Plant hormones

Drought creates water stress in a plant. Abscisic acid, or ABA, acts as an important stress-related hormone; it inhibits growth under unfavorable conditions and promotes stomatal closure, which helps reduce water loss. Its role therefore fits slow growth and drought responses, making option A correct. Gibberellin generally promotes growth, while testosterone and insulin are animal hormones unrelated to this plant response.

Gibberellin is a plant growth hormone that promotes stem elongation and can increase the length of dwarf plants in suitable conditions. Applying it may therefore make the stem taller, so option A is correct. The remaining options describe effects on animal physiology: blood-sugar regulation, heartbeat and spinal-cord activity are not functions of gibberellin. The word “may” is appropriate because the response depends on the plant and dose.

Phototropism requires unequal growth on the two sides of a young stem. Light causes auxin to become distributed unevenly, and the resulting difference in cell elongation bends the stem towards the light. If auxin distribution remains equal, this differential growth will not develop, so bending may greatly decrease or stop. Therefore option A is correct; the other choices do not follow from auxin action.

The governing concept is that plant hormones regulate plant growth and development in characteristic ways. Cytokinin promotes cell division, particularly in actively growing tissues, so option B gives a correct hormone-function pair. Abscisic acid generally inhibits growth and mediates stress responses; gibberellin and auxin are plant hormones but do not regulate blood sugar or heartbeat. Those distractors incorrectly assign animal functions to plant hormones.

The governing concept is the role of plant hormones in growth regulation. Cytokinins primarily promote cell division and help form new tissues, whereas gibberellins are more strongly associated with stem elongation. Since the stated effect is increased division without direct stem lengthening, the chemical acts like cytokinin, so A is correct. Adrenaline and insulin are animal hormones and do not explain this plant response.

The governing concept is hormone specificity through receptors. Hormones may circulate widely in the blood, but they produce a response only in cells that possess the appropriate receptor and the biochemical machinery linked to it. The receptor recognizes the hormone’s shape and starts a cellular response. Therefore A is correct. Blood does reach target organs, hormones are not universally excluded from organs, and the brain does not always destroy them, so B, C, and D are false.

The governing concept is positive phototropism caused by unequal auxin distribution. In a shoot, auxin promotes cell elongation. When light comes from one side, more auxin accumulates on the shaded side, so cells there elongate more than cells on the illuminated side. This unequal growth bends the stem toward the light. Thus A is correct; the other choices do not describe the normal auxin response in a shoot.

The governing concept is thigmotropism, a directional growth response to touch. When one side of a tendril contacts a support, growth on the contacted side is reduced or altered relative to the free side. This unequal growth produces bending and eventually coils the tendril around the support, so option A is correct. Plants do not use a brain, spinal cord, blood, or insulin-based animal response for this movement.

The governing concept is the role of plant hormones in regulating growth. Gibberellins promote stem elongation and can contribute to increased plant height. Cytokinins stimulate cell division and also support the growth of younger tissues. Thus option A correctly pairs their major school-level functions. Options B and C describe unrelated animal processes, while D is false because both substances are plant hormones, not animal-only hormones.

The governing concept is hormonal regulation during water stress. During drought, abscisic acid helps the plant conserve water, especially by promoting stomatal closure; it can also inhibit growth and seed germination when conditions are unfavorable. Therefore option A is the best answer. Blood, brains, and insulin belong to animal biology and are not mechanisms by which a plant responds to drought.

The governing concept is plant hormonal coordination through a directional growth response. In phototropism, light causes unequal distribution of auxin in the shoot, so cells on the shaded side generally elongate more. This unequal growth produces bending toward the light; bending is therefore the visible result, not the complete mechanism. Option A is correct. The other choices incorrectly invoke muscles, blood flow, or a brain.

The governing concept is directional growth coordinated by plant hormones and environmental stimuli. Roots usually grow downward, helping them anchor the plant and absorb water and mineral ions from soil. Shoots generally grow upward toward light, allowing leaves to photosynthesise effectively. Together these responses support nutrition, stability, and survival, so option A is correct. The other choices assign impossible functions such as making light, eating soil, hearing, or blood circulation.

The governing concept is the stress response controlled partly by the plant hormone abscisic acid, or ABA. During drought, excessive evaporation through open stomata can reduce the plant’s water content. ABA promotes stomatal closure, which decreases transpiration and conserves available water, helping the plant survive. Therefore option A is correct. Insulin, blood circulation, and hearing are not plant processes and do not explain drought protection.

The governing concept is hormonal coordination of plant growth. Gibberellins promote processes such as stem elongation, whereas cytokinins stimulate cell division and support the formation of new cells. Comparing these roles shows that growth is not one single process controlled by one substance; it involves coordinated regulation of both cell division and cell elongation. Hence option A is correct. The other choices wrongly invoke animal nerves, deny chemical regulation, or misidentify plant hormones as human blood hormones.

The governing concept is directional information flow in a reflex arc. A sensory or afferent nerve carries impulses from a receptor toward the central nervous system, where the information is processed. A motor or efferent nerve then carries the command from the central nervous system to an effector such as a muscle. Reversing their roles would send information in the wrong direction and prevent the correct reflex response. Thus, option A is correct.

A plant hormone acts according to its concentration, the tissue receiving it, and that tissue’s sensitivity and developmental state. Roots and stems have different structures, functions, and optimal hormone concentrations. Consequently, the same hormone may stimulate growth in one organ but inhibit or alter growth in another. Option A expresses this principle. The remaining choices are incorrect because roots and stems have different functions, plants are cellular, and hormones are not restricted to air.

The governing concept is phototropism, in which unequal auxin distribution causes unequal growth on the two sides of a young stem. Normally, auxin accumulates on the shaded side and promotes greater elongation there, so the stem bends towards light. If this unequal distribution is prevented, the difference in growth is reduced and option A follows. The other choices describe impossible or unrelated animal processes.

The governing concept is auxin-controlled phototropism. In a young shoot, auxin collecting on the shaded side promotes greater cell elongation there than on the illuminated side. Because one side becomes longer, the stem curves towards the light, making option A correct. Immediate cell death, pulling by roots and spinal-cord signalling are not explanations for plant phototropic bending.

The governing concept is differential sensitivity to plant hormones. Roots and stems do not respond identically to the same auxin concentration: a level that promotes elongation in a stem may inhibit elongation in a root. Therefore option A correctly concludes that hormone action depends on both the responding organ and concentration. Auxin is a plant hormone, roots do respond to stimuli, and the stem does not always grow towards water.

The governing concept is the role of cytokinins as plant hormones that promote cell division and support the formation of new tissues, especially in actively growing regions. If cytokinin action decreases, the rate of cell division may decline first, so option A is correct. Heartbeat, blood-sugar regulation and breathing rate are animal physiological functions controlled by different systems, not direct cytokinin targets.