Control and Coordination

A reflex arc follows this route: receptor in the skin detects heat, a sensory neuron carries the impulse to the spinal cord, and a motor neuron carries the response to the muscle, the effector. If the motor nerve is damaged, the spinal command cannot reach the muscle, so withdrawal becomes weak or absent even though the receptor may still detect heat. Thus option B is correct.

The governing concept is the interaction of tropic responses. Roots commonly show positive geotropism, growing in the direction of gravity, but they also show positive hydrotropism, growing towards water. In the stated experiment, the root bends towards water even against gravity, so the water stimulus has the stronger observable influence under those conditions. Option A is therefore correct; the other choices confuse light response, deny the response or make a biologically impossible claim.

The governing concept is the difference between nastic movement and tropic growth. Mimosa leaves fold rapidly after touch because changes in water content and turgor pressure occur in specialised pulvinus cells; this does not require directional growth. A stem bends toward light because auxin causes unequal elongation on its two sides, making it a slower growth response. Thus option A is correct; plants have no brain or blood circulation like animals, and both movements involve stimuli.

Tendrils show thigmotropism, a directional response to touch. Contact with a support changes growth patterns on the two sides of the tendril; the side away from the contact may elongate more, producing unequal growth and coiling around the support. The response is therefore a physical growth change, not a conscious sensation. Tendrils have no brain, and blood or hearing is irrelevant, so option A is correct.

Running demands immediate energy and oxygen from skeletal muscles. The sympathetic response and adrenaline increase heart activity and redirect a greater share of blood toward working muscles, while digestive activity is temporarily reduced. This prepares the body for rapid action, often called the fight-or-flight response. Therefore option A is correct; sleep, hydrotropism, and thyroid enlargement do not explain this blood-flow redistribution.

A synapse is the junction or narrow gap between two neurons. When an impulse reaches the end of one neuron, chemical messenger molecules help carry the signal across this gap to the next neuron. If their amount becomes very low, the next neuron may receive insufficient stimulation, so message transmission becomes weak or may fail. Option A follows this mechanism; the other choices describe impossible or unrelated changes.

Chemical messengers released at a synapse stimulate receptors on the next neuron. Normally, removal or breakdown of the messenger helps terminate the signal and prevents continuous stimulation. If a drug blocks its breakdown, the messenger can remain active for longer, so excitation in the next neuron may be prolonged. Option A is correct. The other options describe effects that do not follow synaptic physiology and confuse neural, sensory or endocrine functions.

Feeling touch indicates that the skin receptor and sensory pathway can detect the stimulus and carry information toward the central nervous system. Lifting the hand, however, requires a motor command to travel from the nervous system through a motor nerve to the muscles, which act as effectors. Difficulty in movement therefore points to the motor nerve or effector, making option B correct. The sensory and skin receptor choices would better explain loss of sensation, not weakness of movement.

Roots can show hydrotropism, a growth response to water, and geotropism, a growth response to gravity. When both stimuli are present, their directions and strengths may jointly influence the final growth pattern. Thus the question tests the combined effect of multiple stimuli, making option A correct. Plants do respond to stimuli, roots are not guided only by sound, and they do not use animal-like nerve cells.

A receptor begins a response by detecting a specific stimulus and sending information through a sensory pathway. If it is activated by an incorrect stimulus, the nervous system may receive misleading information and select an unsuitable response from the effector. Therefore option A is the logical consequence. The other choices do not follow from receptor activity and describe unrelated or impossible changes.

Smell is detected by specialised olfactory receptors in the nasal lining, and the information travels through an olfactory neural pathway to the brain. Since vision and hearing remain normal, the problem is more likely restricted to the smell receptors or their pathway rather than all sensory systems. Thus option A is the best conclusion. Photoreceptor damage would affect vision, while the medulla and pancreas do not specifically explain isolated loss of smell.

The governing concept is receptor-specific hormone action. A hormone produces a response only when it binds appropriately to receptors on or inside its target cells. If the target organ remains present but its receptors are altered, binding or signal transmission may become ineffective, so the response can decrease even though the hormone is circulating. Thus A is correct; B and C are biologically meaningless, while D confuses hormonal action with nerve conduction.

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 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 reflex arc normally follows the path receptor → sensory neuron → spinal cord or relay centre → motor neuron → effector. A functioning sensory nerve can carry information from the receptor to the spinal cord, so detection is possible. However, a damaged motor nerve cannot transmit the outgoing command to the muscle or gland. Hence A is correct; B describes sensory damage, and C and D are biologically impossible.

A synapse is the functional junction between two neurons. When an impulse reaches the presynaptic ending, neurotransmitter chemicals are released into the synaptic cleft and bind to receptors on the next cell. This transfers the signal across the gap. If chemical release or reception is blocked, the next neuron may not be excited, so coordination is interrupted. Thus A is correct; B, C and D are unrelated biological claims.

In a young shoot, one-sided light causes auxin to redistribute toward the shaded side. In the stem, auxin promotes cell elongation, so cells on the shaded side may elongate more than those on the illuminated side. This unequal growth produces curvature, making the stem bend toward the light. Therefore A is correct. Blood flow and root pulling are irrelevant, and option C does not describe the controlling mechanism.

Mimosa leaf folding is a rapid, non-growth movement caused by changes in turgor or water pressure in specialized cells after touch. A tendril coils around support more slowly because contact causes unequal growth on its two sides; this is a growth movement. Both responses involve a stimulus, but their mechanisms and speed differ. Thus A is correct, while B, C and D incorrectly apply animal processes or deny stimulation.

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.

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.

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.