Control and 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.

Nervous control uses electrical impulses along neurons and chemical transmission at synapses, so it is generally rapid, precise, and directed to particular effectors. Endocrine control releases hormones into the bloodstream; their transport and action are usually slower, but they may influence several target tissues for a longer duration. Therefore option A gives the most accurate comparison. The other options reverse the speed relationship, exchange the transport routes, or wrongly restrict both systems to plants.

A coordinated response normally follows a pathway: a receptor detects a stimulus, a sensory route carries information to a coordinating centre, and a motor route activates an effector. If the receptor works but the effector fails, the pathway must be analysed to locate the breakdown, so option A is correct. Merely memorising a hormone name does not require pathway analysis, and options C and D do not describe a meaningful control-and-coordination problem.

The governing concept is receptor specificity: different sensory receptors and neural pathways detect different stimuli. Normal vision and hearing indicate that the photoreceptors and auditory pathway are functioning, but they do not prove that the olfactory system is normal. Therefore, difficulty in smelling most reasonably suggests damage or dysfunction in smell receptors or their associated nerve pathway. Options B, C, and D involve unrelated or scientifically incorrect explanations.

Receptor specificity means that a particular receptor is adapted to detect a particular type of stimulus, such as light, sound, heat, or chemicals. Detection is the first step in coordination; the nervous system or another control system can produce an appropriate response only after the correct stimulus has been identified. Option A expresses this sequence. Options B, C, and D incorrectly describe receptor function or location.

A tropic movement is a directional growth response to a specific external stimulus. Light produces phototropism, gravity produces geotropism or gravitropism, and water produces hydrotropism. Chemotropism, however, is a response to chemical substances, not to sound. Therefore option D is the incorrect match. The other three pairs correctly connect each stimulus with its corresponding tropism.

The shoot tip is an important site for production and distribution of auxin, a plant hormone involved in growth. In phototropism, light causes unequal auxin distribution, so cells on the shaded side elongate more and the shoot bends towards light. If removing the tip reduces bending, the experiment supports the conclusion that the tip contributes to auxin-mediated phototropic response. The other options are unrelated biological claims.

Plants do not possess an animal-like nervous system, brain, or spinal cord, so their coordination is generally slower and depends on chemical signals and changes in growth. Nevertheless, plants respond to light, gravity, water, touch, and other stimuli through hormones and specialised growth movements. Option A correctly states both the limitation and the compensating feature. Options B, C, and D contradict basic plant physiology.

Hormonal effects are carried through the blood and often develop more slowly, but they may last for a long time. The physical and reproductive changes of adolescence occur gradually over months or years under the influence of endocrine hormones. In contrast, foot withdrawal, pupil contraction, and hand withdrawal are rapid nervous or reflex responses. Therefore option A best demonstrates a long-lasting hormonal effect.

A hormone produces an effect only when it reaches a suitable target cell and binds to a functional receptor. Consequently, measuring a normal hormone level does not prove that the target tissue can respond. If the receptors are defective, the hormone may be present in the blood but its message will not be transmitted properly. Thus option A is correct; the other options describe normal plant or reflex situations rather than a failure of hormone action.

The governing concept is control and coordination: an organism detects a stimulus, transmits information through nerves or chemicals, processes it at a coordinating centre, and produces a response through an effector. Option A includes this complete sequence. Digestion, red-blood-cell colour, and remaining fixed in soil are biological features or processes, but they do not describe stimulus-response coordination.

The governing idea is coordination across organisms. Animals may use a reflex arc for a rapid response and hormones for slower chemical regulation, while plants respond through directional growth and chemical signals such as plant hormones. Option A therefore expresses the chapter’s unifying purpose. Memorising hormone names, proving plants are animals, or calculating blood sugar is too narrow and does not explain the common theme.

A reflex response follows the sequence receptor, sensory nerve, spinal cord, motor nerve, and effector muscle. The spinal cord is stated to be normal, but the command does not reach the muscle, so the motor nerve carrying the outgoing impulse is the most likely affected part. The receptor and sensory nerve belong to the incoming pathway, while the forebrain is not required for this immediate withdrawal response.

A nerve impulse is transmitted along a neuron as an electrical change in the cell membrane. At the synapse, there is a small gap between neighbouring neurons, so neurotransmitter chemicals are released by one neuron and act on the next one. Option A correctly describes this electrical-to-chemical transfer. Food, light, blood, bone, and hormones do not represent the normal mechanism of synaptic transmission.

A synapse is the junctional gap between two neurons. Although an impulse can travel electrically along one neuron, neurotransmitter chemicals are needed to carry the signal across this gap and activate the next neuron. If those chemicals are not formed, transmission to the next neuron will be interrupted. Therefore option B follows directly; insulin production and root growth are unrelated distractors.

Seeing an object requires sensory input, but understanding its meaning, interpreting information, forming memories, and making decisions are higher mental functions. These functions are mainly associated with the forebrain, especially the cerebrum. Therefore option C is correct. The cerebellum coordinates balance and movement, the medulla regulates vital involuntary actions, and the spinal cord conducts impulses and mediates reflexes rather than performing complex interpretation.