in specialized cells at the Mimosa leaf base, so the leaf folds without directional growth. Phototropism requires unequal growth in response to light and develops more slowly. Therefore option A is correct; the other choices incorrectly invoke seeds, blood, a nervous system, or bones.

The governing concept is coordination without a specialized nervous system. Plants use chemical messengers such as hormones, together with changes in cell growth, water balance, and turgor, to coordinate responses. Tropisms are common examples in which unequal growth changes direction. Thus option A is correct. Plants do not possess a brain, spinal cord, heart, or animal sensory organs as their central control system.

The governing concept is that different plant organs can have different sensitivity and growth responses to the same stimulus. A shoot commonly shows positive phototropism and bends toward light, whereas a root commonly shows negative phototropism and grows away from light; gravity also influences the root strongly. Therefore option A is correct. The other options wrongly attribute the difference to brains, blood, or the origin of light.

The governing concept is stimulus-response coordination. A stimulus is a detectable change, either outside the organism, such as light or heat, or inside it, such as a change in water level. A response is the action or adjustment produced after detection, such as bending or withdrawing a hand. Hence option A is correct; the other options confuse stimuli with hormones or body parts and make false absolute claims.

Thyroxine is produced by the thyroid gland and regulates the body’s metabolic rate. It influences how cells use nutrients and release energy, and it also supports normal growth and development. Therefore, a low thyroxine level can reduce or disturb metabolism and energy use, making option A correct. Pollen-tube growth, Mimosa movement, and tendril coiling are plant responses controlled by different mechanisms.