5.5Unit 5: Respiration, Internal Environment, Coordination and Gene Technology

Response to the Environment — IAL Edexcel Modular Revision Notes & Topical Questions

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5.5 Response to the Environment — IAL EDEXCEL MODULAR notes: Describe neurones, reflexes, nerve impulses, synapses, drug action, the detection of light, and plant and animal coordination.Nervous system — the CNS is the brain and spinal cord, and the PNS is all other nerves, conne…

Neurone features
Spinal reflex
Cranial reflex
Pupil reflex
Resting potential
Action potential

Part 1 of 11Nervous system and neurones

Nervous system and neurones

Nervous system

Nervous system the CNS is the brain and spinal cord, and the PNS is all other nerves, connecting receptors to the CNS and the CNS to effectors.
Pathway
1stimulus2receptor3sensory neurone4CNS5motor neurone6effector (muscle or gland)
The autonomic system has sympathetic ('fight or flight', noradrenaline) and parasympathetic ('rest and digest', acetylcholine) divisions.

Neurone features

Neurone features an axon, a cell body with the nucleus and axon terminals with nerve endings.
Myelinated neurones have Schwann cells wrapped round the axon with uninsulated gaps called nodes of Ranvier.

Type	Structure and role
Sensory	Carries impulses from receptors to the CNS; the cell body branches off the middle of the axon and there are no dendrites
Relay	Short, entirely within the CNS, with highly branched dendrites, connecting sensory and motor neurones
Motor	Large cell body in the CNS with many dendrites; carries impulses to effectors

Part 2 of 11Reflexes

Reflex

Reflex a rapid automatic response without conscious thought, which minimises damage, and awareness comes afterwards.

Examples are blinking, coughing, the pupil reflex and the knee reflex.

Spinal reflex

Spinal reflex the relay neurone is in the spinal cord, whose grey matter holds cell bodies of motor and relay neurones and whose white matter holds long myelinated axons.
Pulling a foot from a sharp object
1skin receptor2sensory neurone3relay neurone4motor neurone5leg muscle contracts
Neurones are linked by synapses where a neurotransmitter diffuses across.

Cranial reflex

Cranial reflex the relay neurone is in the brain, as in the pupil reflex.

Pupil reflex

Pupil reflex two antagonistic sets of iris muscles control the amount of light reaching the retina.
In bright light, retinal photoreceptors signal through the CNS and circular muscles contract, constricting the pupil (parasympathetic).
In dim light, radial muscles contract and the pupil dilates (sympathetic).

Part 3 of 11Resting potential and action potential

Resting potential

Resting potential the inside of the axon is about −70 mV, so the membrane is polarised.
Sodium–potassium pumps use ATP to move 3 Na⁺ out and 2 K⁺ in.
The membrane is more permeable to K⁺ than Na⁺, so K⁺ diffuses out faster than Na⁺ diffuses in, leaving the inside negative.

Action potential

Action potential a brief reversal of the membrane potential from about −70 mV to about +30 mV.

A stimulus opens a few sodium channels, Na⁺ enters and the inside becomes less negative.

At the threshold of about −55 mV voltage-gated sodium channels open and more Na⁺ enters, depolarising the membrane.

After about 1 ms the sodium channels close and voltage-gated potassium channels open, so K⁺ leaves and the membrane repolarises.

The membrane briefly becomes more negative than resting (hyperpolarisation), the refractory period, then the pumps restore resting potential.

Refractory period

Refractory period the membrane cannot respond, so action potentials are discrete and travel in only one direction.

Propagation

Na⁺ diffuses along the cytoplasm into the next section, depolarising it and opening voltage-gated channels,
so a wave of depolarisation travels along the axon.

All-or-nothing

All-or-nothing an action potential occurs only if the threshold is reached and has the same size every time.
Stimulus strength is coded by the frequency of action potentials.

Blocking impulses

Blocking impulses drugs such as painkillers and anaesthetics bind sodium channels, stopping Na⁺ entry and depolarisation.

Part 4 of 11Myelination and speed of conduction

Unmyelinated axons

Unmyelinated axons depolarisation must occur along the whole length, so conduction is slow (about 1 m s⁻¹).

Myelinated axons

Myelinated axons myelin insulates and stops ions crossing the membrane, so action potentials occur only at the nodes of Ranvier.
Local circuits of Na⁺ diffusing inside the axon depolarise the next node.
The impulse appears to jump from node to node (saltatory conduction), reaching up to about 100 m s⁻¹.

Calculations

Calculations conduction velocity = distance ÷ time and frequency = number of action potentials ÷ time.

Part 5 of 11Synapses and summation

Synapse structure
- the presynaptic neurone ends in a synaptic knob containing vesicles of neurotransmitter, separated by the synaptic cleft from the postsynaptic neurone.

An action potential depolarises the presynaptic knob and voltage-gated calcium channels open.

Ca²⁺ diffuses in and vesicles fuse with the presynaptic membrane, releasing acetylcholine by exocytosis.

The neurotransmitter diffuses across the cleft and binds receptors on the postsynaptic membrane.

Sodium channels open and Na⁺ enters; if the threshold is reached an action potential starts.

Acetylcholinesterase breaks down the neurotransmitter to stop continued stimulation.

Unidirectional

Unidirectional neurotransmitter is released on one side and receptors are on the other.

Divergence

Divergence one neurone can connect with several others.

Summation

Summation a single impulse may not reach the threshold, so effects are added together.
Spatial summationseveral presynaptic neurones converge on one postsynaptic neurone.
Temporal summationmany impulses arrive in quick succession.

Part 6 of 11Drugs at synapses

Increasing transmission

Increasing transmission more neurotransmitter made or released, mimicking the neurotransmitter, blocking its enzymatic breakdown or blocking reuptake.

Decreasing transmission

Decreasing transmission less neurotransmitter made or released, leakage from the knob, or blocking postsynaptic receptors.

Drug	Action
Nicotine	Mimics acetylcholine at nicotinic receptors, then leaves them unresponsive; stimulates dopamine release, reinforcing smoking; raises heart rate and blood pressure
Lidocaine	Blocks voltage-gated sodium channels, so no action potential forms (local anaesthetic, heart rhythm control)
Cobra venom (alpha-cobratoxin)	Binds acetylcholine receptors, blocking Na⁺ entry and causing paralysis and death by respiratory failure; small doses relax muscle
L-dopa	Converted to dopamine in the brain by dopa-decarboxylase, treating Parkinson's disease (dopamine cannot cross the blood–brain barrier)
MDMA	Blocks serotonin reuptake and triggers serotonin release, causing euphoria

Part 7 of 11Detection of light

Eye

Eye light enters through the pupil, the lens focuses it on the fovea of the retina, and the iris controls the amount.
- (shape set by ciliary muscles and suspensory ligaments)
The optic nerve leaves at the blind spot, which has no photoreceptors.

Rod cells

Rod cells mostly in the outer retina, sensitive to light intensity and giving black-and-white images.

Cone cells

Cone cells grouped in the fovea, with red-, green- and blue-sensitive types that give colour vision.

Rhodopsin

Rhodopsin the rod pigment that bleaches in light, with cis retinal converted to trans retinal and opsin separating.

In the dark, Na⁺ is pumped out but diffuses back through open cation channels, so the rod is depolarised (about −40 mV).

The depolarised rod releases an inhibitory neurotransmitter (glutamate) that stops the bipolar neurone generating an action potential.

In light, rhodopsin bleaches and the sodium channels close, but pumping continues, so the rod hyperpolarises.

The rod stops releasing inhibitory neurotransmitter, so the bipolar neurone generates an action potential that passes to the optic nerve.

Part 8 of 11Plant responses and phytochrome

Plants have no nervous system, and chemical growth factors (plant hormones) are made in growing regions and move to other tissues.

Tropism

Tropism a directional growth response, positive towards a stimulus and negative away, such as phototropism (light) and geotropism (gravity).

Phytochrome

Phytochrome a pigment existing in two forms.
P_R is inactive and absorbs red light (660 nm), converting to P_FR.
P_FR is active and absorbs far-red light (730 nm), converting back to P_R, and in the dark P_FR slowly reverts to P_R.

Germination

Germination red light makes P_FR and triggers germination, and far-red light reverses it.

Flowering

Flowering night length changes the P_R and P_FR balance.
Long-day plants flower when nights are short and P_FR is high, as P_FR acts as a transcription factor activating flowering genes.
Short-day plants flower when nights are long and P_FR is low.
P_FR enters the nucleus and binds phytochrome-interacting factor 3 (PIF3) to start transcription.

IAA (an auxin)

IAA (an auxin) it is made in growing tips and moved by diffusion, active transport and the phloem, and acts by changing gene transcription.
In shoots, higher IAA increases cell elongation by loosening cell walls.
Light moves IAA to the shaded side, so the shaded side grows faster and the shoot bends towards light.
In roots, high IAA inhibits elongation, so IAA on the lower side slows growth there and the root bends downwards.
IAA also causes apical dominance and promotes root growth at low concentration but inhibits it at high concentration.

Gibberellins in barley seeds

Gibberellins in barley seeds water uptake makes the embryo release gibberellin,
- which diffuses to the aleurone layer and increases transcription of amylase genes.
Amylase hydrolyses starch in the endosperm to maltose, which is converted to glucose for the embryo.

Other hormones

Other hormones cytokinins (cell division), abscisic acid and ethene (fruit ripening and flowering).
- (leaf loss and seed dormancy)

Part 9 of 11Practical: gibberellin and amylase in barley

Prepare gibberellic acid solutions of different concentrations.

Remove husks, cut each grain and discard the half containing the embryo, which would make its own gibberellin.

Sterilise the grain halves in 3% sodium hypochlorite for 5 minutes and wash with distilled water until no chlorine smell remains.

Soak halves in each gibberellin concentration for 12–48 hours, covered lightly.

Place halves cut side down on starch agar plates, tape the lids lightly and leave for 24–48 hours.

Flood each plate with iodine, which stains starch blue-black, and pour off the excess.

Measure the diameter of the clear zone around each grain and plot it against gibberellin concentration.

Result
- a larger clear zone means more amylase was produced.

Part 10 of 11Nervous and hormonal coordination

Coordination
- receptors detect stimuli, signals pass by nerves or hormones to coordination centres, and effectors respond.
  - (muscles or glands)

Feature	Nervous	Hormonal
Signal	Electrical impulses along neurones	Chemical hormones in the blood
Speed	Very fast	Slower
Duration	Short	Long-lasting
Target	Specific effectors	Target organs with specific receptors

Hormones
- chemicals made by ductless endocrine glands and carried in the blood.
- They bind to receptors on target cells and are used for functions not needing instant responses.
- Endocrine glands are stimulated by another hormone or by a nerve impulse.

Part 11 of 11More on this topic

Hormones

Hormones endocrine glands release hormones into the blood and only target cells with suitable receptors respond.
Hormonal effects are often more widespread and longer lasting than a single nerve impulse, and the two systems can work together.

Plant responses

Plant responses hormones such as auxin are distributed unequally, causing unequal growth on opposite sides of an organ.

Key terms

Term	Meaning
Action potential	A brief reversal of the membrane potential that travels along an axon.
Threshold potential	The membrane potential (about −55 mV) at which voltage-gated sodium channels open.
Saltatory conduction	Jumping of the action potential between nodes of Ranvier.
Synapse	A junction between neurones where neurotransmitter crosses a cleft.
Summation	Adding together of the effects of several impulses to reach the threshold.
Rhodopsin	The light-sensitive pigment in rod cells.
Phytochrome	A plant pigment that exists as P~R~ and P~FR~ and controls responses to light.
IAA	Indoleacetic acid, an auxin that controls cell elongation.

Avoid these mistakes

Saying an impulse is a current flowing like electricity in a wire — it is a reversal of membrane potential.

Saying a larger stimulus gives a larger action potential — all action potentials are the same size and frequency increases.

Saying rod cells depolarise in light — rods are depolarised in the dark and hyperpolarised in light.

Saying IAA has the same effect on roots and shoots — high IAA stimulates elongation in shoots but inhibits it in roots.

Last updated: 12 October 2026