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3.6.1 Stimuli: internal and external responses

AQA A-level Biology 3.6.1 practice on Stimuli: internal and external responses, with free MCQs, clues and worked explanations drawn from the Organisms respond to changes in their internal and external environments section of specification 7402.

Specification route
3.6.1
Question bank
30 questions
Course stage
Year 13 / A-level only

Sample questions

QUESTION 1 · 3.6.1.1 · LEVEL 2

A woodlouse is placed in a dry environment. It begins to move more rapidly and changes direction more frequently. How is this simple response classified?

  • Kinesis
  • Positive phototaxis
  • Negative chemotaxis
  • Tropism
Show clue

This response is non-directional; the organism simply moves faster to increase the chances of entering a new, favorable environment.

Show answer and explanation

Answer: Kinesis

A kinesis is a non-directional response to a stimulus where the rate of movement or rate of turning changes in response to the intensity of the stimulus. Taxis is a directional response.

QUESTION 2 · 3.6.1.1 · LEVEL 3

How does Indoleacetic acid (IAA) affect the growth of a plant shoot exposed to unidirectional light?

  • IAA diffuses to the shaded side of the shoot, stimulating cell elongation and causing the shoot to bend towards the light.
  • IAA diffuses to the illuminated side of the shoot, inhibiting cell elongation and causing the shoot to bend towards the light.
  • IAA diffuses to the shaded side of the shoot, inhibiting cell elongation and causing the shoot to bend away from the light.
  • IAA is destroyed by the light, forcing the plant to grow vertically to escape the light source.
Show clue

In shoots, a higher concentration of IAA promotes elongation.

Show answer and explanation

Answer: IAA diffuses to the shaded side of the shoot, stimulating cell elongation and causing the shoot to bend towards the light.

Control of tropisms by indoleacetic acid (IAA). In shoots, IAA accumulates on the shaded side and stimulates cell elongation, resulting in positive phototropism.

QUESTION 3 · 3.6.1.1 · LEVEL 2

What is the specific effect of a high concentration of IAA on plant root tissue?

  • It inhibits cell elongation.
  • It strongly stimulates cell elongation.
  • It triggers the rapid onset of mitosis.
  • It causes the active transport of water out of the cells.
Show clue

Roots respond to IAA in the exact opposite way to shoots.

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Answer: It inhibits cell elongation.

Control of tropisms by indoleacetic acid (IAA). While high concentrations of IAA stimulate elongation in shoots, they inhibit elongation in roots, which is essential for positive gravitropism.

QUESTION 4 · 3.6.1.2 · LEVEL 1

What type of stimulus does a Pacinian corpuscle specifically respond to?

  • Mechanical pressure
  • Changes in light intensity
  • Chemical concentration gradients
  • Changes in environmental temperature
Show clue

These receptors are located deep within the skin and joints.

Show answer and explanation

Answer: Mechanical pressure

The Pacinian corpuscle acts as a receptor specifically responding to mechanical pressure, acting as a transducer.

QUESTION 5 · 3.6.1.2 · LEVEL 3

When mechanical pressure is applied to a Pacinian corpuscle, how is a generator potential established?

  • The pressure deforms the membrane, stretching stretch-mediated sodium ion channels, which open and allow sodium ions to diffuse into the neurone.
  • The pressure actively pumps potassium ions out of the neurone, reversing the resting potential.
  • The pressure destroys the myelin sheath, causing a massive influx of calcium ions.
  • The pressure forces acetylcholine to be released directly into the sensory nerve ending.
Show clue

The physical change in the membrane's shape directly affects the ion channels embedded within it.

Show answer and explanation

Answer: The pressure deforms the membrane, stretching stretch-mediated sodium ion channels, which open and allow sodium ions to diffuse into the neurone.

The Pacinian corpuscle illustrates that receptors respond only to specific stimuli. Deformation of the lamellae stretches the membrane, opening stretch-mediated $Na^{+}$ channels and causing an influx of $Na^{+}$ which creates a generator potential.

QUESTION 6 · 3.6.1.2 · LEVEL 4

Why do rod cells in the human retina provide high visual sensitivity but low visual acuity?

  • Multiple rod cells synapse with a single bipolar cell (retinal convergence), allowing spatial summation but preventing the brain from distinguishing exactly which rods were stimulated.
  • Each rod cell synapses with its own bipolar cell, meaning they require massive amounts of light to reach the threshold.
  • Rod cells only contain iodopsin, which requires a very high light intensity to be broken down.
  • Rod cells are concentrated in the fovea, where light is most scattered by the lens.
Show clue

Sensitivity is about detecting dim light; acuity is about seeing fine detail.

Show answer and explanation

Answer: Multiple rod cells synapse with a single bipolar cell (retinal convergence), allowing spatial summation but preventing the brain from distinguishing exactly which rods were stimulated.

Differences in sensitivity and visual acuity are explained by differences in connections. Many rods connect to one bipolar cell (convergence). This allows spatial summation to reach the threshold in dim light (high sensitivity) but means the brain cannot resolve the exact source of the light (low acuity).

QUESTION 7 · 3.6.1.3 · LEVEL 1

Which specialized group of cells acts as the natural pacemaker of the mammalian heart?

  • Sinoatrial node (SAN)
  • Atrioventricular node (AVN)
  • Bundle of His
  • Purkyne tissue
Show clue

This node is located in the wall of the right atrium.

Show answer and explanation

Answer: Sinoatrial node (SAN)

Control of heart rate involves the sinoatrial node (SAN), which generates electrical impulses that initiate the heartbeat.

QUESTION 8 · 3.6.1.3 · LEVEL 4

If baroreceptors detect an abnormally high blood pressure, what is the autonomic nervous system's response?

  • More impulses are sent via the parasympathetic nervous system to the SAN, releasing acetylcholine to decrease the heart rate.
  • More impulses are sent via the sympathetic nervous system to the SAN, releasing noradrenaline to increase the heart rate.
  • The parasympathetic nervous system entirely shuts down the AVN to stop ventricular contraction.
  • The sympathetic nervous system releases acetylcholine to constrict the arteries and lower the pressure.
Show clue

The body needs to slow the heart down to reduce the pressure. Which branch of the autonomic system handles 'rest and digest'?

Show answer and explanation

Answer: More impulses are sent via the parasympathetic nervous system to the SAN, releasing acetylcholine to decrease the heart rate.

High blood pressure is detected by baroreceptors (pressure receptors). They send impulses to the medulla oblongata, which sends impulses down parasympathetic neurones to the SAN. Acetylcholine is released, slowing the heart rate.

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