A-Level Biology Topic 6 – Organisms Respond to Changes in Their Internal & External Environments

What is a stimulus?

A change in an organism’s internal or external environment.

Why is it important that organisms can respond to stimuli?

Because responding to stimuli increases an organism’s chance of survival.

Define ‘tropism’.

Growth of a plant in response to a directional stimulus (positive = toward the stimulus, negative = away).

Which plant growth factor is most often linked to tropisms and where is it produced?

IAA (an auxin), produced in growing regions such as shoot and root tips.

How does a high concentration of IAA affect cells in shoots?

It stimulates cell elongation.

How does a high concentration of IAA affect cells in roots?

It inhibits cell elongation.

Outline the sequence that produces gravitropism in roots.

IAA moves to the lower side of the root, inhibits elongation there, so the root bends toward gravity.

Outline the sequence that produces phototropism in shoots.

IAA moves to the shaded side of the shoot, stimulates elongation there, so the shoot bends toward light.

Differentiate taxes and kinesis.

Taxes are directional movements toward/away from a stimulus; kinesis are non-directional changes in speed or turning rate depending on stimulus intensity.

Give an example of taxis in woodlice.

Woodlice move away from light into darker areas to avoid predators.

Give an example of kinesis in woodlice.

Woodlice move faster and make more turns in dry areas to locate higher humidity.

Why are simple (three-neurone) reflexes protective?

They are rapid (few synapses) and automatic, so they quickly remove the organism from harmful stimuli.

In Required Practical 10, what piece of equipment is used to test woodlice response to humidity?

A choice chamber with one humid and one dry compartment.

Why are mesh platforms used above drying agents in humidity experiments?

To keep woodlice a safe distance from the drying agent.

Which statistical test is appropriate for analysing choice-chamber results and why?

Chi-squared test because the data are categorical frequencies compared with expected equal distribution.

Describe the basic structure of a Pacinian corpuscle.

A single sensory neurone ending surrounded by concentric layers of lamellae (connective tissue).

How is a generator potential produced in a Pacinian corpuscle?

Pressure deforms lamellae, stretching Na⁺ channels so Na⁺ enters the neurone causing depolarisation.

What two principles does the Pacinian corpuscle illustrate?

Receptors are specific to a single type of stimulus, and stimulation produces a generator potential leading to action potentials when threshold is reached.

Why are rod cells more sensitive to low light than cone cells?

Many rods synapse with one bipolar neurone, allowing spatial summation to reach threshold.

Why do cone cells give higher visual acuity than rod cells?

Each cone connects to a single bipolar neurone, so signals from neighbouring cones remain separate and the brain can distinguish points.

Which photoreceptors enable colour vision and how?

Cones; three types (red-, green-, blue-sensitive) with different pigments absorbing different wavelengths.

What does it mean that cardiac muscle is myogenic?

It can contract and relax without nervous stimulation.

Name the heart’s natural pacemaker.

The sinoatrial node (SAN).

Why is there a delay at the atrioventricular node (AVN)?

To allow atria to finish contracting and empty before ventricles contract.

Where are chemoreceptors and baroreceptors that regulate heart rate located?

In the aorta and carotid arteries.

Which autonomic pathway increases heart rate?

The sympathetic neurones to the SAN.

Which autonomic pathway decreases heart rate?

The parasympathetic (vagus) neurones to the SAN.

Describe resting potential of a neurone.

Inside of axon is negative relative to outside due to Na⁺/K⁺ pump and membrane permeability differences.

How many Na⁺ and K⁺ ions does the Na⁺/K⁺ pump move per cycle and in which directions?

3 Na⁺ out of the axon and 2 K⁺ into the axon (active transport).

What initiates an action potential?

A stimulus opens Na⁺ channels; if depolarisation reaches threshold, more voltage-gated Na⁺ channels open (positive feedback).

State the all-or-nothing principle for action potentials.

If threshold is reached an action potential of fixed magnitude occurs; stronger stimuli increase frequency not size.

How does myelination speed nerve impulse conduction?

Depolarisation occurs only at nodes of Ranvier producing saltatory conduction, so the impulse effectively ‘jumps’ between nodes.

What is the refractory period and why is it important?

Time after an action potential when Na⁺ channels are inactivated; ensures unidirectional impulses, discrete signals and limits maximum frequency.

List three factors that increase speed of nerve conduction.

Myelination, larger axon diameter, and higher temperature (within limits).

Outline the events at a cholinergic synapse from Ca²⁺ entry to post-synaptic depolarisation.

Ca²⁺ enters pre-synaptic knob → vesicles fuse and release ACh → ACh diffuses, binds to receptors → Na⁺ channels open → Na⁺ influx depolarises post-synaptic membrane.

How is acetylcholine removed from the synaptic cleft?

It is hydrolysed by acetylcholinesterase and the products are reabsorbed into the presynaptic neurone.

Define spatial summation.

Many pre-synaptic neurones collectively release enough neurotransmitter onto one post-synaptic neurone to reach threshold.

Define temporal summation.

One pre-synaptic neurone releases neurotransmitter repeatedly and rapidly so that cumulative release reaches threshold.

How do inhibitory synapses reduce the likelihood of an action potential?

They open Cl⁻ and K⁺ channels so the inside becomes more negative (hyperpolarised), needing more Na⁺ influx to reach threshold.

Give two structural differences between a neuromuscular junction and a typical cholinergic synapse.

Receptors are on muscle fibre sarcolemma (more numerous) and the muscle stores extra acetylcholinesterase in clefts.

What effect do stimulatory drugs often have at synapses?

They increase frequency of action potentials by mimicking neurotransmitters, triggering more release, or inhibiting breakdown enzymes.

In skeletal muscles, what is an antagonistic pair?

Two muscles that pull in opposite directions so that when one contracts the other relaxes (e.g., biceps & triceps).

Name the two main protein filaments in a myofibril.

Actin (thin) and myosin (thick).