Control and Coordination

The governing concept is geotropism, a directional growth response to gravity. The word positive means growth in the same direction as the stimulus. Since gravity acts downward, roots generally grow downward along the direction of gravitational pull; therefore they show positive geotropism. Growth toward light describes phototropism, upward growth is opposite to gravity, and roots do respond to environmental stimuli.

The governing concept is the direction of growth relative to gravity. Gravity pulls downward, whereas the stem generally elongates upward toward the surface and light. Growth opposite to the direction of gravity is called negative geotropism, so option A is correct. Downward growth would be positive geotropism, movement relative to water concerns hydrotropism, and a stem does grow rather than remaining inactive.

The governing concept is thigmotropism, a directional growth response to touch. When a tendril touches a support, auxin distribution and cell elongation become unequal on its two sides. The untouched side generally grows more, so the tendril bends toward and coils around the support. Thus option A is correct; the other choices do not explain plant growth movement or involve blood, food production, or complete loss of light.

The governing concept is nastic movement caused by changes in cell turgor, not permanent growth. Mimosa leaves fold within seconds after touch because water shifts in specialised motor cells at the leaf base, changing their pressure. No lasting increase in cell length is required. Therefore option A is correct; the movement is not restricted to roots, and it is unrelated to darkness or seed formation.

The governing concept is coordination through two complementary control systems. The nervous system transmits electrical impulses rapidly for immediate, local responses, while the endocrine system releases hormones into blood for slower effects that may last longer and reach several target tissues. Therefore option A is correct. The other options deny their established roles or incorrectly restrict them to plants.

The governing concept is specificity in endocrine coordination. Hormones may travel widely through the bloodstream, but they affect only cells or organs containing the appropriate receptors. Such responsive organs are called target organs. Hence option A is correct. The distractors are biologically meaningless: hormones do not act on stones, merely colour skin, or become defined by being lost in air.

The governing concept is sensory reception. Receptors in the nose respond to chemical molecules responsible for smell, and receptors in the tongue respond to dissolved chemicals responsible for taste. Thus both detect chemical stimuli, making option A correct. Light is detected by photoreceptors in the eye, sound by auditory receptors in the ear, and gravity is not the specific stimulus for these nose and tongue receptors.

The governing concept is sensory transduction in vision. Retinal photoreceptors, mainly rods and cones, absorb light and convert its energy into electrical changes that begin nerve signals. These signals are processed in the retina and carried toward the brain through the optic nerve. Therefore option A is correct; filtering blood, joining bones, and producing taste are functions of other body structures.

The inner ear contains balance-related structures that detect head position and movement. These structures generate nerve signals about orientation and motion, and the brain combines them with information from the eyes and muscles to maintain posture. Therefore, option A is correct. The other options describe digestion, blood-sugar regulation, and hormone secretion, which are not functions of the inner ear.

The cerebrum is the largest part of the forebrain and controls higher mental activities such as thinking, learning, memory, reasoning, and conscious actions. Damage to it can therefore affect thinking and memory, making option A correct. Heart-muscle formation, thyroxine production, and downward root growth are controlled by developmental, endocrine, or plant processes rather than the cerebrum.

Seeing attractive food stimulates the eyes, which send sensory information to the brain. The brain then activates the salivary glands through nerve signals, causing saliva secretion. This demonstrates coordination between a sense organ, the nervous system, and a gland, so option A is correct. Bone growth, plant growth, and blood clotting do not explain this immediate response to food.

A voluntary action is performed consciously and according to a choice, such as raising a hand, writing, or walking. The cerebrum evaluates information, makes a decision, and sends motor commands through nerves to the appropriate muscles. Therefore option A is correct. The other statements are biologically false or unrelated to conscious movement.

Plants do not generally have muscles or a nervous system that can produce rapid coordinated movement. Many plant movements result from unequal growth on two sides of an organ, as in a shoot bending toward light. Since cell division and elongation take time, such responses are usually slow. Therefore option A is correct; the other statements are factually incorrect.

Phototropism is the directional growth response of a plant to light, while geotropism, or gravitropism, is the directional growth response to gravity. Although the stimuli differ, both involve growth in a particular direction and are regulated by plant growth mechanisms such as unequal distribution of auxin. Thus option A states the correct similarity.

Hydrotropism means directional growth in response to water, so a root growing toward moisture demonstrates hydrotropism. Phototropism means directional growth in response to light, so a stem bending or growing toward light demonstrates phototropism. Therefore option A correctly names both responses. The other options confuse growth responses with life processes or deny the observed responses.

Hormones are chemical substances released by endocrine glands into the blood. The circulation carries them to particular target cells or organs, where they bind to receptors and change the activity of those targets. Because they transmit regulatory information chemically rather than through nerve impulses, they are called chemical messengers. Thus option A is correct; the other options misrepresent their nature.

Thyroxine is a hormone secreted by the thyroid gland. Its major general effect is to regulate the body's metabolic rate, meaning how quickly cells use energy, and it also supports normal growth and development. Iodine is needed for its production. Thus option A is correct. Taste, hearing, and sweating are not the primary general functions controlled by thyroxine, so options B, C, and D are unsuitable.

During running, skeletal muscles contract more frequently and perform greater work than they do at rest. Their cells therefore need increased oxygen and glucose for aerobic respiration, which releases ATP for muscle contraction. Blood delivers these materials and removes carbon dioxide and other wastes. Hence option A is correct. The other options do not explain the increased circulation to active muscles.

Control and coordination enable an organism to detect internal or external stimuli, process the information, and produce an appropriate response. A suitable response helps maintain safety, balance, and normal activity; an unsuitable response may cause injury or waste energy. For example, withdrawing from excessive heat protects tissues. Therefore option A expresses the governing purpose, while the other options contradict basic facts about responses and coordination.

The chapter shows that organisms detect stimuli and coordinate responses through different control systems. Animals use electrical nerve impulses and chemical hormones, while plants rely mainly on chemical messengers and regulated growth movements. These mechanisms help organisms respond, survive, and maintain internal functioning. Therefore option A gives the most complete conclusion. Options B, C, and D deny established responses or reduce them incorrectly to digestion alone.