14. Coordination and response

Exam analysis

Most tested topics

14.1 Coordination and response

Electrical impulses travel along neurones.

A neurone is a nerve cell specialised to carry these electrical signals from one part of the nervous system to another.

The signal along a neurone is an electrical impulse; neurotransmitter molecules carry the signal across a synaptic gap, not along the neurone.

The mammalian nervous system has a central nervous system and a peripheral nervous system.

Division	What it consists of
central nervous system (CNS)	brain and spinal cord
peripheral nervous system (PNS)	nerves outside the brain and spinal cord

The spinal cord belongs to the CNS; nerves entering or leaving it belong to the PNS.

The nervous system coordinates and regulates body functions.

Job	Meaning
receive information	receptors detect changes inside or outside the body
coordinate	the CNS integrates information and organises an appropriate response
regulate	impulses control effectors so body functions change when required

Coordination links information to a response; regulation adjusts body functions. The nervous system does more than detect a stimulus.

Identify a neurone in a diagram by the direction and connections of its pathway, not only by its shape.

Neurone	Direction of impulse	Diagram cue
sensory	receptor → CNS	enters the brain or spinal cord from a receptor
relay	within the CNS	links neurones inside the brain or spinal cord
motor	CNS → effector	leaves the brain or spinal cord for a muscle or gland

Sensory neurones carry impulses towards the CNS; motor neurones carry them from the CNS. A relay neurone lies within the CNS.

A simple reflex arc is the pathway followed from detection of a stimulus to the action of an effector.

Order	Component	Job
1	receptor	detects the stimulus
2	sensory neurone	carries an impulse to the CNS
3	relay neurone	passes the impulse within the CNS
4	motor neurone	carries an impulse away from the CNS
5	effector	produces the response

receptor → sensory neurone → relay neurone → motor neurone → effector

A receptor detects the stimulus; an effector carries out the response. They are not types of neurone.

A reflex action automatically and rapidly coordinates a stimulus with the response of an effector.

Feature	Meaning
automatic	occurs without conscious decision
rapid	the response follows the stimulus quickly
integrated and coordinated	nervous-system input is linked to a suitable output
effector response	a muscle contracts or a gland secretes

A reflex action is the automatic response; a reflex arc is the nervous pathway that produces it. Effectors can be muscles or glands.

A synapse is a junction between two neurones.

The two neurones do not form one continuous cell: a small synaptic gap separates them, so the signal must cross the junction.

The synapse is the whole junction between neurones; the synaptic gap is only the space within that junction.

A synapse has different structures on the sending and receiving sides of a synaptic gap.

Part	Position or contents
vesicles	in the end of the first neurone; contain neurotransmitter molecules
synaptic gap	small space between the two neurones
receptor proteins	in the membrane of the next neurone

Vesicles contain neurotransmitter molecules; receptor proteins are on the next neurone and do not fill the gap.

Transmission across a synapse converts an arriving electrical impulse into a chemical signal and then stimulates a new electrical impulse.

Order	Event
1	an impulse stimulates vesicles to release neurotransmitter molecules into the synaptic gap
2	neurotransmitter molecules diffuse across the gap
3	the molecules bind to receptor proteins on the next neurone
4	an impulse is stimulated in the next neurone

Neurotransmitter diffuses only across the gap; the impulses travel along the neurones on either side.

Synapses ensure that impulses travel in one direction only.

First neurone	Next neurone
has vesicles that release neurotransmitter	has receptor proteins that bind the neurotransmitter

Because release machinery and matching receptors are on opposite sides, the signal passes from the first neurone to the next, not backwards across that synapse.

The one-way rule refers to transmission across a synapse; it is not caused by neurotransmitter diffusing in only one physical direction.

14.2 Sense organs

A sense organ is a group of receptor cells that responds to specific stimuli.

Stimulus	Example sense organ
light	eye
sound	ear
touch or temperature	skin
chemicals	nose or tongue

A sense organ contains receptor cells; it is not itself a stimulus, and different receptors respond to particular kinds of change.

Use position and connection to identify the seven required structures in an eye section.

Structure	Identification cue
cornea	transparent curved front surface
iris	ring of tissue in front of the lens
pupil	opening in the centre of the iris
lens	transparent biconvex structure behind the pupil
retina	light-sensitive layer lining the back of the eye
optic nerve	bundle leaving the back of the eye
blind spot	point where the optic nerve leaves the retina

The pupil is an opening, not a tissue. The blind spot is on the retina at the optic-nerve exit.

Each required eye structure has a distinct job in controlling, focusing, detecting or transmitting light information.

Structure	Required function
cornea	refracts light
iris	controls how much light enters the pupil
pupil	opening through which light enters
lens	focuses light onto the retina
retina	contains light receptors, including receptors sensitive to different colours
optic nerve	carries impulses to the brain

The cornea refracts incoming light and the lens focuses it; the retina detects light, while the optic nerve carries impulses rather than light.

The pupil reflex changes pupil diameter as light intensity changes.

Light intensity	Pupil diameter	Effect
bright light	decreases; pupil constricts	less light enters
dim light	increases; pupil dilates	more light enters

The pupil is the opening that changes diameter; the iris is the tissue that causes this change.

Circular and radial muscles in the iris act antagonistically: when one set contracts, the other relaxes.

Condition	Circular muscles	Radial muscles	Pupil
bright light	contract	relax	constricts
dim light	relax	contract	dilates

Both muscle sets do not contract together. Circular contraction narrows the pupil; radial contraction widens it.

Accommodation changes lens shape so light from objects at different distances is focused on the retina.

Viewing	Ciliary muscles	Suspensory ligaments	Lens	Refraction
near object	contract	slacken; less tension	thicker, more convex	more
distant object	relax	tighten; more tension	thinner, less convex	less

Near: ciliary muscles contract → ligaments slacken → lens becomes thicker → light is refracted more. Distant: every change reverses.

Ciliary-muscle contraction reduces tension in the suspensory ligaments; it does not pull the lens flatter.

Rods and cones are unevenly distributed across the retina.

Retinal region	Rods	Cones
fovea	absent	highest concentration
peripheral retina	numerous	present in lower numbers
blind spot	absent	absent

Overall, rods are more numerous across much of the retina, while cones form a sharp concentration at the fovea.

The blind spot has neither rods nor cones; the fovea has the highest cone concentration and no rods.

Rods and cones are light receptors with different sensitivities and visual roles.

Receptor	Sensitivity and role
rods	more sensitive in low light; provide night vision
cones	three different kinds absorb light of different colours; provide colour vision

Rods are more sensitive for dim-light vision but do not provide colour vision. Colour vision depends on three kinds of cone.

The fovea is a small region of the retina with the highest concentration of cone cells.

In an eye section, locate it on the retina opposite the lens and separate from the blind spot where the optic nerve exits.

The fovea provides the sharpest, most detailed colour vision because images of objects viewed directly are focused onto its densely packed cones.

The fovea is not the blind spot: the fovea is cone-rich, while the blind spot has no light receptors.

14.3 Hormones

A hormone is a chemical substance produced by a gland, carried by the blood, and able to alter the activity of one or more specific target organs.

gland produces hormone → blood carries hormone → specific target organ changes activity

A hormone travels in the blood, but only target organs with the appropriate response are affected.

Identify each required endocrine gland by its position and match it to its named hormone.

Endocrine gland	Position cue	Hormone
adrenal glands	on top of the kidneys	adrenaline
pancreas	across the upper abdomen	insulin
testes	male reproductive organs	testosterone
ovaries	female reproductive organs	oestrogen

The adrenal glands sit above the kidneys but are separate glands; the pancreas secretes insulin, while glucagon is introduced separately in objective 14.3.5.

Adrenaline is secreted in fight-or-flight situations to prepare the body for rapid action.

Effect	Change
breathing rate	increases
heart rate	increases
pupil diameter	increases

For this objective, the required fight-or-flight effects are limited to increased breathing rate, increased heart rate and increased pupil diameter.

Nervous and hormonal control differ in speed of action and duration of effect.

Control system	Speed of action	Duration of effect
nervous	fast	short-lived
hormonal	slower	longer-lasting

A fast response is not necessarily long-lasting: nervous control is the faster but shorter-lived system.

Glucagon is secreted by the pancreas.

The pancreas is an endocrine gland that secretes both insulin and glucagon.

The pancreas secretes glucagon; the liver is a target organ in blood-glucose control but does not secrete glucagon.

Adrenaline prepares the body for increased metabolic activity by increasing blood glucose concentration and increasing heart rate.

Adrenaline effect	Immediate value
increases blood glucose concentration	makes more glucose available for respiration
increases heart rate	circulates blood and glucose more rapidly

The required metabolic-control effects here are increased blood glucose concentration and increased heart rate; detailed blood-glucose homeostasis belongs to the next Topic.

14.4 Homeostasis

Homeostasis is the maintenance of a constant internal environment.

Internal conditions are kept within narrow limits so cells and enzymes can function effectively even when external conditions change.

Homeostasis keeps conditions within limits; it does not mean that every internal value is perfectly fixed.

Insulin decreases blood glucose concentration.

It is released when blood glucose concentration is too high and helps return the concentration towards its normal level.

Insulin lowers blood glucose concentration; glucagon raises it.

A set point is the normal level around which an internal condition is regulated. Negative feedback reverses a deviation from that set point.

Stage	Job
1	a condition moves above or below its set point
2	receptors detect the change
3	a control system activates effectors
4	the response opposes the original change
5	the condition returns towards the set point

Negative feedback opposes a change; it does not amplify the deviation from the set point.

The pancreas detects blood-glucose changes and uses insulin or glucagon to control the liver by negative feedback.

Blood glucose	Pancreatic hormone	Liver response	Result
too high	more insulin	takes up glucose and converts glucose to glycogen	blood glucose falls
too low	more glucagon	converts glycogen to glucose and releases glucose	blood glucose rises

The pancreas secretes the hormones; the liver stores or releases carbohydrate in response. Insulin and glucagon have opposite effects.

Type 1 diabetes is treated by replacing missing insulin and actively managing blood glucose concentration.

Treatment action	Purpose
insulin injections or an insulin pump	supplies insulin
regular blood-glucose monitoring	guides insulin dose and detects unsafe levels
controlled carbohydrate intake and regular meals	limits large glucose changes
appropriate exercise	supports glucose control

Insulin controls Type 1 diabetes but does not cure the destroyed insulin-producing cells.

Identify skin structures by both their shape and their connections.

Structure	Identification cue
hair	shaft projecting through skin surface
hair erector muscle	sloping muscle attached to a hair follicle
sweat gland	coiled gland with a duct to the surface
receptor	sensory ending in the skin
sensory neurone	nerve fibre leading from a receptor
blood vessels	tubes and capillary networks in the skin
fatty tissue	layer of large fat cells beneath the skin

A sweat gland is a coiled secretory structure; a receptor detects change, while its sensory neurone carries impulses.

The brain coordinates responses that balance heat loss and heat production around normal body temperature.

Condition	Response	Effect
too hot	sweating	evaporation removes heat
too cold	shivering	muscle contraction releases heat
cold surroundings	hairs raised and fatty tissue insulates	trapped air and fat reduce heat loss

Temperature receptors provide information to the brain, which coordinates the appropriate effectors.

Sweating cools only when sweat evaporates; shivering generates heat through repeated muscle contractions.

Arterioles supplying skin-surface capillaries change diameter to alter heat transfer from the blood.

Condition	Arterioles	Blood near skin surface	Heat loss
too hot	vasodilate	more	increases
too cold	vasoconstrict	less	decreases

Vasodilation and vasoconstriction describe changes in arteriole diameter, not capillaries moving closer to or farther from the skin.

14.5 Tropic responses

Gravitropism is a growth response in which part of a plant grows towards or away from gravity.

Response	Direction	Typical example
positive gravitropism	towards gravity	roots grow downwards
negative gravitropism	away from gravity	shoots grow upwards

Gravitropism is directional growth in response to gravity, not simply any downward or upward movement.

Phototropism is a growth response in which part of a plant grows towards or away from the direction of a light source.

Response	Direction	Typical example
positive phototropism	towards light	shoots grow towards a one-sided light source
negative phototropism	away from light	roots may grow away from light

Phototropism is directional growth caused by the direction of light; photosynthesis uses light but is not a tropic response.

A tropism investigation changes one directional stimulus and records the direction of root and shoot growth.

Organ	Gravity response	Light response
root	grows towards gravity	commonly grows away from light
shoot	grows away from gravity	grows towards light

Design feature	Purpose
germinating seedlings of similar age	comparable starting material
one-sided light or darkness	isolate light direction
horizontal placement or clinostat control	test or cancel a constant gravity direction
keep water, temperature and time constant	fair comparison
record starting and final root/shoot directions	describe the growth response

A clinostat rotates seedlings so gravity does not act continuously on one side; it is a control for directional gravity, not a source of light.

Phototropism and gravitropism of shoots are chemical growth responses controlled by auxin.

directional stimulus → unequal auxin distribution → unequal cell elongation on opposite sides → shoot curves

In a shoot, the side with more auxin elongates more, so the unequal growth produces curvature towards light or away from gravity.

The shoot bends because its two sides elongate at different rates, not because mature cells move from one side to the other.

Auxin is made in the shoot tip and controls curvature by changing cell elongation below the tip.

Order	Event
1	auxin is made in the shoot tip
2	auxin diffuses through the plant away from the tip
3	light or gravity causes auxin to become unequally distributed
4	auxin stimulates cell elongation in the shoot
5	cells on the side with more auxin elongate more, so the shoot bends

With light from one side, more auxin accumulates on the shaded side; greater elongation there bends the shoot towards the light.

Auxin is produced in the shoot tip but acts on elongating cells below it; it stimulates cell elongation rather than cell movement.