Plant hormones

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 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.

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 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.

The governing concept is plant hormonal control during stress. Abscisic acid, or ABA, inhibits growth and promotes protective responses such as stomatal closure when water is scarce, thereby reducing water loss. Hence option A correctly connects ABA with growth inhibition and water-stress response. Heartbeat, blood sugar, and reflex arcs belong to animal physiology or nervous coordination, so options B, C, and D are unrelated distractors.

Phototropism is a directional growth response to light, whereas chemotropism is a directional growth response to chemical substances. During fertilisation in flowering plants, the pollen tube grows towards the ovule in response to chemical signals released by tissues around it. Thus, blocked chemotropism may affect pollen-tube growth, making option A correct. Option B describes phototropism, while C and D are animal processes.

This is positive phototropism controlled by unequal auxin distribution. Light from one side causes more auxin to accumulate on the shaded side of the shoot. In a stem, that higher auxin concentration promotes greater cell elongation there. The shaded side therefore grows faster and the stem curves toward the light, making option A correct. The other choices describe biologically unrelated or false effects.

The governing concept is the action of plant hormones. Gibberellin generally promotes stem elongation, seed germination, and other growth-related processes, whereas abscisic acid commonly inhibits growth and supports dormancy or stress responses. Therefore, option A correctly describes their contrasting effects. The other choices refer to animal physiology and do not describe plant hormones.

The governing concept is plant response to water stress. During drought, abscisic acid helps the plant conserve water, especially by promoting stomatal closure, and it can reduce growth so limited water is directed to survival. Thus option A gives the best answer. Plants do not make blood, spinal cords, or insulin, so options B, C, and D are biologically unsuitable.

The governing concept is phototropism, the directional growth response to light. In a normal shoot, auxin redistribution causes cells on the shaded side to elongate more, so the stem curves toward the light. If the plant remains alive but this bending fails, unequal growth or its hormonal control is likely disturbed. Therefore option A is correct; the other choices describe animal processes.

The governing concept is the role of plant hormones in growth and development. Cytokinins promote cell division and help stimulate the formation of new cells in plant tissues. Consequently, a decrease in cell division may be associated with insufficient cytokinin, making option A correct. Insulin, adrenaline, and thyroxine are animal hormones, so they are not the appropriate plant-hormone explanation for this observation.

The governing concept is hormonal regulation of plant growth. Gibberellins promote elongation of stems and other growth processes, so reduced stem-length increase points most directly to a possible deficiency or reduced action of gibberellin. Therefore, option A is correct. Insulin, adrenaline, and thyroxine are animal hormones and do not provide the expected school-level explanation for stem elongation in plants.

In Mendel's pea plants, tallness is dominant over dwarfness. Represent the pure tall parent as TT and the pure dwarf parent as tt. Every offspring receives T from the first parent and t from the second, so all F1 plants are Tt. Because T masks t, all F1 plants show the tall phenotype. Therefore option B is correct.

In a monohybrid cross of two heterozygotes, Tt × Tt, the possible genotypes are TT, Tt, Tt, and tt. Three offspring show the dominant phenotype and one shows the recessive phenotype, giving the expected 3:1 phenotypic ratio. This demonstrates segregation of alleles during gamete formation and their recombination at fertilisation. Thus option B is correct.

The governing concept is the difference between phenotype and genotype. If T is dominant, both TT (pure tall) and Tt (hybrid tall) plants show the same tall phenotype. The visible appearance therefore cannot reveal whether the plant carries one or two dominant alleles. A test cross with a recessive tt plant can help identify the unknown genotype. Option A is correct; the other statements wrongly deny genes, confuse inherited and acquired traits, or reverse dominance.