Responses to Stimuli

In a phototropism investigation, the tip of a coleoptile is covered with an opaque cap, then the coleoptile is illuminated from one side. The most likely outcome is that the coleoptile:

• Abends sharply towards the light
• Bbends away from the light
• Cgrows straight up with no bending✓
• Dstops growing completely and dies

Answer: The tip is the region that detects light and produces and redistributes auxin (IAA). Capping the tip with an opaque cover prevents light detection, so no asymmetric auxin distribution occurs and the coleoptile grows straight up rather than bending.

Indoleacetic acid (IAA) causes a positive phototropic response in a shoot because it:

• Aaccumulates on the illuminated side, where it inhibits cell elongation so that the lit side grows more slowly
• Baccumulates on the shaded side, where it promotes cell elongation so that side grows faster✓
• Cis broken down by light (photo-oxidised) on the shaded side, so that side elongates more slowly than the lit side
• Daccumulates evenly on both sides of the shoot, so the two sides elongate at the same rate and the shoot stays upright

Answer: IAA is redistributed laterally so it becomes more concentrated on the shaded side of the shoot. There it stimulates cell elongation, so the shaded side grows faster than the illuminated side and the shoot bends towards the light.

When a young root is placed horizontally, it shows positive gravitropism. The accepted explanation is that IAA accumulates on the lower side of the root, where it:

• Apromotes cell elongation, so the lower side grows faster and lifts the root
• Binhibits cell elongation, so the upper side grows faster and the root bends downward✓
• Cpromotes cell division only, with no effect on elongation
• Dhas no effect because roots do not respond to IAA

Answer: IAA collects on the lower side of a horizontal root. In roots, relatively high IAA concentrations inhibit cell elongation, so the lower side elongates less than the upper side. The root therefore curves downward, towards gravity (positive gravitropism).

Apical dominance, in which the growth of lateral (side) buds is suppressed, is primarily maintained by:

• Agibberellins produced in the lateral buds
• Bauxin (IAA) produced by the apical bud✓
• Cabscisic acid produced in the roots
• Dethene produced by the leaves

Answer: Auxin produced by the apical (terminal) bud is transported down the stem and inhibits the growth of lateral buds, maintaining apical dominance. Removing the apical bud lowers auxin levels and allows the side buds to grow, which is why pruning encourages bushier growth.

Gibberellins promote the germination of cereal seeds such as barley by:

• Ainhibiting the production of amylase in the aleurone layer, so that the stored starch is conserved for later growth
• Bstimulating the aleurone layer to produce amylase, which hydrolyses stored starch to maltose✓
• Cclosing the stomata of the emerging shoot to reduce water loss, which conserves the seed's limited water reserve
• Ddirectly providing glucose (a respiratory substrate) to the embryo

Answer: The embryo releases gibberellins that diffuse to the aleurone layer surrounding the endosperm. This stimulates synthesis of amylase, which hydrolyses stored starch into maltose. The soluble sugar is then used by the growing embryo as a respiratory substrate.

The phytochrome pigment exists in two interconvertible forms, Pr and Pfr. Which statement is correct?

• APr absorbs far-red light and is rapidly converted to Pfr, which then builds up in darkness to trigger flowering
• BPr absorbs red light and is converted to Pfr, which is the physiologically active form✓
• CPfr absorbs red light and is converted back to Pr in daylight, so very little Pfr is present at the end of a long day
• DPr is the active form, accumulates during the day, and Pfr is the inactive form stored overnight

Answer: Pr absorbs red light (around 660 nm) and is rapidly converted to Pfr. Pfr is the active form that influences responses such as flowering; it absorbs far-red light and is slowly converted back to Pr in darkness, allowing plants to measure night length.

In a long-day plant, flowering is promoted when the night length is short. The form of phytochrome that accumulates by the end of a short night and promotes flowering in such plants is:

• APr
• BPfr✓
• Cchlorophyll a
• Dabscisic acid

Answer: During daylight Pr is converted to Pfr. With short nights there is little time for Pfr to revert to Pr, so a high level of Pfr remains at dawn. In long-day plants this high Pfr level promotes flowering; long-day plants are effectively short-night plants.

When a plant experiences water stress, abscisic acid (ABA) helps reduce water loss by:

• Astimulating guard cells to take up K⁺ (potassium ions) and water, so that they become turgid and the stomata open
• Bcausing loss of solutes from guard cells so that they lose turgor and the stomata close✓
• Cincreasing the rate of transpiration through the open stomata, so that water is drawn up the xylem more quickly
• Dpromoting cell elongation in the leaf mesophyll, so that a thicker leaf holds more water in its tissues

Answer: ABA accumulates in leaves during water stress and acts on guard cells, triggering the loss of K⁺ and other solutes. Water then leaves the guard cells by osmosis, they become flaccid, and the stomata close, reducing transpirational water loss.

Which of the following is an example of a chemical plant defence against herbivores or pathogens?

• Athe waxy cuticle on the leaf surface
• Bproduction of tannins that make leaf tissue unpalatable or toxic to insects✓
• Cthe presence of thorns on a stem
• Dfolding of leaves in response to touch

Answer: Tannins are chemical defences: they are bitter-tasting and can be toxic or reduce digestibility, deterring herbivores from eating the plant. Thorns and a waxy cuticle are physical defences, and leaf folding is a rapid movement response, not a chemical defence.

A motile single-celled alga swims directly towards a region of higher light intensity. This directional movement of a whole organism in response to a stimulus is best described as a:

• Apositive kinesis
• Bpositive taxis✓
• Ctropism
• Dnastic response

Answer: A taxis is a directional movement of a whole motile organism towards (positive) or away from (negative) a directional stimulus. Movement towards light is positive phototaxis. A kinesis is non-directional, and a tropism is a directional growth response of a fixed plant.

Woodlice placed in a choice chamber move more rapidly and turn more frequently in dry conditions, but slow down and turn less in humid conditions, so they tend to aggregate in damp areas. This behaviour is an example of:

• Aa taxis, because movement is directed towards humidity
• Ba kinesis, because the rate of movement and turning changes with conditions but is non-directional✓
• Ca tropism, because the woodlice grow and orient their bodies towards the more humid part of the chamber
• Da reflex arc involving the spinal cord

Answer: In a kinesis the organism changes its speed and rate of turning in response to the intensity of a non-directional stimulus, rather than moving straight towards or away from it. Faster movement and more turning in dry air mean woodlice spend more time, and thus accumulate, in damp regions.

What is a tropism?

• AA non-directional growth response to a stimulus
• BA directional growth response to a stimulus✓
• CA rapid movement of the whole organism
• DA change in the rate of an organism's movement

Answer: Tropisms are growth responses whose direction depends on where the stimulus comes from — phototropism towards light, gravitropism with gravity. A non-directional response is a nastic movement, and rate changes describe kinesis.

How does IAA cause a shoot to bend towards light?

• AIAA accumulates on the lit side and lengthens those cells
• BIAA accumulates on the shaded side and shortens those cells
• CIAA accumulates on the lit side and shortens those cells
• DIAA accumulates on the shaded side and lengthens those cells✓

Answer: Auxin moves laterally away from the light, so the shaded side has more, its cells elongate more and the shoot curves towards the light. In roots the same hormone inhibits elongation, which is why roots bend the other way.

What is the difference between taxis and kinesis?

• AKinesis is directional; taxis changes movement rate randomly
• BBoth are directional but taxis is faster than kinesis
• CBoth change movement rate but taxis is more precise
• DTaxis is directional; kinesis changes movement rate randomly✓

Answer: A taxis moves the organism towards or away from a directional stimulus. In kinesis the organism cannot tell direction, so it changes speed or turning rate — moving slowly in favourable conditions and quickly in poor ones until it finds better.

What is the advantage of a simple reflex arc?

• AIt is rapid because it requires conscious processing
• BIt is slow but allows a more considered response
• CIt allows the response to be learned and modified
• DIt is rapid and does not require conscious processing✓

Answer: Few synapses and no involvement of the higher brain make reflexes very fast, which protects the body from damage. The trade-off is that the response is fixed and cannot be adapted to circumstances.

What is the role of the sinoatrial node?

• ADelaying the impulse before it reaches the ventricle walls
• BInitiating each heartbeat by generating an electrical impulse✓
• CCarrying the impulse down the septum between the ventricles
• DPreventing backflow of blood from the ventricles to the atria

Answer: The SAN is the pacemaker, setting the rhythm myogenically. The atrioventricular node introduces the delay letting atria empty; the bundle of His carries the impulse down the septum; valves prevent backflow.

How does the sympathetic nervous system affect heart rate?

• AIt increases heart rate by acting on the sinoatrial node✓
• BIt decreases heart rate by acting on the sinoatrial node
• CIt increases heart rate by acting on the atrioventricular node
• DIt decreases heart rate by acting on the atrioventricular node

Answer: Sympathetic stimulation of the SAN raises the rate, preparing the body for activity; the parasympathetic system, via the vagus nerve, slows it again. Both act on the pacemaker.

What do baroreceptors detect?

• AChanges in blood pressure✓
• BChanges in blood carbon dioxide
• CChanges in blood glucose concentration
• DChanges in blood water potential

Answer: Baroreceptors in the aorta and carotid arteries respond to pressure, signalling the medulla to adjust heart rate. Chemoreceptors monitor carbon dioxide and pH, and osmoreceptors monitor water potential.

Why do rod cells give greater sensitivity but lower visual acuity than cones?

• ASeveral cones share one neurone, so signals summate but detail is lost
• BEach rod has its own neurone, giving high sensitivity and low detail
• CSeveral rods share one neurone, so signals summate but detail is lost✓
• DRods contain three different pigments and cones contain only one

Answer: Retinal convergence lets several rods summate onto one bipolar neurone, so dim light can reach threshold — but the brain cannot tell which rod fired. Cones mostly have their own neurone, giving sharp detail and colour at the cost of sensitivity.

Why is the pupil reflex described as a cranial reflex?

• AThe reflex arc passes through the spinal cord rather than the brain
• BIt involves conscious processing in the cerebral cortex
• CThe reflex arc passes through the brain rather than the spinal cord✓
• DIt is learned during early development rather than innate

Answer: The sensory and motor pathways run via the midbrain, so it is cranial rather than spinal, but it remains involuntary and rapid. It protects the retina from damage by bright light.