Plant hormones

The governing concept is the role of growth hormone in childhood development. This hormone, secreted by the pituitary gland, supports the growth of bones and other body tissues. If its secretion is very low during childhood, normal physical growth, especially increase in height, may be reduced or delayed. Excess sweating, eye-colour change and automatic food production are not the expected effects of this deficiency.

The governing concept is chemical and growth-based coordination in plants. Plants lack a nervous system, bones and muscles, but they produce hormones that move through tissues and influence cell division, elongation and responses to light, gravity or water. These slow chemical signals and directional growth responses coordinate plant activities. A spinal cord, brain, heart and animal-style blood circulation are therefore not the explanation.

The governing concept is hydrotropism, a growth response directed by water availability. Roots absorb water and mineral salts from the soil, so their growing regions can bend or extend toward a moisture source. Therefore the root is the correct answer. Flowers, fruits and cotyledons may have other biological roles, but they are not the plant part classically identified as showing growth toward water in this example.

The governing concept is the division of functions among plant hormones. Gibberellin promotes stem elongation and can support other growth processes, whereas cytokinin stimulates cell division and supports the growth of developing tissues. Thus option A gives the correct comparison. The remaining choices assign animal functions to plant hormones or incorrectly claim that these hormones occur only in animals.

The governing concept is hormonal response to environmental stress. During water shortage, abscisic acid promotes stomatal closure, reducing transpiration and helping the plant conserve water. This protective action explains why it is called a stress hormone, so option A is correct. The other choices describe unrelated animal functions and do not represent the role of abscisic acid in plants.

The governing concept is phototropism, the bending of a plant shoot toward light. In a shoot illuminated from one side, auxin is redistributed toward the shaded side. Its higher concentration there promotes greater cell elongation, so that side grows faster and the shoot bends toward the light. Therefore option A is correct; the other locations do not explain directional shoot growth.

Cytokinin is the correct answer because it promotes cytokinesis and supports cell division in actively growing plant tissues. It works with other plant hormones to regulate growth and development. Auxin mainly promotes cell elongation and root-related growth, while abscisic acid generally inhibits growth and helps plants respond to stress. Adrenaline is an animal hormone, not a plant hormone, so it cannot be the suitable choice here.

Abscisic acid, commonly abbreviated as ABA, is associated with growth inhibition and plant responses to stressful conditions such as water shortage. During stress, it promotes stomatal closure, reducing water loss, and is therefore linked with wilting and dormancy-related responses. Gibberellin generally promotes stem growth, and cytokinin supports cell division and delays leaf ageing. Thyroxine is an animal hormone, so option C is the only suitable answer.

Gibberellin is a plant growth hormone that promotes stem elongation and can increase the length of internodes, the regions between successive leaves. Therefore, a plant treated with suitable gibberellin may develop a taller stem. Insulin, adrenaline, and thyroxine are hormones associated with animals rather than plant stem elongation. Auxin also affects plant growth, but it is not included among the choices, making gibberellin the unambiguous answer.

A shoot normally shows positive phototropism, meaning it grows towards a directional light stimulus. Auxin becomes distributed more on the shaded side of the stem, where it promotes greater cell elongation. Because the shaded side grows faster, the stem curves towards the illuminated side. It does not bend downward, move away from light, or remain straight when the light is one-sided.

The governing concept is positive phototropism controlled by auxin. With one-sided light, auxin becomes concentrated on the shaded side of a young shoot. In shoots, this promotes greater cell elongation on that side. The shaded side therefore grows faster than the illuminated side, producing unequal growth and bending the stem toward the light. Option B is correct; plant stems do not have blood flow, and neither cell death nor root pressure explains this directional curvature.

Phototropism is a directional growth response to light. In a shoot receiving light from one side, auxin is redistributed and usually promotes greater cell elongation on the shaded side. That side grows faster, so the shoot bends toward the light. The plant does not move by muscles or run, and leaves do not pump blood. Thus option A correctly identifies unequal growth as the cause.

Gibberellin and cytokinin are plant hormones, but they promote different growth processes. Gibberellin mainly stimulates stem elongation, whereas cytokinin promotes cell division and supports the formation of new cells. Therefore, combining their functions shows that plant growth depends on both an increase in cell number and an increase in cell length. Options B, C, and D incorrectly describe these hormones.

During severe water shortage, a plant must reduce transpiration to conserve its limited water. Abscisic acid, or ABA, is a plant hormone that promotes stomatal closure under water stress. If ABA is deficient or the guard cells do not respond to it, stomata may remain open and excessive water loss can continue. Therefore option A is correct; the other hormones are mainly associated with animals.

Hormones are released into the bloodstream and can influence many target tissues. Their effects generally develop more slowly than nerve impulses but may persist for a longer time, making them suitable for growth, development, metabolism, and other sustained processes. Immediate withdrawal from heat and rapid pupil changes require nervous control. Therefore option A is the only appropriate choice.

In a young shoot, the tip detects directional light and coordinates the distribution of auxin. Auxin becomes more concentrated on the shaded side, where it promotes cell elongation, causing the shoot to bend towards the light. Covering the tip can reduce light perception and disrupt this unequal distribution, so bending may decrease. Thus option A is correct.

Phototropism results from unequal growth on the two sides of a shoot. If cells on the shaded side elongate more, that side becomes longer and acts like the outer side of a bend. The shorter illuminated side becomes the inner side, so the shoot curves towards the light. Therefore option A is correct; the other choices describe unrelated or unsupported directions.

Auxin regulates plant growth, but different organs have different sensitivities to its concentration. A level that promotes cell elongation in a stem may inhibit elongation in a root. This organ-specific response explains why the same hormone can produce opposite effects, so option A is correct. Auxin is a plant hormone, roots do respond to hormones, and stem growth is not always toward gravity.

Cytokinin is a plant hormone whose important functions include promoting cell division and supporting growth in actively dividing tissues. If cytokinin activity becomes very low, cell division in regions such as meristems may be reduced. Therefore option A is correct. Emergency responses are associated with animal nervous or hormonal systems, while blood-sugar control and breathing rate are not cytokinin functions.

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 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.