Plant hormones

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.

The governing concept is the specific function of plant hormones. Cytokinins stimulate cell division and support growth in actively dividing tissues, so option A gives a scientifically correct hormone-function pair. Abscisic acid generally inhibits growth and helps plants respond to stress, while gibberellin affects plant growth rather than blood sugar. Auxin regulates plant growth and tropic responses, not heartbeat; therefore B, C, and D are incorrect.

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 differential sensitivity to a plant hormone. Roots and stems contain different tissues and respond to auxin at different concentration ranges. A level that promotes elongation in a stem may inhibit root elongation, because roots are generally more sensitive. Hence option A is correct. Roots do not contain brains, stems do not transport blood, and auxin is a plant hormone rather than an animal-only hormone.

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.

Growth of a plant part in the direction of light is called positive phototropism. Stems usually bend towards light, making this a common example. Geotropism is a growth response to gravity, not to light. Exam tip: associate growth movement caused by light with phototropism.